ENGIN. LIBRARY UC-NRLF DlfcEC W$M fit :$p liXNJ ':: MS JS8 if 5 Mi M LABORATORY MANUAL Direct and Alternating Current LABORATORY MANUAL DIRECT AND ALTERNATING CURRENT Prepared to accompany Timbie's Elements of Electricity By CLARENCE E. CLEWELL \\ Sheffield Scientific School of Yale University SECOND EDITION NEW HAVEN, CONN. 1913 COPYRIGHT, 1913, BY CLARENCE E. CLEWELL T A- i 'I ' Engineering Library PRESS OF THE TIMES PUBLISHING COMPANY BETHLEHEM, PA. PREFACE This book of laboratory directions is designed for the use of students in courses other than that of Electrical Engineering, who take a brief amount of work in the fundamental principles of electricity. The object of the laboratory work in such cases is to aid in the understanding of the theoretical and practical items given in the recitation and lecture room. It is the purpose, therefore, to elimi- nate in as far as possible all features in the actual work of the laboratory which would tend to lessen or detract from concen- tration upon the underlying principles involved in the experi- ment in hand. To facilitate the work, each experiment is prefaced where possi- ble by an assignment of articles in the text book which bear di- rectly or indirectly on the experiment in question. Further, in a part of the experiments, diagrams of the electrical connections will be found, together with simple forms which may be followed in the recording of observations. These forms are somewhat more complete in the first than in the advanced experiments in order to serve at the outset as a guide in the preparation of the data sheets. In the latter experiments this general scheme of rul- ing up the data sheet is to be followed by each group of men in the preparation of data sheets before the laboratory exercise. After teaching with a text book which was too advanced for the students and a laboratory manual which was not suited to the text book, the students nor the equipment, it was thought best to write a Manual which should be adapted to the specific require- ments of the coming year. The difficulties encountered by the student have been observed and the aim of the Manual is to pre- sent the subject from his standpoint. The emphasis placed on such practical items as constant potential supply mains in the wiring diagrams, and the practical points connected with direct and alternating current generators and motors and power trans- 271653 vi PREFACE mission in the text, are intended to familiarize the student with the principles underlying the operation of standard apparatus which he may encounter after graduation. Appreciation for many suggestions and helpful advice in the make-up of this book is due Professor Chas. F. Scott, Sheffield Scientific School of Yale University. CLARENCE E. CLEWELL. NEW HAVEN, CONN., July, 1913. CONTENTS PAeed (that is, the regulation) are the resistance of the armature wind- ing together with slight magnetic reactions in the armature which tend to modify the effective magnetic field produced by the field winding. The percentage regulation of the speed is defined as the differ- ence between full load and no-load speed divided by the full load speed, supply voltage and field current remaining un- changed. ( Obviously this result must be multiplied by 100 to ex- 50 LABORATORY MANUAL press it as a percentage.) Thus, if the full load and no-load speeds are 1,500 and 1,650 revolutions per minute respectively, the regulation is equal to 10 per cent. If the speed at full load falls off more than indicated in this case, the numerical value of the regulation will be greater and, hence, will indicate a certain inferiority in the construction of the machine. Current Supply. 110 or 220 volts Direct Current. Apparatus Required. (1) Shunt motor; (2) starting box; (3) field rheostat; (4) 'armature rheostat ; (5) speed indicator; (6) brake to be used for loading the motor; (7) ammeter for the field circuit; (8) ammeter for the armature circuit; and (9) volt- meter. No. Position of Field Rheostat Speed Armature Amperes Position of Armature Rheostat Speed Armature Amperes Armature Amperes Speed 1 1 (All Out) (For No Load) (Out) (Zero Load) (Zero Load) 2 2 1 (In) (Quarter Load) 3 3 (Out) (Full Load) (Half Load) 4 4 (In) - Form 10. Order of Work. 1. Connect the motor to the supply mains as shown in Fig. 15. With the field rheostat all cut out, start the motor by means of the starting box and cut out all the armature rheostat resistance. With the motor unloaded, observe and re- cord the speed for this and for 4 other positions of the field rheostat, gradually increasing its resistance until the motor speed is considerably above normal. Use Form 10. 2. With the motor unloaded, adjust the field rheostat for nor- mal speed, and observe the speed for no resistance and for a fairly high resistance inserted in the armature circuit, leaving the field rheostat untouched after the initial adjustment. 3. Same as 2 except that the motor is to be loaded by the brake until the armature current is a fair proportion of the full rated current. (Note : To be most instructive the resistance inserted in the armature circuit for the second observation in items 2 and 3 should be the same value in each case.) DIRECT CURRENT 51 4. Adjust the motor for normal speed at no load by the field rheostat after reducing the armature rheostat resistance to zero, and observe and record the speed for zero, %, %, % and full load armature currents in turn (assume full load armature cur- rent as equal to the current rating on the name plate of the ma- chine). Leave the field rheostat untouched throughout these observations after the initial adjustment. Written Report. 1. In item 1, Order of Work, why does the speed increase as the field rheostat resistance is increased ? "What is the range of speed control by means of the field rheostat method as observed ? 2. In items 2 and 3, Order of Work, why is the speed affected more by the armature rheostat when the motor is loaded than when unloaded? 3. From the observations in item 4, Order of Work, plot a curve using speed as ordinates and armature current as abscissas. 4. Calculate the percentage speed regulation of the motor. 5. The torque of a motor depends on the field magnetism ef- fective in the armature and on the armature current. When the field magnetism was weakened in item 1, Order of Work, the motor speeded up. Should not a weakened field cause less torque and, hence, less rather than more speed? Explain. (See Ar- ticle 135 in the text book.) 6. To what is the decrease in speed with increased load due as observed in item 4, Order of Work? EXPERIMENT 15. Efficiency of a Shunt Motor by the Brake Method. See Article 158 in the text book. The object of this experiment is to determine the efficiency (output divided by input) of a shunt motor by measuring the mechanical output and the electrical input at various loads. Theory. The efficiency of the electric motor like that of other machines is defined as the ratio of output to input. In a deter- mination of the efficiency, therefore, it is necessary to measure the mechanical output as indicated by a brake attached to the 52 LABORATORY MANUAL pulley, and for each value of output thus observed to measure the electrical input. In the actual calculation of the efficiency, the mechanical out- put in horse-power may readily be transformed by multiplying the horse-power by 746, the number of watts in one horse-power, and the output is thus expressed in the same units as the input. Since the losses in a motor are partly constant (or nearly con- stant) and partly variable, and since the variable losses (RP) in the armature winding vary as the square of the current, it will be obvious first, that the constant losses play a much larger part for low loads, thus reducing the efficiency at low loads as com- pared to full load, and second, that the RP loss in the armature, increasing as the square of the current, becomes quite large in proportion for high current values and, hence, the efficiency tends to fall off after a certain maximum value has been reached near full load. Current Supply. 110 or 220 volts Direct Current. Apparatus Required. (1) Shunt motor; (2) starting box; (3) field rheostat; (4) speed indicator; (5) brake to be used for load- ing the motor; (6) ammeter; and (7) voltmeter. Order of Work. 1. Connect the motor to the supply mains in the usual manner, arranging the ammeter to measure the entire current input to both field and armature. 2. Start the motor and bring it to normal speed at no load, that is, with the brake detached completely from the pulley. Ob- serve and record the input (volts and amperes). 3. Attach the brake and tighten until the ammeter indicates % the full rated current (see name plate on the machine). Ob- serve and record the torque exerted at the pulley and the volts and amperes input. 4. Same as 3, for y 2 , %, full load, and 1*4 and, if practicable, P/2 of full load current in turn. Written Report. 1. Calculate the efficiency of the motor in per cent, from the observations in items 2, 3 and 4, Order of Work. 2. Plot a curve, using the efficiency as ordinates, and the input current as abscissas. DIRECT CURRENT 53 3. Explain briefly the general shape of this curve, that is, why it follows the form taken. 4. Name the constant and variable losses in a generator or motor. (See Article 155 in the text book.) 5. Why should the constant losses in a motor cause the effici- ency to be low at low loads ? 6. Why does the RP loss in the armature cause a reduction of the efficiency after a certain maximum value near full load ? 7. How could the armature current be found mathematically for a given machine at which the efficiency is a maximum ? EXPERIMENT 16. Series Motor Speed Features. See Articles 140, 141, 142, 143, 144 and 145 in the text book. The object of this experiment is to make a study of the speed of a series motor as affected by the load. Theory. The torque of a series motor is proportional to the field magnetism and to the armature current. Since the field winding and the armature are connected in series and, hence, the same current flows through each, the torque is roughly pro- portional to the square of the armature current. Under a light load, the series motor takes but little current to produce the torque required and, hence, the resistance in series with the motor on starting is made large enough to allow only a small current to pass through the motor. The motor under these conditions speeds up rapidly and the greater the speed the more counter electromotive force induced and, hence, the less the current through the machine until the resistance is cut out. As shown with the shunt motor (See Experiment 14), to weaken the field increases the speed and, hence, the speed of an unloaded series motor becomes excessive and would damage the machine if allowed to run under this condition. The series motor must, therefore, alw r ays be connected to its load, as in street cars where the motor is geared or mechanically connected to the load. As a precaution, therefore, always see that a load is connected or coupled to the series motor before connecting it to the supply mains. 54 LABORATORY MANUAL Under load, the operation of the series motor is somewhat dif- ferent from that of the shunt motor. For example, if the load on a series motor be doubled, the field current as well as the ar- mature current is increased, so that the speed is reduced much more than in the shunt motor where the field current is practi- cally constant at all loads. Again, if the armature current be doubled in a series motor, the torque is increased nearly four Supply Mains (110 Volts D. C.) A -M f I Ammeter Pig. 16. Series motor. The load, which must be connected to the motor throughout the experiment, is not shown in this diagram. The' adjustable resistance represents the starting controller used with series motors. times, since both armature and field current are doubled, while to double the armature current in the shunt motor merely doubles the torque (the shunt field current remaining constant). Hence, under heavy loads, due to the fact that the large start- ing armature current flows through the series field in the series motor, it has a greater starting torque than the shunt motor where the field current remains sensibly constant irrespective of the value of the armature current. The series motor is, therefore, well adapted to those cases where a large starting torque is desirable and where, under heavy loads, DIRECT CURRENT 55 the speed should fall in order that the power requirements and, hence, the current may not be excessive as in street car opera- tion. Where constant speed at all loads is a necessary require- ment, the series motor is not adapted, as its speed variation be- tween small and heavy loads is very large as compared with that of the shunt motor. Note that a given value of current through the series motor produces the same torque whatever the speed. Current Supply. 110 or 220 volts Direct Current. Apparatus Required. (1) Series motor; (2) starting resist- ance, (some form of rheostat having a fairly large current carry- Lever Arm = No. Force at End of Brake Lever Amperes Volts Speed Shunt 1 Without Shunt Around Series Winding f> 3 4 5 6 "5 G ^ "7) Form 11. ing capacity); (3) speed indicator; (4) brake to be used for loading the motor ; (5) ammeter; and (6) voltmeter. Order of Work. 1. Connect the motor to the supply mains as shown in Fig. 16, and arrange to have the brake permanently at- tached to the motor pulley throughout the experiment. 2. With the starting resistance all in, and the brake mode- rately tight, throw in the main switch and gradually increase the speed by cutting out the starting resistance. Tighten the brake until the current input equals % more than the current rating on the name plate of the machine. Observe and record the torque (the tangential force at the rim of the pulley multiplied by the radius of the pulley), current, volts at terminals of motor, and speed. Use Form 11. 56 LABORATORY MANUAL 3. Reduce the load until the input current equals the full load rating of the machine and repeat the observations of item 2. 4. Same for %, %, and 14 load current values in turn. 5. Place a low resistance around the series field terminals as a shunt. Operate the machine at its full load current value and observe the torque, current, volts and speed. Written Report. 1. Explain briefly why the load must be per- manently connected to a series motor. 2. Plot a curve using speed as ordinates and torque as ab- scissas from the observations of items 2, 3 and 4, Order of Work. 3. Explain the decrease in speed with increasing load as shown by this curve. 4. What changes were observed in the speed and torque in item 5 as compared with the corresponding observations in item 3, Order of Work? Explain. 5. A street car equipped with series motors, running at con- stant speed on the level, approaches an up grade. Explain the action of the motors in propelling the car up the grade, as re- gards speed, torque and current in-take, assuming that the motor- man leaves the controller untouched. 6. Sometimes on climbing a steep grade a motorman throws the controller to the series notch. What is the "series notch", and why should this be an advantage under the circumstances ? EXPERIMENT 17. Efficiency ; Stray Power Test ; Brake Test. See Articles 155, 156 and 158 in the text book, also the Theory under Experiment 15 in the Manual. Theory. The losses in a generator or motor may be classed under the head either of losses which may readily be calculated, or of losses which are not subject to calculation. Thus the resist- ance losses (RP) may easily be calculated after measuring the resistance of field and armature, and from the currents involved. The friction losses in the bearings and at the brushes of the ma- chine and in windage and the losses in the iron of the machine (usually called hysteresis and eddy current losses) cannot easily be calculated and are referred to as stray power loss. That is, DIRECT CURRENT 57 the stray power loss includes all the losses in a generator or motor except the RP or resistance losses, and is practically constant independent of the load. A simple experiment to determine the stray power loss in a generator or motor is to drive the machine as an unloaded motor and to measure the power input under this condition. Obviously this input is all loss, since there is no useful power being deliv- ered at the pulley. If, from this input at no load, the RP losses in both field and armature be subtracted, the remainder represents the stray power loss in watts. Inasmuch as the stray power loss is sensibly constant at all loads, the efficiency of a generator, for example, may be calcu- lated when the stray power loss is known for any assumed load by the use of the equation for effiicency : Efficiency = Out P"L_ Output -f alJ losses Output Output -j- stray power loss -{- RI 2 losses Suppose, for example, it was desired to calculate the efficiency at half load. If the generator is rated at 10 kilowatts, and the stray power loss is found by experiment to be 500 watts, the ef- ficiency may be calculated by a substitution in the equation as follows : . 5000 5000 + 500 + RP loss in field and armature The RP loss in the field and armature are easily calculated from the resistance of the two windings, the terminal voltage and the armature current corresponding to the assumed load. Thus the RJ 2 loss in the field is equal to R l X(E/R l ) 2 , and the RP loss in the armature is the resistance of the winding (R 2 ) multiplied by the square of half the full load current as indicated on the name plate of the machine. (Note : While the stray power loss varies slightly for different loads and speeds, it is treated as constant in this experiment for simplicity. ) Current Supply. 110 or 220 volts Direct Current. Apparatus Required. (1) Shunt machine; (2) starting box; (3) field rheostat ; (4) speed indicator ; (5) ammeter for the field 58 LABORATORY MANUAL circuit; (6) ammeter for the armature circuit; and (7) volt- meter. Order of Work. 1. Connect the machine to the supply mains through the starting box, arranging an ammeter in both field and armature circuits. 2. Run the machine as a motor at normal speed and, with no load, observe and record the amperes to both field and armature and the volts at the motor terminals. (Note : Since the starting current of an unloaded motor is apt to be larger than its nor- mal running current after starting, be sure to close the short cir- cuiting switch about the armature ammeter before starting the motor. This makes possible the use of an instrument of low range for observing the rather small currents of the motor while in operation at no load.) 3. Record the full rated current and voltage of the machine as indicated on the name plate. 4. Shut down the machine and measure the resistance of the armature by the voltmeter-ammeter method as described in Ex- periment 2. 5. Arrange to load the machine by a brake, and observe and record the torque, speed, current in field and armature, and ter- minal volts at % % an d full rated load in turn. Written Report. 1. From the observations in items 2 and 4, Order of Work, calculate the stray power loss, and the field and armature resistance of the machine used. 2. From item 5, Order of Work, calculate the output and in- put to the motor in watts, and calculate the efficiency of the motor by the brake method for these observations. 3. Calculate the efficiency of the motor for 14, l /2, % and full load in turn by the stray power method, using the stray power loss as found in item 1, Written Report, and using the terminal volts and input current at full load as given on the name plate of the machine. Compare these calculations of efficiency with those found by direct measurement in item 2, Written Report. 4. Calculate the efficiency of the machine by the stray power method if run as a generator at full load, assuming the terminal volts and output current at full load to be the value given on the name plate of the machine. DIRECT CURRENT 59 EXPERIMENT 18. Static Torque Test on a Motor. See Article 125 in the text book, also the Theory under Ex- periment 4 in the Manual. The object of this experiment is to make a study of the torque of a motor in terms of the field and armature current, while the machine is at rest. Supply Mains (110 Volts D. C.) -4 A 1 -S f T A Uii t i Fig. 17. Study of the torque produced in an armature for various values of field and armature currents. Note that the currents in field and armature can be adjusted independently. Theory. The mechanical force which turns an electric motor is produced by the action of the magnetic field on the current in the armature conductors. The simplicity of the elements which produce motion in the motor are sometimes lost sight of on ac- count of the conditions which determine the armature current, such as load, speed and the like. In this experiment, these sec- ondary conditions are eliminated by taking the observations on the motor when at rest, and the definite relation of field and armature current to the torque produced is thus emphasized. The magnetic field is not directly proportional to the current which produces it, because as the field magnets become saturated, the magnetism ceases to increase in direct proportion to the cur- 60 LABORATORY MANUAL rent (see Article 98 in the text book). Hence, in this experi- ment, if the torque is not found to vary directly with the field current throughout the observations, it must be remembered that the torque is varying directly with the magnetic field, but the field is not varying directly with the field current due to satura- tion of the iron. Obviously the saturation effect will not be very noticeable for small values of the field current. Current Supply. 110 or 220 volts Direct Current. Apparatus Required. (1) Shunt motor; (2) resistances for both field and armature circuits ; (3) Prony brake; (4) ammeter for field circuit; (5) ammeter for armature circuit; and (6) voltmeter. Order of Work. 1. Connect the field winding through a field rheostat, which should possess a wide range of adjustment, to the supply mains ; also, the armature through a suitable rheostat to the supply mains; each to have its own switch as shown in Fig. 17. 2. Clamp the brake tightly to the pulley of the motor and with all the field and armature resistance cut in, throw in first the field arid then the armature switch. See that the torque acts against the opposition of the brake, and that the armature does not rotate. 3. With a constant field current, that is, with normal volts across the field terminals, adjust the armature current to % full load value and observe and record the field and armature cur- rent and the torque produced. 4. Same as 3, using %, %, full load and l 1 /^ load currents through the armature in turn. 5. With, say, full load armature current, reduce the field cur- rent to % its normal value, and observe and record the field and armature currents and the torque produced. 6. Keeping the armature current constant, repeat the obser- vations of item 5 for %, normal and l 1 ^ normal values of the field current. 7. Maintaining constant field current and constant torque, de- crease the resistance in series with the armature and allow the motor to run. Find and record the relation between armature current and torque for a number of different speeds; also the relation between the speed and the volts across the armature terminals for each of these values of speed. DIRECT CURRENT 61 Written Report. 1. From the observations in items 3 and 4, Order of Work, how does the torque (or mechanical force) at the pulley vary with the armature current for a constant value of field current f 2. From items 5 and 6, Order of Work, how does the torque vary with the field current for a constant value of armature cur- rent? 3. If the torque did not vary directly with the field current in the experiment, to what is such irregularity due ? 4. From the general observations of this experiment, explain why a shunt motor must slow down when an added load is thrown on its pulley if it is to carry this added load ? 5. From item 7, Order of Work, what is the relation between armature current and torque at different speeds; and what is the relation between speed and volts across the armature in each of these cases ? Explain briefly. EXPERIMENT 19. Shunt Generators in Parallel. The object of this experiment is to observe the factors which enter into the operation of shunt generators in parallel, first, as regards the necessary conditions for throwing one generator in parallel with another machine, and second, as to the items which are involved in the equal or proportionate sharing of the total output of a power station by the various generators connected in parallel for supplying this total output. Theory. In many electric stations it is the practice to supply power from bus bars to which are connected a number of genera- tors, each delivering its share of the total load supplied from the common bus bars. In this way, when the load requirements are low, say during the day in a lighting station, a few of the gen- erators may be operated at or near full load and, hence, at high efficiency, and as the total output of the station increases, one after another of the remaining machines may be connected to the bus bars in parallel with those already in operation. Obviously the positive terminal of each machine must be connected to the positive terminal of the bus bars. 62 LABORATORY MANUAL If before connecting a machine to the bus bars its voltage is just equal to that of the bus bars, no current will flow. If the voltage induced in the armature be slightly higher, a current will flow. Suppose the machine, when carrying no load, has a voltage 3 per cent, higher than the -bus bars and that before it is con- nected to the bus bars it is loaded until the terminal electromo- tive force decreases (due principally to the RI drop in the arma- ture) and becomes equal to that on the bus bars. If now the load be thrown off quickly and the machine be connected to -the bus bars, it is in condition to continue delivering the same current to the bus bars that it formerly delivered to its independent load. Hence, if the bus bar voltage be much below that of the volt- age induced in the armature connected to it, the current supplied by the armature will rise when the two are connected, until the volts (RI) drop in its armature winding and the leads from the armature terminals to the bus bars equals the difference between the bus bar and the induced armature voltage. If this difference be zero, no current will flow from the machine; while if the dif- ference be such that the accompanying RI drop in the leads and armature involves a current greater than normal for the machine, the generator will, of course, be overloaded. As the induced voltage of a generator depends on the field magnet strength for constant speed conditions, the load may be increased or decreased on a given machine connected in parallel with others, by the simple variation of its field rheostat resist- ance, assuming that the driving engine delivers a corresponding increased or decreased load. Where two similar machines of the same capacity are arranged for parallel operation, the output from each should equal one- half of the total power supplied by the bus bars. If it should be desirable, however, to reduce the load on one of the machines and yet maintain the total output constant at a constant bus bar voltage, it would be necessary to increase the field resistance of the one generator to lower its part of the total load, and to reduce the field resistance of the other generator to increase its part of the total load, thus maintaining the voltage and the total output at a constant value. In this way the total load may be shifted from one machine to another. Since the terminal voltage of a shunt generator varies with the load (see Experiment 9), and further, since this change of volt- DIRECT CURRENT age with load is not apt to be exactly the same with any two ma- chines, even after two or more shunt generators are adjusted to give their share of the total load, they may not continue to share the total load in this exact proportion for all bus bar or total loads, on account of this variation in the voltage changes for dif- ferent machines. This will give rise to slight fluctuations in the sharing of the loads, which, if sufficiently noticeable, can be off- set by hand regulation of the field rheostats when necessary. Voltmeter Flexible Leads J ri <><)<)(j 3 j Lamp Bank Used as Load 1 Bus Bars L 1 } L A b : i * Ammeter Fig. 18. Study of the parallel operation of shunt generators. A voltmeter, not shown in the diagram, is to be available for measuring the voltage of the individual machines. Current Supply. From the Shunt Generators assigned. Apparatus Required. (1) Two shunt generators; (2) lamp banks to be used as a common load supplied from bus bars; (3) field rheostats for each machine; (4) a double-pole single-throw switch for each machine and for the total load (3 in all) ; (5) three ammeters, one for each machine and one for the total out- put current from the bus bars; (6) two voltmeters, one for the bus bars and one for the on-coming machine. 64 LABORATORY MANUAL Order of Work. 1. Arrange the connections of the two assigned shunt generators as shown in Fig. 18. 2. With all switches open, start up the two generators and ad- just the voltage of each to its normal value. 3. Connect one of the generators ("A") to the bus bars (two lengths of wire) and turn on enough lamps to load the machine to its full capacity. 4. Adjust the voltage of the other generator ("B") to the same value as the bus bar voltage and connect it to the bus bars, being sure that the positive terminal of the machine is connected to the positive terminal of the bus bars. 5. Vary the field rheostat of machine "B" until the current in the two machines has the same value, at the same time adjust- ing the field rheostat of machine "A" so that the bus bar voltage remains constant. Then turn on enough lamps to load each of the machines to its full rated capacity. 6. With both machines fully loaded, observe and record the bus bar voltage, current delivered by each machine and total current taken from the bus bars. 7. Leaving the field rheostats untouched, repeat the observa- tions of item 6 for %, % and % of the total load current and for zero current from the bus bars in turn. 8. Turn on the lamps and vary the field rheostats and lamps until each machine delivers one-half of its rated load. Adjust the field rheostat of the two machines until the machine "B" is delivering all the current and machine "A" is unloaded, main- taining constant voltage at the bus bars throughout this adjust- ment. Now disconnect machine " A " from the bus bars. Written Report. 1. What would be the result if the negative terminal of machine "B" was connected by mistake to the posi- tive terminal of the bus bars in item 4, Order of Work ? 2. From the observations in items 6 and 7, in which of the two machines is the armature voltage reduced the more as the total load is decreased? 3. In item 8, why must all the load be shifted to machine " B " before machine "A" is disconnected from the bus bars? 4. What would have been the result if in item 4, Order of Work, machine "B" had been connected to the bus bars when DIRECT CURRENT 65 its induced voltage was much above that of the bus bars? If it was lower ? 5. Explain any inequalities observed in the sharing of the total bus bar load by the two machines as the total output was reduced in item 7. If any inequalities existed, to what were they due? Explain. EXPERIMENT 20. Compound Generators in Parallel. See Article 122 in the text book, also the Theory under Experi- ment 19 in the Manual. The object of this experiment is to observe the factors involved in the parallel operation of compound generators, first, in regard to the conditions necessary before throwing one generator in parallel with another machine, and second, in regard to the items which influence the equal or proportionate sharing of the total output of a power station by the various generators connected in parallel for supplying this total station output. Theory. As stated in experiment 19, the general practice in electric stations is to supply power from bus bars to which are connected a number of generators in parallel with each other, each delivering its share of the toal load supplied from the com- mon bus bars. In this way it is possible to operate the machines at a relatively high efficiency even when the station output is low, by disconnecting certain machines and thus keeping the remain- ing machines in operation at or near full load. In practice the generators thus used are compound wound machines. As in the parallel operation of shunt generators, where the cur- rent delivered by a given machine depends on the difference be- tween its induced voltage and the bus bar voltage, so in the parallel operation of compound generators the difference between the induced voltage in a given machine and the bus bar voltage determines the amount of current supplied by the machine. The induced voltage in a shunt generator depends primarily on the field strength and on the speed of the machine. In the compound generator, however, the induced voltage depends not only on the field strength produced by the shunt winding and on the speed, 6 66 LABORATORY MANUAL but also on the additional field produced by the series winding. Hence, the equal or proportionate sharing of the load with com- pound generators depends on an additional factor, namely, the strength of the field magnetism produced by the series field cur- rent. If two compound generators are connected in parallel (in gen- eral as shown in Fig. 19, except that a series winding is inserted between the upper armature terminal and the machine ammeter in each case), each machine may be made to deliver its share of a given total bus bar load by adjusting its shunt field rheostat. If the machines are over-compounded, however, and one machine speeds up due to an increase in the speed of its driving engine, this means an increase in the induced voltage (due to the in- crease in speed). The output of this machine is then increased (meaning, of course, that the output of the second machine falls off by a corresponding amount) and the action of the series wind- ing is to increase still further the induced voltage in the first machine. This condition being cumulative, results in an ex- cessive overload for the first machine and obviously in an Unbal- anced condition of operation. (This effect of instability is more noticeable with over-compounded than with flat or under-com- pounded generators.) To prevent this unstable condition an equalizer connection is made between the two machines, at the junction of the series winding and the armature terminal in each case (for the short shunt connection), and this places the two series windings in parallel with each other as regards the output current from the machines, thus insuring that the series field current will be in- versely proportional to the resistance of the windings at all times, irrespective of any tendency for the armature currents to be unequal. If the compounding of the machines is different, thus causing an unequal sharing of a given total bus bar load, the resistance of the series field circuit must be changed by connecting an auxili- ary resistance in series with the series winding, thus reducing the part of the total current which flows through the series winding of the first machine and allowing more current to flow through the series winding of the second machine. Adjustments of the compounding by means of a shunt around the series field, as explained in Experiment 11, will not serve the DIRECT CURRENT 67 purpose in this case on account of the parallel condition of the two series windings. (Note: The student should make a dia- gram, similar to Fig. 19, on the data sheet with the series fields arranged for the short shunt connection, and verify, theoretically, the instability of operation without an equalizer connection. Al- so, with an equalizer connection on the diagram, the statements in the two preceding paragraphs should be verified before the experiment is undertaken. ) Current Supply. From the Compound Generators assigned. Apparatus Required. (1) Two compound generators (prefer- ably over-compounded) ; (2) lamp banks to be used as a common load supplied from the bus bars; (3) field rheostats for each ma- chine; (4) a double-pole single-throw switch for each machine and for the total load (3 in all) ; (5) ammeters, one for each ma- chine, one for the total output current from the bus bars, and one to be connected in the equalizer circuit between the two machines (the latter instrument preferably a double-throw ammeter to in- dicate the current no matter in which direction it flows) ; (6) two voltmeters, one for the bus bars and one for the on-coming machine; (7) equalizer connection (a wire to be connected be- tween the two machines at the junction of the series field and the armature in each case, where the short shunt connection is used ) . Order of Work. 1. Arrange the connections of the two assigned compound generators similar to Fig. 19, except that the series field is to be connected between the upper armature terminal and the ammeter in each case, and the equalizer is to be connected between the upper armature terminals of the two machines through an ammeter. 2. With all switches open, start the two generators and adjust the voltage of each to its normal value. 3. Connect one of the generators ("A") to the bus bars (two lengths of wire) and turn on enough lamps to load the machine to its full capacity. 4. Adjust the voltage of the other generator ("B") to the same value as the bus bar voltage (or a trifle lower) and connect it to the bus bars, being sure that the positive terminal of the machine is connected to the positive terminal of the bus bars. (Remember that as soon as machine "B" is thrown on to the bus bars, a current will flow through its series field from machine 68 LABORATORY MANUAL 11 A" which will tend to increase the induced voltage in the arma- ture of "B". This is the reason for adjusting the voltage of "B" a trifle lower than the bus bar voltage before connecting the two. Watch the ammeter of machine "B" carefully until the ad- justments of load have been made according to the following item.) 5. Vary the shunt field rheostat of machine "B" until the cur- rent of the two machines is the same in value, at the same time adjusting the shunt field rheostat of machine "A" so that the bus bar voltage remains constant. Then turn on enough lamps to load each machine to its full rated capacity. 6. With both machines fully loaded, observe and record the bus bar voltage, current delivered by each machine, total current taken from the bus bars, and the equalizer current (noting in which direction the equalizer current flows, that is, whether from machine "A" to "B", or from "B" to "A".) 7. With the field rheostats untouched, repeat the observations called for in item 6, for %, %, and a /4 of the total load, and for zero current from the bus bars in turn. 8. Turn on the lamps and vary the field rheostats and lamps until each machine delivers one-half of its rated load. Adjust the field rheostat of the two machines until machine "B" is de- livering all the current and machine "A" is unloaded, maintain- ing constant voltage at the bus bars throughout the adjustment. Now disconnect machine "A" from the bus bars. 9. Same as items 6 and 7, with a resistance inserted in one of the series field circuits, so that when the load from the bus bars equals the total rated capacity of the two machines combined, machine "A" is delivering l 1 /^ of its rated capacity and machine "B" % of its rated capacity. Written Report. 1. Explain why the operation of compound generators in parallel is unstable without an equalizer connec- tion. 2. Why is this unstable condition less noticeable for flat and under-compounded than for over-compounded machines ? 3. What current was indicated by the equalizer circuit am- meter in items 5, 6 and 7, Order of Work? If any, to what was it due? DIRECT CURRENT 69 4. Explain any inequalities observed in the sharing of the total bus bar load by the two machines, as the total output was reduced in items 6, 7 and 9. If inequalities existed, to what were they due? Explain. 5. Explain the use of resistance in series with one of the series field windings in item 9. How did it affect the proportionate sharing of the total output by the two machines as the total out- put was reduced ? 6. Why cannot a shunt around the series field be used for the purpose of adjusting the sharing of the total output in item 9 in- stead of a resistance in series with the series field winding ? What is the function of the resistance as here used? Explain. Experiments 21 to 30, inclusive, constitute the Alternating Current portion of the Manual. ALTERNATING CURRENT EXPERIMENT 21. Resistance and Reactance in Series. See Articles 261, 269, 270, 271, 272, 277, 278 and 279 in the text book. The object of this experiment is to make a study of the volt- age, current and power relations in a simple alternating current circuit containing both resistance and reactance in series. Theory. Ohm's law states that in a circuit, or portion of a complete circuit, where all the voltage goes to overcome resistance only, the current (7) equals E/R, where E is the electromotive force across the terminals of the circuit and R is the resistance of the circuit in ohms. This law holds true for both direct and al- ternating current circuits as regards that portion of E which overcomes resistance (R) only. In a direct current motor, the voltage (E) across the termi- nals of the armature is partly used to overcome resistance and partly to overcome the counter electromotive force induced in the armature by the rotation of the armature wires in the magnetic field, that is, E^ (the impressed electromotive force) =RI-\-E 2 (the counter electromotive force of the armature). Note that the RI drop and E 2 are added numerically. In an alternating current circuit consisting of a coil of wire, the voltage (E r ) across the terminals of the coil is partly used to overcome resistance and partly to overcome the counter electro- motive force induced in the coil by the rapid reversals of the mag- netic field in the coil due to the alternating current, that is, E^ (the impressed electromotive force) =RI added vectorially to E 2 (the counter electromotive force in the coil). Note that the RI drop and E 2 are added vectorially (not numerically) because they are 90 apart in phase. E 2 , the counter electromotive force, is usually expressed as XI, that is, the reactance (X) of the cir- 71 72 LABOR A TORY MANUAL cult times the current (7), X being the opposition due to the in- ductance of the coil, but for convenience expressed in ohms like the resistance R. (Read carefully Articles 165 and 171 in the text book. ) Since the reactance of a circuit depends on the frequency of the alternating electromotive force across its terminals, the oppo- sition due to reactance is high for high frequencies and low for low frequencies in a given circuit. Obviously the frequency has nothing whatever to do with the RI drop in a circuit. Hence, if an alternating current of 10 amperes flows in a circuit contain- ing both R and X when 110 volts at 60 cycles are applied at its terminals, if the frequency is reduced to 30 cycles, and the cur- rent maintained at 10 amperes by reducing the electromotive force, the RI component of E will, of course, remain the same, while the XI component will be i/2 as great as before because X is % its former value. A coil containing both resistance and re- actance produces the same result as a separate resistance and a separate reactance connected in series. Since the voltage required to overcome R and X is made up of two components 90 apart in phase, the combined effect of the two (R and X) may be expressed as Z, which equals the square root of (R 2 -{-X 2 ). Z (expressed in ohms) is usually termed the impedance of the circuit, and it represents the total opposition to the flow of current in an alternating current circuit due both to resistance and to counter electromotive force (or reactance). In a direct current circuit containing several resistances in series when a current (/) flows, the volts drop across the vari- ous resistances added together numerically determine the total voltage of the circuit. Similarly, in an alternating current cir- cuit, containing both resistance (R) and reactance (X) in series, the volts drop across the various resistances and reactances are added vectoriaUy to determine the total voltage of the circuit. In the direct current circuit the power in the circuit equals the electromotive force (E) times the current (7) because both E and 7 may be thought of as in the same direction at all times. In the alternating current circuit, the power in the circuit also equals El in those cases where E and 7 are in the same direction at all times (that is, in phase with each other), as in a circuit containing resistance only. Where E and 7 are not in the same direction at all times, (that is, out of phase) as in circuits con- ALTERNATING CURRENT 73 taining reactance as well as resistance, account must be taken of the fact that E and 7 are out of phase by an angle ''a", and the product El must be multiplied by a factor called the power fac- tor (cos a) of the circuit. A wattmeter, however, indicates the true power (El cos a) at all times, even in those cases where the voltmeter reading (E) and the ammeter reading (/) taken to- gether do not indicate the true power. Supply Mains (110 Volts 60 Cycles A. C.) A r i r I Voltmeter C=? "R" (Lamps in Parallel) Adjustable Resistance Fig. 19. Study of the voltage and current relations in series cir- cuits, made up, in this case, of a reactance coil and a lamp bank in series. The adjustable resistance shown to the left is an auxiliary to the apparatus under test. In a circuit like that of Fig. 19, where a coil, with both resist- ance and reactance is connected in series with a resistance (lamp bank), the current (7) is obviously the same throughout the cir- cuit and the voltage E 3 across the entire circuit is equal to the RI drop (EJ across the lamps added vectorially to the voltage (E 2 ) across the coil. The relation of these 3 voltages, each of which may be measured separately by a voltmeter, is shown in Fig. 20. This diagram also shows graphically how the reactance (X) of the coil may be determined from the voltage readings. Current Supply. 110 volts 50 and 60 cycle Alternating Cur- rent and 110 volts Direct Current. Apparatus Required. (1) Reactance coil; (2) circuit contain- ing resistance only, for example, a lamp bank; (3) voltmeter 74 LABORATORY MANUAL with rather large range; (4) voltmeter with low range (for meas- uring the RI drop when Direct Current is used) ; (5) ammeter; and (6) wattmeter. Order of Work. 1. Connect the coil and the resistance (R) in series as shown in Fig. 19 (the auxiliary resistance is put in to permit of current adjustments). Adjust the auxiliary resist- ance until a fair value of current flows from the 110-volt, 60- cycle Alternating Current mains. Keeping this current constant. (Resistance R) RI in Coil a = Phase Difference Between ES and I Fig. 20. Vector relations of the voltages and current in series cir- cuits, as in Fig. 19. observe and record the volts across the coil, across the resistance (R), the total volts, not including the auxiliary resistance, cur- rent, frequency, and total watts, not including the auxiliary re- sistance. Use Form 12. 2. Connect the same coil and resistance (R) to the 110-volt Direct Current mains, and reduce the voltage across the coil and resistance (R) by the auxiliary resistance until the current is the same value as in item 1. Observe and record the same readings called for in item 1. 3. Same as item 1, except that R is to have'l 2/3, 1 1/3, 2/3 and then 1/3 the original value in turn, adjust to the same cur- rent as in item 1 in each case, and repeat the observations called for in item 1. 4. Connect the coil and the resistance (R) as in item 1 to 50 (or less) cycle Alternating Current mains and adjust the current ALTERNATING CURRENT 75 until it is the same value as in items 1 and 2. servations called for in item 1. Take the same ob- Written Report. 1. From the observations in item 1, Order of Work, draw a diagram similar to Fig. 20, and from this deter- mine graphically the reactance volts (XI) drop of the coil and the phase difference between E s and I in degrees; also calculate the reactance (X) in ohms and the power factor of the entire cir- cuit not including the auxiliary resistance. The power factor= watts (indicated by wattmeter) /EJ, that is, true watts divided by apparent watts. No. Supply Current Volts Amperes Frequency Watts (Total) "R" Coil Total 1 (A. C.) 2 (D. C.) 3 (A. C.) Form 12. 2. Is E Z I in item 2, Order of Work, the same in value as the wattmeter reading in item 1, Order of Work, for the same value of current (/) in each case? If so, why? If not, why? 3. Draw a vector diagram, similar to Fig. 20, for the voltages observed in items 3 and 4, Order of Work, and repeat the require- ments under 1, Written Report. 4. How do the reactance volts drop in items 1 and 4, Order of Work, compare? (Note: The impedance of an alternating current circuit often involves reactance due to capacity as well as to inductance, but for simplicity this experiment has been limited to the effect due to resistance and inductive reactance only.) 76 LABORATORY MANUAL EXPERIMENT 22. Resistance and Reactance in Parallel. See Article 273 in the text book, also the Theory under Ex- periment 21 in the Manual. The object of this experiment is to make a study of the volt- age, current and power relations in a simple alternating current circuit containing both resistance and reactance in parallel. Supply Mains (110 Volts 60 Cycles A C.) r nb Fig. 21. Study of the current and voltage relations in parallel cir- cuits, made up, in this case, of a reactance coil and a lamp bank. Note that the two ammeters to the right measure the current in the coil and in the lamp bank, respectively. Theory. In direct current circuits where a number of resist- ances are connected in series, the current is the same throughout the circuit, while the volts drop across the separate resistances are added numerically to determine the total voltage of the cir- cuit. In the alternating current circuit, where resistances and reactances are connected in series the current is also obviously the same throughout the circuit, while the volts drop across the various parts of the circuit are added vectorially to determine ALTERNATING CURRENT 77 the total voltage. These have been investigated in experiment 21. In direct current circuits where a number of resistances are connected in parallel, the voltage is obviously the same across each of the resistances, while the total current is equal to the numerical sum of the individual currents in the various resist- ances. In the alternating current circuit, where resistances and reactances are connected in parallel, the voltage is the same across the terminals of each portion of the circuit, while the total current is equal to the vector sum of the individual currents through the separate parts of the circuit. a = Phase Difference Between E and Is Fig. 22. Vector relations of the currents and voltage in parallel cir- cuits, as in Pig. 21. In a circuit like that of Fig. 21, where a coil with both resist- ance and reactance is connected in parallel with a resistance (lamp bank), the voltage (E) is the same for both parts of the circuit, and the total current (7 3 ) is equal to the current 7 2 in the coil added veetorially to the current 7 X in the lamp bank. The diagram shown in Fig. 22 indicates the relations of these three cur- rents, each of which may be measured separately by an ammeter. From this diagram the phase difference between 7 X , 7 2 and 7 3 may be determined graphically. Current Supply. 110 volts 50 and 60 cycle Alternating Cur- rent and 110 volts Direct Current. 78 LABORATORY MANUAL Apparatus Required. (1) Reactance coil; (2) circuit contain- ing resistance only (lamp bank) ; (3) ammeters, one for each cir- cuit and one for the total current; (4) voltmeter; and (5) watt- meter. Order of Work. 1. Connect the coil and the lamp bank in par- allel to the 60-cycle mains, as shown in Fig. 21, with due care that the current through the coil is not excessive. Observe and record the current in the coil, in the lamp bank and the total current, volts, frequency and total watts. 2. Connect the coil and lamp bank to the 110-volt Direct Cur- rent mains, adjusting the voltage if necessary to the same value as used in item 1. Observe and record the same readings called for in item 1. (Note: If the current through the coil is exces- sive with the Direct Current, insert a protective resistance in series with it to bring down the current to a normal value, and repeat item 1 with this extra resistance in circuit so as to have the voltage conditions the same in both items 1 and 2.) 3. Same as item 1, except that R is to have 1 1/3 and 2/3 the original value in turn, use the same voltage as in item 1 in each case, and repeat the observations called for in item 1. 4. Same as item 1, except that a reactance coil with a different power factor from the original coil is to be substituted for the lamp bank. Use the same voltage as in item 1, and repeat the ob- servations called for in item 1. 5. Connect the coil and the lamp bank to the 50-cycle mains, and adjust the voltage to the same value as in items 1 and 2. Take the same observations called for in item 1. Written Report. 1. From the observations in item 1, Order of Work, draw a diagram similar to Fig. 22, and from this deter- mine graphically the phase difference between E and 7 3 in de- grees and calculate the power factor of the entire circuit (true watts divided by apparent watts). 2. Does EI^ (Direct Current) equal EI^ (Alternating Cur- rent) in items 1 and 2, Order of Work? If so, why? If not, why? 3. Same as item 1, Written Report, for items 3 and 4, Order of Work. ALTERNATING CURRENT 79 4. Draw a vector diagram similar to Fig. 22 for the current values observed in item 5, Order of Work, and repeat the require- ments called for under item 1, Written Report. 5. How do the current values through the coil compare in items 1, 3 and 5, Order of Work? 6. Does EI. A in item 2, Order of Work, equal the wattmeter reading in item 1, Order of Work? If so, why ? If not, why? (Note: The impedance of an alternating current circuit often involves reactance due to capacity as well as to inductance, but for simplicity this experiment has been limited to the effect due to resistance and inductive reactance only.) EXPERIMENT 23. Study of Three-Phase Circuits. See Article 282 in the text book. The object of this experiment is to afford an opportunity for observing the voltage and current relations in three-phase cir- cuits. (Note: Two-phase circuits are somewhat simpler and the relations between phases perhaps more readily understood, hence, where the laboratory apparatus is two-phase, the following ex- periment may be carried out with the two-phase instead of the three-phase apparatus as here suggested.) Theory. As three-phase alternating current transmission of electric power is very generally used over long distances, and further, since many three-phase induction motors are in service, the general relations of voltage and current in such circuits are of special interest. A single winding on the armature of an alternating current generator with two collector rings for delivering the current, is called a single-phase machine. Two electrically separate wind- ings may be used instead of one, each winding terminating in a set of two collector rings and thus making a two-phase machine. The two windings in such a case are wound with a definite angu- lar displacement between corresponding wires, this displacement being such that the electromotive forces in the two coils differ by 90 in phase. Similarly, three electrically separate coils may be wound on the armature for making a three-phase generator, 80 LABORATORY MANUAL the displacement of the wires of the three windings being such that the electromotive forces differ in phase by 120 from each other. The principal advantages of the three-phase as compared with the single-phase current are first, the economy in the amount of copper wire required for the transmission of a given amount of power by the three-phase scheme, and second, the improved con- Fig. 23. Study of the voltage and current relations in a three-phase "Y" (or Star) connected receiving circuit. ditions afforded by three-phase currents in the operation of in- duction motors, and other apparatus. The six terminals of a three-phase armature may either be con- nected for the so-called Y (or Star) or the Delta connection, and similarly the three component parts of the receiving circuit for the three-phase line may either be connected for Y or for Delta operation. From Fig. 406, page 512 of the text book, showing a T con- nected system, it can readily be seen that the electromotive force between any two of the outside line wires, X, Y and Z (the wire ALTERNATING CURRENT 81 connected to the middle point is generally omitted in practice) is the vector sum of the electromotive forces induced in the two coils connected in series between these two line wires. Further, as the electromotive forces in series are 60 apart in phase, the vector sum of the two is equal to the V3 (=1.73) times that in one, since the electromotive forces are the same in each of the three armature coils, and as explained in the text book. The cur- Fig. 24. Study of the voltage and current relations in a three-phase "Delta" connected receiving circuit. rent in any one of the outside line wires, however, is supplied by one armature coil only, as will be seen by an inspection of the illustration. Hence, the current in the line wire and in the arma- ture coil to which it is connected, is the same in value. From Fig. 408, page 513 of the text book, showing a Delta connected system, it will be seen that the electromotive force be- tween any two of the line wires, X, Y and Z, is equal to the elec- tromotive force induced in the armature coil, shown in the upper part of the figure, connected between the terminals of the two line wires. Further, since any one of the line wires receives current 7 82 LABORATORY MANUAL from the two coils terminating at its connection to the armature circuit, the current in any one line wire is the vector sum of the currents in the two coils or the \/3 times the current in one coil (since the current has the same value in each of the three arma- ture coils). Hence, if E and 7 be the voltage and current respectively in a single armature coil of a Y connected generator, the voltage be- tween any two line wires is V3#, while the current per line is simply I. "With a Delta connection, this same machine would produce a voltage E between any two line wires and a current per line equal to \/3L Thus a given number of turns on the ar- mature of a three-phase generator results in a higher line volt- No Connection Used Amperes Volts Lamp Bank "1" Lamp Bank 2" Lamp Bank "3" Line Lamp Bank "1" Lamp Bank "2" Lamp Bank "3" Line 1 "Y" 2 ,, Y , 3 "Delta* Form 13. age if Y than if Delta connected. Note that the total power is the same for the given machine no matter which way it is con- nected, since it is expressed by ^3EI in each case (assuming unity power factor). Current Supply. Three-phase Alternating Current. Apparatus Required. (1) Three similar receiving circuits (lamp banks) ; (2) four ammeters, one for each receiving cir- cuit and one for a single line; (3) voltmeter. Order of Work. 1. Arrange the three lamp banks as a Y con- nected receiving circuit to be supplied by a three-phase generator as shown in Fig. 23. Adjust the current so as to be equal in each of the three lamp banks, and observe and record the current in each of the three lamp banks and in one of the lines, also the voltage between any two line w^ires and across each of the three lamp banks. Use Form 13, ALTERNATING CURRENT 83 2. Same, for twice the current value in each of the lamp banks. 3. Arrange the three lamp banks as a Delta connected receiv- ing circuit, as shown in Fig. 24, and repeat the observations called for in items 1 and 2. 4. Ascertain whether the generator armature windings are connected Y or Delta. 5. Observe the number of terminals in one of the three-phase induction motors in the laboratory. Written Report. 1. From the observations in items 1 and 2, Order of Work, calculate the numerical relation between line and individual receiving circuit voltage and current. Record these calculations in the Form on the data sheet. 2. Same, for item 3, Order of Work. 3. When the armature terminals of a three-phase generator are arranged for the Y connection, where are the connections made in the machine ? Explain. 4. Same, for Delta connected armature winding. Explain. 5. In Fig. 406, page 512 of the text book, a fourth wire is shown tapped to the common intersection of each of the Y con- nected armature windings. Why can this fourth wire usually be omitted in practical cases ? 6. In the three-phase Delta connected armature winding, why is it that there is no circulation of current about the three wind- ings which are in reality connected in series as a closed loop ? 7. Why is the current per line in a Delta connected circuit equal to the \/3 times the current in one armature winding? EXPERIMENT 24. Study of the Transformer. See pages 268, 269, 270, 271 and Articles 161, 164 and 165 in the text book. The object of this experiment is a study of the principles of construction and operation of the transformer. Theory. If two electrically separate coils of wire be wound on the same iron core, one of which is connected to alternating cur- rent supply mains, the rapid reversals of magnetism produced by the alternating current in the one coil (the primary) set up 84. LABORATORY MANUAL an induced alternating electromotive force in the other coil (the secondary) . The reversals of magnetism induce an electromotive force in the primary as well as in the secondary, and this induced elec- tromotive force (sometimes called counter electromotive force) opposes the impressed electromotive force, thus keeping the pri- mary current down to a low value when no current is being de- livered by the secondary coil. If the second coil delivers current to lamps, the magnetic field produced by it passes through the primary coil and the counter electromotive force of the primary coil is reduced, thus permitting the necessary increase in primary current to maintain the load on the secondary. Note that the No. Primary Secondary No of Coils Series or Parallel Ei Ii No. of Coils Series or Parallel E 2 Ij 1 2 3 Form 14. primary current, with no load on the secondary, is merely that required to magnetize the iron core. Since the magnetic field in the primary coil is roughly pro- portional to the number of turns of wire for a given current, and since the electromotive force induced in the secondary coil by the given rapidly changing magnetic field is proportional to the number of turns of wire in the coil, the induced electromotive force in the secondary winding (E 2 ) is related to the impressed electromotive force (E^ in the primary directly as the number of turns of wire in the two coils. Obviously the power delivered by the secondary winding (E 2 I 2 ) is equal to the power received by the primary winding (EJ.J (ignoring the slight losses in the transformer), hence, the current in the secondary (7 2 ) is related to the current in the primary (7J inversely as the number of turns of wire in the two coils. ALTERNATING CURRENT 85 Current Supply. 110 volts Alternating Current. Apparatus Required. (1) An iron core with a permanently wound fine wire coil in four equal parts with taps to be used as a primary, and four rather coarse insulated wire coils, to be used as a secondary, wound with the same number of turns each as the primary coils, and with taps; (2) an uninsulated copper ring to fit loosely over a reactance coil with an iron core ; ( 3 ) two ammeters; and (4) voltmeter. Order of Work. 1. Using one of the heavy wire coils, connect one of the primary coils to the supply mains, and observe and re- cord the number of turns in each coil, the primary volts (E^, and secondary volts (E 2 ). Use Form 14. 2. Same, with 2, 3 and 4 primary coils in series, in turn. 3. With the four primary coils in series, use two of the sec- ondary coils in series and repeat the observations called for in item 1. 4. Same, for 3 and 4 secondary coils in series, in turn. 5. With the four primary coils in series and one of the sec- ondary coils, connect the secondary terminals to a lamp bank and observe and record E lf E 2 , 1^ and 7 2 . 6. Same, using 2, 3, and 4 secondary coils in series and 2 and 4 secondary coils in parallel, in turn. 7. With the reactance coil connected to the supply mains, place the copper ring over the iron core, holding it with a pair of pliers. 8. Inspect the windings, the iron core and the case of a com- mercial transformer, making a rough sketch with dimensions. Written Report. 1. Calculate the relation of primary to sec- ondary turns, and E^ to E 2 in item 1, Order of Work. Explain. 2. Same, for items 2, 3 and 4, Order of Work. Explain. 3. Calculate the relation of primary to secondary turns, E^ to E 2 , and I to 7 2 in items 5 and 6, Order of Work. Explain. 4. Explain the phenomena observed in item 7, Order of Work. 5. In a step-down transformer (E l high and E 2 low) why is the primary coil wound with fine wire, and the secondary with heavy wire ? 6. Why is the iron core of a transformer laminated ? 7. Has the nature of the space between the coils and the outer case, and the construction and size of the outer case anything to do with the power rating of a transformer ? Explain. 86 LABORATORY MANUAL EXPERIMENT 25. Electrical Features of the Transformer and the Transmission of Power. See Article 69 in the text book, also the Theory under Experi- ment 3 in the Manual. The object of this experiment is (a) to gain facility in the handling of a commercial transformer; and (b) to study the effect of the voltage value on the transmission of power. Theory. The coils of a commercial transformer usually end in a terminal board which is accompanied by a diagram of connec- tions for the various available voltage combinations. While these connections are ordinarily shown on this diagram, it is advisable to know that in connecting the two secondary coils in series, the two coil terminals connected together must be of opposite po- larity at the same instant ; also in connecting the two secondary coils in parallel, the terminal of one coil must be connected to the terminal of the second coil possessing the same polarity at each instant. (This also applies to the two primary coils.) In the transmission of a given amount of power (El) over long distances, the volts (RI) drop and the watts (RP) loss in the transmission lines obviously depend on the resistance of the lines and on the current. Hence, for a given size and weight of wire (that is, for a given resistance of the lines) the losses of trans- mission are less the smaller the current. Since El represents the power, the transmission losses are, therefore, reduced for a given amount of power transmitted, by using a high voltage (E) and a small current (I). The use of high voltages for transmission is made possible by the transformer, which is arranged to step-up the voltage of the generator at the beginning of a long line, and a second transformer at the end of the line steps down the voltage to a practical working value, thus securing all the advantages of high voltage (that is low current) throughout the length of the transmission wires. Current Supply. 110 volts Alternating Current. Apparatus Required. (1) Two commercial transformers with four coils each (that is, two primary and two secondary coils each), and preferably with a one to one ratio; (2) lamp bank ALTERNATING CURRENT 87 to be used as a receiving circuit; (3) three ammeters; and (4) voltmeter. Order of Work. 1. Connect the primary of one of the trans- formers to the supply mains (with the two primary and the two secondary coils each in parallel), and connect the secondary to the line. Arrange the second transformer in the same way as re- gards the connection of its coils, attaching the terminals at the end of the line to its primary and the lamps to its secondary as shown in Fig. 25. Supply Mains (110 Volts 60 Cycles A. C.) Supply Transformer Usually Step Up Transformation) Lamp Bank Used as Load Fig. 25. Study of the transformer and the transmission of power at different voltages. The short transmission line should be of such length and size as to give a voltage and line loss equal to, say, ten per cent, of the power transmitted in the first case. 2. Observe and record E lf E 2 , 7 X and 7 2 in each of the two transformers. 3. Connect the two transformers so that the line voltage is double the value used in items 1 and 2, recording a diagram of the transformer coil connections, and with the same load on the secondary of the receiving transformer, observe and record E lf E 2t I I and I 2 in each transformer. 88 LABORATORY MANUAL Written Report. 1. Calculate the voltage and power loss in the two lines from the observations in items 1 and 2, Order of Work. 2. Same, for item 3, Order of Work. 3. How do the voltage and power losses compare with the two different line voltages ? Explain. 4. Explain, using diagrams, what transformer connections were used to secure the double line voltage in item 3, Order of Work. 5. Why must the two terminals, connected together in the case of a two-coil secondary, be opposite in polarity at each instant for the series connection of the two coils ? Explain. What would result if these two terminals were of the same polarity at each instant ? 6. Why must the terminal of one coil be connected to a ter- minal of the second coil having the same polarity at each instant in a two-coil secondary for parallel connection of the two coils? Explain. What would result if these two terminals were of oppo- site polarity at each instant? 7. From the observations in this experiment, explain why high voltage increases the efficiency of transmission of electric power over long distances. EXPERIMENT 26. Study of the Induction Motor. See Fig. 385a, page 497 in the text book. The object of this experiment is to gain a working knowledge of the construction, and to observe the principles involved in the production of motion in the induction motor. Theory. If the field poles of a direct current motor were ro- tated about the armature, induced electromotive forces would be set up in the armature conductors. If, further, the armature conductors were so connected that the induced electromotive forces thus set up produced a flow of current, the armature wires carrying current would be acted on by the moving magnetic field thus producing a mechanical force or torque tending to cause the armature to rotate. In the induction motor a rotating magnetic field is produced by alternating current supplied to the stationary field windings ALTERNATING CURRENT 89 (called the stator) and this rotating field acts on short circuited conductors thus inducing currents. This, obviously, fulfills the condition for motor action, namely, wires carrying current in a magnetic field. It will be noted that, while the magnetism is ro- tating, the field windings are themselves stationary, the rotating field effect being made possible by the rapidly changing and re- versing alternating current in the stator. The production of this rotating magnetic field by the alternating current is one of the objects of study in this experiment. Current Supply. 110 volts Direct Current, a low frequency Alternating Current and Alternating Current of the same fre- quency as the motor assigned. Apparatus Required. (1) The field (stator) of a three-phase induction motor with the rotor removed; and (2) a simple iron device corresponding to a compass needle, pivoted in the position of the rotor to indicate the poles of the stator; (3) brake to be used for loading the motor; (4) device for measuring the slip; (5) voltmeter; and (6) ammeter. (If a two-phase induction motor is used, the same instructions apply with obvious modifica- tions.) Order of Work. 1. Mark or label the three terminals of the stator winding as " 1," "2" and "3," in consecutive order, and connect the direct current supply mains through a suitable pro- tective resistance to the terminals 1 and 2, then to 2 and 3, then to 3 and 1 in turn, going through this succession several times. Make a sketch showing the succcessive positions of the pointer as indicating the magnetic field rotation. Observe the number of poles of the stator. 2. Connect the stator to a low frequency supply and observe the action on the pivoted pointer. 3. Make a sketch of the arrangement of several coils in the stator winding as related to each other, that is, showing the an- gular displacement of the various coils. 4. Make a diagram of the rotor winding showing the details of the end connections of the rotor conductors. 5. Place the rotor in position in its bearings and connect the stator to the rated supply mains (alternating current) allowing the machine to come up to its normal speed. Observe and re- 90 LABORATORY MANUAL cord the speed in revolutions per minute and note the frequency of the circuit used. 6. With normal voltage across the motor terminals, load by means of a brake, and observe and record the torque, current, and slip with increasing load, that is, for zero, 14, %, % and full load in turn. 7. Same as item 6, except that % of the normal voltage value is to be used. Written Report. 1. Explain just how a three-phase current produces a rotating field effect similar to that produced in item 1, Order of Work. 2. What determines the number of poles in the stator where the windings are flush with the iron ? 3. What is the object in short circuiting the rotor conductors? 4. Prom the observation of the number of poles in 1 and the frequency in item 5, Order of Work, calculate the number of revolutions per minute of the rotating field. 5. What is the numerical difference between the number of revolutions per minute of the rotor as observed in item 5, Order of Work, and that of the stator as calculated in item 4, Written Report? (Note that the difference between these speeds is the slip of the motor. ) 6. Plot two curves on the same sheet, using the speed of the rotor (found from the observations of stator magnetism speed and the slip) as abscissas and torque as ordinates, from items 6 and 7, Order of Work. EXPERIMENT 27. Electrical Features of the Induction Motor. See the Theory under Experiment 26 in the Manual. The object of this experiment is to observe the conditions which affect the speed and torque of the induction motor. Theory. The induction motor may roughly be considered a constant speed machine under constant load conditions. How- ever, as the load increases, the rotor speed falls and the difference between the speed of the rotating field and that of the rotor, name- ALTERNATING CURRENT 91 ly the slip (see item 5 under the heading, Written Report, in Ex- periment 26) increases. Note that the magnetic field rotates at a speed which is pro- portional to the frequency of the supply current. Thus, if the frequency be reduced to half its original value the magnetic field rotates at half the original speed. The rotor speed depends at no- load on this rotational speed of the magnetic field and, hence, for half frequency the rotor speed (at no-load) is reduced to about half value. On the other hand, the torque of the motor depends on the magnetic field (which is proportional to the impressed voltage E) and on the rotor current (proportional to the magnetic field and, hence, to the impressed voltage). The torque may, there- fore, be said to depend on the square of the impressed voltage (E 2 ). Hence, if the frequency be maintained constant and the im- pressed voltage reduced to half value, the no-load speed of the motor will remain sensibly the same as before (since the magnetic field rotates as before and the torque required at no-load is negli- gible). If, however, when the motor is loaded, the impressed voltage be reduced to half value, the frequency being maintained constant, the slip will be increased because the available torque is reduced by the reduction of E and this must be made up by the larger rotor currents set up in this case by the higher induced electromotive force in the rotor due to the greater relative mo- tion between rotor and field or, in other words, to the greater slip. Note, also, that the frequency of the rotor currents is highest when the rotor is at rest, that is, at starting, hence, the react- ance of the rotor winding is greatest at this time, and the power factor of the rotor at its lowest. This means that the magnetic field and the rotor currents (the two factors which produce the motion) are at their maximum phase difference when the motor is starting. Since the most advantageous condition for the torque is when the field and rotor currents are in phase with each other (or at their least phase difference) it is obvious that the torque at starting is lower than at any other point of the operation of the motor due to the low power factor. The principal means for improving the power factor at start- ing and thus improving the starting torque, is to insert an auxili- 92 LABORATORY MANUAL ary resistance in series with the rotor, which is all cut in at start- ing and gradually cut out as the rotor comes up to normal speed. Current Supply. Three-phase Alternating Current. Apparatus Required. (1) Three-phase induction motor with auxiliary resistance arranged in the rotor circuit; (2) brake to be used for loading the motor ; (3) device for measuring the slip ; (4) ammeter; and (5) voltmeter. Order of Work. 1. Connect the ammeter in one of the leads of the stator circuit and arrange to drive the motor from supply mains with its normal voltage and frequency. 2. Bun the motor unloaded, and observe and record the slip for normal and for % voltage. 3. Starting at zero load with all the auxiliary resistance cut out, and at %, %, %, and full load in turn, observe and record the slip and the torque for normal voltage throughout. 4. Same as item 3, for % voltage, as far as the load can be car- ried. 5. Same as item 3 7 with % the auxiliary resistance cut in. 6. Same as item 3, with all the auxiliary resistance cut in. 7. Tighten the brake, and measure the current in one line and the torque at the pulley at starting, with zero auxiliary resistance in the rotor circuit and using % voltage to prevent the flow of an excessive current. 8. Same as item 7, with % the auxiliary resistance in the ro- tor circuit and using % voltage. 9. Same as item 7, using all the auxiliary resistance in the ro- tor circuit and % voltage. 10. With all the auxiliary resistance in the rotor circuit, re- peat the observations called for in item 7, first, at % voltage, and second, at normal voltage. Written Report. 1. How does the slip compare for the two voltages in item 2, Order of Work? Explain. 2. Plot four curves on the same sheet from the observations of items 3, 4, 5, and 6, Order of Work, and explain from these curves how the speed and torque vary with the amount of auxili- ary resistance in the rotor circuit. 3. What effect on the speed and torque is produced in item 4, Order of Work, by the reduced voltage? Explain. ALTERNATING CURRENT 93 4. Compare the starting torque for items 7, 8 and 9. How does the auxiliary resistance in the rotor circuit affect the start- ing torque ? Explain. 5. How does the slip at normal, and at % voltage, in item 2, Order of Work, compare with the corresponding values in items 3 and 4, Order of Work, at full load? Explain. 6. In item 10, how does the starting torque compare at */>, and at normal voltage ? Explain. EXPERIMENT 28. Study of the Synchronous Motor. See Fig. 411, page 515 in the text book. The object of this experient is to secure an idea as to the gen- eral operation of the alternator when run as a motor (usually re- ferred to as the synchronous motor) . Theory. In an alternating current generator (commonly called an alternator) the magnetic field is produced by direct current from a separate source of supply, while the current in the armature conductors is, of course, alternating current. If the alternator is used as a motor, the field winding is supplied by direct current as before, and, as alternating current is sup- plied to the armature conductors, it will be apparent, after a little study, that the current in a given conductor must reverse in direction in exactly the time taken for the conductor to pass from one pole to an adjacent pole in order that the mechanical force on each conductor may always be in the same direction as the motion. In other words, the motor must run at the same frequency as the alternator which supplies it with power, and for this reason the motor is called the synchronous motor. When the synchro- nous motor is started, it must, in general, be brought up to syn- chronous speed by some outside mechanical means (an auxiliary motor), that is, to the same frequency as the alternator, and the motor must further be connected to the supply mains at that in- stant when the electromotive force of the armature (called counter electromotive force after the machine is running as a motor) is equal to that of the alternator. This operation is termed synchronizing. 94 LABORATORY MANUAL After being synchronized, the motor continues to run at syn- chronous (same frequency) speed at all loads, unless, due to an overload its speed be momentarily brought lower than synchro- nous speed, in which case it stops and must be started as before. Note that the synchronous motor is a constant speed motor and that its speed cannot be adjusted. Very often the synchronous motor forms a part of what is termed a synchronous converter (or rotary converter), namely, a Supply Mains (110 Volts 60 Cycles A. C.) JL Synchronous Motor Fig. 26. Synchronous motor. Note that the main switch is to be thrown in before the synchronizing switch, and the latter not until the conditions for synchronizing are fulfilled. The lamp should be 220 volts, or two 110-volt lamps may be used in series. machine arranged with collector rings at one end and a commu- tator at the other end of the armature, each connected to the same armature winding. The common use of the rotary con- verter is to operate it as a synchronous motor from the collector ring side, for example, in a railway sub-station, and to supply direct current to the railway line from the commutator end of the machine. The rotary converter can, however, be used to run as a direct current motor and supply alternating current, in which case it is sometimes referred to as an inverted rotary con- verter. ALTERNATING CURRENT 95 Current Supply. 110 or 220 volts Alternating Current, and 110 or 220 volts Direct Current (for field excitation). Apparatus Required. (1) An alternator to be run as a motor; (2) field rheostat; (3) incandescent lamp to be used for syn- chronizing; (4) speed indicator; (5) two ammeters; (6) watt- meter; and (7) voltmeter. Order of Work. 1. Connect the alternator for synchronous motor operation as shown in Fig. 26. 2. Bring up the motor to speed by an auxiliary motor (or by direct current if a rotary converter is used), and adjust the volt- age to the value of the supply mains. Determine, by the lamp, when the conditions for synchronizing are realized and connect the motor to the line. Disconnect the driving motor (or the di- rect current supply to the rotary converter as the case may be). 3. Measure and record the speed of the motor, and observe the number of poles and the frequency of the supply alternator. 4. "With the synchronous motor in operation (unloaded) keep the applied electromotive force constant, and measure and record the armature volts and amperes, field amperes and watts deliv- ered to the armature with various field currents, above and be- low that value giving minimum armature current. Written Report. 1. Explain in detail the necessary conditions for synchronizing. 2. Compare the speed of the alternator as calculated from the observations in item 3, Order of Work, with the observed speed of the motor. 3. Just why must the current in a given synchronous motor conductor reverse each time the conductor passes from a north to an adjacent south pole? 4. In a direct current motor, the field is of course furnished by direct current and although the machine, as a whole, is supplied with direct current, the commutator causes the current in the armature conductors to be alternating. Explain just how the mechanical force in the direct current shunt motor armature con- ductors is always automatically in the direction of motion. 5. For each of the sets of observations in item 4, Order of Work, calculate the apparent watts (El) input to the armature and the power factor (true watts as indicated by the wattmeter 96 LABORATORY MANUAL divided by the apparent watts). Explain any variations in the power factor for the different values of field current. 6. Why is the armature current greater when the field current is above or below that value which gives minimum armature cur- rent ? EXPERIMENT 29. Alternators in Parallel. See the Theory under Experiments 19, 20 and 28 in the Man- ual. Theory. The conditions to be met in the operation of direct current generators in parallel must also be met in the parallel operation of alternators, namely, the polarity of the terminals connected to a given bus bar and the voltage must be the same, and, further, the two machines must have the same frequency. This means that if one alternator is connected to the bus bars, and a second one is to be connected in parallel with the first, the second machine must be synchronized with the first, like the case described in Experiment 28. Further, if the two machines have the same number of poles, they must obviously run at the same speed to have the same frequency. The conditions to be met in throwing one alternator in parallel with another may be summarized as follows : (a) Each machine must have the same terminal voltage. (b) The two terminals to be connected to the same bus bar must be of the same polarity at each instant, namely, they must be of the same phase. (c) The machines must have the same frequency. Current Supply. 110 or 220 volts Direct Current (for field ex- citation), and the Alternating Current from the alternators as- signed. Apparatus Required. (1) Two alternators; (2) lamp for syn- chronizing; (3) speed indicator; (4) lamp bank to be used as a load for the bus bars; (5) three ammeters, one for each machine and one for the total bus bar load; and (6) two voltmeters. ALTERNATING CURRENT 97 Order of Work. 1. Arrange the machines and instruments as shown in Fig. 27. Starting up the machines, bring them to nor- mal speed and voltage, and connect one machine to the bus bars (two lengths of wire). Synchronize the second machine with the first and connect it to the bus bars at the proper instant as indi- cated by the lamp (used for synchronizing). Alternator "1 Alternator "2" Synchronizing Switch Fig. 27. Parallel operation of alternators. A voltmeter, not shown in the diagram, is to be available for measuring the voltage of the in- dividual machines. 2. Load the machines from the bus bars until each delivers its full rated load (or some fraction of full load), making needed ad- justments of load between the machines with the field rheostats. Observe and record the current delivered by each machine and by the bus bars. 3. Same, for % the value of current used in item 2, leaving the field rheostats untouched in the positions as in item 2. 8 98 LABORATORY MANUAL 4. With each machine delivering about ^2 its rated load to the bus bars, throw all the load to one of the machines and discon- nect the unloaded machine from the bus bars. 5. Start the machines, and load the bus bars until each ma- chine is delivering, say, full load (or some fraction of full load). Raise the field current in one of the machines and lower it in the other, keeping the bus bar voltage constant. Observe and re- cord the effect on (a) the armature currents of the two machines and their arithmetical sum; (b) the total load current from the bus bars; (c) power output (El in the case of lamps) ; and (d) power delivered by each machine. Written Report. 1. Explain briefly just why the three condi- tions for throwing two generators in parallel must be met, as de- scribed under the Theory. 2. Why are these conditions fulfilled when the synchronizing lamp is dark (or light, depending on which method was used) ? 3. Do the machines share the total bus bar load equally when the load is reduced to half value in item 3, Order of Work? Ex- plain. 4. Why is it necessary to reduce the load on the one machine to zero before disconnecting it from the bus bars ? 5. What would be the effect if one of the machines were discon- nected from the bus bars while delivering its share of the total bus bar load ? 6. From the observations obtained in item 5, Order of Work, state and explain the effect produced on the armature currents of the two machines and their arithmetical sum ; the total load current from the bus bars; the power output (total) ; and the power output from each machine, as the field currents are ad- justed. EXPERIMENT 30. Study of the Mercury Arc Rectifier. The object of this experiment is to afford the opportunity for observing the construction and operation of the Mercury Arc Rectifier. Theory. The function of this device is to make a uni-direc- tional (one direction) current from alternating current. Its ALTERNATING CURRENT 99 principal uses are in connection with arc lighting systems where direct current arc lamps of the Magnetite or Metallic Flame type are used (not operative on alternating current circuits) and where the advantages of distribution of the power by alternating current make it an economy to install this auxiliary piece of ap- paratus for transforming the alternating to direct current ; also where storage batteries are to be charged (with direct current) and alternating current is the only available supply. The principle of operation depends on the fact that mercury placed in a vacuum bulb with one terminal in contact with the mercury and one terminal above the line of the mercury, permits current to flow through it in one direction only. An ingenious device permits both alternations in each cycle to be used in con- nection with the rectifier. Current Supply. 110 or 220 volts Alternating Current. Apparatus Required. (1) Mercury arc rectifier; (2) lamp bank to be used as a load ; (3) two ammeters; (4) two voltmeters ; and (5) a wattmeter. Order of Work. 1. Connect the rectifier switch to the alternat- ing current supply mains and arrange to load the outfit with the lamps, inserting an ammeter and voltmeter on each side of the rectifier, and a wattmeter on the alternating current side. 2. Start up the rectifier and connect in the alternating cur- rent supply. Turn on lamps up to the capacity of the rectifier, and observe and record the current and voltage on each side of the rectifier and the watts on the alternating current side. Note carefully the tilting device for starting the rectifier action. 3. Same, for %, % and % loads, in turn. Written Report. 1. Describe briefly the action of the Mercury Arc Rectifier. 2. From the observations in items 2 and 3, Order of Work, cal- culate the efficiency of the rectifier at full load and at %, % and % loads, also the power factor for each load on the alternating current side and the relation of the alternating voltage to the di- rect current voltage in each case. 100 LABORATORY MANUAL 3. What regular maintenance item must be considered in the operation and up-keep of the rectifier ? 4. Although the current can flow in one direction only through the rectifier bulb, the statement was made in the Theory that both alternations in each cycle are utilized. Explain how this is possible. UNIVERSITY OF CALIFORNIA LIBRARY This book is DUE on the last date stamped below. Fine schedule: 25 cents on first day overdue 50 cents on fourth day overdue One dollar on seventh day overdue. ENG1NEEHINU LI NOV JS47 DEC MAR 2 T 1948 MAY 3 1943 LD 21-100m-12,'46(A2012sl6)4120 271653 Engineering Library UNIVERSITY OF CHT/TFORNIA LIBRARY k