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Now ready. Crown 8vo. 7s. 6d.
ELECTED -DYNAMICS
THE DIRECT-CURRENT MOTOR
BY
CHARLES ASHLEY CARUS-WILSON
M.A. CANTAB.
MEMBER OF THE INSTITUTION OF ELECTRICAL ENGINEERS
PROFESSOR OF ELECTRICAL ENGINEERING AT
JIcGILL UNIVERSITY, MONTREAL
CONTENTS
CIIAI'.
I. The Induction Factor
II. Conditions of Uniform
Motion
III. Equations for the Induc-
tion Factor
IV. Shunt-wound jMotors
V. Series-wound Motors
VI. Efficiency
CHAP.
VII. Acceleration
VIII. The Force Factor
IX. Control
X. Time Curves
XI. Design of Railway Motors
XII. Armature Reaction
Problems
Index
LONGMANS, GREEN, AND CO.
3!) PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY.
1898
PREFACE
TiiH Electrical Engineer is now confronted with a variety of new
problems for whose solution he must look to a full development of
the science of electro-dynann'cs. This in the near future will stand
in the same relation to the electric-motor that the science or
thermo-dynanncs stands in now to t'.ie steam entwine. And since
no branch of dynamics has a better claim to be called an exact
science tlian electro-dynamics, it will be able to otter not only
convincin<f but final solutions of these problems. To aoply tlie
principles of electro-dynamics to the direct-current motor is the
aim of this book.
Writing' for Electrical Engineers particularly, I take for granted
a certain acquaintance with the use and design of motors, but, as
the book is intended to be of service to engineers generally, unex-
plained technicalities have been avoided as far as possible.
There are now so many excellent text books on electricity and
magnetism, that I offer no apology for omitting discussion of
elementary principles here.
I have not considered it necessary to allude to the subject of
self-induction, except in connection with the question of sparking.
The advanced student will perceive the analogy between the law of
acceleration given in Chapter VII. and that for the rise of current
in an inductive circuit, and may be tempted to pursue the subject
for himself.
The numerical accuracy attempted has been limited to that
attainable with an ordinary ten-inch slide rule, on which all the
examples have been worked out. Importance is attached to the
graphic method of solution, and the diagrt-'ms -e intended to serve
as exercises for the student, who should worl, at similar problems
with different data by the same methods,
I have to thank many friends for assistance, particularly Mr.
H. S. Hering, for allowing me to use the results of his tests on
electric cars ; Mr* L. H. Parker, for providing me with particulars
of the construction and performance of the electric locomotives on
the Baltimore and Ohio Railroad ; Mr. H. P. Curtiss, for placing
at my disposal the outcome of his experiments on the Buffalo and
Niagara Falls Electric Railway ; and the Railway Department of
the General Electric Company, for furnishing me with valuable in-
formation and data.
C. A. CARUS-WILSON.
McGiLL University, Montreal :
February 1H98.
1
94
SPECIMEN PAGE
THE DIIlECT-CURllEXT MOTOR
CII. V
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1
CHAPTER V
sp:hies-wound motors
When tlie magnets of a dynamo are connected in
series with the armature, the induction factor will
vary with the current. In Fig. 20 distances measured
along oh represent current in the armature, and also in
the magnets. We will take 500 volts as the tension of the
line, 5 ohms as the resistance of the motor, the maximum
current being thus smaller than 100 amperes. Let the
distance 01 represent 100 volts, 10 amperes, 100 r.p.m.,
1,500 inch-pounds of torque, and an induction factor of 15.
Let oa represent the tension of the line, ob the maximum
possible current. Join ha, and produce it. We shall call
ha the loss line, since the intercept he of any ordinate
between it and a horizontal line through a gives the volts
lost in heat. On oh construct the induction curve. Thus,
if for a current of 70 amperes, the induction factor is
(37 '5, we must set up on the point representing 70
amperes in the armature, a vertical distance equal to
4-5 inches, and this will be a point on the induction
curve.
If there is no residual magnetism the induction factor
is nothing when the current is nothing. When the
current is reversed the induction factor will also be
I
SPECIMEN PAGE
CIT. V
SERIES- WOUND MOTORS
95
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SPECIMEN PAGE
THE DIlfKCT-CURRENT 3I0T0R
CH. IX
S 3 W ] d M V
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('II. IX
SPECIMEN PAGE
CONTROL
203
speed iij) is well ^howii. The liiial current is IS fiinpereH
per motor.
The !iie;in current lor the first four seconds from the
moment when the circuit is made appears to be 7-")
amperes. Tiiis would ^'ive us a total tonpie per motor
(see Fig. 21) of 2,4;30 inch-iiounds. Deductinpf the frictional
toi"(|uecorres])()ndiii<:^ to a current of 18 amperes |)er motor
— nanu'ly. 7'>o inch-j)ouuds — we liave 1 ,()7o inch-pounds
available for acceleration. Since the weio-ht to be
accelerated is ■']• !■ tons ])er motor, the acceleration is
2-()o f.p.s. per second, ^fhe acceleration curve has been
drawn as if the acceleration were constant from the momiMit
of making the current ; this is not strictly correct. The
curve should cut the time base about one second from the
origin, bnt it gives a fairly accurate measure of the mean
initial acceleration, which by measurement appears to
be about 2*1 f.p.s. per second.
Fig. 52 is the record of a test in which the motors
weie allowed to speed np in series. The current taken
does not appear to have been quite so much as in the
case repi-esented in Fig. 51. The form of tlie current
curve is well shown, both with the series and with the
jnirallel connection, and indicates the jerk experienced
when the motors are thrown into parallel, the accelera-
tion at this point being greater than at the moment
of starting. The energy required to attain a speed of
o5 f.p.s. is less than that required by the method illustrated
in Fig. 51.
The diaoTams show a considerable increase in the
current taken from the line at the moment when the
motors are thrown into parallel connection. It is in-
structive to inquire if this increase is necessary, and to
SPECIMEN PAGE
232
TIIH DIIMCCT-iTKUKNT MoTnl;
err. xi
Lt't lis take the case of a tramcar \v.M(r|iiiicr |i) tons,
driven by two niolors workiii*,' on a line liavin<,r u tension
of oOU volts. Suppose tliat we liav.' to desiLrn an
ananirenient by wJn'eli the ear will start up from rest and
travel oUO feet in 3U seconds. The motors are to be series
wound.
First Knd the least possible accelerating,' current per
motor by Equation 107, remendje.-ing that ]\'=o tons.
We get (•„ = 29-5 amperes. Take SO amperes to allow for
the resistance of the motor. From E^juation Id.} we find
that the best value of '/ is 5"15. For the i)resent we
(t *
may take v— 1.-78 and <( = SS inches, giving .lA=35-r).
The maximum speed is 25 feet per second or 1 7 miles
an hour. If the frictional and other resistances retardin*'
the motion amount to ;3,:)80 inch-pounds of torque on the
car axle, the corresponding current will be 15 amperes,
and the resistance of each motor must therefore be
O'G ohm.
The initial acceleration will be 1*25 fp s. per second
and the current of 45 amperes will be constant until the
starting rheostat is all out, at which point the speed of the
motor will be given by ?/ =
500-J.5xO-(;
oo o
= S00 r.
p.m,
The speed of the car will therefore be 2 [•2 feet per second.
Thus we see that if the induction factor is constant, the
acceleration can be maintained constant up to a speed
of 97 per cent, of final speed; after this point the motor
will speed up according to the law already given in
Chapter VII. ; the error involved in assuming that the
acceleration is constant up to full speed will be small, and
CH. XI
SPECIMEN PAGE
UESIGN 01' UAILWAY MO'l'OliS
233
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