Page 216 - Solutions Manual to accompany Electric Machinery Fundamentals

(d) This motor must have 8 brushes, since it is lap-wound and has 8 poles. Since it is duplex-wound,
each brush must be wide enough to stretch across 2 complete commutator segments.


(e) There are a total of 64 10 640 turns on the armature of this machine, so the number of turns per
path is
N  650 turns  40 turns/path
P
16 paths
The total resistance per path is R  P  0.011  40  0.44  . Since there are 16 parallel paths through
the machine, the armature resistance of the generator is
0.44 
R   0.0275

A
16 paths
7-8. Figure P7-2 shows a small two-pole dc motor with eight rotor coils and 10 turns per coil. The flux per
pole in this machine is 0.006 Wb.

(a) If this motor is connected to a 12-V dc car battery, what will the no-load speed of the motor be?

(b) If the positive terminal of the battery is connected to the rightmost brush on the motor, which way
will it rotate?

(c) If this motor is loaded down so that it consumes 600 W from the battery, what will the induced
torque of the motor be? (Ignore any internal resistance in the motor.)

SOLUTION
(a) At no load, V  T E  A K . If K is known, then the speed of the motor can be found. The constant
K is given by
ZP
K 

2 a
On the average, about 6 of the 8 coils are under the pole faces at any given time, so the average number of
active conductors is
Z = (6 coils)(4 turns/coil)(2 conductors/turn) = 48 conductors

There are two poles and two current paths, so

 ZP 48 cond 2 poles 
K    7.64

2 a 2  2 paths
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