Page 222 - Solutions Manual to accompany Electric Machinery Fundamentals

8-3. If the motor is operating at full load and if its variable resistance R adj is increased to 250 , what is the
new speed of the motor? Compare the full-load speed of the motor with R adj = 175  to the full-load
speed with R adj = 250 . (Assume no armature reaction, as in the previous problem.)

SOLUTION If R is set to 250 , the field current is now
adj
V 240 V 240 V
I  F T    0.739 A
R  adj R F 250   75  325 

Since the motor is still at full load, E is still 218.3 V. From the magnetization curve (Figure P8-1), the
A
new field current I would produce a voltage E of 212 V at a speed n o of 1200 r/min. Therefore,
F
Ao
 E   218.3 V 
n   A  n o     1200 r/min  1236 r/min
 E Ao   212 V 
Note that R has increased, and as a result the speed of the motor n increased.
adj
8-4. Assume that the motor is operating at full load and that the variable resistor R adj is again 175 . If the
armature reaction is 1000 Aturns at full load, what is the speed of the motor? How does it compare to the
result for Problem 8-2?
SOLUTION The field current is again 0.96 A, and the motor is again at full load conditions. However, this
time there is an armature reaction of 1200 Aturns, and the effective field current is

AR 1000 A turns
I *  I   0.96 A   0.59 A
F
F
N F 2700 turns
From Figure P9-1, this field current would produce an internal generated voltage E of 185 V at a speed
Ao
n o of 1200 r/min. The actual internal generated voltage E A at these conditions is

E  A V  T I R  A A 240 V   109 A 0.19   219.3 V
Therefore, the speed n with a voltage of 240 V would be
 E   219.3 V 
n   A  n o     1200 r/min  1423 r/min
 E Ao   185 V 

If all other conditions are the same, the motor with armature reaction runs at a higher speed than the motor
without armature reaction.

8-5. If R adj can be adjusted from 100 to 400 , what are the maximum and minimum no-load speeds possible
with this motor?
SOLUTION The minimum speed will occur when R = 100 , and the maximum speed will occur when
adj
R = 400 . The field current when R = 100  is:
adj
adj
V 240 V 240 V
I  T    1.37 A
R  F adj R F 100   75  175 

From Figure P9-1, this field current would produce an internal generated voltage E of 271.5 V at a
Ao
speed n o of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be

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