Page 221 - Solutions Manual to accompany Electric Machinery Fundamentals

8-1. If the resistor R adj is adjusted to 175  what is the rotational speed of the motor at no-load conditions?

SOLUTION At no-load conditions, E  V  240 V . The field current is given by
T
A
V 240 V 240 V
I  F T    0.960 A
R 
adj R F 175   75  250 
From Figure P9-1, this field current would produce an internal generated voltage E Ao of 241 V at a speed
n o of 1200 r/min. Therefore, the speed n with a voltage E of 240 V would be
A
E A  n
E Ao n o
 E   240 V 
n   A  n o    1200 r/min  1195 r/min
 E Ao   241 V 
8-2. Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation
of the motor?
SOLUTION At full load, the armature current is

V 240 V

I  I  I  I  T  110 A  109 A
A
R  L F L adj R F 250 
The internal generated voltage E is
A

E  A V  T I R  A A 240 V   109 A 0.19   219.3 V
The field current is the same as before, and there is no armature reaction, so E is still 241 V at a speed
Ao
n o of 1200 r/min. Therefore,

 E   219.3 V 
n   A  n o     1200 r/min  1092 r/min
 E Ao   241 V 

The speed regulation is

n  n 1195 r/min 1092 r/min
SR  nl fl  100%   100% 9.4%

n fl 1092 r/min

215