Page 245 - Solutions Manual to accompany Electric Machinery Fundamentals

This field current would produce a voltage E of 195 V at a speed of n = 3000 r/min. The actual E is
Ao o A
240 V, so the actual speed will be

E 240 V
n  A  n   3000 r/min  3692 r/min
E Ao o 195 V
If R is minimum at no-load cond itions, the total resistance is 200 , and
adj
V 240 V
I  T   1.2 A
R  F F R adj 200 0 


This field current would produce a voltage E of 282 V at a speed of n = 3000 r/min. The actual E is
A
Ao
o
240 V, so the actual speed will be
E 240 V
n  A  n   3000 r/min  2553 r/min
E o 282 V
Ao
8-17. This machine is now connected as a cumulatively compounded dc motor with R = 120 .
adj
(a) What is the no-load speed of this motor?

(b) What is its full-load speed?
(c) What is its speed regulation?
(d) Plot the torque-speed characteristic for this motor.

S OLUTION
(a ) The field current will be

V 240 V
I  T   0.75 A

R  F F R adj 200 120 

At no load, I  0 A , and
A
E  A T I A  V  A R S R   240 V

and the effective field current will be

N 15 turns
I *  I  SE I  0.75 A   0 A  0.75 A
F
F
N F A 1500 turns

This field current would produce a vol tage E Ao of 245 V at a speed of n o = 3000 r/min. The actual E
A
is 240 V, so the actual speed at full load will be
E 240 V
n  A o  n   3000 r/min  2939 r/min
E Ao 245 V

(b) The field current will be
V 240 V
I  T   0.75 A
R  F R adj 200 120 

F


At full load, I  I  I  100 A 0.75 A  99.25 A , and
A L F
E  V  I R  R   240 V   99.25 A 0.14   0.05    221.1 V
A T A A S
239