Page 223 - Solutions Manual to accompany Electric Machinery Fundamentals

E n
A 
E Ao n o
 E   240 V 
n   A  n o     1200 r/min  1061 r/min
 E Ao   271.5 V 

The field current when R = 400  is:
adj
V 240 V 240 V
I  T    0.505 A
F
R  adj R F 400   75  500 
From Figure P8-1, this field current would produce an internal generated voltage E of 167 V at a speed
Ao
n of 1200 r/min. Therefore, the speed n with a voltage of 240 V would be
o
E n
A 
E Ao n o
 E   240 V 
n   A  n o    1200 r/min  1725 r/min
 E Ao   167 V 

8-6. What is the starting current of this machine if it is started by connecting it directly to the power supply
V T ? How does this starting current compare to the full-load current of the motor?

SOLUTION The starting current of this machine (ignoring the small field current) is

V 240 V
I  T   1260 A
,start
L
R A 0.19 
The rated current is 110 A, so the starting current is 11.5 times greater than the full-load current. This
much current is extremely likely to damage the motor.
8-7. Plot the torque-speed characteristic of this motor assuming no armature reaction, and again assuming a
full-load armature reaction of 1200 Aturns. (Assume that the armature reaction increases linearly with
increases in armature current.)
SOLUTION This problem is best solved with MATLAB, since it involves calculating the torque-speed
values at many points. A MATLAB program to calculate and display both torque-speed characteristics is
shown below.

% M-file: prob8_7.m
% M-file to create a plot of the torque-speed curve of the
% the shunt dc motor with and without armature reaction.

% Get the magnetization curve. Note that this curve is
% defined for a speed of 1200 r/min.
load p81_mag.dat
if_values = p81_mag(:,1);
ea_values = p81_mag(:,2);
n_0 = 1200;

% First, initialize the values needed in this program.
v_t = 240; % Terminal voltage (V)
r_f = 75; % Field resistance (ohms)
r_adj = 175; % Adjustable resistance (ohms)
r_a = 0.19; % Armature resistance (ohms)
i_l = 0:1:110; % Line currents (A)

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