Page 246 - Solutions Manual to accompany Electric Machinery Fundamentals

and the effective field current will be
N 15 turns
I *  I  SE I  0.75 A   99.25 A  1.74 A
F
F
N F A 1500 turns

This field current would produce a voltage E Ao of 292 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 221.1 V
n  A n   3000 r/min  2272 r/min
E o 292 V
Ao
(c) The speed regulation of this motor is

n  n 2939 r/min 2272 r/min

SR  nl fl  100%   100% 29.4%

n fl 2272 r/min
(d) A M ATLAB program to calculate the torque-speed characteristic is shown below:

% M-file: prob8_17.m
% M-file to create a plot of the torque-speed curve of a
% cu mu taively compounded dc mtor.
l

% Get the magnetization curve. Note that this curve is
% defined for a speed of 3000 r/min.
lo ad p86_mag.dat;
if_values = p86_mag(:, 1);
ea_values = p86_mag(:,2);
n_0 = 3000;

% First, initialize the values needed in this program.
v_t = 240; % Terminal vol tage (V)
r_f = 200; % Field resis tance (ohms)
r_adj = 120; % A djustable resistance (ohms)
r_a = 0.14; % A rmature resistance (ohms)
i_l = 0:1:100 ; % Line currents (A)
n_ f = 1500; % Number of turns on shunt field
n_se = 15; % Number of turns on series fiel d

% Calculate the armature current for each load.
i_a = i_l - v_t / (r_f + r_adj);

% Now calculate the internal generated voltag e for
% each armature current.
e_a = v_t - i_a * r_a;

% Calculate the effective field current for each armature
% current.
i_ f = v_t / (r_f + r_adj) + (n_se / n_f) * i_a;

% Calculate the resulting internal generated voltag e at
% 1200 r/min by interpol a ting the motor's magnetization
% curve.
e_a0 = interp1(if_values,ea_values,i_f);

% Calculate the resulting speed from Equation (9- 13).
n = ( e_a ./ e_a0 ) * n_0;

240