voltage E, is therefore fixed. illaChime op- erates at no-load, terminal voltage E12 is equal to the induced voltage E, because the voltage drop in...
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1.     A separately excited DC

generator with an equivalent circuit similar to Figure 4.23 in textbook would produce a terminal voltage of 125 V at a speed of 3000 rpm at no load. The generator is observed to be operating with a terminal voltage of 124 V when under a load of 24 kW. The armature resistance is 0.02 Ω. Calculate the actual operating speed of the generator, assuming armature voltage is linearly proportional to the generator speed.

IMG_0882.jpg


2.     The equivalent circuit of a shunt DC motor is shown. If the input voltage = 120 V, input current = 10 A, shunt field resistance = 80 Ω, and armature resistance = 0.3 Ω, calculate:


(a) Field current


 (b) Counter emf


 (c) Total mechanical power developed by the motor (that is: total input power minus the loss in the motor).



3.     A 25-hp, 240-V shunt motor (with the same equivalent circuit as in problem 2) operating at 850 rpm draws a total current of 91 A when operating at rated conditions. A 2.14-Ω resistor inserted in series with the armature causes the speed to drop to 634 rpm. The respective armature resistance and shunt field resistance are 0.221 Ω and 120 Ω. Determine the new armature current, assuming output speed is linearly proportional to the armature voltage.

IMG_0882.jpg

voltage E, is therefore fixed.
illaChime op-
erates at no-load, terminal voltage E12 is equal to the
induced voltage E, because the voltage drop in the
armature resistance is zero. However, if we connect
a load across the armature (Fig. 4.23), the resulting
load current / produces a voltage drop across resist-
ance Ro. Terminal voltage E12 is now less than the
induced voltage E.. As we increase the load, the ter-
minal voltage diminishes progressively, as shown
in Fig. 4.24. The graph of terminal voltage as a
function of load current is called the load curve of
the generator.
Hang esuit
blud ovi
Ro
F1
o
+
Eo
R
10
F2
guna
NO
Figure 4.23
Separately excited generator under load.

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