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Example 6: A series-wound dc motor has an<br />
internal resistance of 3 W. The 120-V supply<br />
line draws 4 A when at full speed. What is the<br />
emf in the motor and the starting current?<br />
E b<br />
Recall that:<br />
V – E b = IR<br />
I<br />
V<br />
120 V – E b = (4 A)(3 W<br />
The back emf<br />
in motor:<br />
E b = 108 V<br />
The starting current I s is found by noting that E b = 0<br />
in beginning (armature has not started rotating).<br />
120 V – 0 = I s (3 W I s = 40 A
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