Question 128Electricity & Electronics - QMED
70% to passIf an unloaded DC compound motor's shunt field were weakened by excessive rheostat resistance or by an open circuit, how would the motor respond?
Acontinue to run at base speed
Bstop because of low flux
Cslow down and overheat
Doverspeed due to reduced CEMF
AI Explanation
The correct answer is D) overspeed due to reduced CEMF. In a DC compound motor, the shunt field provides the main magnetic flux. If the shunt field is weakened by excessive rheostat resistance or an open circuit, the motor's back EMF (CEMF) will be reduced. This allows the armature to spin faster, resulting in an overspeed condition. The reduced CEMF is unable to counter the applied voltage, leading to the motor overspeeding. The other options are incorrect because: A) the motor would not continue to run at base speed with a weakened shunt field, B) the motor would not stop due to low flux, and C) the motor would overspeed rather than slow down and overheat.
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