When testing the rotor of a synchronous motor for short circuits, a low voltage source is applied across the rotor coils through the collector rings. What would be the indication of a short- circuited coil?

Correct Answer: Option B — low or zero voltage drop reading, while the other coils will have higher readings

Applying a low voltage across synchronous rotor coils creates a voltage drop proportional to coil resistance; a short-circuited coil drops fewer turns, yielding a low or zero voltage reading compared to healthy coils. IEEE 45 rotor testing guidelines utilize this drop method for insulation integrity.