Shorted turns in the exiter winding can be detected by periodic measurement of rotor impedance using AC power supply. These tests should ideally be performed while the machine is running at synchronous speed. Because shorted turns may only exists when centrifugal forces are acting on the turn conductors. When the machine is shut down, there may not be any contact, or the fault resistance measurement.

The induced backward current in a single shorted turn, which opposes the magnetic motive force (mmf) of the entire coil, resulting in a significant reduction in the reactance. This technique is particularly effective in salient pole rotors, where one short circuited turn eliminates the reactance of the complete pole. There is a sudden change in impedance when a turn is shorted during run-up or run down. A sudden change of more 5% is needed to verify for shorted turns.

A normal winding will exhibit, reduction in impedance up to 10% between stand still and operating conditions due to effect of eddy currents on the rotor. The test can only be performed if the field winding is accessible through collector rings because ac power should be applied while the machine is running. The power supply should be ungrounded.