The Rotor voltage drop test is used to identify the presence of shorted turns in the salient poles of the rotors of synchronous machines. The test works by measuring the impedance of each pole on the rotor. If there are short-circuits in the rotor winding, the impedance of the rotor poles will be less.
The equipment required is relatively simple. A fixed AC voltage is applied across each end of the rotor winding and the voltage drop across each pole of the rotor is measured. The voltage values are tabulated. The average of the values is calculated. Each individual value should be within a limit of 10% from the average value. Any further variation would indicate shorted turns in that particular pole.
While the rotor voltage drop test is useful in identifying inter turn faults in the rotors, they are not considered comprehensive. Since the rotor is a rotating object, there are shorts which may form only during running due the action of the centrifugal force on the rotor.