The first step in test running a wound rotor motor is to apply approximately half-rated voltage to the stator, with the rotor circuit open (leads open or brushes lifted).

Check the rotor ring-to-ring voltage. It should also be approximately half-rated rotor voltage. Typically it will be slightly higher than the ratio of rated stator to rotor volts.

For example, if the stator is rated 460 volts and the rotor 300 volts, with 230 volts applied to the stator, the open circuit rotor voltage should be about 157-160 volts.

With the rotor open and energized for the above test, the rotor may “crawl” or most often will remain stationary.

If the rotor immediately accelerates to speed when the stator is energized, the rotor is either shorted or misconnected internally (or the rotor has an unusually high number of parallel circuits).

To test run the motor, short the rotor ring leads and apply reduced voltage to the stator. If the rotor remains stationary, disconnect power to the stator.

Next, hand-rotate (spin) the rotor and energize the stator with the rotor rotating. It should then start.

The reason that the wound rotor may tend to lock-up or not rotate (i.e., cog) is that the stator-rotor slot combination makes it sensitive to rotor position.

In many cases, simply slightly rotating the rotor will allow it to start.