Use an ohmmeter to measure the resistance between the slip rings.
- Rated resistance (cold) - 2.7-3.1 ohms
If the resistance tends to infinity, i.e. circuit is open, replace the rotor.
2. Check if the excitation winding is shorted to "mass".
Using an ohmmeter, measure the resistance between the rotor pole and the slip ring.
If resistance is 0 (circuit is closed), then replace the rotor.
3. Check slip rings.
A) Check the working surfaces of the slip rings. They should not have scuffs or chips.
b) Using a caliper, measure the diameter of the slip rings.
- Nominal diameter - 14.2-14.4 mm
- Minimum allowable - 12.8 mm
If the diameter of the contact tracks is less than the minimum allowable, then replace the rotor.