Causes of failure of voltage build-up or armature heating:
No residual magnetism
Without any residual magnetism, voltage build-up cannot start. Due to again in transportation, the machine may lose residual magnetism. This difficulty can be overcome by excitation of the field winding from a separate dc source for some time. After this voltage build-up takes place.
High field circuit resistances
If field resistances is more critical resistance then the voltage will not build up.
Field resistance will be more than critical resistance due to
- Open circuit field winding
- Large external resistance in the field circuit
- Dirty commentator
Speed less than the critical speed
The voltage will not build up if armature speed is less than critical speed.