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What is a failure of shunt generator to build up?

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

  1. Open circuit field winding
  2. Large external resistance in the field circuit
  3. Dirty commentator

Speed less than the critical speed

The voltage will not build up if armature speed is less than critical speed.