Often caused by a failure in the residual magnetism. Check if the residual voltage is at least 5V .
Used to set the knee point for under-frequency protection, typically protecting the exciter if the engine speed drops below rated levels. an-5-203 avr wiring diagram pdf
If voltage does not build, swap the two field wires (terminals 3 & 4) – phase reversal is a common issue with generic AVRs. Often caused by a failure in the residual magnetism