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Generator Field Flashing Review
Generator field flashing is required when generator voltage does not build up and the generating system (including the voltage regulator) does not have field flash capability. This condition is usually caused by insufficient residual magnetism in the exciter and generator fields. In some cases, a generator that has been out-of-service for an extended period may lose its residual magnetism and require flashing. Residual magnetism can be restored by flashing the field thereby causing a current surge in the generator. Refer to the voltage regulator manufacturer’s literature for procedural instructions.
Solid-state or semiconductor components may be included in the voltage regulator. Perform field flashing according to the manufacturer’s instructions to avoid equipment damage.
If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery , a house unit providing direct current , or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart.
The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.