Field Regulator

Maybe this explains it:

pdfstream.manualsonline.com/7/7079df42-4464-427b-b181-4496e6281f13.pdf

page 2-21:

"Field Voltage and Current Regulators - FVR & FCR The Field Voltage Regulator (FVR) is the typical manual regulator supplied on most applications and uses the generator field voltage as the feedback input. While FVR does permit the current to vary as a function of the field resistance, the FVR makes the manual regulator completely independent from the over excitation limiter. FVR uses the voltage from the generator field as feedback, with a setpoint from the MANUAL REF block. A PI regulator with integral windup protection generates the output. During operation in AVR mode, the output of the AVR is passed directly to the FVR output with no signal conditioning. On units that operate with an inner field voltage regulator loop such as compound exciters and some high ceiling exciters, the FVR uses a setpoint from either the AVR or the MANUAL REF block, and is always operational whether in manual or automatic operation.

The Field Current Regulator (FCR) is a special application of the manual regulator and uses the generator field current as the feedback input. The current setpoint is generally switched between a high level and lower level to provide transient forcing capability as well as steady state operation within the capability of the generator. Generally the setpoint is larger than expected field currents and the integral preset is operational. The FCR output is held at positive ceiling until enable becomes true which allows the output to follow the P+I regulator. The bridge firing command is the smaller of the FVR and FCR outputs. While it does regulate constant field current over varying field temperature, FCR is not the standard manual regulator."