A Field regulator is a phrase I’m used to hearing around DC motors and drives. If this is what your asking about the regulator is usually built into the drive but can be a separate module in some brands especially in the larger horsepower sizes. So it actually controls the field current as the motor heats up voltage will have to increase slightly to compensate for field resistance increases from hot wire in the field coils. Also, if the drive has TACH feedback the Field regulator can be used for high speed operation above Base speed of the motor.
Ok sure Scott glad you asked.
If the drive is commanded to go above base speed, first of all it has to be tuned to do so. The Armature voltage increases directly proportional to the speed of the motor up to Base speed but not above it. Now we are going into what is called the Constant Horsepower range of operation. To allow the speed to increase above Base we have to decrease the Field current so the CEMF stays at motor rated from Base speed to MAX speed, (always verify the actual max speed rating with the motor mfg). So if your motor is rated 240Vdc for a 230Vac system, the 240volt does not change above Base speed (Base speed is usually 1750RPM on most DC motors). If you have a 480Vac system the motor will usually be rated 500Vdc for the Armature and 300Vdc for the Field. But it is the field current that is more closely regulated than the voltage in a field regulator.