When the load torque rises, the automatic speed-regulating governor increases the fuel input to maintain the speed.
However, it does not fully compensate for the load increase, and the prime mover speed drops slightly in approximately linear manner.
This can also be explained in terms of the mechanical reaction torque (back torque) on the rotor, which is proportional to the stator current.
As the load is increased, the rotor speed drops under increased back torque on the rotor. The generator speed regulator (governor) will allow more fuel to the prime mover to maintain the rotor speed constant.
A practical speed governor must have a certain dead band (tolerance band), or else it would go through hunting oscillations in response to any load change. The dead band allows some decrease in speed without a response. Thus, as load increases on the generator, the steady-state speed of the rotor will decrease slightly due to the dead band in the prime mover speed governor.
So, even with an automatic speed control, practical prime mover governors cannot maintain perfectly constant speed.
As a result, the prime mover speed drops slightly with increasing load.
The speed regulation of a mechanical power source: steam or gas turbine, diesel engine, or motor is defined in a manner similar to the voltage regulation of an electrical power source:
Speed regulation = nnoload – nratedload / nratedload
Since the ac generator frequency is directly related with the prime mover speed, power engineers who working with ac generators usually define the generator frequency regulation (GFR) in terms of the governor speed regulation (GSR):
Frequency regulation on ship generator
The frequency droop rate (FDR) or the governor droop rate (GDR) is defined as rate of frequency droop per kilowatt or megawatt of output load, that is: