Could anyone explain why the speed of rotor decrease when the load increased. As the rotation decreased the output frequency is going lower too. I think that more load means more current through stator winding which create more magnetic field which is directed opposite to the field of rotor and hence decreasing its speed. Is it correct? Please give some advice or source to read. Thanks in advance

 

The rotor creates a magnetic field which opposes the field in the stator. As load is increased the rotor lags further and further behind the rotating field of the stator. The result is that the stator draws more power from the input source and increases it's field to keep the rotor following along with its rotating field. Each load point has a corresponding rotor speed which requires a specific amount of input power. The rotor speed graph will show a very gradual slowing until max load is reached. At that point the motor is at the magnetic saturation point for its stator, which is closely related to its max input power point. Increasing the load further stalls the motor because it cannot increase its field strength to keep the rotor following.

SMOKE is the common outcome of stalled motor.

 

The rotor creates a magnetic field which opposes the field in the stator. As load is increased the rotor lags further and further behind the rotating field of the stator. The result is that the stator draws more power from the input source and increases it's field to keep the rotor following along with its rotating field. Each load point has a corresponding rotor speed which requires a specific amount of input power. The rotor speed graph will show a very gradual slowing until max load is reached. At that point the motor is at the magnetic saturation point for its stator, which is closely related to its max input power point. Increasing the load further stalls the motor because it cannot increase its field strength to keep the rotor following.

SMOKE is the common outcome of stalled motor.

Sorry I did not point what machine I am speaking about. As I understand from your explanation you talked about motors. My question is related to generator frequency output. You mean that generator rotor is rotated by motor shaft may be?

 

It works almost the same with generators but instead of your motor load slowing the rotor. In the generator the output is voltage and current. As the output power is increased, the magnetic field is stronger and that field opposes the field of the rotor which is your input power element. Therefore more input power is required to drive that rotor, and when the RPM of that input gets lower, the output freq. (and also voltage) gets smaller

 

I think in the same way, thanks now I am sure