If the rotor of a motor is held fixed (blocked from rotating), and the stator is allowed to rotate freely, the “stator” would rotate at the same speed that the rotor would have in the opposite direction.
This is due to the reaction torque on the stator. It can be viewed as the electrical equivalent of Newton’s third law:
the action and reaction are equal and opposite, both of which can be electrical or mechanical.
If the generator stator coil is pushing electrical current outward, a back torque (reaction) is produced on the rotor coil, which prevents the rotor from turning forward.
Megger testing ship generator insulation resistance
The prime mover must overcome this back torque by using fuel to maintain a constant speed. We feel such a back torque when turning a megger for testing the insulation resistance.
Back voltage on ship generator
In the motor operation, if the rotor coil is pushing a mechanical torque outward, a back voltage (reaction) is produced in the stator coil, which opposes the current from going inward. This back voltage must be overcome by the applied source voltage at the stator terminals in order to maintain constant speed.
The mechanical analogy of the back voltage in a ship generator motor and the back torque in a generator is the back pressure in water pipes and air compressors. The pump must overcome the back pressure in order to push the fluid forward in the desired direction.
Thus, there is a back torque in the rotor of an ship electrical generator, and there is a back voltage in the stator of a motor. Both the back torque and the back voltage are the reactions that must be overcome by the primary power source to keep the machine delivering steady power outward.
Stated differently, there is always a motor reaction in a generator, and a generator reaction in a motor.