Electrical – Why does the stator field rotate at the same speed as the rotor field in a synchronous generator

generatorinduction motorsynchronous

In the case of the induction motor the rotor never catches up with the rotating field of the stator because if it did the induced voltage would be zero as there is no relative movement between the rotor and the stator field.

What changes in the synchronous generator that makes the stator field rotate as fast as the rotor field?

(!)If they rotate at the same speed,there is no relative movement.So how is the voltage induced and the current that creates the revolving stator field produced?(!)

However, there is relative moment between the rotor and the windings. Is this what causes the current?

Edit: I completely understand how the induction motor works. What I'm trying to work out is the synchronous generator and why isn't there a problem if the rotor and stator field are synchronized as there is in the case of an induction motor leading to the 'slip'. Why don't we have a slip in the synchronous generator?

Best Answer

In an induction motor, the speed of the rotor structure is always less than the speed of the stator field. However the rotor field rotates faster than the rotor structure so that the rotor and stator fields are synchronized with each other.

In a synchronous motor, the rotor magnetic field is produced by permanent magnets or by DC current in the rotor winding. In either case, the rotation of the magnetic field of the rotor is mechanically fixed to the motion of the rotor. For uniform torque to be produced, the both the rotor structure and the rotor field must move synchronously with the rotor field.

In other words, both synchronous and induction motors have synchronously turning magnetic field with torque produced in proportion to the angular displacement between the stator and rotor magnetic fields. In the induction motor, the rotor structure must turn at a slower speed than the magnetic fields while in a synchronous motor, the rotor structure must move synchronously.

Re: Question Edit

In a synchronous generator, the stator magnetic field rotates behind the rotor magnetic field with respect to torque angle. It is the relative motion between the rotor magnetic field and the stator windings that allows the magnetic field of the rotor to produce current in the stator. The current produced produces a rotating magnetic field in the stator that is synchronous with the rotor magnetic field but has a torque angle displacement.