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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?

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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.

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The synchronous machine has a field winding or permanent magnets to generate the d-axis flux, where the induction machine relies on the changing magnetic field to induce flux in the rotor. If the rotor speed in an induction machine equals the synchronous speed then there's no induction and no flux.

In the synchronous machine the windings or magnets provide flux regardless of rotor speed.

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  • \$\begingroup\$ One could add the synchronous machine isn't a special case of the asynchronous machine, but instead the inversion of the DC machine, with the field winding rotating and the "rotor" winding affixed. The asynchronous machine is more similar to a transformer than to the synchronous machine. \$\endgroup\$ – Janka Mar 18 '17 at 22:03
  • \$\begingroup\$ They provide the flux but a revolving field is created in the stator windings because of the flowing current( pretty much like the revolving field of an induction motor). If this field is synchronized with the rotor field-permanent magents' field how is voltage induced in the generator? No voltage means no current and no stator revolving field. It's just like the case with the slip of the induction motor, the field should never catch up. \$\endgroup\$ – John Katsantas Mar 18 '17 at 22:42
  • \$\begingroup\$ I think that the stator field doesn't affect voltage induction and that's why I can't work this out. \$\endgroup\$ – John Katsantas Mar 18 '17 at 22:44
  • \$\begingroup\$ The synchronous machine's torque is proportional to the angle between the stator rotating field and the rotor's field, though they are rotating at the same angular velocity. (Until pull-out torque.) The difference in this angle provides the Q-axis (torque producing) flux. So no slip is necessary in the case that the rotor produces it's own flux. \$\endgroup\$ – John D Mar 19 '17 at 2:28
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The synchronous and the asynchonous both need a magnetic field to make and keep the rotor rotating thereby transforming electric energy into mechanical energy. In the asynchronous motor the magnetic field is created due to the slip of the rotor in respect to the stator. This slip creates a magnetic field in the rotor thereby creating the magnetic field required. Such a magnetic field can not be generated without the slip as is the case with the synchronous motor. Never the less a magnetic field is required. Therefore the only remaining option is to create a magnetic field with a field winding or permanent magnets.

Thereafter the only task of the rotating field magnets is to remain in sync with the rotating field in the stator. Here you also find a complication. A synchronous motor normally can not start without additional measures.

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  • \$\begingroup\$ This is not correct. The rotor field is not created by slip, it is created by magnetic induction aka transformer action between the stator and rotor. Slip happens because the rotor field is constantly decaying due to resistive losses, and being recreated by induction from the stator, causing the net rotor field to slowly move relative to the mechanical axis of the rotor. Also, synchronous motors can start just as easily as an induction motor. \$\endgroup\$ – Evan Mar 18 '17 at 22:29

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