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In synchronous machines rotor is excited with DC and rotated so that due to change in mf falling on the conductors in stator an emf is generated.if we use Ac as an exciting means to excite rotor and then rotate rotor so that we get more change in flux due to which we can generate more emf. But we r not doing like that. can any one explain me?

Hazem
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vamsi krishna
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A synchronous motor cannot have an AC supply on the rotor as the rotor is a single-phase element designed just to provide magnetic flux. Since the stator is driven from a three phase source, you would need to provide a three phase, but OUT OF PHASE supply to the rotor in order to generate and maintain magnetic flux. Tough to provide with two wires. For low power applications, synchronous motors often have simple permanent magnets in the rotor and do not need an external supply.

High power synchronous motors can be used to control the power-factor presented back to the line, but there are usually better options for power-factor management.

What you are describing isn't a synchronous motor, but a wound-rotor motor. Wound-rotor motors have a three phase wound rotor (hence the name). External resistance is switched into the rotor windings to inefficiently control speed, and change the startup loading characteristics of the motor.

R Drast
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  • There also used to be a very cool trick used in early three phase elevator controls, in which a three phase wound rotor machine was allowed to idle at sync speed and then had a mechanical brake applied slowing the rotor gradually to a halt (At which point the thing is functionally a transformer), this caused the output frequency and voltage to smoothly ramp from zero to line voltage and frequency, giving a smooth start to the elevator motor that was connected to one of the sets of windings. The early electrical machines used some fascinating tricks in the days before high power active devices. – Dan Mills Aug 31 '17 at 10:49