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How To Control Field Excitation Of A Synchronous Motor For Pf Correction


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#1 mie

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Posted 03 August 2007 - 01:28 AM

I need help on this topic from the professionals out there..
what is the method used to control the synchronous motor DC field excitation to control the power factor of the motor itself.
can you guys help me with the basic circuitry of the controller?
Please advice me on this topic..
thanks

#2 jraef

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Posted 03 August 2007 - 06:02 AM

It depends upon the design of the synchronous motor. Brushless or brushed, self exciting or externally excited, mechanical or electronic exciter supplies.

Generally;
Brushed (slip-ring) synchronous motors take a DC power source and run it through a variable resistor to change the excitation current in the circuit. The altered DC voltage is fed across the rotating shaft via brushes and a slip ring. The source of the DC for the excitation can either be a small DC generator (dynamo) attached to the shaft of the synchronous motor, or a solid state DC power supply.

Brushless motors have 2 flavors. Either they have an external variable AC voltage system that alters the AC going to an induction coil, which then creates an induced AC on the rotor. A rectifier built-in to the rotor changes that to DC and feeds the altered voltage to the windings. The other version just induces a fixed voltage across the rotating gap, then a control system built-in to the rotating core does the rectification and variation of the excitation current.
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