AC synchronous motor power factor

  • Thread starter girts
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  • #1
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Is it true that AC synchronous motors (the ones with the DC slip rings and excitation current running into rotor poles) have a power factor of 1.0 when they run in synchronous mode?
Would that be because with sufficient rotor DC current he rotating AC magnetic field is precisely counterbalanced so there is almost no induction and the incoming stator winding AC "sees" an almost resistive load instead of an inductive load like in a transformer or an induction motor?
can I say that in a sense the synchronous motor is like a magnetic amplifier (saturable reactor)


thanks
 

Answers and Replies

  • #2
anorlunda
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The power factor is adjustable by changing the exciter voltage. So you can have almost any power factor you want.

The picture shows the capability curve for a synchronous generator. It's the same for a synchronous motor. The point where VARs are zero and power factor is one is neither overexited or underexcited. It doesn't have a name but you can call it "just right" or maybe "excited to perfection". :biggrin:


loss-of-excitation-22-638.jpg
 

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  • #3
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so I guess due to all the capacitive filters in power supplies and smps , the generators of today are rather overexcited then underexcited? to make the sine wave with a steeper climb in order to balance out the lagging current right?
 
  • #4
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You need to overexcite enough to compensate for the typical lagging PF on the grid (total load) as a whole.
 
  • #5
anorlunda
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so I guess due to all the capacitive filters in power supplies and smps , the generators of today are rather overexcited then underexcited? to make the sine wave with a steeper climb in order to balance out the lagging current right?

For the bulk power grid (think continental scale), the inductance and capacitance of the transmission lines are more significant than the L or C components of the loads.
 

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