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Power Factor Of Vector Controled Vfd


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#1 subrao

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Posted 28 August 2009 - 06:29 AM

I am told that a vector controlled vfd maintains power factor close to 0.95 even when the motor is loaded lightly because it computes load and reduces excitation as required. this is achieved even when vfd is operated in regenerative mode.
I wonder if it is true ,if so how?

#2 marke

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Posted 28 August 2009 - 08:56 PM

Hello subrao

The power factor at the input to a VFD is always independent of the power factor of the motor, so I assume that in this case, you are talking about the power factor of the motor and not the power factor of the input to the VFD. i.e. the power factor as seen by the output of the VFD and not the supply.

Certainly, if you reduce the voltage applied to a lightly loaded motor, the magnetising current will drop. This reduces the inductive current and improves the power factor. I would not accept the 0.95 value though, rather the power factor could increase to closer to the full load power factor of the motor.
This is the principle that the NASA (Nola) energy saver for induction motors works on. When the power factor is low, reduce the voltage, reduce the magnetizing current and the power factor will improve. Reducing the magnetizing current will reduce the iron loss and save some wasted energy.

Best regards,
Mark.

#3 kana

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Posted 29 August 2009 - 12:04 PM

Hi subrao,

The volt/frequency control available in VSD certainly helps to improve power factor to a certain level because by reducing the voltage applied to the lightly loaded motor, the voltage seen by the magnetizing branch of the Induction motor is reduced as well, thus reducing the magnetizing current.

When operating in regenerative mode, the power is fed back to the VSD from the motor. Why is there a concern on the power factor when the motor is operating in regenerative mode?

Regards,
kana




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