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POwer Factor versus speed with VfD


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#1 Guest__*

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Posted 08 December 2005 - 12:30 PM

POwer Factor versus speed with VfD



Hello,

I am trying to assess resulting displacement PF on VfD driven motors (with 6 pulse SCR DC bus).
I have found many documents indicating that my PF should be close to 0.95-0.97 at full speed, but I have motors running at 70Hz, 60Hz, 50 Hz, 40Hz etc...

Does anyone know how I could assess PF of the motors that do not run at grid frequency?

LG

#2 marke

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Posted 09 December 2005 - 04:02 AM

If you are looking at the power factor that is seen by the supply, it will see the input of the drive only. The motor is totally isolated from the supply in terms of power factor.
The input to the drive is a bridge rectifier feeding a large storage capacitor with, hopefully, an inductive input filter.

The displacement powerfactor is always better than 0.95, but the distortion power factor can be down round 0.7 due to the discontinuous current flowing through the rectifiers.

The displacement power factor does not change with load on the motor, or with the speed of the motor.

Best regards,

#3 Guest__*

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Posted 09 December 2005 - 10:24 AM

Hello Marke - This is LG again,

I thank you for the reply.
I am afraid my concern is that I don't know if I have a VSI or a LCI inverter, and I gopt confused when I read that LCI had "a poor power factor at loaw speed".

The only thing I can read in the Drive document is that:
1. it is a quasi signwave "six step" Variable Frequency controler.
2. 6 pulse SCR bridges recitifer ... converts... to adjustable DC voltage
3. That the inverter is made of "3 pairs of power electronic switches"

I am still confused as inverter makes me say VSI, bt variable voltage makes me wonder if it is not something else.

What I have after the drive is a step-up transformer, 3000ft of cables and a MV induction motor.

Hope someone can get me out of this headache.
Regards.

LG

#4 marke

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Posted 09 December 2005 - 10:40 AM

Hi LG
You should register on the forum, you can get email notification of replies.

If it is a modern inverter with an IGBT output stage, I would expect a VSI.
If it is a CSI, I would expect a large DC reactor and little or no DC bus capacitance across the output stage. The series reactor provides the current source where as capacitors provide a voltage source.

Best regards,
Mark.




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