VFD issue 1

[davidqyuan]

I have some questions about the VFD, does anyone here could help? thanks.
1. Is power factor (input side of VFD) in VFD's name plate? if it is, I just want if the motor's PF is different with VFD's PF, which one should be used to calculate P and Q in the VFD's input side?
2. does VFD produce/absorb the Var to/from the system?

thanks again

David

 

[dpc]

Maybe this will help:

thread237-207697

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[cesgibis]

VFDs do not require a lot of reactive power from the upstream network. P.F. is typically around 0.97.
The VFDs have capacitors that provide the reactive load required by the motor.

 

[ScottyUK]

Displacement PF is about 0.97. Distortion PF certainly isn't.

The capacitors on the DC link have nothing to do with power factor or reactive power - they are in a DC circuit, so how do you propose they have an effect on power factor? The reactive power demand is delivered by the inverter.


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If we learn from our mistakes I'm getting a great education!

 

[royclh]

is there any information about VSD and its reactive power demand? I hope to get more understanding about the VSD. The directly connected drive would stall when the terminal voltage dips more than 80%. What about the VSD? What happen if it stalls and what is the power factor at that time, does it maintain the same power factor of 0.97 as mentioned? i need to build up my knowledge in VSD. I read some info about the VSD from wikipedia but still not able to get my head around it.

thanks,

 

[ScottyUK]

The wiki article on power factor might be a starting point, especially the bit about non-linear loads. You don't need to worry too much about it being a VFD particularly - the front end is normally just a 6-pulse rectifier and capacitor unless you are either paying for a fancy drive or are talking about something really large.

Any decent power electronics text will cover power factor, displacement factor and distortion factor in detail.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

Reader's Digest version:
A VFD is a 3 part power conversion system.

1) It rectifies the AC to DC, called the "Converter Section". This section uses the incoming AC power as the "raw material" for creating a completely new power source for the motor. So from that standpoint, anything other than raw power needs by the motor is isolated from the AC line, i.e. power factor, terminal voltage etc.. The only limit is that the Converter cannot create voltage that is not present on the AC line terminals. It rectifies to the peak voltage value of the sine wave (as opposed to the RMS value) but for the most part, that's somewhat irrelevant because when the motor uses it, it uses RMS anyway, so the net effect is for all intents, the same or less as the incoming.

2) The "DC Bus" section smooths and conditions the DC with inductors and/or capacitors to remove the ripple resulting from rectification. It also usually supplies the control power for the electronics, although that varies from one mfr to another. Some have a separate SMPS to do that.

3) It recreates a pseudo-sine wave AC output of variable voltage and variable frequency in the "Inverter Section" by firing high speed transistors in a PWM pattern. To provide a constant level of output torque from the motor, the VFD maintains a constant ratio of RMS voltage an frequency, referred to as the V/Hz ratio. This can be tweaked to get specific performance out of the motor, but in general the motor's operation is dependent upon this being maintained and controlled.

So when your input voltage drops to 80% of normal, the VFD can no longer provide 100% voltage at full speed. If by chance your commanded speed was no more than 80% of full speed, then no problem. But if you commanded more than 80% speed when the line dropped to 80%, the VFD was no longer able to maintain the proper V/Hz ratio and the motor lost torque. When that happens, slip increases and the motor draws more current, possibly to the point of overloading.


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