General VFD Question

[7JLAman4]

General question regarding PWM VFD (inverter)operation. Using the constant torque setting (V/f), does the input current of the inverter increase as the output frequency increases? and if so, then is it proportional to the output frequency?

 

[DickDV]

Drive output kva has to be supplied from input kva. Thus, there is no correlation between output frequency and input amps. Since input voltage is nearly constant, you could expect input amps to rise and fall in fairly close relationship to output load.

 

[waross]

Using the constant torque setting (V/f), does the input current of the inverter increase as the output frequency increases?
It may but this depends on having a true constant torque load rather than the VFD setting.

Bill
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"Why not the best?"
Jimmy Carter

 

[Marke]

If your output current is constant with changing frequency, and the drive has a constant V/F, then the input current will increase linearly with frequency (in theory).
As DickDV stated above, the output VA is supplied by the input VA and as the voltage on the output changes, the ratio of input to output voltage changes, so the ratio of the currents must also change to keep the input VA almost equal to the output VA.

With a vector drive or a drive that does not have a constant V/F, the output voltage still changes with frequency, but not in a linear fashion, so the input current still changes with frequency, but not linearly.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[ScottyUK]

Dick,

Can I politely disagree with you, on choice of terminology at least? Input kW and output kW have a definite relationship with the difference being the drive losses, but input and output kVA_r aren't so closely related as one depends on the motor magnetic design and the other depends on the front end rectfier, and by implication the input and output kVA have a less obvious relationship. I know you know that I know that you know.


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

 

[itsmoked]

Scotty; I too left scratching my head on that very point.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

I used the term "kva" because of the non-linear waveforms at both input and output but I suppose "kw" would be more precise. In fact, I hardly gave it any thought! Probably should have!

Thanks for the correction, DickDV