# VFD Input power vs. output power



## BBQ (Nov 16, 2010)

Is the amp clamp accurate at the frequencies the output puts out?


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## Electrorecycler (Apr 3, 2013)

BBQ said:


> Is the amp clamp accurate at the frequencies the output puts out?


 
The ammeter is accurate at those frequencies, yes. We've determined that the issue with the machine isn't the VFD, it's working just fine....but I'm still curious as to why this happens.


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## Big John (May 23, 2010)

Electrorecycler said:


> The ammeter is accurate at those frequencies, yes. We've determined that the issue with the machine isn't the VFD, it's working just fine....but I'm still curious as to why this happens.


 You cannot get more power out of the VFD than you're putting into it, so there has to be a measurement error somewhere. The ammeter may have a frequency range above 60Hz and still be unable to accurately measure the output current of the VFD because of the distorted waveform. Do you find that the error in the reading changes significantly with voltage?


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## Jlarson (Jun 28, 2009)

I doubt your amp clamp is capable of accurately measuring on the PWM output. What does the drive HMI read current wise.


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## Tsmil (Jul 17, 2011)

Jlarson said:


> I doubt your amp clamp is capable of accurately measuring on the PWM output. What does the drive HMI read current wise.


Just throwing this out there.... Power factor from motor to drive is much worse than the power factor or the drive to supply.


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## varmit (Apr 19, 2009)

Even a "true RMS" clamp on amp meter will not be accurate on a VFD output. Some VFDs can generate so much electrical noise back into the supply that a clamp on meter will be inaccurate even on the line side of a VFD.


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## JRaef (Mar 23, 2009)

Electrorecycler said:


> I'm getting some odd things happening on one of my machines and hoping that someone could help.
> 
> I have a 250HP VFD supplied with 472VAC. At 1400rpm (full running speed), and no load, my output current is about 80 amps and measured output voltage is about 370V while my input current is approximately 35 amps. When under load the input current increases to about 200A, while the output current shoots up well over 300A.
> 
> ...


First, you are equating current with power, they are not the same. Current is a _COMPONENT _of power, but power also includes voltage and power factor (and in a motor, efficiency).

As Tsmil said, there will be a big difference in the power factors from line side to load side of the VFD. The VFD will be presenting a power factor to the line side that is corrected to typically .95 regardless of motor mechanical loading, whereas the load side (motor) power factor is totally dependent on the mechanical loading of the motor at any given speed. So if the PF of the motor is .70, the load side current will be 300A and the line current into the drive at .95 PF will be 200A (all irrespective of efficiency).


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