VFD Power Factor Angle

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David Castor said:
Yes, the analog Simpson would give you a better indication on the output side of the drive. A digital meter on the output will not provide much useful information. The data provided by the drive itself will be better than either meter.
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Which is why I have kept my old Triplet and Simpson analog meters. I get this a lot with people assuming the VFD is screwed up because they read the output with their DMM. I break out an ancient analog meter and it generally (+-3%) agrees with the VFD display.
 
wwhitney said:
From a theoretical viewpoint, how does the mechanical averaging an analog meter performs compare to RMS?

Cheers, Wayne
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A moving magnet or moving coil analog meter will deflect against the return spring in direct proportion to the instantaneous voltage or current. The mass of the moving parts will average out the applied torque over time. So the meter deflection will be directly proportional to the average rectified AC current.

If you assume a sine wave, then a simple scale factor will allow the meter to read RMS voltage rather than average. For non-sinusoidal waveforms there will be an error. This error will be comparable to that of a digital meter that also senses the average rectified current.

A moving iron meter, where the torque is proportional to the square of the instantaneous current will average the square of the current or voltage, the MS part of RMS. From there a non-linear scale calibration can read out the square root of that value.
 
GoldDigger said:
A moving magnet or moving coil analog meter will deflect against the return spring in direct proportion to the instantaneous voltage or current. The mass of the moving parts will average out the applied torque over time. So the meter deflection will be directly proportional to the average rectified AC current.

If you assume a sine wave, then a simple scale factor will allow the meter to read RMS voltage rather than average. For non-sinusoidal waveforms there will be an error. This error will be comparable to that of a digital meter that also senses the average rectified current.

A moving iron meter, where the torque is proportional to the square of the instantaneous current will average the square of the current or voltage, the MS part of RMS. From there a non-linear scale calibration can read out the square root of that value.
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Isn't a lot of what you just described kind of the reason why a motor will run on a VFD output? I know you can't use a VFD output to successfully drive other load types. "Inverters" use similar base methods for conversion but they put more emphasis on acquiring a more true sinusoidal output.
 
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