VFD Output Current?

[Strahan]

I have recently run into a problem that I'm having trouble explaining to my men. We have a 3HP Allen Bradley 160 variable frequency drive controlling a 1.5HP motor rated at 2.4amps. The drive rated output current is 6A continuous. The problem lies in my current readings. If I use the HIM on the drive to display output current it reads 3.0 - 3.2A and when I put my Fluke 322 clampmeter on each leg it reads 2.4-2.8A? Each leg reads the same with my meter. Why the big difference?

I have noticed this throughout our plant with all our 160 AB drives. Now I have also compared these drives to our powerflex 40's and they display very close to my meter readings. Is it something just with these drives?

Next question would be how to set my trip current setting? Service factor of motor is 1.15 so I can size at a max of 125%. 2.4 * 125% = 3A. Well of course the drive trips. What reading do I use? Now I do presume a mechanical issue with this machine because either way the motor is running more loaded than it should, but this circumstance has brought this problem to my attention. According to the clamp I'm still good on current draw, but according to the drive I am not

 

[Jraef]

I have recently run into a problem that I'm having trouble explaining to my men. We have a 3HP Allen Bradley 160 variable frequency drive controlling a 1.5HP motor rated at 2.4amps. The drive rated output current is 6A continuous. The problem lies in my current readings. If I use the HIM on the drive to display output current it reads 3.0 - 3.2A and when I put my Fluke 322 clampmeter on each leg it reads 2.4-2.8A? Each leg reads the same with my meter. Why the big difference?

I have noticed this throughout our plant with all our 160 AB drives. Now I have also compared these drives to our powerflex 40's and they display very close to my meter readings. Is it something just with these drives?

Next question would be how to set my trip current setting? Service factor of motor is 1.15 so I can size at a max of 125%. 2.4 * 125% = 3A. Well of course the drive trips. What reading do I use? Now I do presume a mechanical issue with this machine because either way the motor is running more loaded than it should, but this circumstance has brought this problem to my attention. According to the clamp I'm still good on current draw, but according to the drive I am not

The PWM output of a VFD is not a real AC sine wave, it is a "pseudo sine wave" meaning it makes the motor ACT like a sine wave would, but it really is a very high speed set of + and - DC pulses. Only very expensive filtering meters can accurately interpret a PWM output and all of it's harmonics to give you any kind of useful reading. The VFD already has to be capable of doing that for its internal feedback loops, so the display on the VFD is much more accurate than what the average hand-held portable multimeter can read. Bottom line, trust the VD, not your meter. The protection scheme inside the VFD is based on that anyway, so it makes sense even from that standpoint.

As to why you don't see that on the other drives? Probably better filtering on the output of the drive. The 160 is the cheapest thing they make, the PowerFlex is the most expensive. You get what you pay for...

It's generally considered a bad idea to use the SF on a motor being fed by a VFD, the VFD operation already puts more thermal stress on the motor. Set the OL protection to the motor nameplate FLA, forget the SF even exists.

PS: The attached chart from Fluke shows that the 345 is the only clamp-on they recommend for reading the output of a VFD.

 

[Besoeker]

The PWM output of a VFD is not a real AC sine wave, it is a "pseudo sine wave" meaning it makes the motor ACT like a sine wave would, but it really is a very high speed set of + and - DC pulses.

The voltage pulses go positive and negative. The current doesn't.
The motor inductance and the high carrier frequency means that, in most cases, the current is reasonably sinusoidal.