testing VFD motor currents 1

[mixg]

Can the current draw on three phase motors run off VFD's be checked the same as line powered? I have a number of small motors (2 to 15 hp) run off VFD's I'd like to monitor as part of a preventative maintenance program. At times I have to contend with a slight supply voltage imbalance and would like to check each phase but my drives don't display current draw that way. Can I use a conventional amp probe with a digital multimeter (true RMS)? Any help is appreciated.

 

[buzzp]

As long as the meter is true RMS and both can handle the 'crest' factors of the current your measuring (see manual on drive and should mention this or you may have to measure this if you have the properly rated oscope for the voltage your dealing with - high voltage probe is most likely required). Typical rms meters can handle crest factors of 3 and some go to crest factors of 5. Crest factor is the ratio of peak current to rms current.

If you measure the currents you can get the current unbalance but it will not include the negative sequence currents generated from the voltage imbalance. In any case, you can get a good idea of potential motor overload due to the imbalance.

 

[dpc]

On a typical PWM drive, the output current from the drive to motor bears little in common with a sine wave. Measurement of the current with a typical "true RMS" meter probably won't tell you much. At least, I've never had much luck trying to measure it. Even a power quality type device, such as Dranetz, gives very ambiguous results.

BTW, a supply voltage imbalance into the drive shouldn't have much impact on the motor since the drive converts the incoming ac to dc before converting it back to the PWM pulses it sends to the motor.

 

[buzzp]

Yep I missed that DPC. The input to the drive will not affect the output of the drive as far as imbalance.

However, I have had great luck with measuring current with RMS meters and drives. I have checked them with using a shunt and a scope.

 

[busbar]

I have no specifics on what is different about them, but there is a {new} meter series intended for motor-drive electrical measurements… “Fluke 80 Series V meters… selectable filter for accurate voltage and frequency measurements on motor drives.”

One effect of line-side voltage imbalance on the traditional 3ø full-wave bridge rectifier is increased line-side current harmonics with increasing line-voltage imbalance.

 

[DickDV]

My advice, based on years of experience, is to forget trying to read any data from the motor leads. DPC's advice is right on.

Let the drive give you the measurements off the keypad or if you want continuous monitoring, simple use an analog output mapped to the output parameter desired. There is no better way.

 

[jbouverette]

to further busbar's point-

amp draw is only 1/3 of the total picture. you can get most of the picture with 1.amp draw 2.voltage (line to line and line to ground, carefull, some VFD's don't like you taking these measurements while it is running) 3.frequency

excluding a scope of figure harmonics and such, i have had great results with this. And....making sure you have the right equipment installed...inverter duty motors, chokes, etc....

 

[jraef]

I agree with most of the above, especially the comment about how usefull VFD output current is taken alone. But I want to point out a simpler solution to reading output current (if you must). Use an old-fashioned analog clamp-on meter, like an Amprobe. For the size of motors you mentioned, they will work fine.

That said, did you understand the points made about the non-existant relationship between input and output in a VFD? This is probably the most commonly misunderstood concept. The VFD uses input power simply as a "raw material" for it to produce an output that will make the motor respond in the desired way. Think of a VFD as an entirely new power source, one that just happens to get it's supply from the same utility as the rest of your plant. Whatever is happening on the line side of the VFD, i.e. voltage imbalance, spikes, phase reversal etc., has no effect on the motor connected to the load side. It only affects the VFD itself (with one exception as follows). In fact with regards to "imbalances", other than an increase in input current most VFDs will work fine with only 2 of the 3 phases feeding it! The one exception is that a VFD cannot create a voltage potential beyond that of the input, so if you have 440V voltage on the input, you cannot have 460V on the output.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mixg]

Thanks, everybody, for your input. I spaced on the fact that the output is generated off the DC bus, and therefore unnaffected by input voltage imbalances. My focus has been on monitoring the long term heat effects of the imbalance shortening the life of my line driven motors, and I carried that thought over to the inverter driven ones.
As a quick test, using the meter with an amprobe looks to be relatively accurate, at least in comparison to what the VFDs' displays indicate. I don't know if this would be a useful tool in monitoring the condition of these motors or if the drives actively balance current draw on the three legs if the motor is not presenting the same reactance on each (the drives only display the one - averaged? - current reading).
I do meg the motors but had thought each phase's current draw would give me a more complete picture of the motors'conditions.
Once again,thanks for the help!

 

[buzzp]

Given the small HP rating of your drive, the current displayed can not likely be used to protect your motor because it probably only shows average current. If it shows all three phases, chances of this small drive having 3 analog outputs are slim to none so you will still need to monitor the current with a seperate monitor, unless you want to trip based on average current (not ideal).

Drives are a voltage source, not current, so any current draw will be based on the load, balanced or not.

If you really want to use this for PM, you should consider monitoring the power instead of just current. With these smaller motors, this will give you a much clearer picture of potential problems, depending on your application, than merely looking at only the current. Granted, your drive is relatively steady with its output voltage but dont forget about PF.