testing VFD motor currents
testing VFD motor currents
(OP)
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.





RE: testing VFD motor currents
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.
RE: testing VFD motor currents
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.
RE: testing VFD motor currents
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.
RE: testing VFD motor currents
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.
RE: testing VFD motor currents
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.
RE: testing VFD motor currents
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....
RE: testing VFD motor currents
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"
RE: testing VFD motor currents
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!
RE: testing VFD motor currents
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.