Measuring amps off a Flex70
- Thread starter mister x
- Start date
I'm trying to decipher info from electrician saying amps measured, on output I believe, measure lower than drive indication. But I'm not sure about accuracy of clamps on outputs. Am I right to think internal lloss should be negligible enough that 60hz feed is the place to measure amps here?
Bit_Bucket_07
Member
It's a reasonable method for measuring power consumption, but since VFDs operate at variable output voltages, you wouldn't really be measuring motor amperage.
OkiePC
Lifetime Supporting Member
I used to be of the mindset that you can't trust your clamp meter with VFD outputs, but I have seen some meters make claims that contradict my belief. I would want to know the make and model of the electricians instrument before making a judgement. I have also been lied to by a VFD display which apparently had an internal CT go haywire.
Bit_Bucket_07
Member
That would be a power measurement. Besides, you'd never see a current imbalance on the motor by measuring the incoming line rather than the output.
jraef
Member
Drives can see things on the output much faster than a typical electricians meter can. But if the problem went away by changing the drive, it was likely a current transducer inside of the drive starting to fail.
milmat1
Member
Is this a drive that has been in service or a new install ?
milmat1
Member
It may be that the drive is seeing an high load at certain times, and you would not catch that with the clamp meter.
Also long cable lengths on VFD outputs can cause all kinds of crazy readings...
Old install where issue went away with replacement by other used unit. But now might be repeated on another old drive. Just trying to make sure current troubleshooting is sound, or as sound as possible.
Bit_Bucket_07
Member
A true RMS meter won't give you an accurate reading, but you can still detect current imbalance with a clamp on ammeter as well as an overall increase in motor current under similar speed and loading conditions between scheduled measurements.
mister x said:
The reason the HIM on the drive (PowerFlex 70/700) may display a different output current than the measured output current at the motor leads is because the drive is only accounting for its produced torque current and does not include harmonic distortion of the waveform. This is known as the RMS fundamental current.
A calibrated true RMS clamp meter will display the total RMS i.e. the fundamental current plus the total harmonic distortion of the waveform. This should be a reasonably accurate measurement of the total current consumption of the load.
I would measure the motor load at the leads over a period while also holding the maximum current consumed (most multimeters have this feature). This should record spikes that may occur, indicating any actual overloading.
The drive is correctly sized for the load, I take it?
Regards,
George
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