Electrical – VSD (Inverter) current measurement on 3 motor phases

currentinduction motorinvertermeasurementvfd

I'm working on a project to build a rig for testing various sized Variable Frequency Drives (VSD's / inverters), the kind that you'd use to run a squirrel cage induction motor. The range would be from 10 to 140A (75kW max), 0 – 65Hz @ 415VAC. There will also be a variable mechanical load on the motor shaft.

The test I need to perform is to measure the current in each of the motor output phases, U, V & W through the VSD's speed range of 0 to 60Hz. At first this sounds easy, just measure the RMS current of each phase with a CT or hall effect current sensor. But the devil's in the detail.

The output of a VSD is not a pure 3 phase sinusoidal waveform but is a PWM square-wave that is smoothed out into a rough sinewave by the inductance of the motor windings. This means there's a lot of harmonics that spoil the readings of most RMS digital current meters. There are some meters that will now compensate for this using a built in low-pass filter. Link

Also the VSD output is variable voltage (Hence current) and variable frequency. AC current transformers become useless as the frequency approaches zero. A hall effect based sensor would probably be better here.

So, I'm looking for some kind of current sensor, probably something that has a built in low-pass filter for reasonably accurate (5%) readings and gives out a proportional signal, e.g. 0-10V / 0-20ma / 4-20ma. The lowest frequency I would expect it to work at – from my experience – would be about 10Hz.

Ideally, something like an industrial sensor version of the Fluke 87V DMM referenced in the link above.

If all else fails, then just a current shunt and analogue meter would suffice but ultimately I'm looking to capture the data into a PC for graphing.

Any advice would be appreciated… 😉

Edit: Although this sounds like a shopping question – it probably is – it may end up as an electronics design question because I think there's nothing on the market that exists to solve this problem.

Best Answer

Something does, in fact, exist on the market, and it's not too expensive, either. Take a look at the Allegro Microsystems ACS770. – Felthry

Hi Felthry, Thanks for the info on that device, it looks like the solution, so I'm going to:

  1. Order the ACS770 evaluation board
  2. Buy a low pass filter and a signal isolator
  3. Hook it up to a single motor output phase on a 7.5kW inverter and motor
  4. Analyse the signal with a PC capture card & digital meter

Thanks again!

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