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.

  • \$\begingroup\$ Welcome to EE.SE. Shopping questions get closed very quickly (see Help) so you need to edit this into a design question if you want it to survive. \$\endgroup\$
    – Transistor
    May 2, 2018 at 14:03
  • \$\begingroup\$ @Transistor Hi, good point, though I suspect that nothing exists on the market so it will end up as a design question anyway. I'll edit it though. ;) \$\endgroup\$
    – user186848
    May 2, 2018 at 14:19
  • \$\begingroup\$ Something does, in fact, exist on the market, and it's not too expensive, either. Take a look at the Allegro Microsystems ACS770. \$\endgroup\$
    – Hearth
    May 2, 2018 at 14:27
  • \$\begingroup\$ There are three phase power analyzers on the market, eg: Keysight PA2203A and Yokogawa WT3000E, but these are probably lightyears outside of your budget. \$\endgroup\$
    – Jeroen3
    May 2, 2018 at 14:27
  • \$\begingroup\$ The most widely used name for variable speed drive inverters is variable frequency drive (VFD). There is a VFD tag on this site. That is a good term to use in searching for products. There are a lot of products that might suit your purpose. Look at lem.com/en. \$\endgroup\$
    – user80875
    May 2, 2018 at 15:51

2 Answers 2


Good quality VFDs already have hall effect sensors built-in to the output circuits, the VFDs need them in order to determine the vector control algorithms they now use. That then means that you can access individual phase currents in the electronics of the VFD, and it is going to be MUCH more accurate than anything you can add after the fact. If you are using a VFD that does not provide that information, you might want to consider using a better quality VFD.

  • \$\begingroup\$ Hi, All the VFDs that I've worked on, (10+ years.. a lot!) have internal phase current monitoring for phase O/C / loss faults etc and for vector calculation, but don't output those measurements as output signals. \$\endgroup\$
    – user186848
    May 10, 2018 at 7:12
  • \$\begingroup\$ The test rig I'm making is for testing various random VFDs so I'm not guaranteed any those will have the functionality of outputting a current value signal for each phase. :-( \$\endgroup\$
    – user186848
    May 10, 2018 at 7:45

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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