CM3 for higher current 3 phase logging

Post any questions you may have about our current range of USB data loggers
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Joined: Tue Dec 10, 2019 4:26 pm

CM3 for higher current 3 phase logging

Post by AndrewC2 »


I would like to log higher currents in the range of around 500 to 1000 A/ph, all three phase supplies.

We have always bought your CM3 kits with inclusive current clamps which have a 200 A max current measuring limit so I asked if other clamps were available.

I was told that the current clamps need to give an RMS output because of the way the CM3 works. I find this a little odd because the clamps are just passive devices and I know from other comments that the CM3 is processing the mV input values to display the RMS current value, so it seems to me that the CM3 is not being given anything RMS related by the current clamp.

The spec of the CM3 states it requires 0 to 1V. Your current clamps state they output 1 mV/A.

Please can someone explain why I cannot use any other current clamp with a known output mV/A ?

If I am not so restricted by your current clamp range, I can justify buying more CM3 loggers.


Posts: 279
Joined: Tue Oct 21, 2014 3:07 pm

Re: CM3 for higher current 3 phase logging

Post by AndrewA »

The CM3 input range is 0-1VAC rms, for each channel.
The CM3 converts the AC signal on its inputs and returns a rms value. This over the rated input frequency (20 Hz to 1 kHz)
These conversions are done in the logger itself.
So the logger could be used with any sensor where you are only interested in logging the rms value of the signal.

So yes you can use any other clamp with a known output mV/A.

Our current 200A clamp that can be brought with the CM3 gives 1 mV/Amp, but only has max. current rating of 200A, which would give 200mV for a sinewave on the CM3.
Also this clamp is passive and does not require batteries. But can only measure AC and not DC current, which is fine the CM3.
So you can't use passive clamps with a scope for example, to measure DC currents, only AC current.

You can use powered AC/DC current clamps with the CM3. But note the battery life is normally around 24 hours.
Some offer an external power input using a power adaptor.
Most use a 9v battery, which you could use a 9vdc power adaptor connected to the clamps 9v battery clip via another 9v battery clip.
9v battery clip
Searching for higher current clamps 1000A and above, I did find some passive ones but these seem to be more expensive than powered AC/DC clamp of the same current rating.
I will email you the current clamps I found.
Regards Andrew
Technical Specialist

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