measurement of 3 phase input currents and voltages 2

[MotorPump]

Hi,

I'm planning to measure 3 phase 230 VAC input voltages and currents to a motor with Data Acquisition Board which accept -10V ~ +10V analog signal.

The total number of collecting data will be 6 analog signals(3 voltages and 3 currents).

I would like to know if there are commercial devices or equipments (including sensors, transducers, or signal conditioning units) designed for measuring 3 phase AC signal conveniently. If any, please recommand equipment and company.

Or let me know ways of measuring (or monitoring) AC power source.

Thank you in advance.

 

[Skogsgurra]

Hello motorpump,

This is one of the more common measurements in the industry and a lot of equipment ranging from dedicated power/energy analysers to simple transducers with 4-20 mA or +/-10 V output is available.

Doing it with a Data Translation board and a PC involves some work - both hardware and software. The simple way is to buy a three-phase transducer with +/-10 V output and input it to one channel on the DT board.

The more complex way of doing it is to install voltage and current transducers. One would normally think that three voltage transducers and three current transducers would be needed and there are such cases - but for a symmetric load like an induction motor, you could do with just one voltage and one current transducer. All you need to do is multiply the result with three to get the total power.

The measurement per se needs that you sample voltage and current fast enough to avoid artefacts, multiply the samples and accumulate the products. Doing this will take care of the power factor (cos(phi)) automatically.

How fast is fast enough? If you have a clean sine, then ten or twenty samples per cycle is sufficient. That translates into 500 - 1200 Sa/s for each channel.
If you have a VFD with PWM output, then you need to be careful. Either filter the PWM or sample very fast. I would say that you need to sample at least five times the PWM switching frequency. This would mean something in the 10 - 100 kSa/s range. So filtering is the better choice.

There is much written about this. Have a look at these:

http://www.m-system.co.jp/english/service/measure_index.html

http://www.secondwind.com/phaser.htm

http://www.lee-dickens.co.uk/process/sigcon_powmes.htm

http://www.electricityforum.com/products/current-transformer.htm

http://www.crmagnetics.com/

There are many more. Google "current transformer" and "voltage transformer"

 

[BigLe]

I would suggest using a digital scope or daq unit instead doing a high-power to logic-power conversion on your own.

some scopes, check if you have some Yokogawa ones, can store quite a lot data (2G and more... ) and some advanced multimeters (fluke.. ) get the interface to the computer... check if you have some of those...

otherwise you need to design the isolation circuit to get the low voltage signal safely..