measurement of the current in a motor


Thread Starter


I need to measure the current in an AC motor,I'm using a transformer (donut) with a ratio 10/1, so as the motor is 2Amp. I have 0.2 Amp in the transformer. HOW CAN I CONVERT THIS CURRENT IN A 0 TO 10V OR 4 TO 20 mA SIGNAL? I need to send this to a siemens PLC and after from the PLC to a computer.
Rather than re-engineering a current transducer
why don't you check out They already solved current measuring problems. For 10A they have one that puts out 5V, you can then take this and may be in your software calibrate the 2A output voltage equivalent to 10V.

If you must make it yourself, here are suggestions though I have not tried this so no
guarantees. I think you need rectify the output [for dc], put a resistor divider. Then build an +15V opamp circuit and gain up the output to match your intended value. You'll have to make sure that you compensate for offset errors and keep those gain resistors tolerance tight. Better yet, get an opamp textbook or application notes from one of those IC companies.

Good luck.


At our facility we usually use a Moore Industries (not to be confused with Moore Products,) ACT. AC Current Transmitter. These come as stand alone units or rack mount if you have several applications. The output of your ct is connected to the ACT which is ranged and calibrated for a 4-20mA output. You application is lower current than I have seen but I sure Moore can provide the proper unit.

Remember never to open the circuit of a ct under load. They can make fire and smoke. (Safety Hazard)
Moore Industries offers signal converters for this purpose. For information, go to Or, feel free to telephone or e-mail me directly and we will help you specify an instrument.

Steve Todd
Moore Industries
[email protected]

Hunter Farris

Several ways.

First you can try using a signal conditioner such as the SlimPak from Action Instruments.

Or even easier and cost effective is to use current transducer such as offered by Neilson-Kuljian. They even come in split core with 0-5VDC or 4-20mA outputs.
There are several current to current transmitter manufacturers out there. Try Ohio Semitronics, Rochester Instruments,or API to name 3. Try and build one yourself would be mucho more expensive - isolation could be fun.


Gilles Allard

I've used two different models with Siemens PLCs.
One is from Riley Instrument
The other is from Entrelec
Both units include the toroidal transformer (donut!) and the 4-20mA converter.

You do not specify if you use a VFD (variable speed drive). If you do, you will need to check manufacturer specifications. I doubt they would be accurate (I never tried it).

1. See posting in the list for the last two weeks which describe various options for doing this.
2. If you aren't firmly wedded to the idea of using the current transformer you already have, then check the web for vendors that sell current
transformers with 4-20 ma output electronics built-in. This is easier and 'cleaner' than putting the circuitry together yourself.


Johan Bengtsson

Better yet (if you build it yourself, otherwise skip this) is to feed the opamp with +- voltage and rectify after the opamp (you loose less information due to voltage drop in the diodes), it is even possible to put together opamps and diodes to completely get the voltage drop dissapear by feeding back the signal after the
diodes instead, but that might be overkill for you.

/Johan Bengtsson

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Robert Dusza

Try CCT series signal conditioners from Omega. They have devices that will convert the amp readings to voltage or milliamps. We use them and have good success.

Bob Dusza

Robert J. Dusza, Jr.
Project & Technical Support Manager
(V) 1-860-647-3219
(F) 1-860-647-3150
E-mail - [email protected]
Manchester Water & Sewer Dept.
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Manchester, CT 06045-0191
A company called Load Controls Incorporated makes tranducers that derive power, not just current, from an AC motor. They can transmit this data to
another control with an analog output.

Bill Sturm