measuring the AC current from Induction motor


Thread Starter


I'm measuring the AC current from Induction motor.
How can I do it ?
Do i have to use Current to voltage converter?
IS there any reference circuit ?

Pls Help

Nasser Mouadeb

You can consulte with Siemens automation they carry a good collection of interface modules for the power industry.
Good luck.
Typically you will use a current transformer (CT).
It steps the motor current down to a standard range. For instance, if the motor is 25 HP, with a full load current of 30 amps, you would
specify a CT, let's say for a primary current of 0-50 amps, and an output of 0-5amps. So if the output of the CT is 2.5 amps, the current to
the motor is 25 amps. If you need to tie the current reading to a PLC or DCS, you can use a signal converter that accepts the 0-5 amp CT
signal to 4-20 ma, or whatever. Try this web page for some additionalinfo. ""
Heed the warning at the end of the article. Leaving the secondary leads of the CT open with current flowing in the primary will generate
some truly dangerous high voltages on the secondary (essentially the CT becomes a step-UP voltage transformer! Be careful, do some
research...This is very standard stuff..
This is a very common requirement and almost everybody uses a CT+transducer
Use CT on the circuit where you need to measure current.
Give the o/p of CT to a current transducer, which will accept 0-5 amps and give out 4-20 ma. You can use this 4-20 ma signal to Analog i/p of plc.
The only CAUTION IS:
Secondary of CT should NOT be open. This is dangerous.
Please read the above line as bold/italics/underlined.
If you are interested in the RMS value, you need a method that takes the waveform RMS over specified multiples of 60Hz (assuming ac line operation) and outputs dc voltage proportional to RMS. If time interval too long relative to thermal time constant, may risk overtemp [if] using for motor protection function. If time interval too short relative to thermal time
constant, may introduce nuisance (premature) overload indication.

If only interested in the waveform e.g. peak detect, any device that outputs voltage waveform that represents current waveform will do. Here, the required signal bandwidth is an issue.
It is very simple. you have choose and connect correct non contact type Current Transformaer(CT) in all phases, then output of CT shoud be
connected to convertor that will convert currnt to voltage(o-10vdc) or 4 - 20 ma.

P. Dave
I assumed you meant to measure the ac current going into (being consumed by) the induction motor. All you have to do is to get an AC current meter also known as a clamp meter and clip the current clamp on one of the motor leads and you can read directly the RMS current value. Most lower price digital multi-meters come with such capability.