Curent Transformer to Monitor Amperage 2

[Ripcord13]

I want to monitor amps on a 75 hp 480 volt 3 phase motor with a remote digital ammeter. Does it matter if I install the current transformer (doughnut) on the load side or line side of the VFD? Thanks.

 

[davidbeach]

VFD current on the line side, motor current on the load side. You would need a very good ammeter to get a good reading of motor current. Probably better just to get a remote annunciator for the VFD and read the current from that.

 

[Skogsgurra]

Yes. It matters a lot.

An ordinary CT doesn't work well below rated frequency (50 or 60 Hz, depending on where you are). So, if you go down with motor frequency, it is very probable that the CT will saturate and introduce significant errors. On the other hand, the mains current is rather distorted and an RMS meter will probably show too high a value if you put the CT there.

Most frequency inverters have have an output where you can read the motor current as a 4-20 mA or 0-10 V analogue signal. That is what I would use in the first place.

Gunnar Englund

http://www.gke.org

 

[LionelHutz]

The current on the line side of the VFD does not equal the motor current. The line current is basically to feed the real power used in the motor and VFD. The motor reactive current circulates between the motor and the VFD. So, you must read current on the motor side to get an accurate reading. However, as Gunner has pointed out, many CT's or transducers don't work right on low frequencies so you'd need one built for use on the VFD output.

As Gunner has also pointed out, use the analog output of the drive to feed the meter, likely a 4-20mA loop.

 

[ScottyUK]

Either use the drive's output as suggested above, or use a Hall effect sensor such as those manufactured by LEM and others.

http://www.lem.com

These sensors will typically work from DC up to a few hundred kHz. This solution would require a low power auxiliary supply to drive the transducer, or you could probably steal a 24V supply at a couple of hundred mA from the VFD internal auxiliary supply.


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