Measuring Current after a frequency converter.

[MrQ]

Hello,

We are about to deleiver a system to the US.

Since we are running at 400V 50Hz here in Sweden I have connected a frequency converter to transformer that is connected to the motor. This is to get 60Hz 480V.

My question is how do I measure the motor current best? I have tried to measure at the motor with a TRMS ampere meter but seem to get a too high result. Is it the frequency that is not a true Sine wave that makes it wrong?

 

[JvdV]

Hello MrQ,


THe sinus is a chopped sinus. In other words the sinus is mad with a blok voltage. And also my experince with a true RMS meter is not very good.
In my case I could do with the amparage indicated on the frequency convertor. Maybe in your case it's also good enough ?
Example:
Get the amparage and the voltage from the convertor. calculate the VA's. Deduct the losses of the transformer (ask the supplier if it is not mentioned on the type plate) and you end up with the amount of VA's of the motor. This divided with 480 Vac gives you the current. The amperage will be a bit off but it gives you a bether indication then measuring the chopped amparage.

Hope this helps you a little.

Jurgen

 

[DickDV]

To read output current from a drive, simply program one of the analog outputs on the drive to current. That will give you either a 0-10V or 4-20ma signal representing output current and it will be accurate.

Trying to measure current on the output leads directly is next to impossible to do with any accuracy.

 

[JvdV]

DickDV,​

​

I think you missed the piece on having a transformer in between the drive and motor to get a 60Hz/480 V "grid".​

The drive current is the current of the electro motor and transformer. If you want to know the motor current you have to deduct the transformer power / current. And like you also said meassuring is next to impossible.​

​

Jurgen​

 

[DickDV]

I didn't miss the transformer but was only addressing the current measuring issue.

But, it is true that more than one mistake is being made here. The transformer belongs in front of the drive to boost the 400V to 480V at 50Hz. Then the drive output can go to 480V 60hz with direct connection to the motor.

With the transformer between the motor and the drive, first, the transformer if a standard unit, will not survive for long on drive PWM pulses, second, the motor will have absolutely no torque below about 15hz so if it is running anything other than a centrifugal fan or pump, the results will be unacceptable. Third, the transformer losses will be much higher will the result being lower system efficiency and more wasted heat to deal with.

 

[MifPT]

I didn't miss the transformer but was only addressing the current measuring issue.

But, it is true that more than one mistake is being made here. The transformer belongs in front of the drive to boost the 400V to 480V at 50Hz. Then the drive output can go to 480V 60hz with direct connection to the motor.

With the transformer between the motor and the drive, first, the transformer if a standard unit, will not survive for long on drive PWM pulses, second, the motor will have absolutely no torque below about 15hz so if it is running anything other than a centrifugal fan or pump, the results will be unacceptable. Third, the transformer losses will be much higher will the result being lower system efficiency and more wasted heat to deal with.

That is you don't get an error on the drive.

DickDV is absolutely correct.

You should parameterize the driver to send you the motor current over a 4-20mA output or over the driver network.


Regards,
Miguel

 

[AltaVoyager]

Hello,

We are about to deleiver a system to the US.

Since we are running at 400V 50Hz here in Sweden I have connected a frequency converter to transformer that is connected to the motor. This is to get 60Hz 480V.

My question is how do I measure the motor current best? I have tried to measure at the motor with a TRMS ampere meter but seem to get a too high result. Is it the frequency that is not a true Sine wave that makes it wrong?

I understand you need to measure actual motor current after
x-mer.
Try to use any standard CT with 4-20 ma output.
Something like that
http://www.westecinstruments.com/Manual%20-%20CT-9112.pdf
http://www.ct-supply.com/images/CT-100.pdf
You are right about frequency, but it's actually "right" data
to use. Since it's not true sine wave, you have to use
"true RMS" meter to protect motor from overload.
Most VFDs, when coupled with step-up x-mers and run unloaded or not at full speed, will show wrong output current.
Check you VFD manual, usually there is three currents: total, torque and flux.
DickDV
It sounds that he is just doing test and 380V power and 400V VFD are only options in Europe. Otherwise why not to use different motor?
By the way you right about x-mers: cheap one will smoke out in no time, good SouthWest or TAPS will last for many years.

 

[panic mode]

true RMS is supposed to give correct reading regardless of shape of the signal. this is improvement over instruments that are designed to take measurements of the sinusoidal signals but be carefull, unfortunately there is also a limit - and usually it is not clearly defined on multimeter specs. most "true RMS" meters are only good up to some 400-500Hz and that is IF the frequency response is listed in the manual (often even if this is mentioned, it is only listed for voltage). some meters (few) do go higher but most drives will easily run on 4 or 8kHz or higher. moral of the story - if you want to know what is the motor current - ask the drive.