I have an application where we need to understand the voltage and current being output from the drive to the motor (one leg of the 480VAC, 3-phase motor). We are using a Reliance Electric GV3000 drive. I started with a simple AC current transducer but that won't work on the output side of the drive since the frequency changes anywhere between 3-200 Hz.

Is anyone out there have a similar application or have done this before? Is there a transducer or instrumentation out there that can do this? What is the best way. I need to be able to monitor continuously and need to monitor that value in the PLC.

 

I am guessing that you don't have this drive networked. Speed or Torque is available using the built-in instrumentation with the drive's analog output on terminals 10 and 11, see parameter P.012. The display can show you voltage and current, but not at the same time. There are true-RMS transducers on the market to take accurate measurements of the drive's output.

> I have an application where we need to understand the voltage and current
> being output from the drive to the motor (one leg of the 480VAC, 3-phase
> motor). We are using a Reliance Electric GV3000 drive.

 

All AC Variable Frequency Drives (VFD) have an electrical output that is a variable frequency and voltage that when passed through the coils of an electric motor "simulate" a sine wave. If you try to measure this with an AC VOM, you will not get a proper RMS measurement. The drive itself is a microprocessor device with digital outputs of RMS voltage and current, but you will need to consult the User Manual to find out how to connect to the drive software. There is a standard RS-232C connector, but you will need software for a PC that you can get from Rockwell Software. There are also DeviceNet options available. Again, as for advice from Rockwell.

 

A true RMS meter.

--
Bob
http://ilbob.blogspot.com/

 

And it depends on what you mean by understand. If you need to understand the relationship of the instantaneous voltage and current, you need a two channel oscilloscope, preferably a DSO, a voltage probe and a current probe. The waveforms can be extremely complex and require much interpretation in the usual case of PWM at kHz frequencies.

Sometimes I find it amazing that a motor acts as if all that hash was a simple sine wave.To get the motor's perspective, low pass filtering to about twice the output frequency helps a lot. If your scope won't do that, you can collect the datasets and apply a software digital filter. Digital sampling can be a beautiful thing.

It all depends on what you need to understand. There is a lot going on on the output of a typical AC drive. it's good that we can normally ignore the details.

Regards

cww