Problem with Reading VFD input current

[richanton]

I looked through the forums here and could not find an exact answer, but here is nmy situation. I have a 5kv chiller motor with a VFD. The VFD has a dsiplay and shows output power and output voltage. I then have CT's and PT's on the input side that I use to read a Powerlogic display on the chiller. The power shown on the Powerlogic meter is always about 15% less than the power shown at the VFD. Is it normal that these values would be different other than for some minimal losses.

So I'm wondering if I could have wired the CT's incorrectly, or if there is some inherent issue with using standard metering CT'a and PT's on the input to the drive. I stress here that the Powerlogic is monitoring the input vs the drive displa that shows the output.

We are using this value in the control scheme to determine if I need a second chiller online, We know the Powerlogic is wrong because we used that value to do the control, and the second chiller did not start when it was supposed to based on the setpoint. Had we been using the output number from the drive itself, the second chiller would have started when expected.

Any ideas?

 

[jraef]

Harmonics created by the drive wreak havoc on common metering unless the meter is capable of properly interpreting the complex waveforms. PowerLogic meters run the entire gamut of capabilities, but it is usually only the top end versions that can do this with any accuracy.

That said, we're it me, I would not base any control on the INPUT power to the drive, because load on the motor is directly measurable as kW output of the drive, so you eliminate measurement errors and you remove drive efficiency from the equation, which mean nothing about motor loading. I do not know of an MV VFD on the market that is not capable of providing output kW in analog form, or made available over serial comms. Who's VFD do you have?

"Will work for (the memory of) salami"

 

[richanton]

It's a Rockwell drive.

 

[jraef]

Assuming it's a PowerFlex7000...

There are 8 Analog outputs ("ports"), 1 isolated, 7 non-isolated. They can each be programmed to a function that is called "Meter 4" which is Output kW of the drive. That is a direct indication of load that the motor is experiencing.

"Will work for (the memory of) salami"

 

[richanton]

It is a Powerflex 7000. Thanks for the info.

But I still wish I could find out why my meter reads lower.

 

[Skogsgurra]

The answer has already been given. You need to use a device that doesn't bother what wave-form you feed it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

The sharp angles of the current waveform inherent on the input of your VFD cannot be properly seen by your current coils. The resulting power derived from those inputs is incorrect and always will be without special equipment. The input power, as jraef has stated, is a lousy number to be using for ANY kind of control.

Keith Cress
kcress -

http://www.flaminsystems.com