Problem with Reading VFD input current
Problem with Reading VFD input current
(OP)
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?
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?





RE: Problem with Reading VFD input current
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"
RE: Problem with Reading VFD input current
RE: Problem with Reading VFD input current
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"
RE: Problem with Reading VFD input current
But I still wish I could find out why my meter reads lower.
RE: Problem with Reading VFD input current
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Problem with Reading VFD input current
Keith Cress
kcress - http://www.flaminsystems.com