Sync Motor PF Trip 7

[T247]

We have a sync motor that is tripping on power factor shortly after it starts. The motor comes up to speed and the trips on power factor after the delay expires. They were able to watch the PF on the relay display during the start event. The PF is initially at 0.3 lag, but then decreases to 0.04 lag by the time it tripped (the PF trip is set at 0.8 lag for 12 seconds). The waveforms show the currents are lagging the voltage by about 90 degrees. What could be causing this to happen? The relay was changed and work was done to the compressor (not the motor) but the same thing was done to a sister motor without any issues.

 

[edison123]

That's a good point, Lionel.

Maybe the CT's or the relays are reverse wired that could show leading PF as lagging.

Muthu

http://www.edison.co.in

 

[jraef]

That’s the first thing that came to mind for me too Muthu. In the relay replacement process, someone didn’t pay attention to CT orientation (or was unaware that it matters), so it’s reading backward. I have seen that on more than one occasion.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[wcaseyharman]

In my experience It’s also a lot cheaper and faster to verify relays and relay configurations first before tearing into the machine. On our brushless exciters it can require a fair amount of disassembly to get to the diodes.

Casey

 

[edison123]

Jeff, I didn't think of reversed CT/relay polarity until Lionel's great point.

I hope the OP updates here as a basic courtesy to those who have given him/her their time and valuable expertise for free.

Muthu

http://www.edison.co.in

 

[T247]

We haven't been able to investigate this further so nothing to report yet. I'll update this thread once we determine the cause or have more information. I do appreciate everyone's inputs.

 

[T247]

They replace the Sync-Rite device on the diode wheel and they were able to get the motor to run.

 

[waross]

The main field is not applied until the rotating "Polarized Field Frequency Relay" or electronic equivalent completes the circuit from the rotating diodes to the main field.
The rotating Polarized Field Frequency circuit may have failed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

So, the rotor was not getting the juice because of a damn switch?

Muthu

http://www.edison.co.in