Current imbalance in three phase chiller motor 7

[Run1on]

I have a current imbalance in a three phase chiller motor (480v 300 hp)Each phase: 1- 200 2 - 215 3 - 252. The voltage imbalance for this is less than three percent. We are also seeing other motors in the facility experiencing current imbalance. We also see the power factor readings for each leg can be more than 1o percent diffeent. If one motor suffered current imbalance I would suspect a motor problem but five tend to lead to a distribution problem. Is it possible several nonlinear loads (277 lighting, etc.) can be a contributing factor?

Run1on

 

[peterb]

This is beginning to sound like a case of a high resistance joint in one or more phases. This could be in a cable termination or in a circuit breaker contact.
One approach to finding the problem, in addition to those noted in the excellent posts above, would be to find where the voltage imbalance starts.
With the system loaded, begin at the line side of the main breaker and move to the load side, then further downstream until the voltage unbalance is found. When you get to this point, the element immediately upstream will be the culprit and appropriate corrective action can be taken. Offline tests to confirm the condition would include joint & contact resistance checks, using a Ducter or high current source (NOT an ohmmeter).

 

[Run1on]

peterb:

I find that conclusion highly plausible. This school renovation was repeated at two other schools with the same design firm and electrical contractor. It is very possible When I have the results of the further investigations I'll post them. It helps greatly when a question is posed and the result is given to everyone.

We found a discolored phase wire on a subfeeder this morning; this feeder fed the kitchen area and we're waiting for the load usage to go up in the area to IR camera the joint to see if it is loose.

Thanks

Run1on

 

[busbar]

With the discussed imbalances, you may be able to smell the joint failure. Depending on how much overtime you need, consider in-service, energized millivolt-drop tests on the electrical-gear busbar joints, breaker/fuse poles and„Ÿcarefully applied„Ÿcable terminations. Use a power-rated multimeter set on the AC-millivolt range, that will not be affected (except for an overrange reading) if unintentionally applied at any phase-to-phase voltage. Load currents must be roughly at their usual for the readings to have meaning. There is the advantage of have three times everything (one per phase) for comparison. A low-end infrared thermometer or multimeter accessory would be helpful, too.

 

[peterb]

busbar -
I hadn't thought of the on line millivolt drop test, but it sounds like a great idea. Just a word of warning to the unwary - only try this if you are fully conversant with all safety implications, as it entails working on energized circuits, possibly defeating door interlocks etc.
Yes, the infrared approach can be a winner here as well - it probably is the easiest test to do under the circumstances.