False ph-ph Fault
False ph-ph Fault
(OP)
We have a large (3500 hp) motor, that sometimes trips on start up on a false ph-ph fault. It often starts on the second attempt. Any ideas on what the problem could be? We have changed the relay, and the CT's have been checked.





RE: False ph-ph Fault
On the second attempt, the windings are warm from the first attempt, and should draw slightly less current.
Can you establish a correlation between ambient temperature and failure to start on the first try?
Mike Halloran
Pembroke Pines, FL, USA
RE: False ph-ph Fault
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: False ph-ph Fault
This can explain an "intermittent" trip in some cases. We often see this with transformer primary protection as well.
Because the inrush current is not equal in all three phases, the motor relay could be interpreting this as a phase-to-phase fault.
If the motor is tripping instantly upon starting, I would suspect this could be the cause. The solution may be to increase the trip setting. The NEC allows up to 13 times the full load current of the motor if necessary to avoid nuisance tripping. I'd check with the motor relay manufacturer and get their opinion.
RE: False ph-ph Fault
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: False ph-ph Fault
Following a little further out the possibility of temperature playing a role:
1 - The small change in resistance associated with tmperature (heatup from first start) would have neglible effect on locked rotor current, since locked rotor current is determined primarily by reactance. I don't agree with the idea that the small change in resistance
2 - The increase in rotor resistance associated with temperature could have a fairly significant effect on the motor torque speed charactersitics, which likely would make the motor accelerate somewhat faster. This might make the difference between time overcurrent trip or not if the setting is marginal to begin with. How long does it take the motor to get up to speed?
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