False ph-ph Fault 1

[chadwiseman]

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.

 

[MikeHalloran]

<no special expertise claimed>

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

 

[jraef]

What kind of relay do you have, and what are the settings?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

I don't know what type of relay you have, but when a motor is started, there can be a signficant asymmetrical (dc offset)current for a cycle or two. The magnitude of the dc offset is a function of the phase angle of the voltage when the breaker, or contactor, closes.

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.

 

[jraef]

That is why I was asking. It is a relatively common phenomenon that your relay may not be set up to properly filter out.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[electricpete]

I agree with jraef and dpc about the random nature of the dc offset, depending on voltage phase angle at closing. If it's a random problem that happens once every five start attempts, then it would only hit once out of every 5 second start attempts.

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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