Motor Tripping part 2

[nightfox1925]

What is the possible cause of breaker tripping of motors when the motor is decelerating (not in full stop) and when you try to start again, the motor protection will trip?



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[davidbeach]

Closing in out of phase with the residual voltage on the motor.

 

[nightfox1925]

If you say residual voltage...you are refering to the motor Back EMF? This is what likely happens during re-start (of a hot motor) or during de-accelleration?

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[LionelHutz]

Immediately after a stop the motor will be generating a back-emf voltage. The voltage decays over time and the frequency follows the motor speed. Typically, this voltage will decay away after a few seconds. So, if you try to re-start the motor right after it was stopped the motor contactor or breaker may close when the mains and back-emf are out of phase with each other. This will cause a much higher current that can trip the protection. Note that the trip will occur at the moment the motor starter closes. The current peak will only last for part of a cycle and then the current goes back to the same level as always seen when starting the motor.

Also, if the motor has been coasting for a minute and still trips on a re-start then it is not the back emf. The back emf should have decayed within about 5 seconds even for a large motor.

If the motor is not tripping on the back emf then you should take a look at the cause of the trip.

 

[slavag]

Yes, it's back EMF. Motor work as generator and you try re-close CB or contactor again in anti-phase. With Residual voltage lower then 0.3Un you can restart motor.
Regards.
Slava

 

[itsmoked]

Yes.

That's within a few seconds of disconnecting the motor. It acts as generator. Then as you reclose the contactor on it while it's still spinning you are hooking up a generator,(sort of), that may be,(likely is), wildly out of phase with the supply.

This can damage things and certainly cause a breaker to trip.

If you mean the motor slows to a stop and then you reclose it and get a trip then were are talking something else.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nightfox1925]

And the number of allowable re-starts for a hot motor as well should be considered so as not to pre-maturely wear out the windings. I'm trying to get details for the scenario that the motor decellerates and OCPD trips...is this a case of motor starting to stall up...just wondering why the Overload did not trip first.

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[waross]

There are a few unusual situations where the residual voltage on a motor may last considerably longer than 5 seconds.
Motors with direct connected capacitors and overhauling loads are one example.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nightfox1925]

Are conveyor motors well considered as "overhauling loads" too?

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[gepman]

Conveyors going downhill (i.e. they won't stop without a brake or take a long time to stop) are overhauling loads. Conveyors going uphill (unless they will roll backwards when stopped) would not be overhauling loads.

 

[nightfox1925]

Thanks gepman

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