Motor Problem 2

[JatTiw]

I have Siemens make 150hp (164FLA), 460V motor (IEEE-841, premium efficiency) is causing main HMCP to trip on start-up. PdMA results shows all OK (PI-10; DDA-1.9; Average Phase resistance & inductance 0.0635ohm, 2.008mH)
Inst. Breaker set at 13*FLA. No problem with cable or starter.
Is motor resistance too low (0.0635ohm) or look for other parameter?

 

[electricpete]

Motor resistance has very little to do with the current drawn.

Premium efficiency, hmmm. You can go higher than 1300% for that one.

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[electricpete]

... provided you have done the coordination calcs etc.

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[electricpete]

NEC allows you to go up to 1300% FLA if you have trips on standard motor and up to 1700% FLA if you have trips on an energy efficient motor. Again providing you have evaluated coordination and protection.

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[JatTiw]

As per manufacturer, LRA-1085.
Instantaneous trip at breaker is already set to maximum, cannot go above what code allows.

 

[electricpete]

It is a rare motor problem that will trip a properly set instantaneous. You would likely see a hard ground fault or a big imbalance on your resistance and inductance. Given your low setting imo, setting is the absolute first thing to look at unless you have something in your test data a little more significant than "low resistance".

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[electricpete]

Instantaneous trip at breaker is already set to maximum, cannot go above what code allows.

Are we talking NEC code here? If so, again max setting is 1700% FLA for your high efficient motor.

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[JatTiw]

From PdMA test, the %age imbalance in resistance & inductance are 0.78% & 3.65%.......Looks like OK to me.

Also please help me under stand why high efficiency motors will have high inrush as compared to normal motor. My thoughts were other way around.

 

[electricpete]

See the bottom of page 4 here:

http://www.danfoss.com/NR/rdonlyres...hEfficiencyMotorsandNuisanceStartingTrips.pdf

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[electricpete]

See also NEC 430.52(c)3 - Exception 1

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[edison123]

pete - nice danfoss article. In a way, OP is right in asking the low winding resistance is the causing the trip.

JatTiw - What is the load ? I'm wondering if a starting resistor in series with the stator for the first few cycles of the start woll solve this problem.

Muthu

http://www.edison.co.in

 

[JatTiw]

Application is for centrifuge pump in refinery.

Electricpete & Edison123 thanks your support & shedding light on the issue.

Best Regards

 

[electricpete]

OP is right in asking the low winding resistance is the causing the trip.

X/R ratio has a role. R includes contributions from stator and rotor but we cannot measure the rotor resistance using pdma. The response of breaker instantaneous to that setting is a little bit obscure (does it respond to true peak, rms of the first half cycle.. how does the time delay damper affect it). In the end selecting a high enough instantaneous settings to avoid trip ends up being very empirical (experienced based) rather than a precise calc (that's why NEC gives us more than one number and tells us to bump it up only if we get trips, rather than giving a recipe to calculate the setting required to prevent trips that everyone would love to have). So the bottom line, I don't think we can realistically look at pdma data and make any prediction about instantaneous trip behavior.

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[Tmoose]

what's it driving? did it not always do this? New installation?

 

[electricpete]

Driven load cannot really have anything to do with an instantaneous trip. It is a function of the motor and the protection, not the load.

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