Locked Rotor Current (LRC)

[rockman7892]

I have a 4.16kV 250 hp motor that has been tripping recently and I cannot seem to figure out why. The FLA for this motor is 29A and the LRC value on the motor datasheet is 192A. After the motor tripped last I went to the multlin 269 protecting this motor and noticed that the motor was tripping at a value of 190A. Does this mean that for some reason the motor is approaching or seeing its LRC? Would this mean that something is jamming or stopping this motor and locking up the rotor? Can a motor ever draw more current than its LRC?

The application this motor is being used for is a fan which is pulling draft through a mill. The draft is controlled by a damper which is upstream of the fan.

If I understand correctly LRC is the current necessary to overcome a rotor from the standstill or locked/stalled position. It is a direct result of the torque required to overcome the inertia of the rotor. Any ideas on why this motor would be tripping at this value?

 

[itsmoked]

A very common cause in ventilation systems is the fan freewheeling backwards due to static pressure differences. This represents a much larger starting load to stop the fan and start it rotating in the opposite direction. Hence tripping the protection.

Can this be occurring?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

It is not unusual to draw locked rotor current or very close to it for a period of time after start. The curve of current vs speed looks something like a parabola with a max at speed=0 so the slope is flat in that area. Current doesn't noticeably decrease until you get up around quarter or half speed.

Also for the first few cycles, there can be an expected decaying dc offset present. The peak current value reached can in theory by as high as 2.82 times the rms value of locked rotor current. 2.82=2*sqrt(2).


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[electricpete]

Actually the peak value 2.82 *LRCrm would be approached at approx the first quarter cycle. After that, the dc decay reduces the peak.

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[rockman7892]

I should have mentioned that this tripping is occuring during normal operation after the motor/fan has been running for a while (hours) This tripping is not associated at all with starting the motor it is happening during running full speed operation. The last trip occured after damper positions were changed and thats when I noticed a value of 190A at time of trip.

 

[itsmoked]

How much power a fan draws is dependent on how much air it is moving. A reduction in restriction brought on by opening a damper can significantly increase the load. If you are increasing the load to the point the motor is slowed down to its 'breakdown torque' (not easy to do BTW), then you could see a starting current as per electricpete.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I was assuming the motor was tripping during starting. My bad.

I assume there is an overload protection provided as well. Typically if the motor is up and running and a mechanical overload occurs, then you will get an overload trip and not an instantaneous trip.

So it seems like the main possibilities would be:
* electrical fault
* relay malfunction
* an extremely rapid severe mechanical overload (lockup) - even then I'm not sure you wouldn't get overload trip before instanatneous.





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[rockman7892]

electricpete

Thanks for the replys. The trip that I am seeing is an overload trip and not an instantaneous trip.

 

[LionelHutz]

You're seeing a pre-trip current of 190A?

To me, that means the motor speed has to be less than about 50% speed. Fans usually do not stall motors but that is what the data is indicating.

Is it possible there is a control problem and the motor is stopping and then re-starting without you knowing it? You probably start with a damper closed to make the start easier. So, if it stopped and then re-started the dampers would be open making the start much more difficult and it could trip while it is re-accelerating.

 

[waross]

With a current of 99% of locked rotor current the the most probable cause is either back spinning as itsmoked suggested or an extremely high pressure in the discharge plenum slowing the fan dramatically.
You could have a failing motor, a failing CT, a failing cable or termination, or a failing multilin.
However the odds of such an impending failure drawing almost 100% of locked rotor current are slight.
Look for an unintended restart as LionelHutz suggests and possible back spinning as itsmoked suggests.
A mechanical problem that causes a motor to draw locked rotor amps is usually noticable by the smoke, sparks and noise that it also generates.
Another possibility is that the multilin is reporting the maximum current history, which would be the starting current, not the current at the time of the trip.

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

 

[jraef]

I don't think there is a "Locked Rotor" trip display in a 269 as far as I know. I think you need to carefully review your 269 settings before you go any further. For instance, are you using a Custom OL trip curve? If so, is the 550% trip point set too low? Is the trip time set too short? Is it set at all? Is it maybe not an "Overload" but a "Mechanical Jam" trip perhaps? Was that set too low and / or too short? There are a number of things that COULD be going on here, you need to start with the settings you have and the EXACT wording of the trip display.

 

[LionelHutz]

The 269 stores the average starting current. I would expect the average starting current to be less than the locked rotor current.

It also stores the pre-trip phase currents. I would expect the trip currents to be close to locked rotor currents if the trip occurs when the motor has only partially accelerated. This is why we're all saying the data has indicating the motor has stalled or has been re-started.

Maybe try clearing the pre-trip data and get a new recording after the next trip. It's supposed to record the pre-trip data for the last trip so the data should be good but just maybe the 269 is doing something stupid.

 

[rockman7892]

Thanks for the responses. I am going to go check the multilin again and make sure that the value of 190A I am seeing is indeed the pre-trip current for the last trip. I am going to clear all existing pre-trip data and start from a clean slate to see what the current value is next time it trips. I am also going to talk to the process engineer about the fan possibly slowing down the motor below 50% as mentioned above.

Just for clarification, is the Locked Rotor Current of a motor the highest current the motor will ever draw aside from a faulted condition. So with a LRC of 192A then the motor will never draw any more than 192A due to an overload condition? Even if the motor is totally bogged down or stalled it will only draw 192A?

From a troubleshooting standpoint using this motor as an example, if I ever see a motor trip on a current value of more than its LRC than can it be deduced that the motor tripped due to a fault? For example if I noticed on the Multilin that this motor tripped on a value of 400A than can it be figured that 400A is greater than the LRC of this motor and therefore must be due to a fault in the motor or feeders?

 

[itsmoked]

I believe about the only way you could exceed the LRA would be a short in the windings or having a motor running in the reverse direction when energizing it. It could then generate out of phase power.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Or if the contactor opens and re-closes very quickly on a spinning load. If the motor field does not have time to collapse before a re-lose of the contactor, it can also still be in regen as itsmoked just mentioned. Such is often the case during open transition wye-delta starting, but it can also happen if you have a power glitch or something in the control circuit that is allowing your contactor to "bounce".

 

[itsmoked]

Ah yes nice one jraef. Same thing almost.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RRaghunath]

I guess it is the Stall / locked rotor condition that caused the trip. The Motor protection relay has different setting for stall condition - that is typically, 50% of expected stall current and the allowable stall time minus 2sec as the time delay for trip.
You can verify from the relay records, if this is the element that has operated.
By now, you would probably have got the feedback from Mechanical people the reasons for stalling (excessive overload, bearings problem, foreign material into the fan casing etc.)

 

[vima]

If motor is tripping while starting, you may have to look into the possibility to install the speed switch. For some fan motors generally the starting current will be more than stalling current.