Please provide your thoughts on a motor under locked rotor and some reasons that support your position. I've searched many different resources and haven't found a great amount of detailed information. I was taught to deenergize as quickly as possible for several reasons, but the other options are to allow the starter OL to drop the circuit but that assumes they are functioning and properly sized. We had a discussion in the shop with different views and there are many knowledgeable posters on here. TIA
Locked rotor
- Thread starter brad609
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zbang
Senior Member
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- Roughly 5346 miles from Earls Court
Yes, if you know the motor is locked, either completely or effectively (moving very slowly), turn it off as quickly as possible. After that isn't this more the purpose of the overcurrent device? More than a few seconds at 300% rating it ought to trip (depending on the particular curve, of course).
Yes I agree, also if the OL is not sized properly or if the starter is not functioning, I've always been taught and concerned that the OCD upstream may not open as quickly as the next one upstream knocking out a major portion of the plant or facility. If the OCD's are not properly coordinated, a small load fault may open a larger subfeed breaker.
I agree, I found it surprising that there wasn't more information out there regarding this, is it because that deenergizing the circuit immediately is an obvious thing that should be done? Does an additional 4-5 seconds make a significant difference in an LR condition?
A standard 3 phase SCIM will have a locked rotor current of approximately 600% of nameplate. It’s possible that is less than 300% over the OCP.
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- Licensed Electrician
Isn't this why we like electronic overloads on a dinky little IEC starter better than a NEMA starter?
I'm not sure I'm included in that "we"
I've seen NEMA starters handle a locked rotor situation as it should and IEC starters disintegrate
xformer
Senior Member
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- Dallas, Tx
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- Master Electrician
IMHO... A Motor under locked rotor is a motor that is energized with proper operating voltage in which the rotor is stationary either momentary or permanently. All motors experience locked rotor for a brief period of time upon startup. The function of the Overloads is to protect the winding of the motor's stator. This all boils down to an engineering issue. The failure point of the circuit design is supposed to be stator coils of the motor since the circuit is designed to fail into a safe mode. We know class 10, 20, and 30 overloads exist. So, choosing an overload that balances economy with safety is important. Several factors affect the decision of the selection of overload device. Factors could include motor accessibility, motor commonality and motor use.
Agree with your points, so are you saying that you would not deenergize a motor under locked rotor for an extended period of time and just allow the OL's to do it?
A NEMA design motor that experiences an extended locked rotor state will normally experience a meltdown of the rotor before the stator windings are damaged. Class 20 eutectic alloy overloads have a trip curve which, when properly applied, prevent the stator windings from damage. They are not always capable of preventing rotor damage.
I agree with that, so what is the time frame here, if the motor is LRA for 5 seconds is it toast? I've read from motor manufacturers that motors can withstand LRA for 10 secs before breakdown, and I have seen motors under LRA for over 5 secs, I can't help to think it created premature failure because the OL didn't open soon enough. So I continue to stand by the fact that if a motor is LRA, deenergize it as quickly as possible and rely on the OL's as secondary backup.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Failure from locked rotor operation depends upon many factors.
If you start with a cold motor driving a locked load, then it might survive quite a while in locked condition.
On the other hand a hot motor (say from continuous running at a full service factor load) doesn't have much headroom prior to damage.
Using the overload to protect the motor is only as good as the OL models the thermal mass and time constant of the motor.
Essentially all motors will experience a transient locked rotor state when they start; large motors are often rated for a maximum number of starts per hour. Some types of small motor are 'impedance protected' and can tolerate sitting in a locked state continuously.
Would I manually de-energize a locked motor or wait for the OLs? That depends on if I thought the locked state was abnormal. If I were manually starting a motor and it didn't feel right, I'd kill it immediately, even if it were spinning.
-Jon
If you start with a cold motor driving a locked load, then it might survive quite a while in locked condition.
On the other hand a hot motor (say from continuous running at a full service factor load) doesn't have much headroom prior to damage.
Using the overload to protect the motor is only as good as the OL models the thermal mass and time constant of the motor.
Essentially all motors will experience a transient locked rotor state when they start; large motors are often rated for a maximum number of starts per hour. Some types of small motor are 'impedance protected' and can tolerate sitting in a locked state continuously.
Would I manually de-energize a locked motor or wait for the OLs? That depends on if I thought the locked state was abnormal. If I were manually starting a motor and it didn't feel right, I'd kill it immediately, even if it were spinning.
-Jon
Some very good points. I had read as well that if a motor is starting cold or if the motor is hot is dependent on how long that it can withstand an OC event. I see this is in your wheelhouse, perhaps you could explain why this doesn't seem to be covered more in either industry, as doing any research on it comes up empty as well as getting any of the manufacturers to take a position on procedures. It was surprising to me that there isn't more of a detailed procedure, albeit that the events aren't typical but a generalization of procedures is not too much to ask..imo.
I don't recall seeing IEC starters disintegrate. The typical cage motor starts at locked rotor at around six times FLC.I'm not sure I'm included in that "we"
I've seen NEMA starters handle a locked rotor situation as it should and IEC starters disintegrate
xformer
Senior Member
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- Dallas, Tx
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- Master Electrician
IF I witnessed an energized overload condition, I would de-energize the motor. Investigate the cause of the overload. Inform powers that be and allow them to make a repair decision.
Well I haven't seen many, but I have to say that I haven't seen a NEMA do this.
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Agree, I would never stand by and wait for the OL's to possibly function properly as they may not. I would compare that to a fireman with a hose who is not extinguishing a burning building because he's waiting for the sprinklers to do it first....as an analogy
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- Wisconsin
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- PE (Retired) - Power Systems
I always considered OL's to be for running motor protection, just like the NEC does.
A LRA condition is more similar to a short circuit conditon than it is to a running overload, therefore it requires different protection.
A LRA condition is more similar to a short circuit conditon than it is to a running overload, therefore it requires different protection.
Interesting take...so you feel that the OL's should be for running protection, and not OCPD to open during a locked rotor...is that correct? So then do you feel that the branch feeder should be the OCD that should take over during a LRA event? But don't we size our branch breaker to withstand the inrush, so wouldn't it take longer for it to open then the OL? I see what you are saying, especially with the older type bi metal OL"s they need to heat up to open, but with the electronics I think the curve is much faster, but IMO I think the OL's should open or will open before the branch OCD just because their threshold has been obliterated where the branch is sized to withstand inrush therefore holding LRA at least for a period of time. Thoughts?
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