Locked rotor

[synchro]

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.

I agree. In a locked rotor condition the motor is effectively a transformer where the rotor cage is the secondary winding with its turns shorted. The resulting LRA is then established by the impedance of this transformer. As the rotor spins up it develops a back EMF which effectively subracts from the voltage applied to the stator windings, and so the current drawn goes down proportionally.

 

[jim dungar]

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?

There have been misapplications of motor protection. Most people don't care because in general it is cheaper to replace a small motor than it is to protect it correctly. It is also a common practice to size branch circuit devices at the maximum allowed setting rather than coordinating them with the motor damage curve and the OL relay curve.

OL relays are usually set for 600% at 10 sec for a fast trip and 20 sec for a typical NEMA application, if they have been selected correctly. We know they do not protect under all running conditions, for example electronic and IEC versions include additional protection for single phase conditions.

I have never seen OL only functions properly protect large motors for conditions like, low voltage, single phasing, stalled/jam conditions, extended locked rotor during starting.

 

[brad609]

There have been misapplications of motor protection. Most people don't care because in general it is cheaper to replace a small motor than it is to protect it correctly. It is also a common practice to size branch circuit devices at the maximum allowed setting rather than coordinating them with the motor damage curve and the OL relay curve.

OL relays are usually set for 600% at 10 sec for a fast trip and 20 sec for a typical NEMA application, if they have been selected correctly. We know they do not protect under all running conditions, for example electronic and IEC versions include additional protection for single phase conditions.

I have never seen OL only functions properly protect large motors for conditions like, low voltage, single phasing, stalled/jam conditions, extended locked rotor during starting.

I'm not sure I can agree with some of that from my perspective. If a motor has a starter, it doesn't cost much more to increase the size of the OL than to replace a motor because it wasn't properly protected. Likewise, to use the maximum breakers allowed is much more costly than to use the lowest in ampacity that still allows the motor to start, as a practice I size my feeder breakers as low as possible because it is cheaper to do so I'd rather install a 40A rather than a 100A or 150A...just my take on it.

 

[Jraef]

Motors have a thermal damage curve. OL relays are designed to trip in what’s referred to as “under the curve”, waning the trip curve of the OL relay is designed to be slightly lower than the damage curve of the motor. Period. Class 20 curves are based on a NEMA designed motor’s thermal damage curve, including the locked rotor time, in this case 20 seconds at 600% of FLA. IEC motors and many submersible motors are designed around a Class 10 curve, so 10 seconds at 600%. Before t these norms are basically going to apply to motors up to and including 250HP, sometime higher. But once you get into 300HP and up, that’s when you need to look into the motor’s damage curve information and make sure your protection scheme is right. That’s why you will see people starting to use “Motor Protection Relays” for larger motors, which allow you to create custom curves if necessary.

The biggest problem with OL relays failing to protect motors is located between the ears of the people who get annoyed when they do their job, so they tweak them by adjusting them or putting in the next size up on heater elements.

If you try using the SCPD (fuse or breaker mag trips) to try to protect against little caked rotor, you will be constantly dealing with nuisance trips.

Don’t confuse “Locked Rotor Current” with magnetic inrush current that takes place the instant you apply power. magnetize get inrush current takes place BEFORE there is ant back emf, for only a cycle or so, and can be 10-20x the FLA depending on circumstances and design. THAT usually happens too fast for the SCPD to react, but not always, especially if it is set too low. But also because it is over so fast, it doesn’t damage the motor and you don’t need extra protection.