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giri
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Don't Bimetaic Relay Gives Protection Against Single Phasing

03/21/2012 4:19 AM

Hi friengs & Gurus,I would like to confirm whether motor bimetallic relay is able to trip the motor in the event of single phasing.

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raghun
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#1

Re: Don't Bimetaic relay gives protection against single phasing

03/21/2012 4:53 AM

It depends. The single phasing protection based on differential bar principle was inherent in bimetal overload relays. But with the electronic or static type relays - you need to confirm with the relay literature. (Since you said bimetal relays in your post, may be the single phasing protection is available).

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JRaef
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#2

Re: Don't Bimetallic Relay Gives Protection Against Single Phasing

03/21/2012 11:07 AM

Not necessarily. It's a common myth, promoted by the relay mfrs. But it does not really sense the specific loss of phase, all it does is hopefully protect the motor slightly better than nothing at all. Here's how it really works:

The Differential Bar feature in IEC bimetal OL relays is there to shift the trip point if the OL setting if a phase is lost, because a lost phase on a RUNNING 3 phase motor will mean additional motor heating effects. So if it were not there and a phase was lost while the motor was running, the motor may burn up even if the running current was below the FLA of the motor. The feature therefor shifts the trip curve to be lower to compensate.

BUT...

If there is very little LOAD on the motor when that happens, an IEC bimetal OL relay may NOT actually trip, because the current is still below even the shifted trip curve. This is where a lot of people get upset when they find out it didn't trip. The issue is, hopefully if that happened, the thrmal damage curve of the motor was not exceeded, so even though it didn't trip, it may not have been a dangerous situation. Notice that there were a lot of maybes and shoulds in there, it is not really an exact science. In other words it is better than nothing, but it is not the best way to protect against single phasing.

The other major thing it does NOT do is prevent the motor from attempting to restart if it was turned off normally but the phase is still lost. So what can happen then is that if a re=start attempt is made and there are only 2 of 3 phases available, the motor will not turn and will see locked rotor current until the OL trips again. Do this over and over in an automatic control system, and you can lose the motor.

Solid State OL relays will prevent that, they truly sense the loss of a phase.

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giri
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#4
In reply to #2

Re: Don't Bimetallic Relay Gives Protection Against Single Phasing

03/22/2012 3:32 AM

-

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giri
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#5
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Re: Don't Bimetallic Relay Gives Protection Against Single Phasing

03/22/2012 3:41 AM

Thanks JRaef.But I would like to ask you that,if single phasing occurs,it will lead to almost double current in other phases,which will heat up the bimetal,eventually tripping the motor.Why this don't occur?.

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JRaef
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#6
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Re: Don't Bimetallic Relay Gives Protection Against Single Phasing

03/22/2012 12:35 PM

The current will always be related to the LOAD on the motor, not the FLC (Full LOAD Current) of the motor, unless the motor is fully LOADED. Plus, the difference is the difference between single phase and 3 phase current, not double.

So let's say that you have a 10HP motor with a FLC rating of 14A, but the LOAD on it at the time of the phase loss is only 5HP and the motor current draw is only 7A. When you lose a phase, you now have the SAME load, 5HP, but it must take all of the power at single phase values. So the current increases by the Sq. rt. of 3 so 1.732 x 7A = 12.12A. If the OL relay is set for 14A, it will not trip. But an IEC bimetal OL relay with a differential bar will shift the trip pickup point to roughly 58% of the 14A setting, so the EFFECTIVE trip is going to be at 8.12A and the OL will trip since the current through it is 12.12A.

BUT!!!.... Let's say the LOAD on the motor at that moment was only 2.5HP and the current draw was only 3.5A at that LOAD. The current would increase to 6.06A and since even with the differential bias change, the OL is set to trip at 8.12A, the motor will continue to run. At that low of a load, it will not likely harm the motor but that bimetal OLR will not trip.

This is why I say that an IEC OL relay will not necessarily TRIP on a phase loss, but it will likely PROTECT a LOADED motor from damage that may be caused by a phase loss. It's a bit of semantic correction, I just think it's important for people to truly understand so that when it does NOT trip, they don't immediately assume it failed.

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TonyS
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#3

Re: Don't Bimetaic Relay Gives Protection Against Single Phasing

03/21/2012 7:39 PM

Most good quality thermal overloads will trip on single phasing. Check with the manufacturers.

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