Single phasing protection

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I'm not sayin there is nothing wrong with some kind of single phasing protection, but some people think motors in general are really sensitive and easy to burn out.

With improper overload protection this is true.

How often is a motor run right about at 100% of its rating. I see more of them all the time - designers are trying to cheapen things so they can sell their product. If this motor or driven load gets a little wear and tear on it and now it is running at 115% - overloads probably holding most of the time but now winding insulation is getting a work out. This will shorten the life a lot faster than one single phasing incident. The single phasing incident on the 115% motor probably trips the overload almost immediately anyway if the load has been running for very long at all.
 
jim dungar said:
I do not know of a major NEMA manufacturer (Rockwell, Schneider, GE, Eaton, Siemens) of overload relays that says their standard thermal offering will protect motors against single phasing. While they all do sell thermal overloads that have single phase sensitivity, for critical applications they usually recommend you use a dedicated type 'phase loss relay'.
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Well Jim, please meet Mr Allen-Bradley:
http://ab.rockwellautomation.com/Ci...n/Motor-Protection/193CT-193T1-Overload-Relay

The pertinent quote on the page:
Motor protection at phase loss (trips at maximum 125% of current setting)

I know that Siemens, Eaton and ABB also have the same functionality on their bimetallic overload relays.

When I used the term 'late designs' I was thinking of the bimetallic relays as they were developed after the euctetic relays. Solid state OL relays, the latest development in overload elements and now widely used for smaller motor protection also, do certainly include loss of phase protection based on thermal modeling.
 
Just had another case today where customer was getting single phased from a utility fuse blowing several times in the past year. Compressor motors with thermal overloads sometimes tripped out, sometimes did not. Those motors have failed in the past.
 
weressl said:
It is a NEMA and Standard offering for Eaton, Siemens and ABB at a quick glance.
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The Eaton marketing literature 'quietly' says their C306 relay meets the single phase requirements of UL508, but I have not found any specific UL508 'loss of phase' protection requirements for thermal (mechanical) overload relays. Eaton makes a big deal about the 'loss of phase' protection provided by their electronic overloads.
 
jim dungar said:
The Eaton marketing literature 'quietly' says their C306 relay meets the single phase requirements of UL508, but I have not found any specific UL508 'loss of phase' protection requirements for thermal (mechanical) overload relays. Eaton makes a big deal about the 'loss of phase' protection provided by their electronic overloads.
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Different types of overload relays of both with or without loss of phase protection now are listed as standard offering. In other words you can still buy overload relays without loss of phase protection but overload relays with LOPP are also available as standard offering.
 
weressl said:
Different types of overload relays of both with or without loss of phase protection now are listed as standard offering. In other words you can still buy overload relays without loss of phase protection but overload relays with LOPP are also available as standard offering.
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If you don't specify, what will you get as their default offering?
 
while we all know a phase loss can cause equipment failure, I've never seen it personally. I manage a big industrial area, and phase losses occur at least once a year.

the HVAC systems handle a phase loss, many times. compressors and blower motors no problems.

air compressors, all the way up to 20-30 hp.
elevator motors. production motors. etc.
all have required a power down, and restart, possibly a breaker reset, nothing more.

just last week, we had a roof top unit single phasing interrmittenly due to a melted circuit breaker. took a while to isolate the power drop out. after 2 weeks of intermittent problems, we isolated the breaker, replaced it, and all is well.

I wouldn't go crazy with phase protection.
 
weressl said:
The most expensive thing you can imagine:p.

If you don't specify, they will include their electronic OL.

All of them; ABB, AB, Siemens, GE, Square D, Eaton.
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Not in my part of the US; here electronic overload relays must be requested.
 
Problem

Problem

This has happened a time or two before I don't know if it a problem with the forum or something with my browser.

I see there is posts on a fourth page but when I go to read them it brings me to page 3. Tried several different ways to get to fourth page but still brings up page 3.

I can see the last posts when composing another post though.

I am curious to see if after adding this post I will be able to see page 4.

edit: After posting I am now able to see page 4. I know I have had this happen a few times before but I just gave up on the thread those times.
 
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kwired said:
This has happened a time or two before I don't know if it a problem with the forum or something with my browser.

I see there is posts on a fourth page but when I go to read them it brings me to page 3. Tried several different ways to get to fourth page but still brings up page 3.

I can see the last posts when composing another post though.

I am curious to see if after adding this post I will be able to see page 4.

edit: After posting I am now able to see page 4. I know I have had this happen a few times before but I just gave up on the thread those times.
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It's a common problem, I think they know about it.
 
Re: bimetal OLs and "Phase Loss Sensitivity". I had to study this as the PM for a mfr a few years ago.

It used to be that the IEC control mfrs sold these standard adjustable bimetal OLRs as having "Phase Loss Protection", but they had to change the wording to "...Sensitivity" because it doesn't really detect a phase loss. What they do is use a compensating spring loaded mechanism, sometimes called a "differential bar" inside that will BIAS the trip point if you lose a phase. What happens is, if current is flowing on each phase, the bimetal stips are applying relatively equal force to the trip bar. The differential bar is applying an equal counter force. This by the way is why you must loop power back through the unused phase on a single phase application on these types of OLRs.

If you lose a phase, the differential bar is now stronger than the force exerted by the remaining 2 bimetal strips, and the trip point is effectively shifted to be lower than it was. In other words the remaining 2 bimetal strips now have to move the trip bar LESS to trip because the differential bar already moved it part way. The PURPOSE behind this was because if a motor is single phased, there are negative sequence currents inside the rotor that are causing additional heating in excess of the thermal model the bimetal strips were selected for. So it takes LESS current under a single phase condition to overheat the motor than if all 3 phases were there. So this differential trip compensation bias to make the trip point lower, compensates for that increased heating effect.

But here's the problem. If the LOAD on the motor is light enough, these bimetal trips may never trip on a single phase situation, because even though it is biased lower, it might still be below the threshold of the trip curve. In most cases this is not an immediate problem because if it's lower, it's lower and no harm no foul. But there is a tolerance issue that allows some motors to burn up anyway, especially unattended applications where the motor runs for a long long time like this. So they had to start using the term 'sensitivity" instead of "protection" because it is NOT true phase loss protection, it is protection against one of the worst EFFECTS of phase loss.

SSOLs are different, what they do now is digitally monitor current. If the current on any one phase is below a setpoint, it trips; plain and simple. For most of the mfrs, that threshold is 20% of the setting. Unfortunately for Sq. D, their Motorlogic unit is factory set for 30% and they have troubles with it nuisance tripping, especially on pump systems where the current builds up slowly as the flow increases. For that reason, they started selling an OPTIONAL version that has a 20% threshold or can be defeated. I don't know of any other SSOL that has that problem.

I happen to know that A-B now gives you SSOLs if you don't ask, because they are essentially the same price and you don't have to mess with the heater element selection any longer. You won't get the superdy duper one that talks to everything else if you don't ask, but the one you get (E1+) will have Phase Loss Trip as I described above.
 
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