# Bi-metallic O/Ls.



## Cow (Jan 16, 2008)

What controls the starters? No way to install a phase monitor in series with the control voltage?


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## frenchelectrican (Mar 15, 2007)

Did ya ever think about using the SqD starter ? 

I have better luck with SqD over the toaster ( aka GE ) units .,, 

The toaster starters I dealt in my area they are slow with single phasing slot.,, 

I get single phasing kinda semi common over here so I dealt with it reguarlly .,


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## JRaef (Mar 23, 2009)

First off, neither bimetallic nor melting alloy NEMA design OL relays will trip on single phasing unless the motor load is relatively high to begin with. In the case of IEC style bimetallic OLs that claim to offer it, all that means is that the motor does not need to be AS loaded as a NEMA design. In both cases if the load on the motor is low enough, the relay will not trip. But because the motor is running with a severe voltage imbalance, negative sequence currents and negative torque is created in the rotor that “fights” the normal torque, so for a given amount of current flowing in the two phases, more heat is being created than normal for that amount of current. So the motor can indeed burn up without causing the OL relay to trip. 

So I’m afraid your “General Bradley” solution is doing nothing toward better single phasing protection. You need a solid state OL relay or a Phase Loss relay.


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## micromind (Aug 11, 2007)

I thought about PFRs but a single-phasing event is pretty rare here. The last one around this location was over 10 years ago, before the building was even built. The controls are 120, transformer in each starter. 

These motors, driving exhaust fans, are always loaded so, in my experience, there is a pretty good chance that a melting-alloy O/L will trip in time. 

Both Square D and Allen Bradley use melting-alloy, GE and a few others use bi-metallic. 

I can't remember seeing a motor burn up due to single-phasing when it was protected by melting-alloy O/Ls. A few years ago, the POCO single-phased a rock crushing plant I designed and installed. I speced Allen Bradley 509s (NEMA), they have melting alloy. 

Every O/L was tripped and every motor survived. About 40 or so.......

I agree though, the electronic O/Ls are quicker but I've seen too many failures to be comfortable with them.


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## frenchelectrican (Mar 15, 2007)

Ya I can see that Mirco., but the issue I am thinking is there is other class overload heaters that you can use. 


I know most peoples are not really aware there is couple class of thermal OL's you can use.

The common NEMA thermals are typically class 30 ( common type )

But if you getting alot of single phasing from time to time I change the thermals from class 30 to 20 some case class 10 due it will trip quicker than class 30 especially on single phasing., 

Oh yuh most IEC OL are adjustable for class 20 or 30 cycle. 

and with exhaust fans you can actually sized the OL closer than what you do normally with other loads. ( I typically sized with unthrottled draft on exhaust fans and once you get the draft adjusted it will be little lower somehow.)


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## telsa (May 22, 2015)

IMHO, phase loss ought to be detected right at the Service.

It wouldn't hurt elsewhere in the system, either.

But losing a high HP motor because of single phasing is ruinous on the economics.


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## paulengr (Oct 8, 2017)

Lots of misinformation here.

The STANDARD overload class is class 20. If the motor is NOT marked for it, the overload class is 20 in a NEMA system. Class 30 is strictly for crusher duty motors and will allow standard motors to burn up. The class 10 stuff is strictly for specialized motors such as submersibles. The number basically sets the curve so that the overload trips in that many seconds at 600% of full load. So a Class 20 overload relay trips in 20 seconds with 600% load. Since motors are supposed to start in under 10 seconds, this is more than enough.

IEC is a little different and the curves are also a little different. Basically IEC Class 10 is the same as NEMA Class 20, and the IEC ratings go way up beyond NEMA Class 30. I have no idea what you'd ever use them for but it's there. The big thing to remember is that IEC 10 = NEMA 20.

As to eutectic vs. bimetallic vs. solid state, this is pretty easy to understand, too. Eutectic overloads rely on the melting point of solder to operate. Over time the solder will evaporate a little at a time, especially if the overload gets hammered and trips frequently so that eventually failures in terms of calibration mean it trips even during a normal startup so the standard troubleshooting technique is to try replacing the overloads. The big advantage of eutectic overloads is that the heat up and cool down is pretty similar to the actual motor that it simulates...it takes a lot longer for the solder to resolidify once it has melted so operators can hammer on the starter and the overload all day long when something is plugged generally without fear of burning up a motor.

Bimetallic overloads rely on a strip of two different metals bonded together with different expansion rates. As the strip heats up it curls until it triggers some kind of hair trigger mechanism in the overload relay. The big advantage of bimetallic overloads is that they are TINY compared to eutectic overloads. A standard eutectic overload is almost twice the width of an IEC style starter so panel builders tend to favor the bimetallics because they fit better in the panel. Also they tend to be a bit cheaper price wise. The big disadvantage is that unlike the others, the cool down rate of the bimetallic strip is MUCH faster than the motor cooldown time. So if operators hammer a starter trying to clear a jam for instance, the motor WILL be burned up where this is basically impossible with the other types. The second disadvantage is that over time the calibration of the bimetallic strip can drift in either direction (trips too early OR too late), so it is not as reliable as the eutectic or solid state relays either.

One of the FALSE claims is that bimetallic and eutectic overload relays will not trip during single phase loss to protect the motor. The first thing that happens during loss of a single phase is that the motor's torque output goes to about 2/3 of normal and/or if system inertia is low enough you will visually see torque oscillation (shaking/shuddering). At rated speed the motor has to produce a lot more torque so it will slow down going towards the peak torque limit of the motor (roughly 200-300% of rated torque). If the torque demand exceeds (now derated) peak torque, the motor will stall out. Either way, current rises drastically. In the man time the transformer is also seeing this so voltage may drop on the remaining two phases. Either way, current will be 187% (almost double) on the remaining two phases, if voltage remains good. Also with the drastically reduced torque if the motor does stop normally it will typically stall out during starting. NEMA motors have to be rated to withstand up to 200% of rated current for 2 minutes which is easily long enough to allow the overload relay to detect the excessively high current and trip out. If the motor is so lightly loaded that it can continue to run and the overload relay doesn't trip during single phasing, that's because the motor isn't in any danger of being burned up...the overload protection relay is doing it's job.

Finally we get to solid state relays. In the 1990's and early 2000's the design that was common was some kind of analog circuit. The worst design that I can think of is the Cutler Hammer Advantage starter series. I can't say enough bad things about this starter other than this. Right in the manual it specifically states that the overload and starter circuits must be protected against voltage surges/spikes but doesn't contain any protection of any kind itself. Say what? Did you notice that these are operating starters that generate all kinds of surges and spikes every time a motor stops? The "solid state" (analog design) overloads were so bad that the IEEE standard for industrial equipment reliability (IEEE 493) has data that shows that solid state analog relays are no better than the electromechanical relays that they replaced in the protective relay world, which is far lower than the track record for eutectic and bimetallic relays.

Enter the new contender: the microprocessor based trip unit. These use CT's instead of heaters and work using digital calculations. To defeat operators turning off power to clear the overload relay (a trick that worked on the analog ones), they use a capacitor or some even simply store the last condition in a memory before they lose power, actually extending cool down times when attempts are made to defeat the relay. They use vastly less power than eutectic and bimetallic relays because they don't need to parasitically generate heat, they are immune to temperature variations in the panel, and generally speaking they don't lose calibration. It is trivially easy to equip them with remote controls for resetting, phase loss or phase reversal protection, anti-jam, built-in networking, self-diagnostics, and reliability actually exceeds even eutectic overload relays in many cases. Here are some extreme examples of what is possible:
1. The Allen Bradley E1Plus is actually cheaper than a full set of eutectic overloads (without the relay base). It has a bunch of plug-in modules that can be used to do almost anything with it. One big downside is that without the button on the front to reset it, you MUST use the ridiculously overpriced reset button optional module and this must also be used with any optional protection modules. The range of settings is so large that for all practical purposes nearly the entire NEMA size range (fractional to 250 HP or so) can be covered with just 3 relays. For the smaller sizes setting it is simply adjusting a dial to match the FLA of the motor. In the larger sizes there is really only one relay that works off CT's so some calculator effort is required to set the relay.
2. The Allen Bradley E3 Plus has built-in Devicenet and it actually contains it's own ladder-logic programming language so that for instance it can be programmed to more or less run a starter or other systems independently if for some reason the network fails. I've never seen this extra programming system actually used but it's there for extreme reliability cases. This one by the way is definitely priced with typical Allen Bradley pricing.
3. The SEL 710. This is in my opinion the ultimate motor protection relay. Better pricing than the GE 369/469 Multilins, completely modular with a ton of features built in. Also does self-diagnostics and is field upgradable as newer software revisions come out. For instance it can be programmed to capture and store motor starting curves as well as waveform level fault logging during trips which make diagnosing failures pretty easy to do in th hands of an expert. That's on top of basic fault logs with time stamps and conditions at the time it tripped. They also make a couple more basic models. Comes with unlimited lifetime warranty and free unlimited technical support. Keep in mind though that this is really what you are more likely to see in high HP (500+ HP) or medium voltage starters, not your common garden variety NEMA Size 0 starter.

These are all the "top of the line" in their class relays, not necessarily the ones that you will typically run into.

As to failures...TYPICALLY the failures that I see with microprocessor based overloads is that they either work or they don't. All of them have some kind of display or at least an indicator light, and all of them have some kind of self-diagnostic function. Failures are typically either the self-diagnostics finds some kind of internal problem, the trip relay burns out, or it just stops working. So troubleshooting them is not much different from eutectics or bimetallics...does it have indicator lights? Does the output relay work (ohm it while tripping/resetting)? Are the indicator lights indicating fault/bad condition? That's about it. The most primitive "self diagnostic" that these relays all have is called a watchdog timer. There is an external/independent timer in the relay. Periodically (say once a second) the programming in the relay resets the timer. If the watchdog timer expires, the relay trips and some kind of self-diagnostic failure light turns on. This prevents any kind of software glitch where the software locks up or gets caught in some kind of infinite loop. The chance of this is pretty slim but we have the same feature in almost every PLC on the market too and it is simple and works extremely effectively for catching the most severe software glitches. There are other checks that can be done as well such as pointing the analog-to-digital converter to an internal calibrated power source to test for failures of the chip or feedback sensors on the various components indicating status but since manufacturers don't specifically document what kinds of diagnostic bells and whistles they use, it's hard to tell which relays are better than others. Suffice to say though that at least in the low voltage market Schneider and Allen Bradley are the standards to beat, and in the medium voltage market all others are compared to SEL.

That being said, I have seen ONE medium voltage starter with a pretty nice relay mounted on I think a Square D starter with the slide-out low voltage compartment. It was indicating massive phase imbalance on the display and tripping out because of it. So this started as a motor failure troubleshooting exercise. I did all the standard motor checks (megger, low ohm bridge) and these were both complicated by the fact that there was a bunch of surge suppression crap hidden in an enclosure next to the peckerhead that plant personnel were unaware existed. It had to be disabled to get valid readings. With that out of the way we could only get the overload relay to allow the motor to run for about 8 seconds so I had to test everything one phase at a time, all the while fighting with the motor cooldown timing which made for a lot of downtime on a relatively simple troubleshooting exercise. By testing current on the input CT wiring and comparing it to the displayed values on the overload relay though it quickly became obvious that the relay was defective. Replacing the relay fixed the issue.

That's not typical, and there is plenty of third party documentation showing that microprocessor relays are more reliable (see IEEE 493), despite the fact that electronic anything tends to be less reliable. I think though that there is a huge difference. Starting with a eutectic relay unless you use a relay test set to test them (I've never done this and never seen it done...so I doubt anyone is doing this), there is no external indication of a problem. Due to design they almost can't trip late and I've only seen a few cases of failure to trip at all (mostly dirt/corrosion related). Most of the time the failures are on screw conveyors and other equipment that tends to jam where operators repeatedly reset it over and over again clearly a jam/plug until so much solder vaporizes that it nuisance trips early. So when you get suspected nuisance tripping the standard response is to replace the overloads. Bimetallic relays are identical except that in my experience you get just as many late/failure to trips as you do nuisance trips so after several motors have burned up, the response is to replace the bimetallic trip units "just in case", hopefully with a eutectic or microprocessor based relay. There is no indication with the analog-based overload relays of any kind so it's kind of like a bimetallic relay and the best way to fix them is to replace the whole trip unit with something better. Finally we get to microprocessor based trip units. The big difference here is that when it's bad, it usually "tells" you it's bad. My suspicion is that there are a lot more failed bimetallic and eutectic overloads out there that simply go unnoticed where the microprocessor relays make it pretty clear when something is wrong (usually a steady or blinking red fault light).

And with either one, there are some truly terrible designs out there. As mentioned, there's the Advantage starters where the only advantage is the bottom line of the manufacturer. Then there are the older Square D's that had the little copper strips and a single relay mechanism off to one side...yuck.


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## John Valdes (May 17, 2007)

The fans being variable torque may not have shown enough current to trip the OL's.
Without phase monitoring, its the luck of the draw. 
I have seen so much of this it makes me sick. How many hours we spent replacing motors from brown outs.
Seems the only motors that survived were on contactors that were on the xfmr leg of the service that dropped out.
No control power, contactors open if the xfmr that feed them looses power.

IMO. Changing contactors is not the answer. Customer not to smart either.


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## paulengr (Oct 8, 2017)

No Codes require detection and tripping on under/overvoltage or phase loss which is really just an extreme example of undervoltage. Ground fault tripping is another area where it is not Code required but even with a 4 wire wye connection, you've got to be kidding yourself if you think that overcurrent protection is going to protect against a ground fault for anthing over about 50 HP.

Again...IF everything is set up correctly it is possible in many cases for the overcurrent protection to do "double duty". If you just follow minimum Code standard only overcurrent protection is ever necessary except with the recent Code requirements for ground fault tripping with high phase currents (over 800 A or so).

As an extreme example IEC starters are designed for Type I or Type II protection. Type II means "no damage"...the starter should survive all fault conditions. Type I means that as long as it trips when needed, it is perfectly acceptable for the starter to self-destruct and melt into slag as long as it trips. This is also the standard for molded case circuit breakers (see NEMA AB-4). NEMA doesn't have the Type I/II stuff but the starters are also built to somewhat higher standards than say IEC AC-1 grade, somewhere between AC-3 and AC-4.


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## gnuuser (Jan 13, 2013)

rather than use bi-metalic overloads use something that is designed for the purpose.
even though it may be more expensive it will pay for itself over and over.
https://www.taylorphaseguard.com/phaseguard/


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## JRaef (Mar 23, 2009)

gnuuser said:


> rather than use bi-metalic overloads use something that is designed for the purpose.
> even though it may be more expensive it will pay for itself over and over.
> https://www.taylorphaseguard.com/phaseguard/


The problem I have with voltage based phase monitors like this is that if there are other 3 phase motors running when the phase is lost, they will regenerate a "phantom" phase voltage on the missing leg, the same way a Rotary Phase Converter does. Many of these voltage based PM devices get fooled by that and fail to drop out their loads. 

I prefer detecting phase loss by monitoring current. That's why I have gone to 100% electronic OL relays. For most of them, a phase loss is "current in any phase is lower than 20% of set current", and on some of them that percentage is adjustable.


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## gnuuser (Jan 13, 2013)

JRaef said:


> The problem I have with voltage based phase monitors like this is that if there are other 3 phase motors running when the phase is lost, they will regenerate a "phantom" phase voltage on the missing leg, the same way a Rotary Phase Converter does. Many of these voltage based PM devices get fooled by that and fail to drop out their loads.
> 
> I prefer detecting phase loss by monitoring current. That's why I have gone to 100% electronic OL relays. For most of them, a phase loss is "current in any phase is lower than 20% of set current", and on some of them that percentage is adjustable.


they just started using them in the factory i just retired from.
most of the current monitoring was done from the vfd and mcc panels.
and many of the guys taking readings during peak hours

our place has been going through major upgrades and i kinda feel sorry for the guys there because i know the manage-mentards will screw them on training


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