PHASE MONITORS VS 3 POLE CIRCUIT BREAKERS

fifty60

Senior Member
Location
USA
I understand why I would use a phase monitor for expensive motors that are protected by individual fuses on each phase. If one of the fuses blow, it would most likely not shut down the motor. To protect the motor in this situation I need a phase monitor to detect phase-sequence, phase-failure, and phase unbalance.

If I use a 3 pole circuit breaker that opens all ungrounded motor supply conductors simultaneously, does this lessen the need for the Phase Monitor, or should a phase monitor still be used to detect phase sequence or phase unbalance? Or are should phase monitors only be used with single pole fuses protecting multi-pole motors?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
The newer overloads for motors do a much better job at phase loss protection due to their ability to detect phase current imbalance. That said we have had to disable that feature on a couple wells this last summer due to the tight tolerance. Maybe to tight.

In the past we had used PLM on almost all three phase motors above 15 HP if the customers budget would allow.
 

iwire

Moderator
Staff member
Location
Massachusetts
If a phase was lost from the utility wouldn't it throw the breaker or trip the overload?
It should trip the overload, but so should a phase loss from a blown fuse.

It is just another way of protecting expensive equipment. I can often protect many expensive motors with a single inexpensive phase loss monitor.
 

kwired

Electron manager
Location
NE Nebraska
If a phase was lost from the utility wouldn't it throw the breaker or trip the overload?
Yes, but generally speaking equipment designed for phase monitoring will respond faster. A motor driving a load that is near the motors full load capacity is probably going to trip faster than one that is lightly loaded also, then you have things with variable torque load like centrifugal fans or pumps that demand much less torque at low speed (or at starting speed of zero), with these type of items standard overload equipment may take much more time to respond to a phase loss condition.

I am not all that convinced additional phase loss protection is necessary on high torque motor applications, as loss of phase will result in pretty quick increase of current on remaining phases and properly selected overload protective devices will do the job.
 

EG3

New member
Location
Georgia, USA
It should trip the overload, but so should a phase loss from a blown fuse.

It is just another way of protecting expensive equipment. I can often protect many expensive motors with a single inexpensive phase loss monitor.
That would depend on the type of protective devices. At the high end, one can install an electronic motor monitor that provides fully configurable protective parameters, phase loss, overload, historical monitoring, etc. At the low end one can install fuses and bimetallic overloads. It comes down to money, as usual. I have very few clients that go high end on motor protection and I have some that specify PLMs for all motors over 5HP.
 

kwired

Electron manager
Location
NE Nebraska
That would depend on the type of protective devices. At the high end, one can install an electronic motor monitor that provides fully configurable protective parameters, phase loss, overload, historical monitoring, etc. At the low end one can install fuses and bimetallic overloads. It comes down to money, as usual. I have very few clients that go high end on motor protection and I have some that specify PLMs for all motors over 5HP.
Most motors can take more abuse from phase loss, or short term overloads than some people give them the credit for. All across the line starting motors will have increased current during starting, and overload protection needs to be able to ignore this current long enough to allow trip free starting, they trick is selecting how long of a duration to allow this temporary overcurrent so you still provide effective overload protection, or go with a VFD where you get much more advanced control and protection.
 

iwire

Moderator
Staff member
Location
Massachusetts
Most motors can take more abuse from phase loss, or short term overloads than some people give them the credit for.
Sure they can and most of us have witnessed it. A 75 KVA transformer can put out more than 75 kva. A 120 volt lamp can run on 130 volts.

But for longest service life it is wise not to run them that way.

Few three phase motors cost less than rudimentary phase loss protection devices, add in downtime and labor costs and it becomes downright foolish not to protect the motor from phase loss in one way or another.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It should trip the overload, but so should a phase loss from a blown fuse.

It is just another way of protecting expensive equipment. I can often protect many expensive motors with a single inexpensive phase loss monitor.
Should? Not really. Technically, it's not part of the job description for a basic overload relay. Some Solid State OL mfrs will ADD that feature, but if they didn't, they could still call their product an overload relay.

IEC bimetal overload relays CLAIM to "protect" motors from phase loss or imbalance, but not really. Here's how that all works:

A phase loss can be described as a full phase current imbalance, as unbalanced as it can get really. When there is any phase current imbalance, there is created, in the stator and rotor, what are called "negative sequence currents" that circulate in the circuits doing no productive work. In fact, the negative sequence currents create negative torque pulsations, so in essence your motor begins to "fight itself" a little bit. That then means that you have more current flowing into the motor adding heat to it, but not producing useful work. That heat is NO LONGER COMMENSURATE TO THE EXPECTED HEATING OF THE MOTOR DUE TO NORMAL CURRENT DRAW. It is superfluous heating.

Normally, an Over Load Relay senses current as a way to DETERMINE the heating going on inside of it, based on motor modeling design specifications. So as a basic rule, an OL relay does not know if the current flowing in is creating MORE heating than is should. Basic NEMA style OL relays are like this, they ONLY look at actual phase current flow. So under a phase loss or imbalance situation, if the current flow is still below the I[SUP]2[/SUP]t trip curve, it's happy and nothing happens. It has no idea that the current is only flowing in 2 phases and therefore is heating the motor up disproportionately to the standard motor model. That means a motor CAN BE THERMALLY DAMAGED under a phase current imbalance, and the OL relay may NEVER trip.

In the IEC Bimetal OL relays that supposedly have "phase loss protection", what they really have is phase current differential compensation. There is a spring loaded balance bar inside and the pressure exerted by all three current sensors is equalized against the spring pressure. If one phase is lost, the pressure is no longer equal, and the trip point of the relay is skewed to be lower than the setting. The point of this is to compensate for the added heating effects caused by the imbalance, but it does NOT actually "trip on phase loss" as they lead you to believe. The problem is, if the motor is lightly loaded and thus not drawing much current, this may not be enough compensation; the motor may still continue running on a phase loss and getting too hot even though these bimetal OLs are skewed to trip earlier.

As to Phase Monitor Relays, Phase Reversal is extremely uncommon unless it involves portable equipment or the use of emergency generators. So usually it's not worth worrying about. But one thing most of them never tell you is that voltage based Phase Monitors can be fooled by the regenerated voltage on the lost phase coming from a motor that was already spinning when the phase was lost. If you've ever heard of "rotary phase converters" it's the same phenomenon. Unfortunately losing a phase while something is running is the case more often than not. So a voltage based Phase Monitor only works reliably to prevent the NEXT restart, not necessarily to take a motor off-line WHEN a phase is lost. Current based Phase Loss / Imbalance protection is far far more reliable.

That's why I like Solid State OL relays better, and recent developments have provided options that make some of them very close to the same cost as bimetal anyway. In SSOLs, they look at the actual current in each phase and if any one phase drops below a threshold, they trip. Simple and reliable. If you use SSOLs you really don't need additional Phase Monitor Relays.
 

templdl

Senior Member
Location
Wisconsin
We install phase loss protection primarily to protect motors from damage due to phase loss from the utility.
I agree. But a number of years ago I provided PVMs for motors at a new waist water treatment plant. The utility didn't advise diggers hotline of some new underground cables, a excavator dig up the cables single phasing the plant and promptly fried (3) 60hp motors.
When called to the sight and asked why my PVMs didn't protect I discovered that all were never touched as the installer was never advised by the engineering firm of their settings. Bach then the cost of replacing the motors was $16k.
 

GoldDigger

Moderator
Staff member
So under a phase loss or imbalance situation, if the current flow is still below the I[SUP]2[/SUP]t trip curve, it's happy and nothing happens. It has no idea that the current is only flowing in 2 phases and therefore is heating the motor up disproportionately to the standard motor model. That means a motor CAN BE THERMALLY DAMAGED under a phase current imbalance, and the OL relay may NEVER trip.
Please correct me if I am wrong, but it seems to me that if you lose one supply phase wire in a delta-connected motor, you will actually lose two phases even though two phase wires are live. The current through the other two windings will be the result of that single phase voltage being applied to the series connected pair of windings.
But if you have a wye connected motor, you can actually energize just two of the windings and have an open circuit on the third.

In the first case, will there be any torque produced at all if the motor is not initially rotating?
In the second case you will, as you said, be building a rotary phase convertor with no output load connected.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
We use better PLMs, over $150 and have never lost a motor from phase loss, or low voltage.

I did loose out on a couple projects because my price was to high for the install. I was called in to fix both. The first still had smoke marks in panel from the installers "oops". I don't know what he did wrong. The second was some years after the install by the same well/pump installer at a different well. Same customer. Large company different tech...maybe. The primary had broken while village well was running. Motor fried. The same company was there replacing the motor, while I was there for the Village looking at the "why". They were wondering aloud about how come the PLM didn't work. It was my pleasure to tell them they had hooked it up wrong. It got real quiet in there, and the village got a free motor.:happyyes:

Moral of the story, don't screw up the install.
 

kwired

Electron manager
Location
NE Nebraska
Sure they can and most of us have witnessed it. A 75 KVA transformer can put out more than 75 kva. A 120 volt lamp can run on 130 volts.

But for longest service life it is wise not to run them that way.

Few three phase motors cost less than rudimentary phase loss protection devices, add in downtime and labor costs and it becomes downright foolish not to protect the motor from phase loss in one way or another.
Well then, across the line starting is hard on them also, do you put soft starters on every motor you connect?

I have seen many many motors with solid state overload and phase loss protection incorporated and they still burn out. Why, many times because some idiot got tired of the overload tripping and set it at the highest setting. I still have more faith in thermal overloads (if sized properly) than some of these accurate but adjustable and tampered with devices.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Well then, across the line starting is hard on them also, do you put soft starters on every motor you connect?

I have seen many many motors with solid state overload and phase loss protection incorporated and they still burn out. Why, many times because some idiot got tired of the overload tripping and set it at the highest setting. I still have more faith in thermal overloads (if sized properly) than some of these accurate but adjustable and tampered with devices.
But what stops the same idiots from putting in larger heater elements? Or just jumping the elements out with pieces of wire? You and I both know that happens too.

Many SSOLs come with an optional clear cover that can be sealed after adjusting the dial. Even that doesn't stop them, but it leaves evidence. AB has one that has no dial at all, it must be set across a network connection or with a hand held programmer. Still, the idiots persist. I just came across one where someone cranked up the setting to max over the network. The good news was, we know exactly who he is because we know exactly when the change was made and who was logged on.

You know the old saying. You can't make anything idiot proof, you just succeed in creating better idiots.
 

iwire

Moderator
Staff member
Location
Massachusetts
Well then, across the line starting is hard on them also, do you put soft starters on every motor you connect?

I have seen many many motors with solid state overload and phase loss protection incorporated and they still burn out. Why, many times because some idiot got tired of the overload tripping and set it at the highest setting. I still have more faith in thermal overloads (if sized properly) than some of these accurate but adjustable and tampered with devices.
Are you seriously arguing against adding a $100 device to protect motors that the loss of may cost $100s to tens of thousands in downtime, replacement and labor?:blink:

To each their own but I will keep putting them in. It is not unusual for me to add one phase monitor that protects 3 to 5 motors.
 

kwired

Electron manager
Location
NE Nebraska
But what stops the same idiots from putting in larger heater elements? Or just jumping the elements out with pieces of wire? You and I both know that happens too.

Many SSOLs come with an optional clear cover that can be sealed after adjusting the dial. Even that doesn't stop them, but it leaves evidence. AB has one that has no dial at all, it must be set across a network connection or with a hand held programmer. Still, the idiots persist. I just came across one where someone cranked up the setting to max over the network. The good news was, we know exactly who he is because we know exactly when the change was made and who was logged on.

You know the old saying. You can't make anything idiot proof, you just succeed in creating better idiots.
I have seen overload elements changed by the "idiot" when in a place were it is common to have spare elements around. If the application is somewhere where they don't have those kind of supplies most of the idiots have no clue those are changeable, and if they do they are coming to someone like me to find out where to get them, then I can step in and tell them why they were selected in the first place. A SSOL with an easily adjustable dial - is no different than the guys that figure out a 30 amp fuseholder will hold a 30 amp fuse even though the original installer put in a 5 amp fuse

Are you seriously arguing against adding a $100 device to protect motors that the loss of may cost $100s to tens of thousands in downtime, replacement and labor?:blink:
No, I am saying properly applied overload devices are more effective than you are saying they are. There may still be times when they don't provide enough protection from phase loss. High torque applications they are pretty effective though.
 

StarCat

Senior Member
Location
Moab, UT USA
Phase Monitors

Phase Monitors

Circuit breakers do not allow the same level of protection as Phase monitors. Phase monitors will give you faster response before you reach as high a stress point with your motor as tripping a breaker. Most well seasoned HVAC professionals do not like to see fuses on 3 phase motor or compressor loads. Breakers are preferred because they open all 3 lines. AS an example this area has fairly " dirty " power. We are running several refrigeration and chiller loads in the 60 amp service range. Before installing phase monitors on these units, power problems would take this machinery down by tripping some of the on board pump starters etc. By adding some simple and basic phase monitors, the power problems are seamlessly dealt with and the machinery resets and re-starts automatically and it does not affect the process operation. Rarely is it even noticed whereas before machines were going down and motors being pushed to the " trip " point. They can save you wasted time, down time, and streamline your operation.

All the best
 

kwired

Electron manager
Location
NE Nebraska
Circuit breakers do not allow the same level of protection as Phase monitors. Phase monitors will give you faster response before you reach as high a stress point with your motor as tripping a breaker. Most well seasoned HVAC professionals do not like to see fuses on 3 phase motor or compressor loads. Breakers are preferred because they open all 3 lines. AS an example this area has fairly " dirty " power. We are running several refrigeration and chiller loads in the 60 amp service range. Before installing phase monitors on these units, power problems would take this machinery down by tripping some of the on board pump starters etc. By adding some simple and basic phase monitors, the power problems are seamlessly dealt with and the machinery resets and re-starts automatically and it does not affect the process operation. Rarely is it even noticed whereas before machines were going down and motors being pushed to the " trip " point. They can save you wasted time, down time, and streamline your operation.

All the best
Circuit breakers are generally for short circuits and ground fault protection only, I would never expect one to open during a phase loss condition, and if it did open the motor has suffered some damage before it opens.

I would expect properly selected "motor overload" to provide fairly significant phase loss protection - especially for high/constant torque loads. Fans and pumps maybe have holes in the protection when it comes to phase loss. They may be well protected from attempted starting with a lost phase, but if phase is lost while running is when you are likely to see more damages from a lack of response by the overload protection.
 

iwire

Moderator
Staff member
Location
Massachusetts
By adding some simple and basic phase monitors, the power problems are seamlessly dealt with and the machinery resets and re-starts automatically and it does not affect the process operation. Rarely is it even noticed whereas before machines were going down and motors being pushed to the " trip " point. They can save you wasted time, down time, and streamline your operation.
A lot of my work is with large refrigeration racks in supermarkets and you have nailed it.

Auto restart is a huge advantage.
 
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