Why does 3-PH AC motor fry when... 3

[paw158]

I've recently seen a 3-Phase motor fry. This happened right after one of the legs was disconnected. I've been trying to understand why the motor would fry if one of the legs of power is disconnected, but I can't think of one.

I would think that the motor would have just not run as efficiently.

Why did the motor fry?

Any help would be appreciated.

 

[davidbeach]

It had more load than the single phase source could supply? Single phasing of 3-phase motors can be nasty.

 

[jraef]

First off, I will assume you had an older style NEMA overload relay, because most IEC relays compensate for this scenario and would have prevented the motor from burning up.

A motor is a "dumb animal" in that it will try to spin at full speed no matter what. When you disconnect one leg, you are running that motor on 1 phase power. That reduces the torque output of the motor by 42% (1/1.732), and if the load remains the same, the slip difference encountered now puts your motor in an overload condition in an attempt to remain at full speed, but not by enough to cause a fast trip in the overload relay. You also now have a severe current imbalance, and the heating effect on the motor winding insulation goes up exponentially to the percentage of imbalance. You have a 100% imbalance!

If your protection system does not account for that phase loss separately, you have overloaded the other 2 windings but the heating effect of that overload is greater than is represented by the current through the OL sensor because of the severe current imbalance. That additional heating effect can burn the insulation before the OL relay can act to take it off line.

IEC style overload relays have a differential bar mechanism that biases the trip point if one phase is missing, thus compensating for the necessary quicker tripping needed if one phase is lost. Solid state overload relays typically monitor phase currents directly and immediately trip on a lost phase. Bottom line, get a better OL protection system or add a phase loss relay to your controls.


Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

Three phase motors rotate because of the vector relationship of THREE PHASES if you remove one then the motor's torque drops, the speed drops, the back emf drops, the current jumps.. the motor frys...

You want real fun single phase a refrigeration compressor.. Then when the motor fails, which is immersed in the refrigerant, the refrigerant decomposes into to acids that destroy the entire system.

 

[paw158]

jraef: I'm a graduate. This is for my job, but as I said initially, I don't have much experience with electricity.

 

[paw158]

By the way, thanks for the help though!

 

[Warpspeed]

paw158, if you disconnect one phase, you lose the rotating magnetic field and end up with a stationary field that just goes up and down in strength.

The motor certainly will not start, and if it is running under any sort of load it will most likely stop.

The winding that is still energised will induce massive currents into the rotor, and in turn draw excessive current from the remaining two still connected phases.

You then have a genuine UFO (unidentified frying object).

 

[14041972]

Paw 158
This is a very common fault in 3 phase electric Motors. You will have dozens of those in the future so it is important that you understand the concept clearly. Assume you are riding a tricycle; remember it is designed to run on three wheels. When on top speed you loose one wheel. You may choose to ignore the break down and keep on riding but you know as much as I do that you will either tip over or keep on going for few meters due to momentum but for sure you will not be able to pedal again. If you force it you will break the bike further or injure your muscles.
Jraef referred the motor as a ‘dump animal’ call it ‘dump rider’ who will keep on riding the bike after loosing the wheel. 3-phase motor is designed to run on 3 phases and must have all of them to operate healthily. These phases are normally Red (R), yellow/white (W) and blue (B). Absence of any of the there phases is an abnormal scenario referred to as single phasing. Single phasing 3-phase motor on load will behave dump and keep on turning the load as if northing happened. The rotor of an induction motor is electrically isolated from the stator. Connecting 3 phase (RYB) to the stator results to rotating magnetic field at 120 degrees phase shift. These fields link with the rotor through the air gap (transformer action) and causes the rotor to turn. If one of the RYB is open, then you will have unbalanced voltage, un-uniform magnetic field linkage to the rotor, high current, frying.
Happy?

 

[ccjersey]

postmortem on the motor stator will show 1 burned winding phase if it was a delta connection and 2 if it was a wye connection.

 

[aolalde]

Three phase windings are geometrically distributed at 120 electric degrees and the MMF’s on each winding have 120 °E out of phase; that combination creates a resultant MMF rotating phasor.

When one line is lost the excitation is no longer 120 °E out of phase for each phase winding and the resultant is an alternating MMF vector.

When the driving magnetic rotating phasor is replaced by an alternating vector, the rotor induced currents do not produce a coordinated torque any more, increasing the slip and reducing the resultant fem on the windings which finally results in unbalanced increased currents. If the load is close to the motor nominal power the rotor stalls.

Coordinated protection should trip the motor off-line.

 

[machmech]

Hi paw158

That line in jraefs post was not directed or intended for you, He places quotes as do many other people on the bottom of postings and this is the appropriate time of the year for this quote. A lot of kiddies out there looking for the easy education!

Chuck

 

[bigbillnky]

paw158
Hello,
This is a great example of the importance of proper coordination of the motor OL device to the motor. It has been my experience that most of those who maintain motors or motor controllers don't fully understand how to properly calculate and select OL devices for NEMA style motor starters. Nor, do they understand the significance of selecting the wrong OL device rating.

Several factors should be considered when selecting OL devices. Differences in the ambient temperature from where the motor is located and where the OL device is located will impact the proper operation of the OL. Also, consideration for the inherent time delay characteristics of the particular type of OL device is an important factor.
All to often OL devices are oversized, and until there exists a real problem, they go unnoticed.

If the overload devices were properly sized, and the motor starter and OL devices were properly wired and functioning correctly, the OL device SHOULD have prevented the motor from failing due to a single phase condition.

You admit that you are lacking on electrical knowledge, so I would like to suggest the book "Electric Motor Maintenance and Troubleshooting" by Augie Hand. It has been a great resource for me over the years. I would also suggest that you consider checking the ratings/wiring of the rest of the OL devices at your facility to prevent similar problems in the future. Best of luck and Happy New Year to all.


Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[paw158]

I appreciate everyone's eagerness to help out in this situation. I learned a lot of useful information. Thanks!

 

[buzzp]

The old overloads (before 1971) only used two elements. These types of overloads might not have picked up the increased current in the one leg due to the single phasing condition. If this is an older installation you might find the OL only has two elements when it should have three. The two element OL's were definatley the norm for a long time until they figured out that a single phase condition might not cause an increase in current in the two phases with OL elements. If I remember correctly, single phasing a three phase motor, generally, only causes one leg of the motor to draw more current. There is a lot of info on the web on this and I believe there is even a word or two about it in the notes in the NEC.

http://www.usmotors.com/products/ProFacts/1-124.htm

http://www.pge.com/docs/pdfs/biz/power_quality/power_quality_notes/open_phase.pdf

(see last page)

 

[dpc]

The OP's question has been pretty well answered, but I just want to add that I am much less confident than others in the ability of standard motor overload relays to protect motors from single-phasing. Because of the negative sequence current caused in the motor by single-phasing, the motor can be destroyed at measured current levels below the OL relay's threshhold - at least the older bimetallic relays - even three-phase ones. I have been in mills where virtually every motor was destroyed by loss of an incoming utility phase, even though all had three-phase NEMA OL relays. I suspect the newer electronic OL relays can provide better protection.

But if it is a motor I care about, I would make sure there was a good quality loss-of-phase protection device somewhere in the system.

 

[itsmoked]

dpc I'm with you on that one.. As I stated in a previous post, many times we have seen refrigeration equipment destroyed BECAUSE of overloads! They get flaky, heaters burn out, etc. They are so bad that now Carrier one of the biggest refrigeration companies,(if not the biggest) void warranties on any compressor "protected" by overloads. Better are phase monitors and there kin, which have gotten a lot less expensive.

 

[jraef]

paw158,
machmech was right, that comment at the end of my post is part of my signature line, it wasn't directed at you personally (thanks machmech for clearing that up). By the way, if you were curious as to why this only occurrs in 3 phase motors, think about it a little more. On a 1 phase motor if you lose 1 phase your motor just stops!

aolalde,
I liked your description of why the imbalance raises the current so much more than just by the OL on the other 2 phases. A PLS for you.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[buzzp]

The overloads are meant to protect against an overload whether it be from a single phase condition or a mechanical load. The key here is, a loss of phase, may or may not cause an overload condition. Single phasing is just gross unbalance which, ideally, would be measured using negative sequence components. Large motors warrant this type of protection but a smaller motor does not warrant such expensive protection (talking about negative sequence currents here) so to handle a loss of phase there are many stand alone relays to handle this or they even have thermal OLs that have single phase protection built in as well.

I was not trying to say the OL would protect the motor in all single phase conditions. Some cases it will, other cases it won't. Depends on the load on the motor and the severity of the OL. This is why they have started to integrate OL and single phase protection in one package. If an OL could always detect a single phase condition, then they would not need the single phase protection. Another factor in all of this is where the trip level is set. If its set to the running amps (which is hopefully lower than FLA), you will be more likely to detect a single phase. If it is set to the FLA and the motor is not loaded much, chances are a single phase condition will be not be detected by the OL (via the current). So, NO an OL will not necessarily protect the motor from a single phase condition as some posters have eluded too.

Just wanted to mention the reference to ambient temperature is only a factor for older thermal OLs which are still used all over. Newer electronic relays do not need temp compensation because they don't work off from heat like a thermal OL.