3 phase induction motor and their failure - Page 1
Forum section: electronics / beginners

author: aiq25
author_rank: Regular Contributor
timestamp: December 07, 2014, 06:00:56 pm
content: Hello. Recently I been doing some research on 3 phase induction motors and I had a question. In the 3 phase system if one of the stator windings breaks, will it still be possible for the motor to run? I would think you could make the motor run by making some hardware changes. I think using two of the stator windings you could create a magnetic field in 2D space in any direction, so you could use this magnetic field to move the rotor.

author: SeanB
author_rank: Super Contributor
timestamp: December 07, 2014, 06:41:21 pm
content: Yes it will run, but will be noisy, and the remaining 2 windings will carry excessive current and heat up considerably, until they eventually have insulation failure and either go short circuit internally ( normally a lot of smoke comes out, or a lot of black goo) or short out to the case and blow the leads open. with an open winding it may not start under close to full load, and will sit there and rapidly overheat till it fails.That is why it is pretty standard for 3 phase motor overload relays to have current imbalance protection built in along with overload protection.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2014, 04:27:40 am
content: Ya know, I don't think it would be that bad. Just one winding failing means the motor still sees a full circle magnetic field each cycle. It's a little imbalanced, but you still have three connections at work. The remaining two windings would see on the order of 2/sqrt(3) times more current I think, which -- if the first winding failed due to thermal overload and burnout, the other two won't be far behind, especially now -- but under reduced ratings rather than a fault condition, I see no reason why it should be destructive.Saturation is not a concern, because each winding must tolerate the line voltage, period, whether driven in full three phase or not. You can drive an induction motor with just one winding powered (as a single phase unit), if you give it an initial starting direction (it will spin just fine in either direction). The third terminal basically serves to provide directional information and smooth torque output.Point in case, there is a two transformer three phase connection called 'open delta': the advantage is only needing two transformers (instead of three independent transformers or one three-legged type), while the disadvantage is extra required capacity, and a load dependent droop between the output terminals (because, the third transformer acts to balance it).Tim

author: kolbep
author_rank: Frequent Contributor
timestamp: December 08, 2014, 05:56:12 am
content: In some cases, when a Winding fails on a 3ph motor, or if one of the supply phases fails,then the motor runs backwards!!!But the previous comments are right,the motor will buzz and overload,it will draw a lot more current on the existing phases,overheat, and eventually burn out (unless the protection device / breaker trips out first)...At some sites where I am, if the overload on the contactor keeps on kicking out (because of failure of a phase or winding),then they just get one of the lesser paid guys to stand there, holding the contactor in with a stick,Until the whole thing is completely messed (smoke pouring out of the motor), by then the Contactor, Overload and Motor all need to be replaced.

author: johansen
author_rank: Super Contributor
timestamp: December 08, 2014, 07:05:01 am
content: the motor will run on two out of three windings when WYE wired, but it won't start very well, if at all, and it will spin in either direction.many folks run a three phase motor on single phase, but the third coil is supplied with starting and run caps to ease the load.some people have a separate motor belted to the idler for startup to solve that problem.i think the general rule is the motor could still deliver half the rated output. 57% i think, but i can't recall exactly.

author: SeanB
author_rank: Super Contributor
timestamp: December 08, 2014, 07:26:06 pm
content: Open Delta is used with very large derating on the transformers. The most common use of a 3 phase motor is in a larger air conditioner, where it will be driving the compressor and will be running at full load all the time. Loss of phase there means that the motor will burn out in a very short time. Running a fan will also do it, but there it might take longer as the motor will have higher slip and thus will deliver less power, so it will draw less from the supply so it might only run at 150C core temperature instead of 200C.

author: aiq25
author_rank: Regular Contributor
timestamp: December 10, 2014, 03:41:43 am
content: Thanks for the replies. I don't think I'm going to experiment with running an induction motor with 2 windings but it's good to know. Man it's been a while since I have heard of OPEN DELTA transformer connection. I used to work for a utility company, and we had a lot of open delta connections, as well as WYE connections.

author: johansen
author_rank: Super Contributor
timestamp: December 10, 2014, 08:51:05 pm
content: re-reading your question, it is rare for a single winding to fail open without burning up and destroying one or both of the other coils.in the case of a wye motor where you have the floating neutral point grounded (which is rarely the case) you can run the motor at about 2/3rds load and it will also start, because there is a rotating field. same way for the open delta with one winding failed.. you still have three phase connected.in the case of a 9 lead dual voltage motor, when wiring the motor for the lower voltage, you have access to the wye neutral point of one of the sets of windings. if you connect that node to neutral, then a loss of any single coil will still allow for the motor to start and depending on which coil is lost you'll either have 16% derating or 33% derating. (unless you bring out all 12 leads, then any single coil will cost you 16% derating) --provided you connect the wye neutral to neutral.most everyone will tell you its a really bad idea to ground the neutral of a wye connected motor to the neutral of your three phase supply... I don't think there should be any serious problem.. might even get more power out of it at full load, at the cost of higher no load draw due to circulating third harmonic currents. -it really comes down to who built the motor.