3-Phase Induction Motor Not Stopped

What a good motor!

I'm sure that when it is connected to whatever it is driving, the problem will go away.

You are right..Problem is resolved with increased time delay between power contactor and winding heating contactor and by connnecting motor to its load.

But How can 24V AC drive it..? thats the point of discussion.

How about setting the time delay to 30 seconds rather than 3? What's the hurry?

It's like HP use in a car, when you are at a slow cruising speed HP usage can drop way down....At 30 mph for an average car to keep it moving at that speed you just need to overcome rolling resistance and wind resistance, you already have momentum of speed....on a motor with no load, rolling resistance is the only force to overcome, and on properly lubricated bearings, well it turns out in your case about 108 watts is enough for that...

What about the rotating magnetic field..?? is 24V AC between two phases enough to rotate a rotor.?

capacitive coupling, even more common single phasing a motor with power factor capacitors, phase difference of current due to capacitive shift, like a badly designed capacitor start motor, keeps it going. I would not have bet that 24V would keep it running, though.

Thank you,!!

Motor is not capacitor start/run,

I am agreed that only 24V is not just enough to keep motor running...

But you observe that it is enough to keep it running, and this is why..

One (or both) of the motor adjacent windings is capacitively coupled to your 24Vac source, so it acts like a single phase capacitor start motor, the capacitor coupled winding is 90 degree phase difference, so that gives you the torque to keep the motor turning. The capacitance completes the circuit to the motor anti-condensation heating power source.

Your motor leads may be long, so enough charging current to keep the motor running. Motors with power factor capacitors can run single phase and carry quite a bit of load, the phase difference of the capacitor supported phases supplies the 2nd magnetic field to keep the rotor pulled around. No actual capacitor needed...

From that description, clearly, the answer is "yes".

with 30 Sec, it absolutely working ok, and come down to zero speed. no issue with that time delay. I am curious to know that how can only 24V AC between two phase(R &Y) can drive a torque for 3-phase 440V AC Induction motor..?? What is that miracles ..?? Is it the number of pole..??

This is why I have always hated using LV AC as the heating circuit, I prefer using DC. No chance of rotation that way.

3 seconds was absolutely pointless, it could be 3 minutes really. All you want is for the motor to stay above the dew point so that condensation doesn't form inside. Residual heat in the motor lasts a long time.

On the flip side, if you turn it in too soon, as in before the magnetic field of the motor has collapsed, you can blow out your 24V circuit. Hopefully it is fused...

If the idle load is 6% of the normal load, then 6% of the normal voltage can drive it.

But winding heating supply given between R & Y phase only( Only two phases are being used in contactor, while third will remain in open ended while motor is in stop condition)..still it will drive..???

Haven't you ever heard of single phasing? That's running a 3 phase motor on 2 phases, and is usually a bad thing in that it overheats the motor and damages it....but with reduced voltage and amperage it's just enough for moisture control...

http://www.usmotors.com/TechDocs/ProFacts/5Cases-Run-1Phase

Agreed, it is like single phasing..but it is still running with reduced voltage 24 V and current.... we have total 08 motors of same rating and with same time delay..identical for moisture controller..but i found such observation in particular one motor only..

Is inductance does make any difference between them..? Any construction dissimilarity..?

This observation is being over-analysed.

As winding the time delay out to 30sec solved the problem, there is clearly nothing else to be done.

Mr. PWSlack,

The all answer are relevant and i express my sincere thanx to all for good participation in this debate,

I am not interested that how we can overcome from this problem, I am interested to know why did it happened..?? can we prove it theoretically..??

We are on right track nothing like over-analysed...

Probably have a more worn bearing than the others....spins more freely...

Well, clearly, from the description given that is what it does, unremarkably.

"Analysis Paralysis" at its finest. A three phase motor once rotating in a particular direction will continue rotating when single phased, it just won't do much work since the available torque is quite low. About the only work it will do is overcoming friction from rotor windage and shaft bearings, both of which are remarkably low especially at a fraction of the rated speed.

Since it is unlikely that the overhauling did anything to affect the windage (unless the cooling fan was modified), the bearing friction has changed. The shop may have replaced the bearings, put in a lighter grease, and/or did a better job of aligning the shaft, all of which would affect the drag due to bearing friction.

The torque should be so low that you can stop the rotation by putting some drag on the shaft. Similarly, you should be able to put 24VAC on any of the other motors and give the shaft a slight twist and watch it accelerate until it reaches steady state, even if it is opposite to its indicated direction of rotation. Remember static friction is lower than rolling friction, so your hand provides the necessary torque to overcome it.

Interesting fun fact: if you were to measure the real power consumed with the single phase 24V supply you would get a measure of the amount of the power losses going into the cooling fan and the bearings. Not exact of course, because the iron and copper consume a small portion, but you could use the test to see if the other motors have tighter or looser bearings under the same voltage and current conditions.

Doesn't anybody take AC Machine Lab anymore??

We have not gone in detail part of grease and cooling fan balancing, thank you for your inputs.. will come back to you after checking the same and verifying in other motors also.

Thank you all participants to make it good debate, please share some more ideas to find the exact root cause.

Why do you think it takes a lot of torque to get a properly aligned, properly lubricated machine to turn? You could easily turn the rotor of a 600MVA alternator with the aid of a 3 ft lever. That's where the term "barring" comes from.