Motor reversing

[neal5105]

Good day all,

I have a 2.2KW motor installed in a milling plant.The motor is 400v 3 phase 4 pole.
The motor starts and stops by means of a level switch that contols a contactor. The switching is frequent ie evey 2 seconds.

Now heres the problem. The motor will run fine for a few day and the the direction will sudenly change. When this happens there is no problems and it will run in reverse as long as we leave it. I then have to change phases to get it to run in the correct direction. All if fine for a few day and then the direction will change again.

There are about 10 other motors on the main board but only the one motor has the problem.

Now please feel free to tell me this cant happen because that is what i told the customer the first time he told me.

 

[rbulsara]

Someone is messing around with the circuit. Look for disgruntled employee or a prankster.

Rafiq Bulsara

http://www.srengineersct.com

 

[neal5105]

I thought about that until it happend wilst i was watching the motor by chance yesterday and saw it with my own eyes.

this is why i decided to join and post

 

[rbulsara]

Look harder. Circuit can be changed from a location other than where you are standing too, like at the panel.

Other possibilities: The motor loses and power and load is reversing it making it look like reversing.

Check the starter it is reversing type with some kinda automatic timer.

I do not believe in mysteries like this. Think and look harder.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

I agree with Rafiq.
But, I haven't seen everything yet. Check the motor connections and megger the windings and check that the winding resistances are well balanced. The connect a small 3 phase motor to the motor leads at the pecker head. If one motor reverses and the other doesn't, you have narrowed the problem to the motor. If both motors reveres, walk the feed cables back to the supply looking for a reversing switch.
What does the motor drive. It may help us to know.
With such severe starting duty, the motor may be failing in a unique fashion. More likely some one or some thing is reversing the phases somewhere.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I agree sounds unlikely and sabotage may be among most likely explanations.

** How about if one phase was open. Then it acts like a single phase motor. When you switch off/on a single phase motor it may start rotating in a different direction.

By the way, what indications are you looking at to infer the direction of rotation?




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[electricpete]

When you switch off/on a single phase motor it may start rotating in a different direction.

Perhaps there is some help from the process that may help it start rotating in one direction or the other (?)


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[electricpete]

Side comment – when you say switching is frequent ie every 2 seconds... do you mean the motor turns on every 4 seconds or so? I have a hard time imagining that but it true I wouldn’t think you would expect very much life from this motor.

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[jraef]

I must say that I have seen the exact thing electricpete suggested. 3 phase motor on a pump, pump feeding into a header with 4 other pumps in a qudraplex pumping scheme. One phase winding on this particular pump motor was open, but because of the connection to the common header if other pumps were running when it turned on, the flow started the impeller moving and even though it was getting single phase power, it would spin correctly. If no other pumps were running, back pressure (from the process head when the MOV opened) would be forcing water through the pump and when energized, it would run in reverse. The way I found it was to look at the running current vs flow and comparing it to the other pumps. Nobody had noticed the reduced flow and increased current because 75% of the time it was running as an add-in to other pumps.

Had it not been a Motor Operated Valve, this pump, when selected as the lead pump by the quadraplex controller, would never have started and therefore never developed pressure to open the check valve and the situation would have been discovered immediately.


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[rbulsara]

jraef:

Ah, that is interesting and makes some sense. I would have thought that the OP has checked his 3 phase motor.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

I still think someone is messing with him though...

Any other possible explanation involves the necessity of several contributing factors, each one contributing to the decrease in the likelihood of it happening.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[neal5105]

Thanks for all the comments,

After some thought I have done the following;

Walked the cable to check for other starting divises found none.

Meggered the motor all tested fine

Cheched power supply with analizer. found plant power factor to be 0.6 Can this have a effect on the motor??

The motor drives a gear box and then drive a bucket wheal via a chian. The direction can be seen becuses of the low speed on the bucket

 

[waross]

If this is a mine mill, you may be connected to the wrong MCC. I have seen mills with reversing contactors on the feeders to entire MCCs. Some equipment was periodically reversed to equalize the wear. It is much cheaper to dedicate an MCC to such equipment and reverse the entire MCC rather than reverse each motor individually.
Did you try connecting a second motor to the circuit as I suggested?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

One scenario is that you have an intermittent open circuit (either in the motor or supply) causing the single phasing. Or one contactor contact intermittently fails to close during start.

Other checks that might be done:
Verify current in all 3 phases (balanced) ?
Verify current in all 3 phases (and balance) when motor was rotating reverse?
resistance check of motor windings with high precision bridge.
Thermography survey of supply circuit

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[rbulsara]

Look at the motor not the bucket wheel to be sure and to eliminate as load or the mechanical as the 'cause'.

The load may be causing the bucket wheel to reverse but gear box may not turn the motor when turning in reverse (disengages like a ratchet).

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Cheched power supply with analizer. found plant power factor to be 0.6 Can this have a effect on the motor??

What is “Plant power factor”? The displacement power factor sensed at supply to entire plant? The motor certainly doesn’t care about that.


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[OperaHouse]

I suspect a motor elsewhere in the plant is reversing from load and phase loss. The one you are watching is just tagging along with the regenerated phase. Start looking for something big upstream.

 

[waross]

Reversing every few days sounds about right for an MCC dedicated to flotation cells. Look for something upstream of the MCC and possibly upstream of the transformer. Take a good second look at other motors on that MCC. They may be quietly reversing at the same time. NOTE: Not any other motors, just motors fed from that MCC.
I am assuming that when you checked the motor you did not miss finding any faults.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[neal5105]

Thanks again for all the input.

After spending some time on the plant, I suspect the following:

The contactor that starts the motor is “failing.” When the contactor closes, one of the phases makes contact milliseconds after the other two contacts. This causes the single phasing of the motor. I suspect that this causes the motor to reverse. This would also explain why once the motor is running in reveres we do not pick up any abnormal reading.

What I am not sure about is if this is the case why does the motor not try to change direction when the “late” phase is applied.

I have replaced the contactor (again) so we will have to wait and see.

PS. The suspected problem was found using a power quality analyzer set to measure inrush current. The meter measured the inrush per phase down to 100th of a second.

Enjoy your Easter weekend

Neal

 

[rbulsara]

Will keep the fingers crossed...but feedback like this is always appreciated.

Rafiq Bulsara

http://www.srengineersct.com