Motor reversing
Motor reversing
(OP)
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.
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.





RE: Motor reversing
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor reversing
this is why i decided to join and post
RE: Motor reversing
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
RE: Motor reversing
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
RE: Motor reversing
** 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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RE: Motor reversing
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RE: Motor reversing
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RE: Motor reversing
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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RE: Motor reversing
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
RE: Motor reversing
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: Eng-Tips.com Forum Policies
RE: Motor reversing
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
RE: Motor reversing
Did you try connecting a second motor to the circuit as I suggested?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor reversing
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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RE: Motor reversing
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
RE: Motor reversing
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RE: Motor reversing
RE: Motor reversing
I am assuming that when you checked the motor you did not miss finding any faults.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor reversing
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
RE: Motor reversing
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor reversing
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RE: Motor reversing
Probably not, that is why my fingers are crossed. But stranger things have happened to me while "solving" apparent mysterious issues.
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor reversing
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RE: Motor reversing
RE: Motor reversing
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor reversing