Slip Ring Motor

[bob]

Hi,

 

Im looking for the expert advices on this forum. We have got on site two 900 kW 400 V slip ring motors driving a common load. We noticed since a couple of days now that one of the motor is slipping with the result that one motor is driving the most part of the load. Neither motor is heating to a point of major concern. The motors are started in turn by a common liquid starter.

When the motor slipped, we noted no current on one phase of the rotor . However, this phenomenon dissapeared when the motor is restarted and appeared back again after days, four to five days of continuous operation. I have removed the air cooler but could not see anything abnormal on the rotor connections. I need to take a decision to dismatle the rotor and look for any rotor bar damage. However, we will need to stop production for about two days.

Could a rotor bar failure exhibit such behaviour ? Could it be on the end connections ?Definitely , it is not on the liquid starter because I shorted the rotor connections on the slip ring with the same behaviour.

Any help welcome.

 

Bob

 

 

[marke]

Hi Bob

 

I would be looking hard at the ring gear and brushes and the shorting contactor. It sounds like it is intermittent and that is commonly indicative of brush/ringgear problems. The would rotor should not be intermittent like that except at the terminations on to the ring gear.

 

Best regards,

[bob]

Dear Marke,

 

Thanks for your usual prompt reply. There are four brushes /phase and the connections to the ring gear have been checked. There is also no shorting contactor. both rotors are shorted by the liquid starter contacts through two independent contactors. You are right it is an intermittent problem but the rotor connection only open after days of operation and stayed opened until the motor is stopped and restarted again.

 

Bob

 

[marke]

Hi Bob

 

There obviously an open circuit developing somewhere, and it is either inside the motor, or the starter.

I find it useful to divide and conquer, so what I would do, is to attach meters or lamps to the rotor terminals on the motor and monitor those. Next time the rotor goes open circuit, you can just check the lamps/meters and see if there is any voltage at the rotor terminals. you can tell your operators to check this when there is a problem. If there is some voltage there, then the problem is in the starter. If there is no voltage there, then it inside the motor- you will know which end to start at!!

 

Best regards,

[jraef]

A wound rotor motor doesn't have rotor bars anyway!

It has windings (hence the name wound rotor).

 

http://www.ewh.ieee.org/soc/es/Nov1997/09/B_WOU1.JPG

 

Most likely you either have a problem of some sort affecting your rotor connections, or you have a problem in the contactor that takes over when the liquid starter is finished. If it's in the rotor it could be a termination problem where your rotor windings attach to the slip rings or even a coating of some sort affecting your brush connections. I think Marke's test method makes a lot of sense. Isolate the problem to discover where it lies first.

[bob]

Thanks Marke, I never thought of measuring the voltage at the slip ring connections.We were only monitoring current. Thanks again Marke. I really appreciate.

 

I will quote a French poet "Appreciation is a wonderful thing: It makes what is excellent in others belong to us as well”

 

I shall keep the forum posted.

 

Warm regards.

 

Bob

 

Jraef- Thanks for your comments as well. The rotor has got bar and we have many such wound rotors on site here.

[mariomaggi]

Dear bob,

using my special "crystal ball", I see a brazing or welding defect in one bar.

In certain conditions this bar could heat something more locally, where the contact is no more perfect, and the single bar will dilate itself, reaching a position where the insulation or oxide add a great resistence on the circuit.

 

After cooling, expansion will end, and the phenomena will be repeated.

 

If the motor don't reach such critical situation, it could run also for weeks.

 

If this is the case, you could hear - using an AM radio receiver - a disturbance when the phenomena will start to happen, the noise will continue up to opening of the circuit.

 

Regards

Mario

[bob]

Hi,

 

Afetr five days of continuous operation, the motor started to slip again. No voltage detected on the slip ring end connections . We will dismantle the motor during the weekend. I will keep all posted.

 

 

Thanks again.

 

 

Bob

[mariomaggi]

Dear bob,

We will dismantle the motor during the weekend

 

week-end is arrived ! No news?

Regards

Mario

[bob]

Hello Mario ,

 

Thank you very much for your concern. We removed the slip ring and rear cover of the motor for inspection. The connection where the flexible cable was soldered to the rotor bar at one point was loosed. We did the repair and the motor is back to operation.

I am sorry for the delay due to the fact that we had a problem on a generator, which problem I would like to share with you later.

Thanks to all.

 

Warmest regards.

 

Bob