HIGH SPEED ROTOR

[QUAVIET]

What side bands do I look for when analyzing a 3600RPM 500HP 480V refrigeration motor. The problem is intermittent but it is an old motor which has been rebuilt at least three times.

 

[Skogsgurra]

What do you want to do? Is it vibration, rotor problems, alignment, foundation or bearing problems you want to analyze?

Gunnar Englund

http://www.gke.org

 

[sms]

What have you found on the rebuilds? What damage have you found. Bearing problems have their own frequency, unbalance is at running speed frequency, electrical problems are associated with line frequency or slip frequency if it is an induction motor. Is it induction, synchronous, variable speed, DC....? You have not really provided enough info to even begin to answer the question.

Oh and by the way, to a rotating machinery vibration guy 3600 is not considered "high speed." I mess with turbo compressors with speeds up to 35000 rpm, now that is high speed....

-The future's so bright I gotta wear shades!

 

[QUAVIET]

Motor is a Wye/Delta with contactor control. The fault is in the electrical and not the motor load. It intermittently has a problem when transfering from Wye to Delta where it makes a lot of noise and trips the local control 1000A circuit breaker. Motor FLA is 560A. Voltage 480VAC. RPM is 3570. Motor has a new winding. When first installed it ran OK under load. Motor was not under load when the last event was observed. I did not witness it. Motor contactor sequence was checked over and over again with the motor electrically disconnected and the motor contactors operated properly every time. The "S" (start) contactor showed some evidence of arcing but both M1 and M2 (Wye and Delta contactors) show no evidence of arcing. The sequence for start is S and M1. For run S drops out and both M1 and M2 are picked up. The motor tap connections in the peckerhead were opened and no arcing was found and connections were tight. Motor megs good. I have suspected the 1000A circuit breaker in the past but do not know how to eliminate it as a source of the fault other than replacement. I asked about the spectrum analyzer because I saw on this forum someplace that a damaged rotor could be found this way. The squirrel cage rotor is old. I think that this is the same problem the unit experienced before with a different motor but its rotor was old and the motor repair shop verified that the rotor was bad. It runs fine with no load in a WYE configuration only and I thought I may be able to determine the condition of the rotor with a spectrum analyzer or I may be barking up the wrong tree.

 

[petronila]

Hello QUAVIET,

I think is better to check the Instantanous current set of main breaker because the trip could be caused by Inrush, another think to check is if the time elapsed from wye to delta is too short.Also tigh all connection screws.

If you suspect something wrong on the rotor ,for a 60 Hz Supply, perform a vibration analysis and look for 7200 CPM Frecuencies and cut off the current, if vibration stop at the instant the motor is de-energized, then the cause could be electrical.

An electric motor with a damage in the rotor´s squirrell cage will drawn higher current and runs slower than Rated values.

Another test you could perform are : Stray Flux pick Up Coil

and Motor current monitoring System.

Best Regards

PETRONILA

 

[QUAVIET]

Thanks PETRONILA, my expertise is with general industrial electrical maintenance but am the one who is called when the refrigeration motors act up. I think that this is due to the valuable assistance provided to me by this forum in the past. I will follow up on your suggestions and when the solution is found post the source of the fault for others.

 

[electricpete]

I would say most likely the fault is associated with the control setup and not the motor.

These type problems are expected to be intermittent because they can depend on the phase angle at time of start.

There are a lot of subtleties in the setup of a wye-delta start which I am not familiar with. They have been discussed on this forum. (open-tranisition, closed transition, resistor value, time delay, how close is motor to being up to speed at time of transisition).

At any rate, I would recommend you re-post this thread with a title something like "motor trips during wye-delta start"

I guarantee you will get a lot of great responses from Marke, jraef, skogsurra and the other control guru's.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[QUAVIET]

I neglected to mention two things, there is a new Calco controller and new CT (all three phases have a CT). Someone, burned both out at the last attempt, reguards, and we have a old Calco man who is in charge of it. It is not timing or current. If the circuit breaker is set too low and has a lot of trips over its life time, I know a smaller breaker can cause a lot of problems but it is a rare event. I think I must eliminate the circuit breaker as a source of the fault. I just need to bounce ideas off qualified people at times.

 

[QUAVIET]

I said I would provide feedback to this thread. The problem was dual in nature. The motor controller would intermittently decide not to connect to Delta and stay in WYE which would cause overcurrent. Plus the rotor is damaged due to old age. It is easy to get a motor stator rewound but hard to get a new rotor. I'll tell them on Monday to dig deep.