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Slip ring motor


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#1 bob

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Posted 06 June 2006 - 04:54 AM

Hi,

I refer to one of my last post regarding the installation of slip ring motor using variable speed drive. We did many installations in the past and which are still working fine. However, I had a bad experience some days ago. A particular workshop serviced an old slip ring motor 750 k W 400 V 750 rpm and the rotor was shorted after the brushes . The motor was tested with one VSD AT THE WORKSHOP and run well THROUGHOUT THE SPEED RANGE. We even did a vibration monitoring at no load which proved very successful.
When tested with OUR VSD ON SITE ( motor alone ), the motor run well until it reached 45 Hz when it started to make unusual loud noise like something vibrating heavily. However,at that frequency the output current was very stable.The motor was also tested with a second VSD but with the same result. We did some tuning on the VSD as per the manufacturers instructions but without any success.
Some people are of opinion that the rotor is faulty but I hardly believe this argument as the motor run well on site at the workshop. The issue of resonance was also raised.
Any hint welcome.

Bob


#2 marke

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Posted 06 June 2006 - 06:00 AM

Hi Bob

The slipring motor with a shorted rotor will generally behave as a standard induction motor except that the effective LRC will be high and the LRT will be low.
If you are having issues when running on a drive, this could be because the motor model is not able to fully simulate the motor characteristics.
If possible, try running on a true open loop V/Hz drive. It is possible that a "vector" drive may have issues with the characterisitcs being out of range, or there may be an issue due to the brush contacts not being secure at that speed.

Best regards,

#3 bob

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Posted 06 June 2006 - 01:54 PM

Hi Marke,

Thanks for your reply. We are actually running on V/F mode and the drive was tuned successfully before operating the motor. The issue of loose brush contact has also been verified.

Regards.

Bob

#4 marke

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Posted 09 June 2006 - 08:26 PM

Hello Bob

I have just returned from a trip away. - hence slow reply.

There are open loop V/Hz drives and vector drives that can be configuered to run in V/Hz mode.
I suspect that the later is still essentially a closed loop drive and could possibly chase it's tail in an abnormal load condition.
I have used open loop v/Hz drives with slip ring motors with no problems.
I am concerned about the statement "the drive was tuned successfully" which sugests a closed loop system. If the motor characteristics are outside of the range that the mathematical model can cope with, it could go unstable which is what you appear to be experiencing.

Just a thought that may be worth following up.
Best regards,

#5 bob

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Posted 16 June 2006 - 06:55 AM

Hi Marke,

It is possible to tune the motor in V/F using the particular drive and the message received after auto tuning was successful. It is also important to note that the motor alone was also tried with another type of VSD but again at 45 Hz , the motor started generating the same kind of noise.
We believe the problem is mechanical, probably resonance.
Any comment welcome.

Bob






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