Delta Star Connection Problem

[MasudDU]

hello,
gentlemen please help me with this motor problem described below...
an electrical motor (400V, 133.5Amp, 50Hz, 75KW, 2960 RPM, COSθ=0.89) was running on delta connection.
after rewinding its getting very hot when running on delta connection.
now if i run this motor on star connection, will there be any problem ?

 

[Artisi]

MasudDU

You should red-flag this post in this forum and repost it in the electric motor section.
I would also check with the motor re-winder what he has changed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[kingnero]

I wouldn't take any chances with a 75kW motor.
weren't any thermal fuses in the windings? cheap security!

 

[electricpete]

Can you give more details about “running hot”
The motor was under load when it ran hot?
Was the current measured? How much in comparison to FLA
Was the current balanced? (unbalance current might point to voltage unbalance or other issues)
Did you verify the voltage supplied to the motor?
Do you have some reason to believe the motor should be connected in wye? What does the nameplate or connection diagram (sometimes in terminal box) say?
I agree also with engaging the rewinder to help solve your problem… he is familiar with the motor, has a stake in understanding whether there is an actual motor problem or some other problem and in my experience usually the rewinder will be more than happy to talk through the situation to help understand it.


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(2B)+(2B)' ?

 

[electricpete]

Also what type of cooling... TEFC? Does the airflow seem normal?

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(2B)+(2B)' ?

 

[Artisi]

The OP said after rewind so can we assume that voltage, connection is probably ok, although connection should be rechecked to make sure. Yes, rewinder would be my first telephone call.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[arunmrao]

Is it a bearing issue or a damaged rotor ? There are endless possibilities. Rewinding such large motors is normally done at company approved work shops, who have all the facilities .

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[MasudDU]

thanks all of you gentlemen for your replies...
@electricpete
yes it ran hot at under full load,
taking more currents than the rated full load value,
rated voltage supplied to the motor, and cooling was good.
the re-winder suggested to run the motor in STAR connection
but my senior engineer said it can't be run on STAR connection..
but when the re-winder asked him the cause, he didn't answer.
now the question is- why we can't run this motor with STAR connection?
please help...

 

[Artisi]

If it was running delta prior to rewind, why can't it run delta after rewind? Seems a pretty simple question to ask the rewinder.



It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[electricpete]

yes it ran hot at under full load,
taking more currents than the rated full load value

How much more?
Did you measure current before the rewind?
How sure are you that mechanical load didn’t change?
Is it possible to run the motor without load for troubleshooting?
* Are the three phase currents balanced? (unbalanced could be indicator of high resistance in a connection, could be indicator of rewind problem… at any rate important to check)

but my senior engineer said it can't be run on STAR connection..
but when the re-winder asked him the cause, he didn't answer.
now the question is- why we can't run this motor with STAR connection?

In general I would not recommend reconnecting as a guess without knowing what is really going on. I would present full details to the rewinder and engage him in methodically working through the problem.... I'd think he would have a personal stake in figuring out whether it is a site application problem (in which case he can clear his name) or a motor rewind problem (in which case he needs to figure out what needs to be improved within his process).
But if you want to give it a try that is your perogative.
In general reconnecting a delta motor into wye should result in reduced flux density and reduced torque capability. If your problem was severe saturation due to a particular rewind error that created severe saturation (reduced series turns) then it might help. But in general the reduced torque capability will prolong the motor DOL start severely… might not even be able to accelerate the load to full speed. And may severely drop speed / increase current during temporary overloads.


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(2B)+(2B)' ?

 

[electricpete]

By the way, what does this motor drive?

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(2B)+(2B)' ?