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Rotor Bar Motor


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Hi all,

We have serious problem about squirrel cage.


Desc of motor :

- 6 kV

- 50 Hz

- 900 kW

- Isolation Type : Class F

- Connection : Y


All of them have crack on rotor bar and also happened on End-Ring.

All protections of motor are not detect the fault.

Could you explain how the rotor bar become crack?


Pls, give us any related document/reference to solve the problem.


Thanks and regards



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Hello gundala


Welcome to the forum


Cracked rotor bars and shorting rings are typically the result of high transient loading (star/delta starting), poor construction (bars too short and/or poor welds), and high start frequency.

If the bars are showing indication of very high heating, this can be due to very high dissipation in the rotor with a high inertia load. Excessive rotor dissipation will cause a high expansion of the bars and this can increase the stresses on the bar due to very high levels of expansion and contraction.


It is difficult to give a definitive answer with out doing a thorough inspection. You need to get an opinion from someone who sufficiently qualified and experienced and is able to do a close inspection.


Best regards,

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  • 3 months later...

Hello Gundala


This cracks are commons in bar-to-end-ring junction due to poor welding.

This failure is reduced when rotor bars are welded in induction method.

The sensitive points on copper rotor bars are weldings


Now you said that the end ring had broken? The end-ring or the bar-to-end-ring juntion only?

If the end ring is broken then the problem is serious one,, maibe mechanical shock on shaft or motor or very high thermal losses (overload or nor starting the load) on rotor.



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please give some info about load. What kind of load do you want to start? What about inertia or starting time? Are such startings too frequent?



Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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