Broken Bar of an Induction Motor

Hello Samac !!

I believe that broken bars will tipically give a long warming before it fails completely. First think will be increasing of vibration and probably increased starting time . Broken rotor bar approve use of the motor for a long time after detection but starting time degrades to point where protection relays take out motor during start . If you ignore this, winding failure will be last stadium of completely broken bars in squireel cage .

There will be unbalance current & heavy vibration.The broken bars may come in between rotor and stator-damaging the winding.Roror gets extreemly heated up.When ever a motor is dissembled,if you notice heated up rotor,then rotor is to be checked for borken bars.

"The broken bars may come in between rotor and stator-damaging the winding."

There really is very little chance of this happening since the rotor bars are typically cast into slots in the rotor which are larger toward the shaft than at the rotor surface. The rotor slots are shaped in that manner to prevent the bars from being ejected from the slots due to centrifigual force.

The bars usually break at one end or the other where they connect to the end rings as a result of a void in the casting.

I am reffering to old rotor design( Siemens Make) of an HT motor where the rotor bars are not casted.They are brazed to the end rings on either side.There were several events of such failure in our plant.Such Rotor designs are obsolete now.In die cast rotore broken bars can not occur as said by joeconed:

Heating of Stator as well as Rotor occur due to unbalance current if i am right.

Why should the rotor warm up? In the bar that is broken, will not flow any current, so there will be no warming up.

When there are multiple bars broken, the resistance of the rotor will increase. This will result in a lower starting torque. When the motor has to deliver the same amount of power at the output shaft, then slip will increase. The motor will turn a little slower and take a bit more current.

The torque will be lower, the slip higher, there will be a tangential vibration on the rotor and the supply current will be distorted. Observing the input current with o'scope is a common way to detect broken cage bars.

Another method for detecting broken rotor bars is with motor current signature analysis data collection equipment and software which can display the results on a PC in the form of a frequency spectrum.

More details please...

What is the effect of broken bar on the performance of induction motors?

How can I detect a broken bar of the cage?

Thanks alot... Samak

Thanks

Do you know any URL or standard code explaining this topic?

If yes, please post.

Regards... Samak