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Rotor Bar Problems

 

An important failure mode of large electric motors is the cracking and subsequent heating and breaking of the rotor bars, especially in motors that experience frequent starts under load. The starting condition places the heaviest stress on the rotor bars because they are carrying the highest current since the rotor is running at much lower than synchronous speed. The high currents cause heating and expansion of the bars relative to the rotor itself, and differences in the electrical resistance of the individual bars result in uneven heating and uneven expansion. This leads to cracking of the joints where the bars are welded to the shorting ring. As soon as a crack develops, the resistance of that bar increases, increasing its heating, and consequently worsening the crack. At the same time, the adjacent rotor bars experience increased currents because of the reduced current in the broken bar.

This scenario results in localized heating of the rotor, causing it to warp. See the paragraph on Rotor Thermal Bow, above.

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Rotor Bar Monitoring via Motor Current Analysis





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