Preventing Rotor Bar Failure: Key Insights on Thermal Expansion and Stress

During startup, the outermost sections of many rotor bar designs can reach temperatures as high as 300 °C in under 10 seconds, while the innermost section nearest the shaft may only climb to about 100 °C. This steep temperature gradient causes the rotor bar to warp, producing a substantial axial pressure equivalent to several tons.

A rotor bar with a high‑resistance connection draws less current, so it expands at a slower rate than its lower‑resistance neighbors. The resulting differential expansion and asymmetric heating place severe mechanical stress on the bar, often culminating in a crack. If left unchecked, this creates a repeatable environment for stress fractures in subsequent slots.

For a detailed analysis of broken rotor bars, see the case study: Case Study CS0402R.

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