AC Induction Motor Problems:
Broken / Cracked Rotor Bars

Figure 1 - FFT Showing Advanced Broken / Cracked Rotor Bar Symptoms
Rotor Bar Problems- The electrical problem that is most effectively diagnosed and monitored through vibration analysis. Broken, cracked rotor bars, bad joints between end rings and rotor bars and end ring problems have unique and easily recognizable symptoms. An accurate assessment of condition and remaining life can also be made with on-line current analysis. The initial diagnoses, however, can easily be made with vibration data. Do NOT send the motor to a motor shop - especially in the early stages. It is doubtful that they will have the equipment to diagnose the problem without running unreliable and potentially destructive tests on the unit. In cast rotors, these symptoms can indicate voids in the casting.
Broken Cracked Rotor Bar Symptoms:
  • FPole sidebands surrounding running speed harmonics. 
  • Advanced problems will exhibit a 'humming' or 'pulsing' sound and feel.
  • Significant is the number of and size of the sidebands. They increase as the unit deteriorates.
  • Amplitude at 1x rpm is relatively unimportant - it will fluctuate greatly as the hot spots being generated cause the rotor to bow unpredictably. It is a result of the problem - not a cause.
Recommended Actions:
  • On-line current analysis to determine severity.
  • Limit starts since they are easily the single, most destructive thing you can do to a motor.
NOTE: The development of sidebands of any amplitude should be noted. This may require the use of a logarithmic scale. Again, do not confuse a running speed harmonic adjacent to 2 x FLine with a pole pass frequency sideband - they are by definition separated by that amount and are not sidebands.

Figure 2 - Motor Construction

Figure 3 - Winding Construction