Broken rotor bars: their effect on the transient performance of induction machines

Around 10% of failures in cage induction motors are on the rotor. Problems result from defective castings in die-cast rotors, or from high resistance joints as a result of poor welding or brazing in fabricated cages. In such cases uneven bar stress is then unavoidable. Often, however, failure occurs due to fracture of bars caused by fatigue. This is common in machines with uneven and heavy duty cycles, which consequently are subject to high stress in the cage during periods of rapid braking and acceleration. Some success has been achieved in the development of techniques to identify bar failure, or high resistance joints, even on operational machines. The majority of techniques depend upon detecting a twice-slip frequency modulation in speed, torque or stator current by analysing an appropriate frequency spectrum. As yet, there is still no universally effective procedure for the early detection of such conditions. In this contribution, rotor parameters are evaluated allowing for current redistribution in the damaged rotor cage, and for the concomitant changes in resistance which result. Based upon this, a modified model of the induction machine, which includes the effect of broken rotor bars, and which will represent both transient and steady state behaviour, is developed. Experimental and calculated results compare the transient performance of the healthy and faulty machines.