Influence of broken rotor bars on magnetic quantities in induction machine

Broken rotor bars (BRB) is a typical type of failure in induction machines (IM), especially in drives where frequent transient operation is common. The BRB occurrence disturbs the air gap flux distribution giving rise to a sequence of interrelated phenomena. In this paper an analytical treatment of the electromagnetic quantities following fault development is presented. By means of superposition principle faulty rotor is modeled as a sum of two symmetrical rotors each of them producing their own MMF. However, the nature of two MMFs is quite different. The first rotor produces rotational MMF, whereas the second can only establish pulsating MMF. The latter can be modeled as a sum of two opposing rotational MMFs with half the original amplitude. Once re-combined with first rotor, the joint MMF can be described as a sum of direct MMF and oscillating MMF with typical fault frequency (1-2s)f₁. The oscillating MMF induces stator current component at the same frequency, which is known to be the basic indicator of BRBs in IM.