O uszkodzeniach klatek wirnika

Rotor cage failures are not regarded as the main cause of the induction motors breakdowns. An induction motor with broken squirrel cage bars can still be exploited, consecutive starts may be executed. At durability tests carried out by Komel Centre some years ago on a prototype 800 kW induction motor, it was stated, that even if 80% of the rotor bars were broken, the motor started fairy successfully and could be loaded. But any rotor cage failure (e.g. single bar or end ring fracture) causes electromagnetic asymmetry of the motor; main technical features of the motor are heavily deteriorated. Sound power level and vibration level become higher, efficiency factor lower, temperature rise of the stator winding may by higher. Starting (locked rotor) and breakdown torque is lower. In the paper special laboratory tests of a standard, 4 kW, 1500 RMP, 3 phase motor with aluminium rotor cast are presented. The tests were carried out on symmetric ("healthy") squirrel cage and on the cage with one, three and six broken bars. Several examples of large AC motors with damaged rotors and a reliable method for rotor squirrel cage diagnostics are presented.