CONDITION MONITORING OF INDUCTION MOTORS: The Detection of Broken Rotor Bars in Variable Speed Induction Motor Drives

The squirrel cage induction motor is the most common means of converting electrical energy to mechanical energy. As such they form a very important part of modem industrial plants. Adverse service conditions may cause faults to develop within a motor that eventually result in the motor failing. If warning of an impending failure can be obtained, the motor can be scheduled for repair or replacement before catastrophic failure occurs, thus avoiding costly excess downtime of plant. A fault which occurs in cage induction motors, is where a fracture occurs between the end ring and a rotor bar, or in an end-ring segment. These faults may be detected by examining the frequency spectrum of the stator current, while the motor is operating under loaded conditions, for the presence of characteristic frequency components. The basic theory is reasonably well known, however little work has been done on detecting faults when the motor is controlled by a variablefrequency drive, which causes extra frequency components to appear in the stator current spectrum. A variable speed drive controls the speed of an induction motor by changing the frequency of the supply voltage. Thus the problem of detecting faults becomes one of analysing a non-stationary signal. One approach to solve this problem is to synchronously sample the stator current waveform, such that the sampling process tracks any change in frequency, producing a useful frequency spectrum. A hardware system based on a phase locked loop circuit is developed in order to implement such a process. In order to determine which frequencies are produced by a pulse-width modulated (PWM) drive, a theoretical analysis of various PWM methods is carried out, with