Transient Analysis and Motor Fault Detection using the Wavelet Transform

Induction motors are the most common means of converting electrical power to mechanical power in the industry. Induction machines were typically considered robust machines; however, this perception began to change toward the end of the last decade as low-cost motors became available on the market. Nowadays the most widely used induction motor in the industry is a machine which works at the limits of its mechanical and physical properties. A good diagnosis system is mandatory in order to ensure proper behavior in operation. The history of fault diagnosis and protection is as outdated as the machines themselves. Initially, manufacturers and users of electrical machines used to rely on simple protection against, for instance, overcurrent, overvoltage and earth faults to ensure safe and reliable operation of the motor. However as the tasks performed by these machines became more complex, improvements were also sought in the field of fault diagnosis. It has now become essential to diagnose faults at their very inception, as unscheduled machine downtime can upset deadlines and cause significant financial losses. The major faults of electrical machines can be broadly classified as follows: Electrical faults (Singh et al., 2003): 1. Stator faults resulting in the opening or shorting of one or more stator windings; 2. Abnormal connection of the stator windings; Mechanical faults: 3. Broken rotor bars or rotor end-rings; 4. Static and/or dynamic air-gap irregularities; 5. Bent shaft (similar to dynamic eccentricity) which can result in frictions between the rotor and the stator, causing serious damage to the stator core and the windings; 6. Bearing and gearbox failures. However, as is introduced in the basic bibliography by Devaney (Devaney et al., 2004), the effect of bearing faults is, in most cases, similar to eccentricities and has the same effects on the motor. The operation during faults generates at least one of the following symptoms: 1. Unbalanced air-gap voltages and line currents 2. Increased torque pulsations 3. Decreased average torque 4. Increase in losses and decrease in efficiency 5. Excessive heating 6. Appearance of vibrations