Condition monitoring of electric motors

The aim of condition monitoring is to recognise the development of evolving motor faults at an early stage, so that the motor can be scheduled for repair or replacement before catastrophic failure occurs. Often in a modem industrial plant, the cost of an unscheduled shutdown of the process is very much greater than the cost of the motor itself. This thesis concentrates on the detection of stator and rotor defects in squirrel cage induction motors. Very little work has been done on the detection of these faults when the motor is supplied at variable frequency. Hardware and software approaches have previously been attempted but with limited success. An alternative method put forward here is to compute the instantaneous input power of the motor from the two line-to-line voltages and two line currents. In order to do this, specialised hardware has been developed, including a study of the suitability of different types of current transducers. The effects of open circuit and short circuit armature coils, and poor commutation of DC motors have been examined. Because the conventional current transformer cannot be used for DC, and the electrical noise inside the control cubicle of a thyristor-controlled DC motor prevents successful operation of a Hall Effect clip-on ammeter, an air core clip-on "Rogowski" coil was designed to monitor fault-related frequencies in armature and field current. In order to study the effect of open-circuit or short-circuit coil, steady-state models for the DC motor were developed using inductively coupled coils theory. Individual field and armature coils can be modelled, and the effect of faulty armature coils on the amplitude of frequency components that are introduced into armature and field current, can be predicted. To summarise, the main achievements described in this thesis are as follows: Voltage, current, and power monitoring of the induction motor • Design of fully shielded, linear phase shift, clip-on current transducer for power monitoring (It was initially designed by Mr Langman and modified by the author). • Design of a voltage and current monitoring hardware system for induction motors with assistance from the technical support group in the Electrical Engineering Department of the University of Tasmania. • Theory and implementation of using input motor power for the overall check of induction motors. • Use of spectrum analysis of current or power signal to attempt to detect bearing faults. • Use of the instantaneous input power signal to detect stator and rotor defect of the motor with variable frequency supply. Current monitoring of the DC motor • Frequency spectrum analysis of armature and field current waveforms of DC motors. • Investigation of the effect of incorrect interpole strength, open-circuit and shortcircuit armature coils on the armature and field currents. • Numerical modelling of DC motor with open-circuit armature coil for steadystate conditions. • Numerical modelling of DC motor with short-circuit armature coil for steadystate conditions