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Condition monitoring of electric motors


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Ho, SYS 1999 , 'Condition monitoring of electric motors', PhD thesis, University of Tasmania.

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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
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
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
• 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
• 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
• 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
• Numerical modelling of DC motor with short-circuit armature coil for steadystate

Item Type: Thesis - PhD
Authors/Creators:Ho, SYS
Keywords: Electric motors, Induction
Copyright Holders: The Author
Copyright Information:

Copyright 1999 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references

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