Voitto Kokko - Condition monitoring of squirrel-cage motors by axial magnetic flux measurements

157 Pages·1.94 MB·English

CONDITION MONITORING OF VOITTO
KOKKO
SQUIRREL-CAGE MOTORS BY
AXIAL MAGNETIC FLUX Department of Electrical Engineering,
Optoelectronics and Measurement
MEASUREMENTS
Techniques Laboratory,
University of Oulu
OULU 2003

VOITTO KOKKO
CONDITION MONITORING OF
SQUIRREL-CAGE MOTORS BY AXIAL
MAGNETIC FLUX MEASUREMENTS
Academic Dissertation to be presented with the assent of
the Faculty of Technology, University of Oulu, for public
discussion in Raahensali (Auditorium L10), Linnanmaa, on
March 14th, 2003, at 12 noon.
OULUN YLIOPISTO, OULU 2003
Copyright © 2003
University of Oulu, 2003
Supervised by
Professor Kalevi Kalliomäki
Reviewed by
Professor Olli Aumala
Professor Tapani Jokinen
ISBN 951-42-6938-1 (URL: http://herkules.oulu.fi/isbn9514269381/)
ALSO AVAILABLE IN PRINTED FORMAT
Acta Univ. Oul. C 179, 2003
ISBN 951-42-6937-3
ISSN 0355-3213 (URL: http://herkules.oulu.fi/issn03553213/)
OULU UNIVERSITY PRESS
OULU 2003
Kokko, Voitto, Condition monitoring of squirrel-cage motors by axial magnetic flux
measurements
Department of Electrical Engineering, Optoelectronics and Measurement Techniques Laboratory,
University of Oulu, P.O.Box 4500, FIN-90014 University of Oulu, Finland
Oulu, Finland
2003
Abstract
The aim of this research work is to develop a tool for condition monitoring of squirrel-cage motors
using axial magnetic flux measurements, and to design a diagnostics system for electrical motors. The
basic theory of the measurements and systems was found through literature reviews and was further
developed from the experimental results of this research work.
Fluxgate magnetometers and Hall effect sensors are not reliable enough for condition monitoring
purposes, but measurements by flux coil sensors can reach adequate reliability. The useful frequency
area of the flux coil sensor is from about 0.2 Hz to 15 kHz, an area is well applicable for condition
monitoring of squirrel-cage motors. Output voltage is frequency dependent, increasing towards
higher frequencies. Sufficient sensitivity is usually reached by a flux coil sensor having a diameter of
about 30 cm and the number of turns of about 200. Sensitivity can be improved by increasing the
diameter or number of turns of the coil. The sensor should be placed axially centred on the end of the
motor, and measurements should be made with the loaded motor in steady operation. Output voltage
is typically from the microvolt to millivolt level, however, installation inside the motor can increase
it from tens of millivolts to some volts. The dynamic resolution requirement of measurement is about
70 dB and the highest line resolution needed for the spectrum analysis is about 3200 lines.
Time base signal can be used to study rapid disturbances of flux caused by mechanical loading or
switching of the frequency converter. Various motor failures cause specific variation to the frequency
distribution of flux, so spectrum analysis is well applicable for condition monitoring. Reference
measurement of each motor is required because stator winding factors, installation tolerances,
operating conditions and mechanical load affect leakage flux. A broken rotor bar failure can be
detected from the amplitude difference between the supply frequency and its rotor bar induced
sideband. A broken rotor end ring failure can be detected by the amplitude difference between the slip
frequency and the supply frequency. However, it was found that the stator current spectrum is a more
reliable method of detecting both these rotor failures. The supply voltage asymmetry can also be
evaluated by specific sidebands of axial flux. Turn to turn failure of the stator winding was most
reliably detected by sidebands around the rotor slot pass frequencies. Equations for frequency
converter supplied motors are the bases for similar equations, but the supply frequency is replaced by
the output current frequency of the converter.
The developed diagnostics system design for condition monitoring of ac motors includes stator
current, flux coil, temperature, vibration, partial discharge, bearing current and voltage
measurements. At the system diagnosis stage these measurable signals are divided to time base and
frequency base signals and for each of them a fault indicator is determined. For flux coil
measurements four fault indicators were found: rotor bar failure ratio, rotor end ring failure ratio,
stator winding turn to turn ratio and supply voltage asymmetry operation ratio. With these failure
indicators we determine failure location, state and cause. From this information a lifetime prediction
of the motor is made.
The results of this work are used to analyse flux coil measurements of squirrel-cage motors. In
addition the research has led to a special application to monitor electric motors using an on-line
condition monitoring system for paper machines and power plants.
Keywords: condition monitoring, diagnostics, electric motors, electrical measurements,
flux coil

Owned to my parents

Acknowledgements
I wish to express my gratitude to my supervisor Professor Kalevi Kalliomäki for his
encouragement and scientific guidance of this work, which started out from my licenciate
thesis.
The experimental work for this thesis was carried out in co-operation with my
colleagues in Fortum Service Oy Technology Unit Oulu and I wish to thank them all for
the support they have provided during the work. My warmest thanks to my colleagues in
the forest industry for their helpful co-operation during recent years. My family deserves
my warmest thanks for their support over the years.
I wish to thank Professor Tapani Jokinen and Professor Olli Aumala for examining this
thesis. I also want to express my thanks to Gordon Roberts for revising the language.
Finally, I wish to thank Fortum Service Oy for allowing me to use software and
measurement data to do this research and development work.
Oulu, November 2003-01-18 Voitto Kokko