| With more than 60 percent of the connected load in an underground coal mine being electric motors, there is a demonstrated need for real-time monitoring of ac induction motors for predictive maintenance applications. In this thesis, the theory, measurement, and analysis of stator vibrations are investigated for failure prediction of ac induction motors. The theory of stator vibrations due to ac excitation is presented, based on the permeance method from the fundamentals, and the general frequency expressions are identified.;A data acquisition system suitable for both time- and frequency-domain analysis of vibration signals from ac induction motors is described. The test system consists of an ac induction motor, a dc drive motor, an accelerometer for sensing vibrations, a preamplifier, a digital oscilloscope, a microcomputer, and associated peripheral devices. The vibration signals were sensed on the stator of the ac test motor, and displayed on the oscilloscope at any desired instant. The digital data were then transferred to the microcomputer. The data are stored in the computer for future analysis. The vibration signals were analyzed in the frequency domain using special software.;Laboratory experimental investigations were conducted on a 1.5-kW induction motor and include base-case data and six simulated failure studies. Field experiments were carried out on a 7.5-hp ventilation-fan motor and on a 305-hp shearer cutting motor. Several conclusions are drawn. The zero-frequency vibrations are significant, and relative amplitudes are found to depend on the mounting arrangements for the motors tested. With stator related faults and low saturation, only certain dual frequencies show appreciable increase in vibration amplitude. With stator related faults and moderate to high saturation, more of the dual frequencies show higher vibration amplitudes. The combined vibrational behavior of the zero-frequency amplitudes and the dual frequencies can be used to identify failures. The results of this thesis will be useful in developing procedures for implementing predictive maintenance programs using vibration monitoring for ac induction motors. |
