Conclusions

The project involved the development of a new analytical method of predicting the behaviour of squirrel cage induction motors subjected to dynamic pulsating loads such as a reciprocating compressor for example. The objective was to develop a design method for determining the rating of industrial induction motors driving a pulsating load. The analytical approach used to analyze the motor was based upon the harmonic coupling inductance method which was capable of accommodating any stator winding arrangement used in industrial motor designs. The coupling impedance approach has been used in the past to develop steady-state models of induction machine operating at constant speed and load. The analytical dynamic model developed in this project was validated initially using a Matlab software model for cage induction motors driving a selection of compressor loads. The simulation results were finally correlated with a detailed experimental validation in the laboratory using an industrial cage induction motor connected to a synchronous permanent magnet motor controlled electronically to simulate a compressor load.

The calculations of the self and mutual inductances based on the harmonic analysis method used in this thesis have shown a good correlation with the simulated results and also with those obtained from the experimental work. A standard numerical integration method, the 4th order Runge Kutta method, was shown to be adequate in determining the transient time solutions for a cage motor driving a dynamic load.

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coefficients that provide a better understanding of the interactions between a cage motor and a general dynamic load.

The general approach enables the designers to quickly determine the total system inertia required to maintain the motor torque pulsations within certain pre-defined limits, and subsequently reducing current and speed pulsations as well. The results obtained from the simulation using the recommended inertia have shown a significant reduction in the current, torque and speed pulsations.

In addition, the dynamic model proposed in this thesis could also be used to predict the resonance hunting frequency for any particular dynamic loading condition at which the largest speed oscillations occur. These speed oscillations can have a significant impact on the motor performance and could result in a major mechanical failure. The dynamic model simulation results for the predicted frequency range that high speed oscillations occur again show good agreement with the experimental results and demonstrate the validity of this approach to predict resonance hunting frequencies. In practice it would be useful to avoid operating the motor at these frequencies.

The simulated model results and the measured results for different hunting frequencies correlate closely. Both set of results show that as the motor approaches the hunting frequency, the phase shift between speed and torque increases. As the hunting frequency increases, the phase shift between the speed and torque also increases. Both the measured and simulated results have shown that the amplitude of speed and torque variations reduce as the hunting frequencies increases above the resonant frequency range.

The dynamic model developed has the additional capability of predicting the motor response with a faulty cage. This is likely to be more common in motors driving dynamic loads. The influence of broken rotor bar for example modifies the motor speed and torque response. The run-up time to reach full load speed increases with broken rotor bars. The

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resultant circulating end-ring current in this case will be non-zero. The rotor bar currents are also distributed non-uniformly compared to a healthy cage. The rotor bar currents adjacent to the broken bar have higher values than the rest of the bars. As a result, the adjacent bars to a broken bar or bars will be subjected to higher thermal stresses. The presence of broken rotor bars introduces, as expected, torque and speed oscillations. The peak speed oscillation was found to reduce for a rotor with broken bars and the average speed oscillation also reduced. That was expected due to the net average increase in the rotor resistance caused by the broken bars. As the number of broken bars increases, the circulating current flowing in the end-ring also increases due to asymmetry of the rotor. Both peak speed and torque oscillations reduce as the number of broken bars increases. The currents that flow in the neighbouring bars have the highest magnitudes but the bar current reduces as you move progressively away from the broken bar.

The work presented in this thesis has demonstrated the validity of the dynamic cage induction motor model developed. The model has been shown to be reliable and to predict accurate results. It is a useful contribution for designers of cage induction motors because it enables the correct specification of motor and inertia required for any dynamic load in a relatively simple manner. It further allows the designer to understand the consequences of a faulty rotor cage on the system performance. Alternative approaches such as the finite-element method are available but these are complex and computationally very expensive.

The harmonic dynamic model developed in this thesis is considered to be the only model that capable of predicting the rotor bar currents for all bars and the end-ring current unlike the dq-model. The application of the model in computing the damping and synchronising toque coefficient is easier than Kron’s networks approach that require a lot of long computations. The model can be used to analyse any cage rotor induction motor regardless of the stator winding arrangement for any dynamic load. It can be used to rate the cage rotor induction motor with the aid of additional inertia when the load is oscillatory such as reciprocating compressor as described in Chapter seven.

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