Transients in Single-Phase Induction Motors: A Study of the Transient Torque Characteristics of an Unloaded Single-Phase Capacitor-Start Induction Motor
The thesis describes a study of the transient torque characteristics of an unloaded single-phase, capacitor-start induction motor. A review of published works on the study of transient torques in induction motors is made in Chapter 1. Though there have been attempts to calculate transient torque characteristics of single-phase Induction motors for zero speed and constant speed conditions, there does not appear to have been any attempt at an experimental study of the problem. Experimental investigation avoids the limitations normally involved in theoretical calculations. A systematic experimental investigation necessitates the exercise of control over the point-on-wave of closing the supply to the motor and equipment to detect transient torques, under both free-rotor and blocked-rotor conditions. After an exhaustive survey of possible methods of detecting transient torques under these conditions (Appendix I), it was decided to use a d.c. excited, drag-cup, two-phase induction generator as an accelerometer in the former case and a stator reaction detecting scheme using a precision load cell for the latter case. The calibration of the accelerometer is described In Appendix II and the design of the stator reaction detecting scheme in Appendix III. The attempt to design and develop a p.-on-w. control and the actual one used are described in Chapter 2. The operational details and calibration of the p.-on-w. switch are also described in detail. It was found to be very important to have the accelerometer rigidly coupled to the shaft of the motor. For the stator reaction detecting scheme, it was necessary that the natural frequency of the stator suspension should be at least about 450 c/s. Results obtained simultaneously by the two detecting methods show satisfactory correspondence. Chapter 3 deals with the investigation of transient torques with free rotor. The effect of pointx-on-wave of closing the supply voltage is studied in detail at a reduced terminal voltage of 140V. The results show that the initiation of the transient torques is governed by point-on-wave. The transient torques have a maximum near 0 point-on-wave and are almost completely absent for 90. Speed effects are discussed and It is found that speed plays an Important part in determining the value of maximum peak torque. Speed also introduces dynamic braking torque which affects the mean torque pattern. Free-rotor investigations with different applied voltages are described, and their results discussed. The importance of speed effects is well demonstrated by these results. The duration of the line frequency component of the transient torque is almost entirely governed by speed except for very long run-up times. The appearance of a double line frequency pulsating torque also seems to be speed dependent. With short run-up times, the machine overspeeds before finally attaining its no-load speed. Variation in speed effects with differing acceleration and the effect of saturation on the torque peaks are discussed. Experiments to study the effect of switching on while the motor is running are reported. The results show that the dynamic braking torque assumes greater importance when re-closing is done near no-load speed and 0 point-on-wave. Investigation of the stalled-rotor transients is presented in Chapter 4. This investigation was carried out both by experiment and by computation. The experimental results show that the initiation of transient torques depends on the point-on-wave in much the same way as in the free-rotor case. In the absence of speed effects the magnitude of torque peaks does not show an Increase over the initial value and the transient torques decay with their 'natural' electrical time constants. The principal component has a very long time constant. Transient current equations are derived based on the actual two-axes arrangement of the windings. Experimentally determined machine constants are substituted in these equations and the resulting current expressions are solved for instantaneous values using a digital computer. Instantaneous values of torque are computed from the Instantaneous values of currents. The various components of the transient torque are discussed. Computed transients compare favourably with experimental results. The approach, systems used, and results obtained are discussed in a general fashion. Blocked-rotor results and free-rotor results are compared with a view to determining the degree to which the former results can be applied to the latter condition. It is thought that the very large dynamic braking torque that can be obtained during re-closing while the rotor Is rotating, my be of some practical significance
