Measurement and assessment of transient torques in a three-phase squirrel cage induction motor

ThesisComputer simulation models have been developed to predict the magnitude of transient torques when starting and reswitching induction motors (De Sarkar & Berg, 1970:1031, Ghani, 1988:106 and Krause & Thomas, 1965:1038). In these models the starting or running parameters are used so that the squirrel cage induction motor has been represented by its single cage model and the effects of deep rotor bars have not been considered. During transient conditions, in an induction machine with deep bars, the deep bar effect can significantly influence the rotor time constant of the machine, which is an important parameter during reswitching operations. A model was developed by McCulloch, landy, levy & Macleod that accommodates the change in rotor resistance and leakage inductance as the rotor speed changes and it takes into account the effect of saturating the stator and rotor leakage reactances. During the implementation of the research project the motor dynamic equations have been simulated using the CASED (Computer Analysis and Simulation of Electric Drives ) software package developed by McCulloch et al for the simulation of the dynamic performance of motors with the addition of variable speed drive systems. It allows one to predict behavior with or without the improved bar models. The equations are formulated in the direct - quadrature axis form. Similar models have been developed ( Klingshirn & Jordan, 1970 : 1038, Lipo & Consoli, 1984 : 180, Smith, 1990 : 48 and Siemon, 1992 : 412) but more experimental measurements where required to validate the models. This would mean developing and uSing methods to measure the transient characteristics of the motor. Therefore, the measuring system forms a vital part of this work. Successful measurements were achieved of starting and restarting transients on a 75 kW induction motor. The very acceptable correlation obtained between measured and predicted results, using the deep bar model, was very encouraging and without any doubt shows the advantage of using these improved models when predicting performance. This research project has met its objectives by showing how important it is to use the improved model. It is shown in chapter 3 that if the fixed parameter model is used the correlation between measured and predicted results is very poor. When starting a loaded induction motor direct on line, its rotor windings are subjected to mechanical and thermal stresses due the sudden inrush of cu rrent. The performance of the rotor windings, under these conditions, is affected if the motor has loose or broken rotor bars. Techniques have been developed to detect rotor winding malfunctions i.e. broken rotor bars ( Tavner & Penman, 1987:259). During completion of the project there was an opportunity to assess whether a broken bar in the cage of the motor affects the transient behaviour. If a deviation in the transient behaviour of the motor indicates the presence of broken rotor bar(s), the torque measurement system can be economically used as a diagnostic tool to detect broken rotor bars. For th is reason, measurements on an identical machine having a broken cage where taken. These results are also shown in this thesis and it is interesting to note that it was found that the position of the broken bar affected the transient torque developed by the motor during run-up. Although no reason for this is offered in th is work, the author believes that this finding can form the basis for further intensive research. In the project the background information on the equations used for the simulation of the dynamic performance of a squirrel cage induction machine and the CASED computer simulation package are discussed. A method used to measure the torque-speed curve of the motors is also described. The simulated and measured results of the transient starting- and reswitching torque on the healthy motor are discussed. The reswitching transient torques on the healthy motor and the motor with the broken rotor bar are compared . Finally, the measured results of the starting torque on the motor with the broken rotor bar are shown and evaluated

Modelling and simulation of induction motors for variable speed drives, with special reference to deep bar and saturation effects.

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Doctor of Philosophy.Variable speed motors are achieved by varying the voltage of a DC machine or by varying the frequency of an AC machine, the former method being the simpler of the two. DC motors have the major disadvantage of brushes and commutators which require regular downtime for maintenance, a fact already recognised by Tesla [1] in 1888. Thus the AC motor, in particular the induction motor, is of a more rugged design and does not suffer from the commutator problem of its DC counterpart. Recent advances in the technology of the power electronics used to supply a variable frequency to the motor has allowed the induction motor to be a viable alternative to the DC motor in variable speed applications. Problems have been encountered in industry when an inverter is injudiciously selected to be combined with a motor. Such problems were highlighted by difficulties being experienced with some 400 kW inverter drives. The inverters had been bought from one supplier and the motors from another. When this system was coupled together, there was excessive heating in the motors and the overall plant was only able to operate well below its capacity, incurring a substantial weekly loss of income. The motor and inverter were evidently incompatible, and since the inverter could not be modified, the motor was redesigned to make it less susceptible to the harmonics present in the inverter waveform, These problems have led to the development of a variable speed drive simulation package at the University for use by the local industry which can accurately model the complete system of inverter, motor and its associated load. It is envisaged that this package could be used to predict the performance of a drive system and highlight problems that may occur. To be able to do this, an accurate model of the motor is required. This investigation gives the development of an induction motor model which is suitable . for variable speed drive system simulations. The model accounts for the deep bar effect by using lumped parameter circuits and includes saturation of the leakage paths using only information which is typically available from motor design data. A complete analysis is given of the different lumped parameter models and their suitability for use in this application. The thesis also shows the utilisation of the deep bar model to simulate reswitching transients and double cage motors. The author hopes that the models used in the simulation package wallow industry to predict problems prior to their occurrence, alter the designs and thereby avoid costly remanufacture of the system.Andrew Chakane 201

The monitoring of induction motor starting transients with a view to early fault detection.

The aim of this work is to investigate the possibility of detecting faults in a 3 phase Induction motor by monitoring and analysing the transient line current waveform during the starting period. This is a particularly onerous time for the machine and the inter-relationships between parameters such as current, torque, speed and time are very complex. As a result two parallel paths of investigation have been followed, by methods of experimentation and computer simulation. Transient line current signals have been obtained from purpose built test rigs and these signals have been analysed in both the time and frequency domains. In order to assist with the comprehension of this data a sophisticated computer simulation of the induction motor during the starting period has also been developed. Computer simulation of the induction motor has been developed initially using the two and then three phase induction motor voltage equations which are solved by numerical integration. Using these techniques it has been possible to detect small degrees of fault level for both wound and cage rotor machines by analysing the line current waveform during the starting period. Good agreement has been found between the real and simulated data. A range of Digital Signal Processing techniques have been utilised to extract the components indicative of rotor faults. These techniques were at first wideband and highly numerically intensive, some originating from Speech Processing. The final processing techniques were far simpler and selected by analysis of the results from experimental data, both real and simulated