ANALYTICAL AND EXPERIMENTAL INVESTIGATIONS ON CYCLOCONVERTER FED CAGE INDUCTION MOTOR DRIVE

Abstract

The technological development of semiconductor devices has made it possible to develop different types of static converters for industrial applications. For variable speed ac motors, a variety of static power converters are available. Cycloconverter and dc link inverters are both capable of providing variable frequency, variable voltage supply and both have been used to control speed of ac motors. However, cycloconverter control of the cage induction motor offers several advantages over the dc link inverters. It is a single stage conversion process with the natural commutation ability resulting in higher efficiency and reliability. Further, a eyeloconverter is inherently capable of bi-directional power flow. The quality of output supply improves naturally as the output frequency is reduced to very low values. Thus a cycloconverter fed cage induction motor offers an ideal proposition for low speed drive applications, such as the gearless ball-mill drive in a cement manufacturing plant. In present investigation, a 3-phase, 2.2 KW variable speed cage induction motor drive fed from a 3-pulse cycloconverter operating in non-circulating current mode is considered. Power diodes are connected in anti-parallel in each line to sense the load current polarity for implementing the non-circulating current mode of operation. In order to analyse the steady state and dynamic behaviour of this system, it is described by means of a mathematical model in terms of a set of five first order differen tial equations. The differential equations are linearised using the d-q transformation of variables and a synchronously rotating reference frame. The effect of saturation in the main flux path and cross saturation effect due to non-linearity of magnetic circuit is also considered in the model. The cosine wave crossing technique is simulated to obtain the cycloconverter output phase voltages which are then transformed into their d-q components. The set of differential equations are then solved numerically by using the fourth order Runge Kutta method to obtain the steady-state and transient response of the system. The mathe- matical model accounts for only copper losses in the induction motor and considers instantaneous switching of the cycloconverter devices. A 3-phase 3-pulse cycloconverter using 18 converter grade thyristors along with associated control circuit has been develop ed in the laboratory for the purpose of experimental investigation on the chosen 2.2 KW, 50 Hz induction motor. This laboratory model has been used to obtain experimental result to evaluate the validity of the computed results obtained from the mathematical model under steady state and transient conditions. The drive's steady state performance has been computed in a frequency range of 5 Hz to 16.66 Hz, for an input supply of 50 Hz. The performance curves are compared with those obtained by considering sinusoidal supply to emphasize the role of harmonics. At any given frequency, the cycloconverter fundamental output voltage is so adjusted that the motor operates on rated flux under full load. Two modes of operation of the cycloconverter are investigated; one, called m-control, in which the output voltage is controlled by adjustment of the amplitude of the sine modula ting wave or in other words by controlling the modulation index and the other, called x-control, in which the output voltage is controlled by directly adjusting the input supply voltage to the cycloconverter, the modulation index remaining unchanged at unity. The drive's characteristics under steady-state operating condition for both x- and m-control are computed at rated full load torque. The performance parameters such as torque, speed, motor current, power input, power output, power factor and effici ency are computed over the complete frequency range. For some typical cases, the computed results are compared with the corresp onding test results and close agreement of the two is established. The drive's transient response, consequent to step changes in load torque and output frequency settings, has also been investigated. It is essential to know the behaviour of the system under such transient conditions, as generation of severe instantaneous torque during transient period imposes undue strain on mechanical parts of the drive. The effect of step change, both positive and negative, in load torque and frequency has been seen in motor line current, motor torque and speed in terms of settling times and peak values of the most significant transient consequent to step change of the above two parameters. It is found that the drive responds eminently to such transient disturbance by quickly settling to new conditions without generating large torques and speed pulsations. The computed results are compared with the test results for certain specific cases. An estimation of harmonics at different operating frequencies has been made by analysing the waveforms of (a) cycloconverter phase voltage (b) cycloconverter line voltage and (c) motor line current through a standard FFT software package. The drive is always considered to operate under full load condition for both xand m-control. Total Harmonic Distortion (THD) is also computed for the motor voltage and current waveforms to assess the total effect of harmonics on the waveform distortion. The computed results are compared with the corresponding test results, again for selected typical cases. Based on the investigation made it is concluded that cycloco nverter fed cage induction motor is ideally suited to low speed applications and the mathematical model developed in this thesis can be used for reliable predetermination of the drive's steady state and dynamic behaviour.