SIMULATION STUDY OF CYCLOCONVERTER FED INDUCTION MOTOR DRIVE

Abstract

Electrical drives play an important role in electromechanical energy conversion in transportation and industrial applications for precise and continuous control of speed, torque, position, good transient response and high efficiency. From the early stages of the development of industrial adjustable speed drives, dc motors (especially separately excited) have been considered the most suitable choice for high level of dynamic performance. However, the dc motor is not the ideal solution to the problem of adjustable speed motor operation. Its mechanical commutator not only requires regular maintenance but is also responsible for restricting the voltage rating of the motor. The induction motor, on the other hand, has robust rotor construction which permits reliable and maintenance-free operation at high speed However, its speed, being closely tied to the supply mains frequency, is more or less constant The introduction of thynstors has heralded the advent of solid state power electronics and revolutionized the traditional methods of electric power conversion and control. The rapid advances in power semiconductor devices, integrated electronics and microprocessors have dramatically reduced the size and cost of power electronic converters With these developments, the picture of induction motor, as a machine with near constant speed characteristics, underwent a major change. The application of induction motors in adjustable speed drives has since progressively increased. In adjustable speed applications, the induction motor is supplied from a power electronic modulator which converts fixed voltage and fixed frequency of supply mains into adjustable voltage and adjustable frequency supply A cycloconverter and a d-c link inverter arc both capable of providing an alternating voltage at adjustable frequency and adjustable amplitude. Rated torque capability throughout the desired speed range can be provided by varying the voltage in coordination with the frequency. Cycloconverter control of induction motor drive offers several advantages over the dc link inverters in the high power rating and low speed applications Outstanding characteristics of cycloconverter fed induction motor drive include: (i) single step processing of power, (it) no commutation equipments are required; (111) high efficiency and reliability, (iv) capability of bidirectional power flow; (v) improved performance as the frequency is reduced and (vi) practically no restriction on power limit. Thus the cycloconverter fed induction motor drive offers an ideal proposition for high power rating and low speed applications, such as gearless ball- mill drive in cement plants. In this work, the performance of a 3-phase adjustable speed cage induction motor, fed from 3-pulse/6-pulse cycloconverter operating in noncirculating current mode of operation, is evaluated. For steady state and dynamic analysis, a mathematical model is developed in terms of d-q variables in synchronously rotating reference frame The model consists of a set of differential equations which are solved numerically using Runge-Kutta method to obtain the steady state and transient behaviour of the system The model takes into account the effects of magnetic saturation and core loss. Mam flux path saturation and cross saturation effects have been included by considering the nonlineanty of the machine's magnetic circuit. The values of magnetising inductance and dynamic inductance are obtained experimentally. The effect of core loss has also been included in the model. This

is done by adding two fictitious windings to the stator. The parameters of the 'core loss windings' are also obtained experimentally The mathematical model of the induction motor consists of five first order differential equations On account of additional 'core loss windings', the number of equations becomes seven. The model of the cycloconverter has been developed by simulating cosine wave crossing principle for determining the triggering instants of the thynstors of the 3-pulse/6-pulse cycloconverter. The voltage waveforms thus obtained at the cycloconverter output consist of a series of segments of the input voltages These voltages are then transformed into d-q voltages using the transformation equations The model treats the thyristors as ideal switches, i.e. the voltage drop across the thyristors and the turn-on and turn-off times are assumed negligible. Steady state performance of the drive has been computed at rated torque for both 3-pulse and 6-pulse configurations. A 3-phase 2.2 kW, 415 volts, 50 Hz cage induction motor is considered for simulation. Performance variables such as torque, speed, motor line and phase currents, input power, output power and supply line current are computed and their nature with respect to time is obtained. Variation in the operating frequency is considered in a chosen range of 0-16.66 Hz for a supply mains frequency of 50 Hz. The machine is considered to operate under constant flux in the entire range of operating frequency. The flux level is chosen, in the present work, corresponding to no-load flux for better utilization of the magnetic circuit of the machine. Voltage and frequency control of the cycloconverter output voltage is achieved by controlling the amplitude and frequency of the reference waves in the control circuit In the present work, this method of control is referred to as 'modulation index control' A generalised expression for the modulation index is formulated to incorporate the effects of operating frequency and load torque under constant flux condition in The performance variables under cycloconverter control are compared with those when the motor is fed from a sinusoidal supply, to bring out the effect of harmonics. The nature of supply current waveforms has also been investigated for both configurations of cycloconverter. The waveforms of motor phase current and cycloconverter input current (supply line current), have been analysed from the point of view of harmonic contents. This is done by computing the order and amplitude of harmonic components in these currents at several settings of operating frequencies A standard FFT software package has been used to analyse the current waveforms for their harmonic contents. For a given voltage/current waveform, this method offers a direct and straightforward means to precisely evaluate the harmonic amplitude and order simultaneously. Effects of modulation index and load displacement angle on the nature of harmonics have been investigated by estimating the harmonic content in the current waveforms at partial loads. Effect of pulse number has also been studied by comparing the harmonic spectrums of current waveforms of 3-pulse and 6-pulse system. Total harmonic distortion has also been computed for the motor phase current and supply line current, for both configurations, to asses the total effect of harmonics on the waveform distortion Prediction of peak transients in the system variables is an important aspect to be studied, as generation of severe instantaneous torques during transient period imposes undue strain on mechanical parts of the drive. Transient behaviour of the drive consequent to step changes in load torque and speed settings has been investigated considering both configurations of the cycloconverter. Variation in motor line current and supply line current, motor torque and speed have been observed in terms of settling times and peak values of the most significant transient consequent to step change of load torque and speed settings. It is found that the drive responds eminently to such transient disturbances by quickly settling to new steady state conditions without generating large torques

and speed pulsations. To highlight the effect of harmonics, the behaviour of the drive has been compared with that when the motor is powered from a sinusoidal supply. For 3-pulse cycloconverter, typical simulated performance under steady state condition, transient condition as well as typical harmonic spectrums are compared with the corresponding experimental results. A close agreement has been found Based on the above investigations, it is found that 6-pulse cycloconverter offers an ideal variable frequency supply for low speed applications In such drive the motor current is naturally very close to sinusoidal one. The mathematical model developed in this work for the cycloconverter fed induction motor drive, 3-pulse and 6-pulse, has been found eminently suitable for predicting the steady state and dynamic performance of the drive. Suggestions for further investigations, which may lead to further improvement of the drive's mathematical model are also included.