EXPERIMENTAL INVESTIGATION ON A TWO STEP REGULATOR FED SINGLE PHASE INDUCTION MOTOR DRIVE

Abstract

The use of induction motor in variable speed applications, has become quite popular in recent times due to the development of control techniques using power electronic devices. Stator voltage control using thyristors provides a simple method for speed control of induction motor especially for fan load. However it affects the performance of the motor adversely due to the introduction of harmonics in load voltage and load current. Also the supply side is affected by current harmonics and poor input power factor of the regulator. Sequence control of ac regulator provides an easy method to overcome to a good extent, these limitations. In this dissertation work a single phase two step ac phase voltage regulator has been developed and its performance is investigated thoroughly. Chapter 1 gives a brief introduction of variable speed electrical drives and the advantages of AC drives over dc drives. Also the literature available on variable speed AC drive using stator voltage control method is reviewed. In chapter 2, methods of speed control of induction motors are listed and the theory of stator voltage control method is discussed in detail. The principle and operation of two step AC voltage regulator is explained in chapter 3. The design and development of the two step AC regulator is discussed in chapter 4. The working of firing circuit is explained with the help of oscillograms obtained at various stages. The performance of the two step regulator is H iii thoroughly investigated, first on R, RL and RC load and then with induction motor coupled to fan load. FFT analysis of voltage and current waveforms for induction motor with fan load is carried out and is compared with the harmonic spectra of the motor under same speed conditions when fed from single step regulator and also with sinusoidal supply. Performance curves of the induction motor during a fan load are also obtained using conventional moving iron instruments to get an estimate regarding the comparative performance of the motor under the three modes of supply. iv