Analysis of an electronic phase shift circuit for starting a single phase induction motor

Abstract

Fractional horse power single phase induction motors have no starting torque of their own. As a result, these motors are started by auxiliary methods. Now-a- days, single phase induction motors having large horse power rating are also in use in rural areas and in industries. These large horse .power single phase induction motors require medium size capacitors for their start and running purpose. In this thesis static cycloconverter and inverter topologies have been proposed to perform the function of capacitors. Static converters are used for conversion of ac and dc power to ac and de having frequency, magnitude and phase other than supply voltage. Several of the conventional static converters can replace the starting and running capacitors of single phase induction motors. When sinusoidal supply is switched at every quarter cycle, switched waveform shifted by 90° can be obtained. This waveform may be utilized in the starting purposes of large hp single phase. motors. This would replace the bulky capacitorsotherwise required for starting these motors alid may also serve the purpose of power factor and other type of controls of the motors .. The quarter . cycle switched waveform though fulfils the required phase shift, pulse width modulated switching is proposed in the thesis for better harmonic performance of output waveform. Analysis and implementation of the PWM phase shift circuit has been carried out and reported in this thesis to justify the feasibility of using these circuitsin the practical field. It has been found by analysis that over modulated sine PWM switched static cycloconverters provide necessary phase shift for motor start and at the same time keep the low order harmonics of the phase shift circuit output voltage within acceptable limit. Motor operation by the proposed circuits has not been studied in detail in this thesis and left for future investigation ..