Abstract
In integral-cycle triggering mode of voltage control, subharmonic as well as higher order harmonic components are generated in the load voltage waveforms of a three-phase system. These harmonic components are found to be unbalanced in phase displacement. The correction of the unbalanced phase displacement angles of a particular subharmonic or higher order harmonic for this type of triggering is investigated to solve the limitation of use of this important type of control as a drive and many other industrial applications. In this research a new phase shifting technique is proposed to correct the unbalanced phase displacement angles in the three-phase system. This technique depends on shifting the waveforms of either phase B or phase C or both by multiples of 2π. A microprocessor-based harmonic phase shifter is designed and tested with three-phase resistive and induction motor loads. It is found that there is a well agreement between the theoretical and experimental results and it is believed that the major problem of harmonics phase unbalances associated with the integral-cycle triggering mode of thyristors when used with three-phase circuits have been solved in the present research.
Highlights
Load voltage control by means of switching a pair of inverse parallel connected thyristors or triac is well established
The three-phase induction motor is loaded by a dc dynamometer available in the laboratory to examine the performance of the motor under load condition
The unbalanced sets of subharmonic and higher order harmonic voltages generated by integral cycle control technique create severe problems for ac machines as they found to cause excessive heat, mechanical vibrations and noise
Summary
Load voltage control by means of switching a pair of inverse parallel connected thyristors or triac is well established. The supply frequency components are found to be balanced, since they are 120o apart in time-phase while the phase displacement angles of a particular subharmonic or higher order harmonic are unbalanced[11,12]. The amplitude cn of nth order subharmonic or higher order harmonic as well as its phase displacement ψn are found as follows: ψ nj tan −1 a nj bnj cos nγ j − cos φcos n
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