Soft starters of I.M. have been extensively investigated and widely used in
industry in the past few years. They offer many advantages over direct on line
(DOL) starting, Y-A starting, autotransformer starting, or other methods.Soft
starters are part of a group of motor control techniques classified as starter
voltage control techniques.
The device circuit of any previous work about Softstarter consists of six
thyristors connected in back to back. In spite of using a modified switching
strategy for the I.M. soft starter to enhance its speed control and starting
capabilities, still the main disadvantages lies in harmonic contents of the
current waveforms.[l],[6].
To avoid harmonic problem this paper presents a new control technique using
PLC by inserting an auto-transformer in the line to the motor, reducing or
increasing the voltage at the motor terminals depend on selected taps according
to a SW program in the PLC .
This Softstarter has the capability to feel load changes during motor operation,
and it changes the driving voltage to a suitable minimum level improved motor
efficiency and energy saving are achieved by applying this
technique Moreover PLC gives wide capability to control more than one motor
in the same or different time for starting or stopping.