"Advanced Control of Soft Starters for Industrial Drives"
Three-phase semiconductor contactors and soft starters are cost-effective alternatives to frequency inverters in powering asynchronous motor drives with pump-, ventilator-, valve-, or conveyor-load. Advanced design of the power electronics topology and of the control algorithms extends the field of applications, increases the drive quality and reduces the reaction on power supply as well as the amount of investment. An economic solution results from using two controlled phases with back-to-back thyristor pairs which is characterised by an asymmetrical currect flow. A symmetrical current flow can be attained by applying an asymmetrical firing angle in the controlled phases which also eliminates the oscillation torque of asynchronous motors during voltage ramp controlled starting or breaking. To eliminate the high oscillation torque in contactor mode of operation a switching strategy will be presented based on the space-vector theory of the asynchronous motor and the power electronics part.