Discrete Variable Frequency Soft Starting on DSP-based Voltage Controller-Fed
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Discrete Variable Frequency Soft Starting on DSP-based Voltage Controller-Fed IM Drive
Abstract

GThyristorized soft starters are being used as inductionmotor starters in fan or pump drives, ere, the principleofwhich is based on the control of variable voltage and constantfrequency. One ofthe demerits of the soft starters is that a smallreduction in voltage produces a considerable drop in electromagnetictorque, which may cause the motor with loads failureto start. A new control strategy is proposed to improve electromagnetictorque at starting. According to the relationshipthat the electromagnetic torque of the induction motors is inverselyproportional to the ac power supply frequency, reducingthe supply frequency can improve the starting torque. Based onthe fixed main circuit of soft starters, the strategy defines thetriggering instants of soft starter thyristors to include or omitpartial half cycles of the ac power supply. Therefore, the discretevariable frequency voltage and current applied to themotor are obtained. In fact, the new discrete frequencies are theones of the fundamental components of the generated voltageand current. For all the subharmonic frequencies, in order toobtain most positive torque, the system is properly unbalanced,using the method of symmetrical components to choose the mustpositive sequence ofthe initial phase angle combination ofthreephases. The strategy similar to variable voltage variable frequencyinverters gets the motor to produce low impact of CUTrentand high torque. Applying this strategy combining with themethod ofsoft start allows the induction motor to start with fullloads. The proposed strategy has been verified experimentallyon a laboratory machine using a 32-b DSP-based soft starter-fedinduction motor drive system, and proved to have good startingtorque and current performance.Index Terms-induction motor; thyristors; soft starter;electromagnetic torque
1. INTRODUCTION
Employing power semiconductors, Motor starters arebeing increasingly used to replace conventional reducedvoltage starters. Among these, thyristorized soft starters thatapply reduced voltage to the motor are cheap, simple, andreliable [I]. By use of them the starting current surge can bereduced remarkably and many advantages like smooth acceleration,ease in implementation of cument control, andenergy savings with a light load can be available [Z-31.The thyristors are initially fired at a big triggering anglewhen sot? start. Thus, the voltage applied to the motor is smallThe starting torque at standstill of an induction motor is approximatelyproportional to the square of the applied voltage.If the voltage is reduced by SO percent, the starting torque atstandstill will be only 25 percent of its normal full-voltagevalue [4]. Fig.1 shows the electromagnetic torque with fullvoltage start and with sot? start. Comparing the two-400 O5 Time(s) 1 1.5Fig.1 soA start comparing with full voltage startcurves, it is clearly seen that a small reduction in voltageproduces a considerable drop in starting torque, which maycause the motor with a load fiiilure to start.Some sot? starters have additional functions such askick-start, but it cannot meet the need of enough startingtorque of induction motors in some conditions, e.g. a conveyorsystem. This implies that the traditional soft startmethod has limited applications where the starting torquerequirement of the mechanicnl load is not high and the mechanicalload is a function of speed, such as with fans orpumps [SI. Unfortunately, many induction motor applicationshave heavy loads at startup and the motor need to start withfull loads. The traditional sot? start method is a nonviablesolution to such starting problem.A new discrete variable frequency control strategy isproposed to improve electromagnetic torque at starting. Thestarting electromagnetic torque is inversely proportional tothe ac power supply frequency of induction motors, reducingthe supply frequency can improve the starting torque. Basedon the fixed main circuit of soft starters, the strategy definesthe triggering instants of three hack-to-back thyristors, so thediscrete variable frequency voltage and current applied to themotor can be obtained, which are the fundamental componentsof the generated voltage and current. The initial phaseangle combination of three phases is more reasonable thanthat in literature [ 5 ] . The strategy simulates inverters by allowingthe motor to start with low impact of cument and hightorque. Therefore, applying this strategy allows the inductionmotor to start with full loads. The principle of the strategy isdescribed in sections 11. Sinlulation has been carried out insections 111. And Theoretical results of thc proposed strategyare verified experimentally on a laboratory machine insections IV.


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