Emotron MSF 2.0 Softstarter

The disadvantage of the star-delta start is that it cannot beadapted to a special application. Both the voltage in star andin delta connection are defined by the supply, the resultingstarting performance depends on the motor’s DOL characteristic.For some applications the star-delta starter cannotbe used as the resulting torque in star connection is too lowto start rotating the load. On the other hand for low loadapplications further savings of starting current are impossibleeven though a big torque reserve is available. Moreover,the resulting abrupt rise of torque first at start and laterwhen changing from star to delta connection may contributeto mechanical wear. The high transient currents duringstart-delta transition create unnecessary excess heat in themotor.Better performance is achieved with a voltage ramp start,which a simple electronic softstarter can provide. The voltageis increased linearly from an initial value to the full supplyvoltage by means of phase angle control. The resultingtorque and current characteristics are shown in the followingfigure.T/T n2,521,510,5TorqueA softstarter i often used to keep the starting current below adesired level. For the example above, setting a current limitof three times the nominal current may be desirable. Thefollowing figure shows the resulting torque and current characteristics.T/T nI/I n2,5876543221,510,50Torque0 0,5 1Currentn/n s0n/n s0 0,5 1100 0,5 1n/n sI/I n8CurrentFig. 9Soft starting – voltage ramp with current limitFig. 876543210n/n s0 0,5 1Soft starting – voltage rampObviously a much smoother start is realized compared to thestar-delta start and the starting current is decreased.Once again the figure illustrates that the resulting performancedepends on the combination of motor and load characteristics.In the example above the motor torque is close tothe load torque at about half speed. This means for someother applications with different load characteristics (forexample a linear torque-speed correlation) this particularmotor would need more than three times the nominal currentto start.The most sophisticated electronic softstarters use torquecontrol, which results in an almost constant accelerationduring the start. A low starting current is also achieved.However, this start method also uses reduced motor voltageand the quadratic correlation between current and torquedescribed in the first section of this chapter is still valid. Thismeans, the lowest possible starting current is determined bythe combination of motor and load characteristics.Emotron AB 01-4135-01r2 Description 11