Emotron MSF 2.0 Softstarter

2. DescriptionT/T nn/n sIn this chapter different starting methods for inductionmotors are explained and compared. The functionality ofsoftstarters with torque control and their advantages andlimitations compared to other starting methods areI/I n 8Currentexplained.First a brief account of the background theory of starting76induction motors will be given in section 2.1. Thereafter thedifferent starting methods based on the usage of reducedvoltage will be described and compared. This chapter willalso cover softstarters with torque control. In section 2.3some common starting methods based on other physicalprinciples are explained. With this information some limitationsof the reduced voltage starters will become clear. Insection 2.4 there is a brief analysis of which applications maybenefit from using a softstarter.54321Torque2,5T/T n 2,5Torque0n/n s0 0,5 12.1 Background theoryFig. 4 Typical current characteristics for the DOL startThe following two sections deal with motors with squirrelcagerotors. In contrast to a wound rotor, the squirrel-cageFor many industrial applications direct on-line starting isrotor consists of straight conductors, which are shortcircuitedtogether at both ends.not convenient, as the supply in this case has to be dimensionedto deliver the unnecessarily high starting current.When such a motor is connected directly to the line voltageit will typically draw a starting current of about 5 to 8 timesits nominal current while the resulting starting torque willbe about 0.5 to 1.5 times its nominal torque. In the followingMoreover, most applications do not gain anything from thehigh starting torque. Instead there is a risk of mechanicalwear or even damage because of the resulting jerk at speedup.picture a typical starting characteristic is shown. The x-The acceleration torque is determined by the differenceaxis represents the speed relative to the synchronous speedbetween motor and load torque. The figure below showswhile the y-axis shows the torque and the current respectively,with those quantities normalized to their nominal val-some typical torque characteristics for constant speed applications.For comparative purposes, the induction motors’ues. The dashed line indicates the nominal values.torque characteristic is added to the diagram.221,51,5110,50,500 0,5 10n/n s0 0,5 1Fig. 3Typical torque characteristics for the DOL startFig. 5Typical load torque characteristicsTypical applications with constant load are elevators, cranesand conveyors. Linear load characteristics are found for calendarrollers and smoothing machines; quadratic correlationbetween speed and torque is typical for pumps and fans.Emotron AB 01-4135-01r2 Description 9