Altistart 48 Enclosed Altistart 48 Soft Starters Page 39 Conventional Starting Curves

39 Altistart 48 soft starters for asynchronous motors Conventional Starting Curves Conventional Electronic Starting With Voltage Ramp And Current Limit 3 M A soft starter with 6 thyristors connected head-to-tail in each line phase is used to power the three-phase asynchronous motor by gradually increasing the voltage on start-up. - Depending on the firing time and angle of the thyristors, it can be used to supply a voltage which will gradually increase at a fixed frequency. - The gradual increase in the output voltage can either be controlled by the acceleration ramp, or by the value of the limiting current, or linked to both parameters. Figure 1 shows the behavior of the torque in relation to the starting current. Limiting the starting current Is to a preset value Is1 will reduce the starting torque Ts1 to a value which is almost equal to the ratio of the square of currents Is and Is1. Example On a motor with the following characteristics: Ts = 2 Tn for Is = 6In, current limiting at Is1= 3 In or 0.5 Is results in a starting torque: Ts1 = Ts x (0.5)2 = 2 Tn x 0.25 = 0.5 Tn. (Tn is motor rated torque; In is motor rated current.) Figure 2 shows the torque/speed characteristic of a squirrel cage motor in relation to the supply voltage. The torque varies like the square of the voltage at a fixed frequency. The gradual increase in the voltage prevents the instantaneous current peak on power-up. (1) Load torque Advantages of starting with the Altistart 48 Conventional electronic starting To rectify problems caused by: - Mechanical stress when starting - Hydraulic transients during acceleration and deceleration in pump applications Conventional electronic starting requires the use of several current limits or the switching of several voltage ramps. The settings become complicated and must be modified every time the load changes. Starting with the Altistart 48 The ATS48 soft starter's torque control enables: - Starting without mechanical stress - Smooth control of hydraulic transients with a single acceleration ramp - Simple and effective settings, whatever the load. Conventional starting curves (continued) The following graphs illustrate the starting curves of commonly used starting methods for starting a motor. 0 0 0.25 0.5 0.75 1 Motor speed (1) Ts1 Ts Is1 Is 0 0 0.25 0.5 0.75 1 Motor speed (1) V 0.85 V 0.6 V T Schematic diagram Firing angle Figure 1 Figure 2

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