Medium Voltage Application Guide

MOTORS
Soft starters
Electronic soft starters control the voltage applied to the motor by means of an impedance in series with each phase
connected to the motor. The impedance is provided by AC switches – reverse parallel connected SCR-diode or
SCR-SCR circuits. The voltage is controlled by varying the conduction angle of the SCRs.
Soft starter control
Main contactor
M
3~
Electronic soft starter
Overload relay
13191.A
1 2 3
The SCR-SCR switch is a symmetric controller, which results in odd order harmonic generation.
The SCR-diode switch is an asymmetric controller, which causes even order harmonic currents to flow in the motor
and supply. Even order harmonics are undesirable for motor control because of the increased losses and heating
induced in the motor and supply transformers.
Electronic soft starters come in two control formats.
Open loop controllers, which follow a timed sequence. The most common open loop system is timed
voltage ramp, where the voltage begins at a preset start voltage and increases to line voltage at a preset
ramp rate.
Closed loop controllers, which monitor one or more parameters during the start period and modify the
motor voltage in a manner to control the starting characteristics. Common closed loop approaches are
constant current and current ramp.
Page 16 Medium Voltage Application Guide 710-12280-00A