Medium Voltage Application Guide

13478.A
13477.A
SOFT STARTERS
3 Soft Starters
3.1 What is a Soft Starter
A soft starter is an electronic motor controller used on three phase squirrel cage induction motors. During motor
starting, the soft starter controls the voltage or current supplied to the motor. Motor start performance is
optimised by reducing the total start current while optimising the torque produced by the motor. Motor stopping
can also be controlled by ramping down the output voltage over a predetermined time period. This is particularly
useful for eliminating water hammer in pumping applications.
Soft starters use SCRs (silicon controlled rectifiers, also called thyristors), arranged back-to-back for each controlled
phase of the soft starter. This provides phase angle control of the voltage waveform in both directions.
Controlling the voltage controls the current supplied to the motor. The stepless control of motor terminal voltage
eliminates the current and torque transients associated with electromechanical forms of reduced voltage starting,
such as star-delta or autotransformer starters.
SCR configuration (per phase)
L1
T1
Voltage waveform
Q1 Firing angle
Q2 Conduction angle
Q1
Q2
A soft starter designed to control motor voltage is referred to as an open loop controller. A soft starter designed
to control motor current is referred to as a closed loop controller.
710-12280-00A Medium Voltage Application Guide Page 19