Medium Voltage Application Guide

SOFT STARTERS
Operating Sequence
NOTE
The phase sequence of the incoming mains supply and the motor winding connections must be
verified for correct motor rotation.
NOTE
The motor must be stopped before changing its operating direction. There is always a short time
delay built into the selected changeover of the phase rotation. This is typically less than 3 seconds
which is enough time to allow motor flux and thus any back EMF in the motor to decay. The soft
starter SST can use the coast-to-stop or soft stop method.
Forward control sequence
Before starting, both the supply side earth switch (Q3) and the motor side earth switch (Q30) must be open and
mains supply must be present.
1. The forward direction circuit breaker Q1 is closed. Electrical interlocking disables the reverse direction circuit
breaker Q2 from closing.
2. The soft starter is given a start command and the main circuit breaker Q10 closes.
3. The soft starter performs a series of prestart checks, then starts the motor in the forward direction.
4. Once the motor has reached full speed, the soft starter SST is bypassed using circuit breaker Q20.
Reverse control sequence
Before starting in reverse, both the supply side earth switch (Q3) and the motor side earth switch (Q30) must be
open and mains supply must be present.
1. The reverse direction circuit breaker Q2 is closed. Electrical interlocking disables the forward direction circuit
breaker Q1 from closing
2. The soft starter is given a start command and the main circuit breaker Q10 closes.
3. The soft starter performs a series of prestart checks, then starts the motor in the reverse direction
4. Once the motor has reached full speed, the soft starter SST is bypassed using circuit breaker Q20.
Page 60 Medium Voltage Application Guide 710-12280-00A