Medium Voltage Application Guide

13571.A
SOFT STARTERS
3.3 Anatomy
Key components
Most soft starters have the following main components:
SCRs (also called thyristors)
Snubber circuits
Heatsink
Fans (optional for increased thermal ratings)
Busbars
Current sensors
Printed Circuit Boards (PCBs)
Housing
Example: MVS IP00
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Printed circuit board (PCB)
Housing (IP00 chassis)
Current sensors (CTs)
Busbars
Snubber circuits (RC network)
SCRs and heatsinks
SCRs
SCRs (silicon controlled rectifiers, also called thyristors) are the primary component of any soft starter. The SCR is
a controlled diode that only allows current to flow in one direction.
An SRC has three terminals. When the gate terminal is triggered with a low voltage signal, the SCR is turned on.
This allows current to pass through from the anode to cathode terminals. An SCR is self commutating and current
stops flowing when it reaches the zero point crossing.
A soft starter has at least two SCRs per phase, connected in reverse-parallel configuration so that current can be
controlled in both directions. The soft starter can control one, two or all three phases.
There are two physical styles of SCR:
modular pack SCRs are a self contained reverse-parallel device. These are often found in low voltage soft
starters with a voltage range of 200 VAC to 690 VAC and a current rating less than 300 A.
disk or hockey puck style devices are a single SCR which needs to be electromechanically configured in
reverse-parallel configuration for soft starter use. This style of SCR is used on higher current rated, low
voltage soft starters with current ratings greater than 300A.
Medium voltage soft starters with an operating voltage range of 2.3kV to13.8kV always use disk style SCRs
connected in series for each half of a phase to obtain the necessary voltage rating.
Modular SCR
Disk style ("hockey puck") SCR
710-12280-00A Medium Voltage Application Guide Page 23