Medium Voltage Application Guide

13490.A
13489.A
SOFT STARTERS
Snubber circuits
Snubber circuits are used to suppress a phenomenon called notching which occurs at the zero voltage crossing point.
Snubber circuits provide SCR control stability and a level of overvoltage protection.
In the simplest form, a snubber is a resistor-capacitor network connected in parallel across each SCR. Resistors used
for this purpose are typically wire wound, for the necessary power rating. In medium voltage soft starters, grading
resistors are connected in a series-parallel configuration across all SCRs. This divides the voltage across SCR in each
phase evenly.
Basic LV snubber arrangement
C
R
SCR
LV supply
SCR
Motor
R
C
Basic MV snubber arrangement
C
R
C
R
SCRs
SCR
SCR
RC snubber network
Grading resistors
MV supply
SCR
SCR
Motor
1
2
R
C
R
C
3
Heatsinks
Heatsinks are designed to efficiently dissipate the heat generated by SCR switching during motor starting and
stopping. Optimum heatsink design maximises the rating of the soft starter by keeping the SCR internal junction
temperature below 130°C.
SCRs are always bonded to a heatsink, using an appropriate thermal paste.
modular SCRs are bonded to an isolated heatsink arrangement.
for disk SCRs, the conducting faces of each SCR are compressed against a conducting heatsink face.
Many soft starters reduce the heatsink size by turning the SCRs off at the end of a start and bypassing the SCR
arrangement during motor running.
Page 24 Medium Voltage Application Guide 710-12280-00A