Medium Voltage Application Guide

13766.A
SWITCHGEAR
Motor Line Inductors on Soft Starter Applications
Long motor cables on the output of a soft starter are seen as a capacitive load which creates a di/dt current transient
at each SCR turn-on event. If the current transient repeatedly exceeds the di/dt rating of the SCR, failure will occur.
Soft starter start current waveform
1500
1000
500
0
-500
300
200
100
0
63.05 63.10
-1000
-1500
55 59 63 67 71
The current transient, peak value and rate-of-rise are are installation dependent, and are determined by many factors
external to the soft starter. The di/dt value of the current transient is proportional to the system voltage (for
example, the value for a 6.6 kV system will be approximately twice as much as a 3.3 kV system).
At a critical cable length, the cable capacitance must be negated by line inductance to avoid damaging current
transients. Air-core line inductors are specified according to the installation and are fitted close to the soft starter
output terminals.
4
5
1
2
M
3
3
13768.A
3-phase supply
Soft starter
Line inductors
Cable capacitance
MV motor
Soft starter SCRs are most vulnerable to current transient damage at two stages of the motor starting procedure:
in the initial ramp-up to the start current (current limit) level.
when the SCRs reach full conduction and just before the motor current falls to the running current level.
The latter stage is potentially more damaging. AuCom medium voltage soft starters are bypassed in run state and
the SCRs are turned off, minimising the risk of current transient damage.
Current transient influences
The di/dt current transients that occur at SCR turn-on are influenced by the following factors:
System voltage and frequency
Total cable capacitance upstream and downstream of the soft starter
Motor characteristics (kW, efficiency, starting power factor)
Soft starter snubber component values (RC network components)
Control stage of the motor start-up period
Start current (current limit) level
Page 130 Medium Voltage Application Guide 710-12280-00A