Soft Starter header

Testing Soft starters


All the AuCom Soft starters use reverse parallel connected SCRs to control the voltage applied to the motor.

There are two SCRs in each phase in all Soft Starters except the CSX and CSXi soft starters where the phases 1 and phase 3 are controlled and phase 2 is not controlled.

If the SCRs fail, there is 99% probability that they will fail with a very low resistance, virtually and short circuit.

Failure of the SCR die is usually due to a voltage transient, or excess current and both failures result in a shorted (melted) die.

In a very small number of cases, there may be a failure in the gate circuit of the SCR that causes the gate to not operate. This is very rare.



Disconnect the supply and the motor from the soft starter,
Measure the resistance of the SCRs using a 500V insulation tester such as a megger.
This can be done with the electronics connected, no damage will be caused by the 500V tester.
Test from L1 to T1, L2 to T2 and L3 to T3.
Test each phase with positive and negative polarity. (reverse the leads on the tester)

Series Electronics Connected Electronics Disconnected
EMS 200K - 470K > 500K
MSS 33K > 500K
MSX 33K > 500K
EMX 33K > 500K
EMX2 33k > 500K
IMS 33k > 500K
IMS2 33k > 500K
CSX 100k > 500K
CSXi 100k > 500K
EMX3 100k > 500K


Operational Testing

Connect a circuit comprising 3 x 100 - 500 watt 230 Volt lamps in a star configuration with the star point isolated from all other circuits.
Apply Line voltage to the input of the soft starter and set it to run mode.
All three phases should ramp up equally to full voltage.
If one phase does not ramp to full voltage, then there is an SCR that is not being controlled.
This could be a faulty connection, a faulty SCR or a faulty pcb.
Unplug the PCB and then plug back in. Repeat this 3 or 4 times and retry. If the problem still exists, replace the pcb with a new one and retest. If the problem still exists, then the SCR on that phase may be faulty and not conducting.