Testing Soft Starters
Testing AuCom soft starters is straightforward and doesn't require expensive or extensive equipment — a 500V insulation tester (megger) and a simple lamp load will cover most fault-finding.
SCRs
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 phases 1 and 3 are controlled and phase 2 is not controlled.
If the SCRs fail, there is a 99% probability that they will fail with a very low resistance, virtually a 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.
Resistance Test
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.
Frequently Asked Questions
How do SCRs typically fail in an AuCom soft starter?
In about 99% of cases, SCRs fail with very low resistance, effectively a short circuit, usually caused by a voltage transient or excess current melting the SCR die. In rare cases, the gate circuit fails instead, leaving the SCR unable to be triggered.
Which AuCom soft starters have all three phases SCR-controlled?
All AuCom soft starters use reverse-parallel SCRs on each controlled phase, except the CSX and CSXi, which only control phases 1 and 3 — phase 2 is not SCR-controlled on those two models.
What resistance should a healthy AuCom soft starter SCR read?
With the electronics disconnected, a healthy SCR should read greater than 500K ohms on any series. With the electronics connected, expected readings vary by series: around 200K–470K for the EMS, around 33K for the MSS, MSX, EMX, EMX2, IMS and IMS2, and around 100K for the CSX, CSXi and EMX3.