The cause of the problem can be either internal to the starter or external, in the installation.
- Ensure that there are no short circuits between input and output on any phase of the soft starter. If there is a shorted phase, there is a failed SCR.
- With the motor connected to the output of the soft starter, apply voltage to the input terminals (L1, L2 L3)
- Measure the voltage between input and output on each phase.(L1 - L4, L2 - L5, L3 - L6) If the starter is three wire connected, the voltage should equal the phase to neutral voltage for the supply and all three phases should be equal. If the motor is six wire connected, the voltage should be phase to phase voltage and be equal.
- If the voltage on any phase is zero, then check the incoming voltage on the supply.
- If the supply voltage is balanced, and the measured input to output voltage is unbalanced, then there is either a leaky SCR, or a pcb fault.
- Disconnect the inputs and outputs from the starter and use a 500volt insulation tester (megger) to measure between the input and output on each phase. The reading should be in the order of 33K. If there is a higher reading, then the control pcb is at fault, or there is a bad connection between the control pcb and the SCRs. If the reading is less than 33k, then an SCR is probably leaky.