Cause of an SCR failure on a soft starter

Note that the vast majority of SCR related faults occurring on the S611/S801+/S811+ soft starters are actually caused by a disconnection between the soft starter and the motor.  Check any disconnects, fuses and junctions on that run, and the connections to the soft starter and motor.  Verify that both mains and load lines are connected and secure. Verify that any isolation and/or reversing contactors are properly engaged prior to the soft starter receiving a START command. If the current draw is significantly less than the frame size of the soft starter (less than 1/16 of motor FLA), not enough current may be flowing to allow the SCRs to fire.

For the controller to have SCR failures, I can offer that these typically come from three main causes:

- too much voltage
- too much current
- too much heat

or any combination of the three.

The cause for failure can be the result of violating the duty cycle limitations; i.e starting too often without proper elapsed time between starts.   

Also, allowing the Soft Starter to source high current for too long a time will raise the heat, and cause SCR degradation and ultimate failure (elevated heat reduces the reverse blocking capability of the devices and they fail).

Also, if it subjected to transient spikes. These transient surges can cause failure of the SCR’s as well.

Lastly, a defective motor or an intermittently defective motor...(i.e., stator winding shorts or grounds) can cause failure.