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What can cause 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.

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.