SHORT-CIRCUIT CONTRIBUTION

[Engels]

We are making a short-circuit calculation now, for industrial plant, and needs information about considerations of contribution of high power motors using soft-starters (without by-pass contactors)during short-circuit time. The question are: We need consider its contribution for short-circuit current in the switchgear bus or not?
Thanks

 

[redtrumpet]

I believe you posted this question before and got no response, so I'll stake a stab at it.

First, it may be as easy as checking with the soft-start manufacturer. Second, most soft-starts have two SCRs in parallel per phase, each conducting in the opposite direction. Once the motor is at full voltage, the SCRs are gated on all the time and in principle there is nothing to stop a reverse current from flowing back to the source, unless the soft-start has internal protection that would prevent this scenario.

 

[jraef]

redtrumpet is correct. If your soft starter is running at full output, it will not hinder a short circuit contributiton from the connected motor appreciably more than if connected by a contactor. If you were attempting to compensate for the 1-1/2 volt drop across the SCR as a limiting factor, it is unwise when considering short circuit calcs. The chances are that the "worst case" scenario that you are looking to protect against would more than likely short the SCRs, making them full time conductors. Besides, it's not much anyway.

I know of no soft starter manufacturers that offer reverse flow protection, nor can I think of how we would do it.