# Soft starter damage



## Wirenuting (Sep 12, 2010)

Only saw one fail after someone pulled the disconnect open while running. It was a 200hp @208 for a fire pump.
After that, we had the scr's fail several times due to low voltage. This was due to the flow loop not opening as the fire pump ran. Dragged the transformer voltage way down.


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## JRaef (Mar 23, 2009)

Tsmil said:


> I'm hoping someone with greater knowledge than me can answer a question. In the soft starter (Allen Bradley) there is a thyrister module. What can cause this module to fail?
> 
> Background: T1, T2, and T3 were hooked up to motor. Line was connected to T4, T5, and T6. We were told this would be safe because T4 thru T6 were solidly connected to L1, L2, and L3 and this is shown on the diagrams in the manual.
> 
> This is a large soft starter.


Who told you this is OK? Although it's correct that electrically T4 = L1, T5 = L2, T6 = L3, you can't actually use it that way. Those terminals are INTENDED to be connected to the other 3 leads of a 6 lead Wye-Delta wound motor. When used in that manner, those terminals would only be expected to carry 58% of the motor current. L1-L2-L3 are rated to carry 100% of the motor current, no matter how it is connected. The tap-off points for T4,5,6 are down stream of L1,2,3, but are sized based on that 58% load maximum. So by connecting your line to those terminals, the current through those connections is going to be more than they, and their connections, are rated to carry. They would over heat and if they didn't cause a severe voltage drop as the resistance increased with heat, the heat itself could damage the SCRs.


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## micromind (Aug 11, 2007)

I doubt if this would matter in this case but a typical 6 lead motor is L1 = T1 and T6, L2 = T2 and T4, L3 = T3 and T5.


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