Soft Start Short Circuit Contribution

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Shaneyj

Shaneyj

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Some say yes and some say no.
Which is it?
If the SCR shorts closed then there is a path for the motor to contribute, no?
Bonus points if you can direct me to a technical paper.
 
Answer is, like many things, it depends. If you have integral or external bypass contactors all bets are off as @petersonra points out. Typically Scheider soft starts do not have integral bypass contactors. Eaton's 811s do. Any case-SCR's are more complicated than diodes, but they behave on the same principals of PN or NP junctions. The SCR is a diode with a third region and terminal to control the conductivity of the sandwiched region. For motors SCRs I would expect a PNP junction.

I'm not an expert in electronics so I don't exactly know. I speculate that If you reverse bias a diode (NP or PN), you need to exceed the breakdown voltage before anything more than leakage current flows. Add the third layer, and the same relationship should remain, with an additional layer that might not matter. I would not expect a regenerating motor be enough to hit the avalanche region and so contribution through a reverse-biased diode would be negligible.

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It's not a matter of whether or not the SCRs short, because that could be exactly the same as a contactor welding, it's an unusual circumstance. But if the motor is running and the soft starter is gating when an upstream fault occurs, power can flow in either direction, even without a bypass contactor. So yes, a motor RUNNING on a soft starter can contribute to a fault on the system.

If someone told you no, they were likely confusing a soft starter with a VFD. Standard "non-regenerative" VFDs cannot let motor energy back onto the line if there is an upstream fault. That is the only technology that will limit motor contribution.
 
Jraef said:
But if the motor is running and the soft starter is gating when an upstream fault occurs, power can flow in either direction, even without a bypass contactor. So yes, a motor RUNNING on a soft starter can contribute to a fault on the system.
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my understanding is if you reverse bias a gated SCR that is powering an inductive load, the reverse current will be limited to leakage current plus a small amount, but the SCR will dissipate power rapidly (heat up) and perhaps fail.
 
drktmplr12 said:
my understanding is if you reverse bias a gated SCR that is powering an inductive load, the reverse current will be limited to leakage current plus a small amount, but the SCR will dissipate power rapidly (heat up) and perhaps fail.
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Yes, that is true of any ONE SCR, but remember in a Soft Starter there are an inverse parallel PAIR of SCRs on each phase so that power can flow in BOTH directions. So if they were still in phase angle control, such as ramping, where they were only gating part of each cycle, then that would be true as well. But once the soft start is done ramping, BOTH of the SCRs in each phase (so all 6) are gated fully on and kept that way, so if current wants to flow in the opposite direction, it just flows in the other fully gated SCR. The control board would have to KNOW there was a short somewhere upstream and stop gating, and none that I know of have that ability.

It’s all kind of a moot point though, it’s very rarely that I see someone use a soft starter without a bypass contactor now. Most of them are now designed with integral bypass, you have to go out of your way to get one without that now.
 
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