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Posted By Topic: Parelling contact pairs in a SSD

Feb 02 2020 23:07

Does anyone know if paralleling the 3 x 15A pairs in is OK?

Basically, I need to switch a 7.5kW single-phase kiln (purely resistive load) that is on it's own 32A supply.

The device I have is a 3 x 15A unit so in theorey, it will become a 1 x 45A unit.

But someone has suggested that timing issues may mean that one pair close before the others and cause a smoke-out. But surely, SSD's are instantaneously going to close together aren't they?

Does anyone know anything about these things?

Feb 03 2020 08:07

Read the SSD manufacturer's instructions carefully to see if it is possible.

In general, I would think that it would unlikely as there is no external method of ensuring equal current distribution in each solid-state pole of the SSD.

There will be SSDs able to do the single-phase current in a single poles device.

Feb 03 2020 08:17

Is the kiln's load all in a single element? Or can the load be split into three groups, keeping the load of each element bank within the rating of the device?

Feb 03 2020 12:17

The switching components are thyristor based (SCR or triac). Those don't evenly share in parallel even when controlled as one. I'm not sure of suitable de-rating to compensate. Omron themselves might be helpful to talk to.

Separately, for the cheap no-name brands at least, you have to de-rate them substantially for long term reliable operation. For 32 Amp heating element I'd select a minimum of 100 Amp SSR when they're a thyristor.

Omron probably have this already factored into their ratings though.

IGBT switching doesn't need such large de-rating in both cases. However, I'm not sure how common these are in SSRs just yet. They mostly get used in VSDs.


Feb 03 2020 12:59

Soft-starters are most likely IGBT based. They could be very effective for inserting between the contactor and the heating elements. And they should operate with single phase, albeit not fully utilised.


Feb 03 2020 13:09

Err, I'll take that back. Soft-starters aren't what I thought they were.


Feb 03 2020 14:24

Thanks all. Alek - it's a series/parallel setup but pretty unwilling to start beggering about with it as my last experience resulted in cracked bricks, fire-cement work etc....!

Buying a bigger one not really an option as the customer has already supplied this one. But based on the above, he'll either need to find someone else to fit it or he'll need to accept the risk of it failing.