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Thread: Solid State Relays
09-09-2011, 03:32 PM #1
Solid State Relays
I had a unit that blew out 3 contactors today. It got me thinking......
Why doesn't anyone make solid state relays to replace contactors? Design them with the terminals and mounting bracket similar to your generic contactors that are used. Hmm....?
I know you can retro something in, but I'm talking a line of general sizes that would easily replace common contactors.
09-09-2011, 03:55 PM #2
In my experience , solid state relays are pilot duty and not able to handle any significant current , such as motor starting.
09-09-2011, 05:09 PM #3
09-09-2011, 07:27 PM #4
Now you mention solid state starters and vfd's ascj , your onto something there , those are able to handle significant current draw. Closing a circuit and allowing amounts of current to flow , with no mechanical components.
I see where your going with this , a solid state relay , but with a higher current capability , for starting your average hermetic compressors , maybe up to 40A or so.
That could be a really neat component that could last longer than silver contacts subject to arcing.
09-09-2011, 08:57 PM #5
The 40A SSRs I have used have a one second maximum overcurrent rating of 120A, which is about half what the locked rotor for a compressor having a 40A RLA would probably be. So it would take an 80A SSR for the inrush, with no safety factor and assuming the compressor would stop drawing LRA in one second. SSRs are prone to fail closed on overcurrent. I think the safe thing would be to size by inrush; 240A LRA = 240A SSR. But a ground fault or short circuit could still take it out. A standard contactor can be taken out, too, but the SSR that size would cost 10 times more than the contactor.
09-09-2011, 09:02 PM #6
They make solid state relays that will handle hundreds of amps and they work just fine starting motors...even motors for pumps and compressors.
Cost is probably the biggest reason you dont see them used more in consumer products. Its cheaper for a manuf to install a contactor and let the contractor replace them every so often.