Using solid state relay in high load applications

Thread Starter

coolro18

Joined Jul 10, 2020
17
What are the potential risks in using a solid state relay for a load between 10-20A? I've found HP rated SSR's with a surge current graph that I believes fits the characteristic of the inrush current of an AC or DC motor, but I also read that the peak current of the motor must be half the amperage rating of the SSR. I'd like to know more about how feasible it is to use an SSR for a high load inductive motor along with dangers like contact shorting and melting. I am still considering using a contactor as I feel an EMR may be more applicable.
 

andrewmm

Joined Feb 25, 2011
574
SSR's are great for this application.
If you know what the motor surge current is , then rate SSR accordingly, if not, rate SR at twice the motors normal current seems as good a number as any.

The primary fault mode in an SSR is heating, The current flowing through the resistance of the SSR , causes heat in the junction, that needs to get out. So "with in reason" if the surge is short, the over current SSRs can take is amazing.

Could be over thinking this here, but I'll mention it.

I don't know what your volts are, but remember, SSR is never fully off, there is always a leekage current,
at a few thousand volts, that can be real "interesting "

We do series / parallel connected SSR / contactors in that case. Let the SSR tak eth ecurrent, the contactor switch out the leekage, and ground .
 

strantor

Joined Oct 3, 2010
5,331
The primary fault mode in an SSR is heating, The current flowing through the resistance of the SSR , causes heat in the junction, that needs to get out. So "with in reason" if the surge is short, the over current SSRs can take is amazing.
To add... Panel mount SSRs in the amperage range you're looking for, typically have a metal backside. This is meant to be fastened (bolted) to a heat sink, with a thermally conductive paste or strip between. Sometimes an extruded aluminum heat sink, but more often simply the subpanel/backplane of the enclosure in which they're mounted. If you want to replace SSRs with regularity, mount them to plastic, wood, or let them dangle freely from wires in the air.

Also being solid state they can be damaged by over voltage conditions, especially across the DC side. They sell snubber boards which bolt straight onto the SSR's terminals and I recommend using them especially for inductive loads.
 
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