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Hey guys, i'm hoping someone can help me decide the best relay to purchase from SF for controlling a 1/2hp inductive load 120v motor. I've spent numerous days researching this and hoping someone with more experience could help me clarify a few things.

Relay will be controlled by some type of microcontroller, probably an ESP32 or maybe even a Pi (not sure yet), but would be 3.3v or 5v GPIO to control the relay.

  • 110-120v 1/2 HP electric motor --- 7.2a at full load
  • Runtime anywhere from 30-60 seconds, twice a day (up to 10 times a day, with ~30 second runtime)
  • Installation will be inside sealed IP65 enclosure with DIN rail
  • Microcontroller or Pi Zero W (undecided as of now) will be mounted on DIN rail
Possible Solutions
  • SPST Electromechanical Relay Lifespan - One concern of mine is the lifespan of an electromechanical relay, given the fact that my setup could essentially have up to 10 on/off cycles per day --- what kind of lifespan could I expect for an electromechanical relay in my type of setup with an inductive load from a motor?
  • Inductive Load on Electromechanical Relay - One other concern I have is that the relay would be driving an inductive load from the motor ... do anybody have any experience, warnings, suggestions, or thoughts on this?
  • Solid State Relay Heat - My concern regarding the solid state relay is the heat generated from it, as I have seen a lot of them that require a heatsink ... my question is, with my load only being a max of 7.2a and runtimes being very low (30-90 seconds max), is this something I would need to be concerned with? I ordered both relays to test with, and some temp strips to get max temp it gets to, but wondering if anybody has any insight or history regarding this?
  • Price Points - The electromechanical relays tend to all be cheaper in price, but in my project is it worth the ~$5-10 savings as in regards to having an SSR which would have a longer lifespan than the EMR? If my prototype turns out to work well, I may eventually want to turn this into a commercial product, so price at that time would be a concern, but I also want to consider quality/lifespan of product based on having a lot of cycles on the relay.
  • DIN Rail Mountable SPST Electromechanical Relays - As my project will be inside an enclosure with DIN rail, my first thought was to look at buying an SPST EM relay, but my concern with that is regarding the protection of the GPIO from the MC ... does anybody have experience using any of these, or what I should look to make sure is included in the relay to protect the MC (like snubber circuit)? I'm not sure if there's a specific term used for DIN rail relays that specifies it has some type of snubber circuit integrated in it.
I'm tending to lean towards going with the solid state relay as an easier implementation method, with a longer lifespan, and not having to deal with building a snubber circuit for MC protection (unless I can find a DIN rail SPST with it built in), or have enough time & money to build out a PCB to handle this.

Hoping that someone with a little more experience in this than I have could provide any feedback based on past experiences or knowledge. Thanks ahead of time for your help, and for taking the time to read this :)
The solid state relay will far outlast the electromechanical relay.
The zero crossing nature os the solid state means it will switch the load on/off when the AC waveform is at zero volts.
the EM relay will not have this - opening the relay contacts will cause arcing and wear on the contacts.
You will find very few EM relays with a 'snubber' (for DC control, this will simply be a diode to constrain the flyback voltage (back EMF)).
The SS relay datasheet says it's good for ~176° F.
Mounting it on the backpanel of your enclosure will provide plenty of heat dissipation (don't think you should even worry about it with the low duty cycle).
The price is incredibly low compared to even discount automation component suppliers! They are $16 for 10A, $25 for 40A.
lyndon wrote: Tue Apr 17, 2018 7:43 am When SSR's fail, they tend to fail shorted, so think about an additional switch or a normally-on relay in the loop. Don't forget to properly heatsink the SSR if you're going to mount it on DIN rail.
Yikes... so you mean that if the one from SF does fail, it would normally be shorted meaning the relay would be turned ON? And if so, you were recommending having a NC (normally closed) relay as well as the SSR? I guess I need to do a bit of research into the failure rate of SSR's before making a decision now
There's cause for some concern about when that relay fails it would fail in the ON state but that's likely true of a mechanical relay as well.
A 40 Amp relay has plenty of capacity to have a huge long life in this sort of application.
Crydom says you should run their SS relays at no more than 75% of max rating.
and their inrush curves show a massive safety factor for a 40A SS relay running a 7.5 A motor load.
100 A for 10 seconds. ... -mount.pdf