i'm hoping someone can help me decide the best relay (in my scenario) for controlling a 1/2hp inductive load from 110-120v motor. I've spent numerous days researching this and hoping someone with more experience could help me clarify a few things, as i'm still new to this and trying to learn as much as I can.

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/SPDT Electromechanical Relay (SPST more than likely)

  • 40a Solid State Relay (i've found clips for DIN rail mounting for these)

  • Din Rail SPST Relay - TBD


  • 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


2 Answers 2


Heat rejection for the SSR will be roughly 1.5W per running load amp, so your 7.2A rated motor will cause the SSR to reject about 11W of heat into your enclosure while the motor is running. That's not a lot, but should be factored in with all of the other heat dissipation going on, along with your worst case ambient scenario to determine if the enclosure needs venting or not, or if the environmental rating requires sealing, then the total exposed surface area of the box.

SSRs are somewhat easier to interface into a digital controller, but are also less "rugged" when it comes to operating induction motors unless you take the proper precautions. I doubt you will have issues with a 40A SSR though, that is more than sufficient "over kill" for this load. But be aware that SSRs do not isolate a load off-line, there is leakage that can be detected by meters and if the product is consumer related, you can get a lot of scared people calling back if they put a meter on it and detect voltage when it is supposed to be off. For consumer related products, I always recommend regular air-gap relays. Just make sure, as mentioned, that it has the correct HP/kW rating, not just an amp rating.

  • \$\begingroup\$ Thank you for your response and providing insights on some of my concerns \$\endgroup\$
    – sMyles
    Commented May 1, 2018 at 19:09

Electromechanical relays that are suitable for starting and stopping motors have horsepower ratings if they conform to USA standards. Good quality relays also have specifications that give the expected life in on/off cycles. To protect the relay DC coil driver circuit, an inverse parallel diode is required. Some drivers may have that built in.


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