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I've got a straightforward question: I'm wanting to control a pump (120V AC) with my Pi. I tried using a Cyrdom SSR (info) but I now realize it is for resistive loads only. Is there a relay rated for motors I can directly control with GPIO pins on the Pi that will work with my pump? Anything else to know before wiring it up?

Pump info:

HP: 1/200 Voltage: 115V Current: 1.1A Wattage: 70W

The pump will be turning on for 1-5 seconds every few minutes (reliability is important).

Thank you!

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  • \$\begingroup\$ How big is the pump, how many amps and what voltage? You should be able to find a 120v relay that can be driven by logic voltages, although you will need to buffer the relay turn on signal with a smaller relay or transistor since the relay will require more then the PIs GPIO pins can provide. You will also want to add a flyback diode across the relays logic leads to keep the inductive kick from toasting the PIs pins. \$\endgroup\$ – MadHatter Nov 14 '15 at 21:05
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    \$\begingroup\$ Why don't you just go for an electromechanical relay. Omron G2RL can handle around 16A. Choose one which is rated for 1.2 to 1.5 times of your motor current. You can use a transistor to turn on the relay using a signal from your GPIO. However, I'd suggest you to use an opto-isolator (PC817) to get complete optical isolation. Don't forget the free wheeling diode on the relay. \$\endgroup\$ – Whiskeyjack Nov 14 '15 at 22:02
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    \$\begingroup\$ Seriously, /do/ go for an electromechanical relay! They're almost indestructible and motors can be horrible, horrible loads. In an application like this you lose nothing in reliability: almost anything evil you might do (eg chatter the relay on and off) will hurt the motor more. With a sloid-state relay, there's much more of a chance of release of the magic smoke. Treat the relay with respect, too, as advised above. Always include a freewheel diode as advised, and drive it with a darlington. \$\endgroup\$ – Dan Sheppard Nov 14 '15 at 22:24
  • \$\begingroup\$ I don't think you need one for this project necessarily, but also maybe take a look at the hybrid solid state/ electromechanical relays just to be aware that there is such a thing. They can achieve superior contact life by avoiding transient arcs when making or breaking the circuit, and still have the benefit of a nice beefy piece of metal to handle the load most of the time. \$\endgroup\$ – Sean Boddy Nov 14 '15 at 23:24
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    \$\begingroup\$ @Whiskeyjack: that's not enough! Relays are derated to 1/5 to 1/10 current for motor use! Either that or you need a snubber. \$\endgroup\$ – Fizz Nov 14 '15 at 23:48
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Besides being an inductive load, an AC motor is also a load that requires a high current during starting. The start time for a small motor will probably be one second or less. In the USA and Canada, motors are usually rated in horsepower and suitable relays have a horsepower rating. Any relay that does not have a horsepower rating is probably not suitable. Elsewhere in the world, motors are rated in kW and suitable relays have a different rating system. If you need that search for IEC relay ratings.

Click here for a PDF from Crydom.

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    \$\begingroup\$ for controlling motor loads you want a contactor, this is basically a special kind of relay. \$\endgroup\$ – Jasen Nov 15 '15 at 6:54

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