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I am working on a project where I would like to power on and off a 240v industrial fan with the raspberry pi. Basically the idea is to tap into the 240v line and add a relay that the pi can control via GPIO outputs or even a 120v line (I can already toggle a 120v line from the pi using a SSR). I am struggling to find a relay that is properly rated for the motor loads (single phase 240v, 30 amp, 7.5 hp) and I am not especially well versed in electronics, so I may be missing something.

Additionally, it would be preferred if the relay could be normally closed so that if my pi goes down for some reason the fan would run as normal.

I talked briefly with an electrician and they suggested a NC Contactor, but I struggled to find one rated properly. They also suggested a “normally open contactor due to the rating and use a holding contact, that would require a by pass switch”. But I really dont understand what this means.

If anyone has any ideas or could simply explain things better for me as to how I could best move forward that would be great. Thanks!

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  • google search NC contactor 240V 30Amp ? Commented Jan 20 at 1:20
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    If you have to ask these questions you should NOT be working on mains power. Get a licensed person.
    – Milliways
    Commented Jan 20 at 1:47
  • @JaromandaX ha I definitely have but unable to find one that is properly rated for a 7.5hp load. The best I find are rated for single phase up to 5hp Commented Jan 20 at 1:49
  • @Milliways I have been working with a master electrician and they have given me the advice in my post. Im aware of the dangers with 240v and Im not going to do anything without consulting someone of that stature. Im simply trying to find the proper piece for this solution. Thanks Commented Jan 20 at 1:52
  • your question is not even about the Raspberry Pi and it is a shopping question, which is a double off topic here ... please find a suitable contactor, and then ask a question about controlling it with the Raspberry Pi
    – jsotola
    Commented Jan 20 at 18:50

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Your question is off-topic here, but I'll offer this:

  1. I'd recommend you try to find a latching relay for your application. I'd also recommend a manual switch bypass (or a circuit breaker). These items will take some stress off your RPi, and give you a means to control the fan without having to fiddle about with your RPi.

  2. Driving the control coil (solenoid) of a 240 VAC relay will require switching a fair amount of power; certainly far more than your GPIO pins are capable of sourcing. If your relay coil is dc-powered, you may be able to drive the coil with a Darlington transistor; for example. Find a Darlington with enough collector current capacity (I,sub>c) to drive the coil.

  3. Since the coil is an inductive load, you will also need to employ a "snubber" circuit to limit the "inductive kick" (L x di/dt).

Finally, and most importantly, know that at 240 VAC you are working with lethal voltages. If you have zero experience with safety procedures for wiring such equipment, you should retain a licensed electrician's support.

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