A question came up on a home automation (HA) forum regarding a pool pump motor. Today currently controlled via a mechanical timer (dial type Intermatic) for so many hours of operation a day. The change desired is to make this controlled under a home automation systems so it can be programed and used more flexibly.
All that said the question came as to whether a 240VAC pump motor needed to switch both phases on and off on the pump motor of if just a single pole switch would be adequate. I thought I'd post this over here where more licensed folks can chime in.
My thinking is a single pole switch to control the motor is fine if some sort of disconnect is local to the pump motor. That disconnect can be a DPST switch or a disconnect like an AC disconnect panel. Whatever disconnect used it would have to disconnect both phases to the motor. Furthermore I would argue, since the HA switch is remote controllable, it should be disallowed as a disconnect since it can be inadvertently switched remotely. Not exactly fail safe....
Most 240V equipment is indeed controlled through the switching of just one leg. Some equipment for reasons of safety only use double pole switching. Of course if you have wired your pool with safely in mind then it would include a gfci breaker, which would take care of the safety side of things.
But if you're talking automation, then you're talking about controlling the system from a distance (not being present to ensure everything is working properly). That being the case, why take a chance. Double pole relays aren't that much more expensive than a single pole.
Pretty much, all the electronic timer systems have gone to switching one leg to save room and keep the system as small and cheap as possible. This includes the HA designed pool timers. I do agree, if you're building you're own HA system, just install the double pole and break both legs. As you pointed out, the price difference is small, sometimes the same.
It comes down to what is the disconnect, a controller only needs to break one leg, but if you are using some device as a disconnect it MUST break both legs...
You'll need a double-pole disconnect (manually activated, not a contactor), but the switching device for control only needs to break the circuit at one point. A solid-state relay is your best bet these days, since they are so cheap and reliable, and much easier to interface to digital devices.
Both types of relays are easy to interface. For the electro-mechanical, you just need a power supply with a little more power. Both will require glue logic circuits and both come in 5VDC coil (gate on SSRs) versions.
I suppose there are some suitable relays with very low coil power requirements. You don't usually see relays capable of switching that much load with that little coil power. It's still enough to always require an intermediate switching device though. I'll stick the my SSR recommendation. They are more inherently reliable and even easier to drive. They are LEDs internally - regardless of what type of thyristor is used (many are photothyristors), every line-voltage SSR I've used has been optically isolated. Most do not require more than 10mA for full turn-on, and it's not hard to find a microcontroller than can drive that directly. Simpler, more reliable, about the same price... solid state relay.
Oddly, around here, people never turn their pool pumps off, they run 24/7... so i always chuckle when people want to integrate their pump into home automation.
For the life of me I can't understand WHY anyone who want's to "automate" their home would take a pool pump or a water heater, devices that are already automatic, and integrate them into a "system." This isn't "home automation" but more like re-integrating automation into the hands of a control freak.
Why not use a small cheap digital timer a wall wart
And a contactor with a 12v coil.
Easy as, and cheap !
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