Project Help Pool Motor

[sprinkfitter]

My nieces tell me all the time I have the coldest pool in July. Outside temp where I live in July around 90 and above and my pool temp never gets higher than the low 80s This winter I would like to work and find a solution to this problem.
I have bought a duel speed 220 volt single phase motor that I plan to use in this project. I have a solar header and I want to max out the length of time I can put warmer water in my pool a day. My pool has to run at least 12 hours a day. I want to put temp sensors on the solar water and the pool water. I plan to use solid state relays a total of 4 to control the switching between high and low speed. Of course I want to add wifi to see the status and length of time the pump has been running and the over all temp of the pool. This is the planning stages so any help or direction would be greatly appreciate.
Thank You for your help
New guy from KY

[inakto]

Neat idea. This type of application is what my MicroPython powered product is supposed to be good for, too bad we don't have a RTD (temperature sensor) module yet.

[pythoncoder]

A forum search on DS18B20 (a waterproof temperature sensor chip) yields promising results. An interesting project

[sprinkfitter]

Great idea Peter with the DS18B20. Looks like I will use these for getting water temp. I was going to use these relays SSR-25 DA 25A 3-32V DC / 24-380V AC Solid State Relay + Heat Sink I was going to mount 2 in a box to control each speed of the pump. I was wondering if I should put a small fan mounted on the box to keep these relays cool. Or do they really need it having large Heat Sink.
I was thinking about using 4 relays but could I just use 3? I have attached a picture of the wiring detail.

51+6DDQQkhL.jpg (52.31 KiB) Viewed 10409 times

Thanks for all the help.
Ray

[pythoncoder]

A couple of issues to consider. Solid state relays don't provide safety isolation: even when in the OFF state, wiring should be considered to be live from a safety point of view.

Secondly I'd be concerned about the possibility, in a fault condition, of power being applied simultaneously to the low and high speed terminals of the motor. I couldn't find any technical data on that particular motor so I don't know what the consequences might be but it's wise to assume they might be bad. Note that solid state relays don't necessarily turn off immediately: it is likely it will continue to conduct until a zero crossing of current occurs. This presents the possibility of an overlap if both relays are switched in quick succession. But there is always the risk, without external electronic hardware, that a code crash might cause power to be applied to both terminals of the motor. There is a simple solution.

I would use a conventional relay with changeover contacts to do the low/high speed switching with a separate relay for on/off - the latter could be solid state subject to the safety note above. A conventional relay with break-before-make contacts guarantees that power can't be applied to low and high speed terminals simultaneously. If you're concerned about wear to the contacts of the relay, arrange your control code to turn the motor off before changing speed. That way the conventional relay never has to break the running current of the motor.

The worst that can happen with this arrangement is that the mechanical relay switches while the power relay is on. So long as the mechanical relay has an adequate current rating, no damage should result.

As for the SSR I'd need to see a datasheet to comment on the heatsink requirements - do you have a webref?

[sprinkfitter]

As usually Peter you bring up some great things to consider. Please help me and correct me if my thinking is wrong. To take care of the voltage leakage I could put in a bleeder resistance AKA resistor running parallel in the load line. I think this would take care of any voltage leakage. I do want to always switch the relay with a good delay in them so no chance to flip them on together.
I think I want to add a small 12 volt fan to blow across the heat sink. Just a small precaution because the pool room will be getting a little hot in the summer time.
The datasheet I found http://www.datasheetcafe.com/ssr-25da-datasheet-relay/
So having a 12 volt fan I think I will do the switching with 12volt
Thanks again for all the help
Ray

[mcauser]

DS18B20 is not the most accurate sensor, but since you can have multiple of them on the same wire, add a few and average the results.
Calibration: https://edwardmallon.wordpress.com/2016 ... nailed-it/

[marfis]

Solid state relays don't provide safety isolation

just wondering. afaik the isolation barrier of an opto isolated ssr is sufficient for safety isolation

[pythoncoder]

@marfis The point is that the semiconductor device which performs the switching (typically a triac) is not considered to be safe in its off state. Semiconductor devices can degrade especially if run outside of their design limits. The presence or absence of optical isolation is irrelevant in this context. Note that in the datasheet referenced by @sprinkfitter, you'll see that even if the triac is off there is a snubber network and an unspecified "zero cross cicuit" both of which which will pass current and could potentially fail.

@sprinkfitter I'm unsure where you plan to fit a bleed resistor or what problem it seeks to solve.

Time delays are all very well, but when designing circuits for switching high voltages and currents you should consider fault conditions. If energising slow and fast motor terminals simultaneously is liable to have a catastrophic outcome you really should ensure that it's impossible even if your code crashes. Even bug-free code can crash if the microcontroller is hit by an event such as a supply voltage spike or a brownout.

Of course if the motor manufacturer's specifications guarantee that energising slow and fast motor terminals simultaneously is harmless then the above advice may safely be ignored. Failing that guarantee, I'd assume the worst.

Re heatsinking the datasheet doesn't give you much to go on; there are no thermal resistance figures. But it's clear that dissipation is an issue - you can work out the power dissipation by multiplying the maximum motor current by the on voltage (1.6V). So if the motor takes (say) 10A then you have 16W to dissipate. The SSR is rated to 80°C. If your maximum ambient temperature is 35°C then you need a heatsink with at least (80-35)/16 = 2.8°C/W performance. In practice there is a thermal resistance between the device and the heatsink; you also need to allow for temperature rise in the enclosure, so a heatsink will be required with lower thermal resistance than the above crude calculation suggests. On the other hand forced air cooling will substantially improve the performance of the heatsink. In practice you take a first shot at the design, then take measurements, then revise if required.

[roland_vs]

As a suggestion I would like to add that you can parallel relays and SCR with the benefit of dealing with the inrush when activating the SCR and have a (smaller) relay for holding. Power dissipation would be minimal and the fan can be omitted.

Also using some Pt1000/Pt500//Pt100 and a little bridge circuit can be placed before the ADC's of the pyboard as alternative to the one wire solution.

Greetz,

Roland