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| Over-temperature circuit for water pump controller |
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| calzap:
I'm designing a circuit to monitor the temperature of the motor and pump-head of a water pump. In the case over-temperature, the pump must be shut down and require a manual restart. And there must an override switch and status indicators. The pump is powered with 240 VAC supplied via a contactor with a 24 VAC coil. The picture below is what I've produced so far. Normal operational position of the latching relay is 2. In this position, its SPST contacts are open, which means the secondary DPDT relay will not be activated. The NC contacts of the DPDT relay allow the contactor coil to operate. The closed SPDT contacts of position 2 of the latching relay allow the thermostats to cause the contacts to move to position 1 if either thermostat closes. When the latching relay moves to position 1, the SPST contacts will be closed activating the DPDT relay. And change of position of the SPDT contacts of the latching relay means the thermostats will no longer be able to change latching relay position until it is reset to position 2 by the reset switch. The reset switch also serves as a test switch because successive presses will change the latching relay position. I'm not working with a blank sheet. There are plenty of the latching relays and DPDT relays on hand. So they are going to be used unless there are really good reasons not to. DPDT relay will have a MOV flyback suppressor. Latching relay will have a TVS diode flyback suppressor. Mike in California |
| duak:
I haven't worked with AC alternate action latching relays and don't think or know of any unusual characteristics. I think your circuit should do what you said. Overtemperature and overcurrent sensors are usually normally closed and open on fault so if a wire is cut, the circuit stops working. To apply that here would require another relay. Do you need a guaranteed off function like an E-stop for safety? I see you're using 5 V for LEDs. Is there are particular reason? One thing to consider with relay contacts is that unless they are designed for dry switching (ie. no voltage or current) they require a mininum load to keep themselves from corroding and developing high contact resistance. This usually means at least 12-15 V and 10 mA or so. 5 V is not usually enough. There are LED Pilot lights for industrial controls that operate on 24 V AC or DC. |
| beanflying:
Are you just wanting loss of prime protection shutdown or are you running a heating loop of some sort? Temperature is not always a good option for protection depending on the plumbing on the suction side of your pump. What we used to fit to pumps for protection was a paddle switch (flow detection) with either an integral timer or one fitted in the panel (generally a 15 second delay). https://mackvalves.com/flowswitch-2/ The cheap and cheerful way out is an evilbay one eBay auction: #261846873840 and either roll your own 555/relay solution or buy one of these sorts of boards eBay auction: #201629716476 |
| calzap:
Thanks for the replies. I was planning on using 5 VDC for LEDs just to cut down on power wastage in the resistors, but I could go to 12 VDC with no problem. I've used 24 VAC for LEDs and still do in some applications. But it's a bit more complicated, requiring an additional diode and higher wattage resistor. It's no big deal or expense to stick a small 5 VDC or 12 VDC PSU in the control cabinet. The pump will have additional controls and protections: loss of prime, flow switch, over/under voltage, overload, output pressure too low, etc. Protection from overheating of the motor or pump-head is only one. Mike in California |
| floobydust:
Latching relays is pretty old tech. The better way to do it is simply monitor motor power and in software you can then tell if it's going to heat up from overload (pressure or jammed) or loss of load (running dry) has occurred. Especially if you know lift height, you can model the motor in MCU firmware to estimate the motor's temperature i.e. power verses run time. Or if lazy, just buy a modern pump protector that uses true power. https://www.loadcontrols.com/wp-content/uploads/2015/02/Monitoring-Pumps.pdf |
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