Over-temperature circuit for water pump controller - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: March 18, 2020, 02:59:54 am
content: 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

author: duak
author_rank: Super Contributor
timestamp: March 18, 2020, 03:46:26 am
content: 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.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2020, 05:23:18 am
content: 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 oneeBay auction: #261846873840and either roll your own 555/relay solution or buy one of these sorts of boardseBay auction: #201629716476

author: calzap
author_rank: Frequent Contributor
timestamp: March 18, 2020, 05:44:59 am
content: 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

author: floobydust
author_rank: Super Contributor
timestamp: March 18, 2020, 06:17:29 am
content: 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 modernpump protectorthat uses true power.https://www.loadcontrols.com/wp-content/uploads/2015/02/Monitoring-Pumps.pdf

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2020, 06:20:52 am
content: If you have a flow switch planned then drop the loss of prime switch (assuming you were planning to fit a diaphragm type) the LOP will be redundant - no flow no prime. Also these generally had a separate mechanical lever to hold in while pressure was established making them a huge PITA.Generally the KISS principle when it comes to pump and motor controls was the most reliable. Motors providing the pump was within operating parameters and not for example open pipe (maximum motor load condition) was near a zero failure condition. The only other exception to this was SWER (single wire earth return) fed rural supplies when brown outs could be an issue.

author: floobydust
author_rank: Super Contributor
timestamp: March 18, 2020, 06:57:28 am
content: It would help to know what type of pump - progressive cavity, centrifugal, piston etc. because they fail in different ways.

author: tautech
author_rank: Super Contributor
timestamp: March 18, 2020, 07:22:58 am
content: Quote from: beanflying on March 18, 2020, 06:20:52 amYep this ^With beans help I installed a flow switch and 2s 555 timer board to a new installation last year. Installed on the suction side it detects loss of prime (my primary concern) and when used in conjunction with the 555 set to 2 secs prevents any damage to the pump from being run dry.Phase failure and overload relays do the rest providing adequate insurance for a long trouble free life.For this installation that's wirelessly controlled we have a halogen spotlight for ON high on the power pole that we can see for some distance that also serves as an indicator if the pump was to lose its prime, Pump ON = light ON but if light goes OFF after 2s we know pump prime is lost.All the LV stuff is 12V cascading through cheap and cheerful relays to then switch the 440VAC mains.

author: calzap
author_rank: Frequent Contributor
timestamp: March 18, 2020, 07:40:03 am
content: Again thanks for the replies.Motors heat-up for reasons other than overload. For example, failure of the cooling fan. And pump-heads can heat-up for lots of reasons too ... bearing failure for one. So in addition to electrical load monitoring, heat sensors directly on the motor and pump-head provide additional insurance.The old tech is robust. The relays will get activated only if there is a problem ... hopefully, not very many cycles for them. Further, the plan is drawn-up, and the relays are on the shelf. I use computers and MCUs when needed ... not here.There is very little suction head. The water is drawn from a pond, the surface of which is less than a meter lower than the pump. Pump is two stage centrifugal.And both loss of prime and the flow switch are needed. I won't go into all the reasons. But one is that because the pump is pushing water into pressurized tanks. Flow goes down as output pressure increases. So the flow switch opens before cut-out pressure is reached. In addition, before the pump starts, the flow is obviously zero, but it's important that the pump have prime before it is started.I'm still struggling with the circuit for the flow switch. At very high flows, such as when a fire hose is being used, the output pressure drops so low that the pressure switch safety shuts-off the pump. At this point, the flow switch needs to substitute for the pressure switch. On the other hand the pressure switch needs to rule at the beginning and end of ordinary pump cycles. The circuit itself will be easy once I get the logic right.Mike in California

author: tautech
author_rank: Super Contributor
timestamp: March 18, 2020, 08:07:37 am
content: Drop your control circuitry to LV and many doors open.Series and NO and NC wiring and then many options are possible.I used a flow switch just like Bean linkedhttps://www.ebay.com/itm/261846873840?ViewItem=&item=261846873840wired NO that if it goes closed means the pump is flowing water but of course the NC 2s timer ensures the pump gets to start at power ON but if the flow switch doesn't immediately close the timer relay goes open and the pump turns OFF before any damage occurs. I could have easily set for 1s but didn't want it hiccuping.This is the timer I used:https://www.aliexpress.com/item/32954793505.html?spm=a2g0s.9042311.0.0.3da24c4dchjEzgFYI this is a 12 stage vertical centrifugal shifting 6+m3/hr from a 2m suction to 60m head.