Can a moisture sensor relay be connected to 5HP centrifugal pump (3phase)

Our project is about automatic switching on and off a centrifugal pump of 3phase input, based on the moisture content of soil.
So we have selected a product from online. But we are not sure whether we can connect it’s relay directly to the pump… Can any one suggest a best way.
This is the product we selected,eto:10486653559732911773_0,pid:10486653559732911773&sa=X&ved=0ahUKEwjP1Jqdvbv1AhVeSGwGHYr-Ct0Q9pwGCAU

Something seems a bit off with the link -

As a start, I know many moisture sensors have (or can be connected to a microcontroller to create) a logic-level output which commands “pump on” or “pump off”.

However, I’m not certain what the correct method for attaching one or more relays inline with the pump power supply is to correctly and safely control its motion.

If you have a pump run from a 3-phase input, you are dealing with fairly high power levels. It would be best that you get someone with the appropriate expertise in working with 3-phase power and pumping applications to work with you in-person. There are likely to be a lot of considerations that you have not mentioned in your post and that you are not aware are significant so advice from this forum will not be able to properly address them.

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you probably need a “contactor”, rated to be used with a 3 phase, 5 hp motor. Then you have to select one that you can drive with the power that the sensor relay is capable of switching.

without a valid link to the sensor you have in mind, it’s not easy to answer.

And besides, this is a robotics competition forum, so your question is off topic. Might see if you can find an industrial controls forum to ask? although I expect there is probably someone here who can help you find the answer you need.


This is a bad idea. You should hire an expert.

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A three phase motor will need a starter or contactor to handle the Voltage and currents involved. A Variable Frequency Drive is an alternative, and can allow you to power the motor off single phase if you need it. The VFD can easily accept a logic or relay input!
You need to make sure you don’t turn the motor on and off every couple seconds or, worse, allow the contacts to chatter rapidly on and off. This will fry your hardware and blow breakers.
A three phase pump is going to move a lot of water!!!


If you going to switch a 3 phase pump directly, the sensor will need three contacts rated for the motor load. Very few sensors will have that. The standard way of doing this is to use a contactor (A relay designed for switching heavy loads) You will also need overload and short circuit protection for the motor.

It depends on the pump. I used mostly 3 ph motors because it is more convenient than making single phase. It also helps to keep my power factor balanced even that is not really a consideration for small motors.

You can describe a technical solution easily enough but the question posed demonstrates a complete lack of knowledge in the field. Which, three phase power is dangerous and any “solution” to their problem isn’t complete without considerations for safety - arc flash especially. The worst thing that we can do is send them off to go buy a drive off ebay and they install it out in the open and someone gets hurt.

I agree. The screw up potential for any line connected 3 phase system includes plenty of “and somebody dies”!

I might hope my response could help shape a discussion with a pro, or that this is a paper only design… our school does a lot of design problems where the output is CAD and PowerPoint!

On the pumping side, it’s difficult to find something even as small as a 1/3 hp 3 phase pump (other than coolant pumps for machine tools). 1 hp is sorta the default starting point, in my experience… Single phase is used for allmost all the small pumps. That means a big mess with kids involved :wink: Of course, kids involved -normally- means a mess…

@194g1a0326 to be very clear, I do agree with the folks above.

You’re in a position where you don’t know what you don’t know.

To fix this, your task is not “find an answer and do it”. Your task is “learn why certain answers are correct or incorrect”.

It seems clear the question itself isn’t directly related to FIRST robotics, unless you’re actively making plans for a water game none of us know about :slight_smile: . While there’s a lot of folks here with both technical experience and desire to teach, you’ll likely not receive the correct level of assistance, since no one has context to what you’re really doing.

If this is a school-related project, I’d poke the professors for help. A crash course in embedded electronics and design, as well as relevant constraints related to safety in your particular situation would likely be far more helpful than the strangers on the internet.

If this is a work-related project, try poking a coworker who has done something similar before. If you don’t have one of those… pull out the yellow pages to look for electrical contractors in your area.

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There are a lot of issues related to implementing the solution that people on this forum cannot teach you, mostly relating to safety.

It is probably best if we stop offering “solutions” other than get in touch with an expert in-person. Otherwise, it is likely that we will encourage the OP to “wing it” and hurt someone.

I also get the sense that “make it work” / cobble it together is what the OP is looking for. Which can be done, but no professional should help you do it without thoroughly understanding the environment or making sure the solution you implement is robust. And connect sensor X to component Y to component Z to the pump motor isn’t a robust solution.

The request should get the same reaction as someone asking chemistry advice on how to make a lot of nitric acid or asking programming advice on how to to get keyboard input without the application having focus. No, I don’t think there’s necessarily a nefarious purpose with any of the 3 requests, but if you don’t know how to do it then you’re also missing the requisite information that will keep you from electrocuting yourself, accidentally blowing something up, or unleashing malware on the world.

In this case, an automation engineer would be a good resource to seek out. Not saying you need a PLC, but you definitely need someone that works in the space of 3 phase power, enclosure design, sensing and controls to get you to a solution that works and more importantly is safe.

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Thanks for your responses. I will search for an expert to solve this…

So uh… what is this project for? I can’t be the only one who is curious now.

Actually our project is that we want to supply water to fields without human effort or Interference.
We need to supply water when ever moisture content in soil is less than threshold value.
So when moisture is less the pump must be automatically switched on. And when it reaches a preset sufficient value the pump must be switched off automatically…
This is all our project about.

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In my job I work with 3 phase 230/460 for air compressors, roller/chain conveyors, hydraulic pumps. I have designed, built, and programmed a few smaller panels.
I have disassembled, moved and reinstalled the following from one warehouse to another, making mechanical, electrical repairs as needed. High Speed Pallet Sorter - YouTube

Here are some of my recommendations, again always seek trained and qualified help when needed.

  1. If you can plan for some sort of plc, never know what may change down the road, like only wanting pump to run during evening or night for example.
  2. VFDs(variable frequency drives as stated above allow for ramping up, this puts less strain of the pump and wires as it starts, also if you don’t need 5hp worth of output, or if pump keeps short cycling, you can reduce speed to lower flow. allowing pump to run longer each time, but cycle fewer times.
  3. Always have proper motor protection, Fuses usually before VFD of Contactor.
  4. a means to lockout the disconnect for maintenance
  5. Phase failure relay, VFDs usually perform this function. When properly connected they immediately stop and prevent pump from running if one of the 3 phase legs fails.
  6. some way to alert when pump trips out, and possibly when its running
  7. Easily assessable Emergency stops, if panel and pump are not close one at panel and one by pump. (If pump is down a well than one near top of wellhead.
  8. A way to monitor if pump has enough water entering it
  9. Pumps only work one way(Most) and can be damaged if spun the wrong way

9b. Depends what’s attached to the pump / motor driving the pump, if the pump is pushing a column of water up make sure to not restart it if it is backspinning (I accidentally did this one once under a weird set of circumstances, was about a second away from destroying a 4160V motor… always test your logic before going live).

  1. Include thought about where the panel will go. If it’s exposed to the elements, make sure it’s in an enclosure rated for it. If large temperature swings, make sure the components are rated for that as well or figure out how to control environment.

  2. Make sure to size conductors correctly for the load & distance the load travels

  3. A sensor you found for cheap might be more expensive to integrate than one made for automation (4-20mA sensor will be easier to integrate with this stuff than a 3-5VDC sensor)

  4. Manual / override controls

  5. Indicator / warning that system is about to run automatically.

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