pumpwater-pressure
I have a water pump that fills up water tank on roof top. I have to turn on water pump manually every time the water supply is on and then have to turn it off when supply is off to prevent dry running pump.
I want a switch that can turn on/off the pump automatically sensing water supply. What that kind of automatic controller switch is called and how to use it ?
There are many different solutions depending on how much you'd want to spend, how much DIY you want to do, and what exactly you are trying to achieve. I would not try to directly switch this load: inductive loads generate voltage spikes when switching them, which can damage electronics and such that are not designed to handle it. They also have high in-rush startup currents which are beyond the normal ratings.
On the cheapest, very DIY side, I would use a definite purpose contactor for each pump (~$20, give or take), and run some smaller (14/2) wires inside to a switch.
The benefit here is the wiring going inside is low-current, and so you can use a regular lightswitch, count-down timer or programmable timer.
Insteon is another way to go. Insteon is a power-line carrier signal, which means it runs over your existing wiring. They have a high-current relay module which would be used to control your pumps.
You can then control from any Insteon-capable controller: there are a number of keypads, light switches, timers, home automation controllers, key chain remotes, motion sensors, and PC interfaces. The benefit of this is your "remote" can be anywhere (and you can even have multiple remotes). For example, here is a 6-button keypad that would work:
You could have a timer/controller set up to run your pump at certain times of day, or have a motion sensor that runs it if someone is in the pool area, or a button to manually trigger it. With a controller you can do very complex things, like have it run if someone is in the pool area for more than 30 minutes, or if it's manually triggered, and also ensure that regardless of that, it runs for at least 4 hours every day. Heck you can go crazy fancy and have it run more after it rains, or after it's particularly windy (to get leaves out).
I've linked to smarthome.com (who is the manufacturer of Insteon) but you can also buy their stuff from many other places, including Amazon.com.
There are equivalents to Insteon in other technologies (both wireless and power-line carrier) such as Z-Wave, UPB and X10 (though X10 is very poor technology, I would recommend staying with something newer).
You can also combine. For example, there is a UPB dry-contact output which you could wire up to a definite purpose contactor.
The current setting is to pump up to 90 psi and shut off. When the pressure drops to 45 it will turn back on and pump to 90. If you wand a more even pressure reduce the differential pressure. Reducing the differential to 20 the pressure will stay between 90 & 70. this will cause the pump to cycle more often than its current settings. If the pump is cycling rapidly the pressure tank may have lost its air charge.
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Edit: I think I misread your question, your only water pressure is from gravity? I assumed your water supply was pressurized. You want something to sense the water level in the tank to cycle the pump, not pressure or flow related.
Sensing water flow is more complicated than just sensing water pressure. A SPDT water pressure switch can be installed at the water pump inlet to power the pump when it senses water pressure at the pump, and cuts power if it doesn't sense water pressure. This way the pump always runs when the water supply is on, and never runs when the water supply is off.
Ideally you'd have an electric circuit that opens a solenoid water valve for your supply and powers the pump at the same time, being activated by a water level float or sensor in your tank.
I use the same switch for my reverse osmosis water filter. I have an AquaTec SPDT pressure switch that prevents the booster pump from running if there is less than 20 psi supply water pressure. That way it never runs if there is a problem with the supply, because it would run nonstop and eventually burn out the booster. Technically it wouldn't run dry, because air would have to be introduced into the system first which would be unlikely.