When I was a child, my parents owned a one room fishing cabin on the banks of the Rogue River in the town of Shady Cove, Oregon. This was a pretty primitive place, and our source of water was a small 120 volt electric pump in the river. On the inlet side of the pump a 5′ long garden hose extended into the river, with a piece of window screen on the end to keep out most of the fish. The pump outlet was connected to a standard garden hose that ran up the bank from the pump to the cabin area. When we needed water we simply plugged in the extension cord that ran to the pump and moments later, after a few gurgles and burps, the water would start flowing from the hose. To turn the water off we simply unplugged the cord. This is an example of the most basic of pump control systems, and yet all pump controls are no more than a variation on this same procedure. The pump is started and stopped by turning on or off the power supply!
The next step up in pump control is to get rid of the plug and outlet and use a permanent switch to turn on and off the power flow. Some small pumps are stopped and started in exactly this way. But with the higher voltage and amperes commonly used for irrigation pumps a simple switch will not be practical. The switch would have to be huge to handle the load. Add to this the safety problems that develop when you add water to the equation. While 110 volts can kill you, contacting it with dry hands will likely only give you a bit of a “bite”. Grab that same voltage with wet hands and there is a good chance your heart will stop, or at the least you will wish you had stayed in bed that day! Water makes your skin a much better conductor of electricity. Remember that little childhood trick your friends pulled, involving the 9-volt battery and your tongue? If you press the battery terminals on your skin nothing much happens. But lick your skin before trying it (I don’t recommend sticking the battery terminals directly on your tongue!) and you will hear and possibly feel the spark jump! That’s what water does with 12 volts and a fraction of an amp. Believe me when I tell you that the 24 volts and 2.5 amps put out by most automatic irrigation controllers will make you say some pretty unnice things if you grab the wrong two wires with wet hands. Imagine, then, the effect if you increase the voltage to 240 volts and 15 amps! (By the way, it’s the amps that kill you, not the voltage!) For this reason most pump control circuits are designed using a relay circuit that isolates the user from the pump voltage. The relay circuit is like a messenger. You tell the relay to start the pump and the relay starts it for you. Most relays use 12 or 24 volts, a few use 120 volts. For example, the controls for spas usually use a 24 volt relay circuit because the manufacturers know you’re going to be turning them on and off with wet hands.
Source: http://www.irrigationtutorials.com
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