This part of the Pump Tutorial Series. The 1st page is at Pumps: Selecting-a pump step-by-step.
The most basic type of pump control system is a simple plug and outlet. To turn the pump on you simply plug it int the outlet, to turn the water off simply unplug it. 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, similar to a light switch. 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 safe or 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 unpleasant shock. 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 prank 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 graphic 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 or 20 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.
Automating the Pump Control Circuit
To automate the control of the pump you simply replace the manual switch in the circuit with an automatic switch. There are several types commonly used.
Types of Automatic Pump Switches:
Variable Speed or “VFD” Drives. These are small computers that control the pump. They don’t just turn the pump on and off, they also control the speed of the pump motor. Variable speed drives are rapidly replacing all of the other methods listed below because they result in a higher operating efficiency for both the pump and the irrigation system. That means both lower electricity costs and better irrigation! The way a typical variable speed drive works is by using a electronic pressure sensor installed on the irrigation system pipe downstream of the pump. The sensor constantly measures the pressure in the irrigation system and sends that information to the computer. The computer then varies the speed of the pump up or down depending on the current pressure reading. You set the desired pressure on the computer. If the current pressure is lower than the desired pressure, then the computer speeds up the pump to increase the pressure. If the pressure is too high, the computer slows down the pump. Since computers can be programmed to do lots of things these systems often have all sorts of other features, they can often monitor lots of different types of sensors, temperature, flow, time of day, etc. and respond to those inputs. If connected online they can send notices like text messages if there is a problem detected with the pump or the pressure is too low and the pump can’t go any faster. New features show up all the time.
Pressure Switch. This is the old school system, used for decades for home well pumps. This system is inexpensive to buy, reliable, and time-tested. A pressure switch is installed on the pump output pipe and measures the pressure. The pressure switch turns on the pump when the pressure drops in the pipes, and turns it back off when the pressure gets too high. A typical installation will have a pressure tank which stores pressurized water so that the pump doesn’t cycle on and off rapidly due to pressure changes. The tank acts as a reservoir to stabilize the pressure in the system. Most pressure switches are adjustable, you can modify the on and off pressure settings within a limited pressure range, usually 20 to 45 PSI.
Irrigation Controller. Most irrigation controllers (the “timers” that turn on and off your irrigation system valves) have a pump start circuit built into them. You simply connect the wires that go to the switch in the diagram above to the controller pump start terminals, and the irrigation controller will turn the pump on whenever it turns on a valve. Do not hook up the controller to an existing relay circuit. In other words if your pump already has a low voltage starter circuit with a relay you will need to install a new one for the irrigation controller. The controller has its own 24 volt AC power source, so if you hook it up to an existing circuit the irrigation controller may be damaged. You can buy ready-to-go relays that connect your irrigation controller to the pump. If you don’t understand electrical circuits get an electrician to wire it for you!
Flow Switch. Flow switches are sometimes used on booster pumps. When the switch detects flow in the pipes it turns on the pump. So if you open a sprinkler valve, the water would begin to flow, and the pump would come on. This only works if the water is already pressurized and you want to use the pump to increase the pressure. An example would be a sprinkler system hooked up to the local water company pipes that needs more pressure than the water company provides. Flow switches seem to have rather short lives and only last a few years in service, so it you use this type of switch be prepared to replace it often.
Timer. Any electric timer can be used to turn the pump on and off at predetermined times. Just a step up from the plug or a manual on/off switch. A relay is used to start and stop the high voltage pump using a low voltage timer, like a standard irrigation controller.
This is the end of the pump tutorial series.