Industrial Motor Control: Float Switches

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  • Describe the operation of float switches.
  • List the sequence of operation for sump pumping or tank filling.
  • Draw wiring symbols for float switches.

A float switch is used when a pump motor must be started and stopped according to changes in the water (or other liquid) level in a tank or sump. Float switches are designed to provide automatic control of AC and DC pump motor magnetic starters and automatic direct control of light motor loads.

The operation of a float switch is controlled by the upward or downward movement of a float placed in a water tank. The float movement causes a rod operated (Ill. 1) or chain and counterweight (Ill. 2) assembly to open or close electrical contacts. The float switch contacts may be either normally open or normally closed and may not be submerged.

Float switches may be connected to a pump motor for tank or sump pumping operations or tank filling, de pending on the contact arrangement.

Ill. 1 Rod-operated float switch.

Ill. 2 Chain-operated float switch with normally closed (NC) and normally open (NO) wiring symbols.

Ill. 3 Mercury bulb type float switch.

Mercury Bulb Float Switch

Another float switch that has become increasingly popular is the mercury bulb type of float switch. This type of float switch does not depend on a float rod or chain to operate. The mercury bulb switch appears as a rubber bulb connected to a conductor. A set of mercury contacts are located inside the bulb. When the liquid level is below the position of the bulb, it's suspended in a vertical position (Ill. 3A). When the liquid level rises to the position of the bulb, it changes to a horizontal position (Ill. 3B). This change of position changes the state of the contacts in the mercury switch.

Since the mercury bulb float switch does not have a differential setting as does the rod or chain type of float switch, it's necessary to use more than one mercury bulb float switch to control a pump motor. The differential level of the liquid is determined by suspending mercury bulb switches at different heights in the tank. Ill. 4 illustrates the use of four mercury bulb type switches to operate two pump motors and provide a high liquid level alarm. The control circuit's shown in Ill. 5. Float switch FS1 detects the lowest point of liquid level in the tank and is used to turn both pump motors off. Float switch FS2 starts the first pump when the liquid level reaches that height. If pump #1 is unable to control the level of the tank, float switch FS3 will start pump motor #2 if the liquid level should rise to that height. Float switch FS4 operates a warning light and buzzer to warn that the tank is about to overflow. A reset button can be used to turn off the buzzer, but the warning light will remain on until the water level drops below the level of float switch FS4.

The Bubbler System Another method often used to sense liquid level is the bubbler system. This method does not employ the use of float switches. The liquid level is sensed by pressure switches (Ill. 6). A great advantage of this sys tem is that the pressure switches are located outside the tank, which makes it unnecessary to open the tank to service the system.

The bubbler system is connected to an air line, which is teed to a manifold and to another line that extends down into the tank. A hand valve is used to ad just the maximum air flow. The bubbler system operates on the principle that as the liquid level in creases in the tank, it requires more air pressure to blow air through the line in the tank. E.g., a 1-square-inch column of water 26.7 inches in height weighs 1 pound. Now assume a pipe with an inside area of 1 square inch is 10 feet in length. It would require a pressure of 4.494 psi to blow air through the pipe.

120 in./26.7 psi = 4.494 lbs.

If the water level was 7 feet in height it would require a pressure of only 3.146 psi.

Since the pressure required to bubble air through the pipe is directly proportional to the height of the liquid, the pressure switches provide an accurate mea sure of the liquid level. The pressure switches shown in Ill. 6 could be used to control the two pump circuit previously discussed by replacing the float switches with pressure switches in the circuit shown in Ill. 5.

Ill. 5 Two-pump control with high liquid level warning

Ill. 6 Bubbler system for detecting liquid level.

Ill. 7 Operation of the microwave gauge.

Ill. 8 Cut-away view of a microwave level gauge.

Ill. 9 Microwave level gauge with meter.

Microwave Level Gauge

The microwave level gauge operates by emitting a high frequency signal of approximately 24 gigahertz into a tank and then measuring the frequency difference of the return signal that bounces off the product (Ill. 7). A great advantage of the microwave level gauge is that no mechanical object touches or is inserted into the product. The gauge is ideal for measuring the level of turbulent, aerated, solids-laden, viscous, corrosive fluids. It also works well with pastes and slurries. A cut-away view of a microwave level gauge is shown in Ill. 8.

The gauge shown in Ill. 9 has a primary 4 to 20mA analog signal. The gauge can accept one RTD (Resistance Temperature Detector) input signal. The gauge can be configured to display the level, calculated volume, or standard volume. A microwave level gauge with meter is shown in Ill. 9.


1. Describe the sequence of operations required to (a) pump sumps and (b) fill tanks.

2. Draw the normally open and normally closed contact symbols for a float switch.

3. What type of float switch does not have a differential setting?

4. What is the advantage of the bubble type system for sensing liquid level?

5. Assume a pipe has an inside diameter of 1 square inch. How much air pressure would be required to bubble air though 25 feet of water?


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