Float Switch Control of a Pump and Pilot Lights (Circuit #3)

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  • Discuss the operation of circuit #3.
  • Troubleshoot circuit #3 using the schematic.

In circuit #3, a float switch is used to operate a pump motor. The pump is used to fill a tank with water. When the tank is low on water, the float switch activates the pump motor and turns a red pilot light on. When the tank is filled with water, the float switch turns the pump motor and red pilot light off, and turns an amber pilot light on to indicate that the pump motor isn't running.

If the pump motor becomes overloaded, an overload relay stops the pump motor only.

The requirements for this circuit indicate that a float switch is to be used to control three different items: a red pilot light, a motor starter, and an amber pilot light.

However, most pilot devices, such as float switches, pressure switches, and limit switches, seldom contain more than two contacts. When the circuit requires these pilot devices to use more contacts than they contain, it's common practice to let a set of contacts on the pilot device operate a control relay. The contacts of the control relay can be used as needed to fulfill the requirements of the circuit.

The float switch in Ill. 1 is used to operate a control relay labeled FSCR. The contacts of the control relay are used to control the motor starter and the two pilot lights.

Ill. 1 Circuit #3. Float switch used to operate a control relay

Ill. 2 Warning light and pump motor have energized.

In the circuit shown in Ill. 2, current can flow through the normally closed FSCR contact to the red pilot light, and through a second normally closed FSCR contact to the coil of motor starter M1. When motor starter M1 energizes, the pump motor starts and begins to fill the tank with water. As water rises in the tank, the float of float switch FS rises also. When the tank is sufficiently filled, the float switch contact closes and energizes relay FSCR (Ill. 3).

When the coil of relay FSCR energizes, all FSCR contacts change. The normally closed contacts open and the normally open contact closes. When the normally closed contacts open, the circuits to the red pilot light and to coil M1 are broken. When motor starter M1 de-energizes, the pump motor stops. When the normally open FSCR contact closes, current flows to the amber pilot light. When the pump motor turns off, the water level begins to drop in the tank. When the water level drops low enough, the float switch opens and de-energizes relay coil FSCR. When relay FSCR de-energizes, all FSCR contacts return to their normal positions as shown in Ill. 1. If the pump motor is operating and the overload relay opens the overload contact, only the motor starter will be de-energized.

The pilot lights will continue to operate.

Ill. 3 Float switch energized FSCR relay.


(Refer to circuit 1.)

1. Explain the operation of the circuit shown in Ill. 1 if float switch FS were connected normally closed instead of normally open.

2. Explain the operation of the circuit shown in Ill. 1 if relay coil M1 were burned out.

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