Schematics and wiring diagrams:Float switch control of a pump and pilot lights

Float switch control of a pump and pilot lights

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 is not 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 is 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 Figure 21 – 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.

In the circuit shown in Figure 21 – 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 (Figure 21 – 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 Figure 21 – 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.