Functions of Motor Control:Selector Switches

Selector Switches

Selector switches are operated by turning a knob instead of pushing a button. A very common selector switch is the MAN-OFF-AUTO switch. MAN stands for Manual and AUTO stands for Automatic. This is

a single-pole double-throw switch with a center off position, as shown in Figure 2 – 29. When the switch is in the OFF position, as shown in Figure 2 – 29A, neither indicator lamp is turned on. If the switch is moved to the MAN position, as shown in Figure 2 – 29B the red lamp is turned on. If the switch is set in the AUTO position, Figure 2 – 29C, the green lamp is turned on. Another symbol often used to represent this type of switch is shown in Figure 2 – 30. A combination START-STOP push-button station, pilot lamp, and HAND-OFF-AUTO switch is shown in Figure 2 – 31.

Selector switches often contain multiple contacts and multiple poles (Figure 2 – 32). A symbol used to represent a selector switch with three poles, each having three terminals, is shown in Figure 2 – 33. This selector

switch contains a common terminal for each of the three poles. The common terminal is connected to the movable contact. A different type of selector switch is shown in Figure 2 – 34. Switches of this type are often supplied with a chart or truth table indicating connections between contacts when the switch is set in different positions. In this example, there is no connec- tion between any of the contacts when the switch is set in the OFF position. When the switch is set in position A there is connection between contacts 3 and 4, and 5 and 6. When the switch is set in position B there is connection between contacts 1 and 2, 5 and 6, and 7 and 8. It is not uncommon to see a combination of selector switches, push buttons, and meters mounted on a single control panel (Figure 2 – 35).

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