In a typical model of a crane, there may be three motors which need to be controlled independently; one motor that rotates the jib, another that raises and lowers the jib, and another that raises and lowers the hook.
Each motor can be controlled by a switch that has three positions for forwards, off, and reverse. Switches of this type are called Double Pole Double Throw (DPDT) switches with centre-off position.
Types of DPDT Switch
DPDT switches are available in 'toggle' or 'slide' types, as shown in the picture below. Both types have six connections - three for each pole of the switch.
The ideal switch for this application (a crane) would have a momentary, or non-locking, action - it would automatically switch back to the centre-off position after the operator has released the switch. These switches are normally a little more expensive.
Connecting up a DPDT Switch
The circuit in figure 1 shows how to connect one DC motor to a DPDT switch.
Connections A, B, and C form one pole of the switch and connections D, E, and F form the other. Connections B and E are the 'common' connections for their respective poles. Figure 2 shows figure 1 as a circuit diagram to illustrate this - note the dotted line between the switch actuators which shows that they are 'ganged' and therefore move together.
How it Works
If the positive power supply (Vs) enters at connection B, and the switch is set to the topmost position, connection A becomes positive and the motor will turn in one direction.
If the switch is set to the lowermost position, the power supply is reversed and connection D becomes positive. The motor will turn in the opposite direction.
In the centre position, the power supply is not connected to the motor and it does not turn.
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