A centrifugal switch is a device designed to open or close a set of electrical contacts when the machine it is attached to reaches a pre-determined rotational speed. These devices make use of a phenomenon known as centrifugal force to achieve this switching action. The most common uses of centrifugal switches are starter cut-outs on gasoline engines and starting winding disconnectors in split phase electric motors. Centrifugal force is a product of rotational motion and requires one half of the centrifugal switch mechanism to rotate along with the crankshaft or rotor and the other to be static.
If a bucket of water is rapidly rotated in a 360-degree arc, none of the water will spill out of the bucket as long as the rotation is maintained. If a heavy ball on the end of a rope is rotated in the same way, the ball lifts until it is level with the center of rotation. A fast cornering car will also tend to drift towards the outside of the turn. All of these effects result from centrifugal force which dictates that inertia in conjunction with rotation will always force a rotating object away from the center of rotation. This effect forms the core of centrifugal switch operation.
A centrifugal switch typically consists of two parts. The first is a spring loaded set of weights connected to an actuator and an electrical switch plate. This assembly is mounted on the crankshaft of an engine or the rotor of an electric motor. The second is a static electrical switch plate mounted in direct contact with the rotating switch plate when the engine or motor is stationary. These two switch plates represent a closed or active electrical circuit while the machine is not turning.
When the motor or engine begins to turn, the two switch plates maintain the electrical contact until the machine reaches a pre-set rotational speed. At this point, centrifugal force begins to drag the weights away from the center of rotation causing the actuator to move the rotating switch plate away from its static counterpart to break the circuit. Once the motor or engine is switched off and stops, the springs pull the weights back into their rest position to reestablish the connection.
The centrifugal switch is most commonly used to energize the starting windings of split phase electric motors. The switch will maintain the connection until the motor has reached approximately 75% of its operational speed then switch the electrical supply to the starter winding off. This allows the motor to operate using its running winding alone. Centrifugal switches are also used in auto-start gasoline engines where they keep the electric starter turning until the engine reaches a pre-set speed.