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A centrifugal switch uses centrifugal force to open or close electrical contacts when a machine reaches a set rotational speed. It consists of spring-loaded weights and switch plates, and is commonly used in starter circuit breakers and split-phase electric motors.
A centrifugal switch is a device designed to open or close a series of electrical contacts when the machine to which it is connected reaches a predetermined rotational speed. These devices use a phenomenon known as centrifugal force to achieve this switching action. The most common uses of centrifugal switches are starter circuit breakers on gasoline engines and starter 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 with the motor shaft or rotor and the other to be static.
If a bucket of water is rotated rapidly in a 360 degree arc, no water will spill out of the bucket as long as the rotation is maintained. If a heavy ball at the end of a rope is similarly rotated, the ball rises until it reaches the level of the center of rotation. A fast cornering car will also tend to swerve towards the outside of the corner. All of these effects arise from centrifugal force which dictates that inertia in combination with rotation will always force a rotating object away from the center of rotation. This effect forms the core of the centrifugal switch operation.
A centrifugal switch typically consists of two parts. The first is a set of spring-loaded weights connected to an actuator and an electrical switch plate. This assembly is mounted on the crankshaft of an engine or on the rotor of an electric motor. The second is a static electrical switch plate mounted in direct contact with the rotary switch plate when the engine or motor is stopped. These two switch plates represent a closed or active electrical circuit while the machine is not turning.
When the engine or motor begins to turn, the two switch plates maintain electrical contact until the machine reaches a pre-set rotational speed. At this point, centrifugal force begins to pull the weights away from the center of rotation causing the actuator to pull the rotary switch plate away from its static counterpart to break the circuit. Once the engine or motor is off and stops, the springs return the weights to their rest position to re-establish 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 engine has reached approximately 75% of its operating speed, then it will cut off electrical power to the starter winding. This allows the motor to run using only its run winding. Centrifugal switches are also used in self-starting gasoline engines, where they keep the electric starter running until the engine reaches a pre-set speed.
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