Machine Design

Switch Tips: Centrifugal switches

Centrifugal switches typically serve as a means of turning on or off circuit functions depending on motor speed. The most widespread use of such switches is as a start winding cut-out for single-phase fractional-horsepower motors. In some clothes driers, the switches can also be found controlling dryer heating elements, allowing the dryer to switch on only when the drum motor is up to speed.

The basic operating principle of the switch is to use a speed-sensing mechanism that consists of a conical spring steel disc that has weights fastened to the outer edge of a circular base plate. Fingers on the spring attach to an insulating spool that rides free of the shaft.

As the motor rotations reach switching speed, the centrifugal force of the calibrated weights overcomes spring force. This causes a snap action that pulls the spool away from a button on a stationary switch mounted to the motor. When the spool pulls away from the button the action allows switch contacts to open. The reverse takes place when the motor shuts off. As rotational speed drops, spring force eventually overcomes the centrifugal force of the weights. The spring then snaps the spool back and depresses the button, closing the stationary switch.

Such mechanisms can detect motor speeds ranging from 500 to 10,000 rpm. To control switching points, the spring is precision made to a thickness tolerance of ±0.00025 in. Weights are also precision calibrated. Switches designed for two-pole motors generally use three weights, and units for lower speed motors tend to use six.

Manufacturers use such switches to handle different capacities and configurations by customizing the stationary portion of the device. For example, the stationary switch might contain different several sets of contacts each having different capacities, multipole switches, special-purpose contacts, and so forth.

Though different manufacturers may offer switches based on the same basic design principle, there can be differences between brands. For example, there can be differences in life expectancy if the spring snap action isn't crisp enough to quickly clear the spool away from the actuator button.

Finally, there are cases where solid-state controls may work better than mechanical centrifugal switches. These would include positioning applications where the motor is repeatedly jogged into position. Jogging would tend to wear out the switch. And switch contacts might be a source of reliability issues in extremely dusty environments. But solid-state approaches tend to be more costly than simple centrifugal switches.

Torq Corp., Bedford, Ohio, provided information for this article.

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