In the early 20th century, induction motors' design and switching mechanisms were quite simple. For instance, these motors had their motor switches within the motor frame, allowing engineers to produce controlled switching operations when the motor attains its required speed. However, this design was unsatisfactory as it caused an accumulation of dust, oil and grease on those switches, making switching operation unreliable.

Centrifugal switches were originally developed during the 1920s. They are mounted on the motor shaft and a fixed stationary switch, providing for better switching control than conventional switches once the motor reaches a specified speed.

But how do centrifugal switches work, why are these switches essential, and what are their applications today?

What is a centrifugal switch, and how does it work?

According to Newton's mechanics, centrifugal force describes an apparent outward force felt by a body rotating or moving in a curved path. This force acts outwardly and tends to drive the body away from the center.

A centrifugal switch is a type of switch that disengages once the centrifugal force generated from a rotating shaft reaches a certain RPM. It features a governor mechanism mounted on the motor shaft, which allows it to rotate along with the shaft.

Understanding how centrifugal switches work in single-phase motors

A single-phase induction motor typically consists of a single stator winding and auxiliary (or startup) winding. The auxiliary winding generates a rotating flux in conjunction with the single stator winding, producing the starting torque required to start the motor. However, this auxiliary winding must be removed from the motor circuit before the motor begins to induce voltage in the startup winding, which would damage the circuit or components, such as a capacitor. This can be achieved using a centrifugally operated switch (or centrifugal switch).

The accompanying video shows a typical centrifugal switch mounted on an electric motor. This device features three essential components: the switch contacts, the moving contact arm and the governor assembly.

The switch contacts are connected in series with the supply and startup winding, while the governor assembly is mounted on the rotating shaft. The governor assembly features a spring-loaded weight.

When the rotor is at a standstill, the pressure of the spring on the weight causes the contact arm to be closed. The startup winding circuit, which is connected across the supply terminals through the centrifugal switch, remains closed. Next, the auxiliary winding generates rotating flux, causing the motor to start.

As the motor starts and the shaft rotates, centrifugal force causes the spring-loaded weight to move outwards against the spring pressure. When the rotor attains about 75% of its synchronous speed, the contact moves, opening the switch contact and disconnecting the startup circuit.

What happens when the centrifugal switch malfunctions?

The effects of centrifugal switch malfunction can be costly. For instance, if a centrifugal switch does not open when needed, the startup winding might overheat and start a fire, or cause a capacitor to rupture. Likewise, if the centrifugal switch is not disconnected before the motor speed reaches 70% to 80% of the synchronous speed, it will cause an overcurrent through the startup winding circuit.

All of these malfunctions cause the failure of the startup winding and motor. Therefore, it is always recommended that engineers test centrifugal switches before using them for an application. As a rule, a centrifugal switch’s process must be consistent across its entire life cycle. The switch’s work cycle must be uniform between open and close states while being able to connect (or disconnect) easily.

Finally, product designers should opt for devices with switches that are accessible. This permits replacement, fixing or cleaning the switch without disassembling the entire device.

[Learn more about switches on GlobalSpec.com.]

What are other applications of centrifugal switches?

1. Centrifugal switches are used in split-phase motors to disconnect the motor’s starting winding circuit as the motor approaches its normal operating speed.
2. Centrifugal switches are also essential components in aircraft for regulating the ignition circuit, speed indication circuit and speed safety circuit.
3. Engineers will also find centrifugal switches in escalators, lifts and conveyors.

What type of split-phase motor does not generally contain centrifugal switches?

Two-value capacitor motors, a special type of split-phase induction motor, do not generally require a centrifugal switch to disconnect the power source from the starting winding. Instead, it uses two capacitors (the start and run capacitors) to operate the motor.

As its name implies, the start capacitor provides the required energy (and torque) necessary to start the motor. Once the motor reaches a specific speed, the run capacitor takes over, causing the motor to continue rotating.

Conclusion

Centrifugal switches are helping to solve many challenges in motors and other devices today. However, the performance of centrifugal switches primarily depends on engineers choosing the ideal centrifugal switch for a particular application. Therefore, engineers are advised to reach out to centrifugal switch suppliers to discuss their application needs.