What is a Centrifugal Switch and Its Working

A Centrifugal switch is an electrical switch usually found in the signal phase induction motors and split-phase induction motors. Patent for the centrifugal switch was granted to Royal Lee during the 1920s. This switch is used to provide controlled switching operation which is required in motors when specified motor speed is generated. Earlier, before the invention of this switch, to simplify the design of motors switches were located inside the motor frame. This design was very unsatisfactory as it led to the accumulation of oil, dust, grease on those switches. This in turn made the contact operation unreliable.

What is a Centrifugal Switch?

A centrifugal switch works on the principle of centrifugal force. It is an electrical switch. These switches are specially designed for single and split phase induction motors. The Centrifugal switch is commonly known as ‘clutch’ as its working is similar to that of a centrifugal clutch used in automobiles.

Diagram

A typical centrifugal switch consists of two parts-

1. A centrifugal mechanism mounted on a motor shaft.
2. A fixed stationary switch.

The centrifugal mechanism mounted on the motor shaft rotates along with the shaft and is attached to the stationary switch using electrical contacts to control the start-winding circuit in the induction motor.

Working Principle

As the same suggests, this switch works on the principle of centrifugal force This is a fictitious force that acts on the rotating bodies.

According to newton mechanics, when a body moves in a circular motion, a force is generated from the center of the circle that tends to push the body away from the center. This force is known as centrifugal force. It is generated due to the body’s inertia. This force acts on the body and drives it away from the center. This principle is also used in washing machines.

Centrifugal Switch in Induction Motors

To understand the working of this switch in induction motors let us first understand the model of induction motors. Induction motors consist of a single stator winding and auxiliary winding. A single-phase AC current is applied to the stator winding. But the single stator winding cannot produce enough rotating field required to generate starting torque. Hence, an auxiliary winding is provided.

This auxiliary winding generates a field that is out of phase with the field generated by stator winding. Hence, the resultant field produces a starting torque and starts the motor. Once the motor is started, the rotor sets up a pulsating field which does not include the stator filed.

When the motor speed reaches a specified percentage of the synchronous speed, the circuit the energizes the auxiliary winding has to be disconnected. This is where the centrifugal switch comes into the picture for induction motors. Here centrifugal switch helps to open the circuit and disconnect the auxiliary winding.

The Function of the Centrifugal Switch in Induction Motors

Let us look at how the centrifugal switch operates in an induction motor. The centrifugal mechanism mounted on the motor shafts consists of a spring dish with calibrated weights attached to its base supported by a steel plate. The switch contacts are closed to provide the auxiliary winding with the necessary power to produce the starting torque.

When the rotor rotates, the calibrated weights experience centrifugal force. At a particular speed when this force overcomes the spring force of the disc, the switch contacts are opened due to the centrifugal force. Here the weights are moved away from the rotor shaft, thereby disconnecting the auxiliary winding from the circuit.

At the critical operating point, three factors can be seen:

1. The spring force reduces at a linear rate.
2. The centrifugal force increases at a rate proportional to the speed of the rotor.
3. The radius of the weights is increased

How to test?

An ideal centrifugal switch should meet the following conditions:-

1. The operation should be uniform throughout its life cycle.
2. The number of parts of the equipment should be minimal for the simplicity of the design and low cost of production.
3. Should have negligible frictional components.
4. Cut-out /Cut-in ratio should be changeable easily without causing any major design modifications.

As the contact unit of the switch is present on the exterior of the motor frame, the switch is easily accessible. So the switch can be inspected, cleaned, and can be replaced without dismantling the motor frame.

Applications

This switch is also used in systems where the detection of speed in systems is essential for the protection and proper working of the device. Some of the applications of the centrifugal switch are as follows:

1. Overspeed protection in engines, generators, etc.
2. Used in DC motors, conveyors, escalators, elevators, etc..
3. These are also used in detecting under-speed in devices such as blowers, fans as well as conveyors.
4. Also used in systems where the loss of speed could lead to damage of devices, material losses.

Compared to the switches used earlier for the same purpose, the centrifugal switch provides good contact reliability. As this switch is not present inside the motor frame it is isolated from dirt, grease, oil which usually gets into the motor frame by circulating air currents. What are the units of centrifugal force?