Introduction to Centrifugal Switch

Catalog

I. Working Principle

A centrifugal switch is an electrical switch commonly used in induction motors and split-phase induction motors for the signal phase. During the 1920s, Royal Lee won a patent for the centrifugal switch. This switch is used to provide the controlled switching operation needed for motors when generating a specified motor speed. Earlier, before the invention of this switch, the motor switches were placed within the motor frame to simplify the construction of the motor switches. This design was quite unsatisfactory because it led to the accumulation of those switches of oil, dust, grease. This in turn rendered the operation of communication unreliable.

A typical centrifugal switch consists of a centrifugal mechanism mounted on a motor shaft and a fixed stationary switch.

Using electrical contacts, the centrifugal device mounted on the motor shaft rotates along with the shaft and is connected to the stationary switch to control the start-winding circuit in the induction motor.

This switch operates on the concept of centrifugal force, as the same implies. This is a fictional force that acts on the spinning bodies. 

When a body moves in a circular motion, according to Newton mechanics, a force is produced from the center of the circle that tends to drive the body away from the center. This force is known as the force of centrifuges. It is created due to the inertia of the body. On the body, this force works and pushes it away from the core. In washing machines, this principle is used as well.

A centrifugal switch is a type of switch and an electronic symbol may be used to represent it. An electronic symbol is a pictogram used to describe different electrical and electronic devices or functions, such as wires, batteries, resistors, and transistors, in the schematic diagram of an electrical or electronic circuit.

In electrical engineering, a switch is an electrical function that can disconnect or link the conducting route in an electrical circuit, interrupt or redirect the electrical current from one conductor to another. 

A switch that is controlled by shaft rotation is a centrifugal switch. By opening only at increasing speed, it is receptive to velocity or direction.

II. Functions

Centrifugal Switch in Induction Motors

To understand the function of this switch in induction engines, let us first understand the induction engine model. A single stator winding and auxiliary winding consist of induction motors. For the stator winding, a single-phase AC current is applied. But the single winding of the stator does not produce sufficient rotating fields to generate the starting torque. Therefore, it provides an auxiliary winding. 

This auxiliary winding produces a field that is out of phase with the stator winding field produced. The resulting field, therefore, induces a starting torque and starts the engine. If the engine is started, a pulsating field that does not include the filed stator is set up by the rotor.

The circuit that energizes the auxiliary winding has to be disconnected when the motor speed exceeds a specified percentage of the synchronous speed. For induction motors, this is where the centrifugal switch comes into the frame. Here, the centrifugal switch helps to open and detach the auxiliary winding circuit.

Functions

Take a look at how in an induction motor the centrifugal switch works. A spring dish with calibrated weights attached to its base, backed by a steel plate, is the centrifugal mechanism mounted on the motor shafts. To provide the auxiliary winding with the necessary power to generate the starting torque, the switch contacts are closed. 

The calibrated weights feel centrifugal strength as the rotor rotates. The switch contacts are opened due to the centrifugal force at a certain speed as this force overcomes the spring force of the disc. The weights are shifted away from the rotor shaft here, thereby disconnecting the circuit from the auxiliary winding. 

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

- The spring force reduces at a linear rate.

- The centrifugal force increases at a rate proportional to the speed of the rotor.

- The radius of the weights is increased.

How to test:

1. The following criteria should be met by an ideal centrifugal switch:

2. Throughout its life cycle, the process should be uniform. For simplicity of design and low production cost, the number of components of the equipment should be minimal. 

3. It should have marginal elements of friction. 

4. Cut-out /Cut-in ratio should be changeable quickly without causing any significant design modifications. 

The switch is readily accessible as the communication unit of the switch is present on the outside of the motor frame. So, without dismantling the motor assembly, the switch can be tested, washed, and replaced.

III. Applications

This switch is often used in systems where speed detection in systems is necessary for the computer to be safe and operate properly. The following are some of the uses of the centrifugal switch: 

1. Defense against Overspeed in motors, generators, etc. 

2. Used for DC generators, conveyors, escalators, lifts, etc... 

3. These are also used in devices such as blowers, fans as well as conveyors to detect under-speed. 

4. Material losses are often used in systems where the loss of speed could lead to device damage.

The centrifugal switch offers good contact reliability compared to the switches used earlier for the same reason. As this switch is not present inside the motor frame, it is separated from dirt, grease, oil, which normally gets through circulating air currents into the motor frame.