History of Single Phase Induction Motor: Its Construction, Working principle and Types

 

Jacobi was a remarkable success when he first invented the first-ever electric motor in May 1834. After four years, he repurposed his motor and used it to pull a boat with 14 people in it. Later, many researchers worldwide started to build motors that were similar to Jacobi's original one. Eventually, many years later, it was Nikola Tesla who invented the first single-phase induction motor while conducting experiments in 1887. Today, we can find the application of single-phase induction motors in many appliances, including fans, dishwashers and washing machines. 

 

What is a Single-Phase Induction Motor?

A motor is a device that converts electrical energy into mechanical energy to perform functional tasks. Motors can be classified into two broad categories: AC motor and DC motor. The single-phase induction motor is an example of an AC motor that uses a single-phase alternating current (a current that changes the direction, polarity, and magnitude). The single-phase induction works on the principle of electromagnetic induction. 

 

Construction of Single-Phase Induction Motor

The construction of a single-phase induction motor is very simple, with two main components, a stator and a rotor. As the name suggests, the rotor is the rotating part, and the stator is the stationary part. Both the stator and rotor have windings of wire that produce a magnetic flux or field when the current passes through it. If you look at the rotor, you can find that it is slightly skewed. The rotor bars are skewed to reduce the noise and vibrations. The rotor lies in the core of the stator, and the stator is laminated to reduce the eddy current loss.

Working Principle of Single-Phase Induction Motor 

The single-phase induction motor works on the principle of electromagnetic induction. Electromagnetic induction is a phenomenon that occurs in a conductor whenever it is placed in a changing or moving magnetic field. Due to electromagnetic induction, an electromotive force is developed in the conductor.

To understand the working principle, you must understand two crucial facts in physics, Faraday's first and second laws.

Faraday's first law states, ' Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced'.

Faraday's second law states, ' The induced emf in a coil is equal to the rate of change of flux linkage.'

 

 

We know that the alternating current varies in nature, providing the necessary flux to induce an emf. 

When we switch ON the motor, an alternating current starts to flow through the stator windings. The single-phase alternating current generates a magnetic flux which is changing in nature. An emf is induced in the rotor winding placed at the stator's centre. The direction of the induced emf can be explained with the help of Lenz law. Lenz law states that ‘the emf induced in the conductor is equal in magnitude and opposite in direction, and the conductor generates a magnetic flux, which opposes the cause that produced it.’ 

We can conclude that the induced emf in the rotor initially tries to oppose the stator's changing magnetic field. When a starting torque is given to the rotor, the rotor starts to rotate along with the stator coil's changing magnetic field. But, what is the starting torque? Torque is the force that is needed to make machinery rotate. At the initial condition, the torque of the rotor is zero. A starting torque has to be applied externally to make the rotor rotate.

 

Why does Single-Phase Induction Motor Need a Starting Torque?

Single-Phase induction motor needs a starting torque because they are not self-starting. This can be explained by another fact called the double-revolving field theory. According to the double-revolving theory, the magnetic field created by single-phase AC in the stator can be resolved into two components. These two components are equal in magnitude and are opposite in direction. Because of this effect, the rotor starts to vibrate instead of rotating. This is the reason why a single-phase induction motor is not self-starting. 

To overcome this, another winding is added to the stator. Thus, the stator has two windings, one is the main winding, which produces the magnetic flux, and the other is the auxiliary winding.

 

What is Auxiliary Winding?

An auxiliary winding is the stator's secondary winding to achieve the rotor's initial torque required to rotate. Usually, an auxiliary winding works until the rotor reaches up to 80% of the full speed, after which a centrifugal switch disconnects it. There are many ways to create the starting torque with the help of auxiliary winding based on which a single-phase induction motor is classified into five types.

 

Types of Single-Phase Induction Motor

 

1. Split-Phase Induction Motor

The auxiliary winding is placed perpendicular to the main winding in a split-phase induction motor. The auxiliary winding has fewer turns and is resistive, while the main winding has more turns and is inductive. Since the auxiliary winding is resistive, the current flowing through it is in phase with the input voltage. Due to the inductive nature of the main winding, the voltage lags. Thus a phase difference is achieved between the flux produced by auxiliary winding and main winding, sufficient to provide the starting torque. After the motor reaches up to 75% of its full speed, the centrifugal switch opens and disconnects the auxiliary winding. This motor requires a high starting current, about 7-8 times equivalent to the motor’s requirement for running at full load.

The phase difference is very small; hence the starting torque achieved by this method is also small. Therefore, you can find split-phase induction motors in applications that require low starting torque, like blowers, fans, etc.

 

2. Capacitor Start Induction Motor

The capacitor start induction motor is an advanced version of a split-phase induction motor. In this, you can find a capacitor kept in series with auxiliary winding to give the necessary phase difference. The phase difference achieved is almost equal to 90 degrees which is the maximum phase difference that can be obtained. Hence, the starting torque is also high, up to 300% of the torque in a full load condition. The capacitor and the auxiliary winding get disconnected when the motor achieves 80% of the full speed.

Due to its high starting torque, you can find this type of motor in applications that require high starting torque, like lathe and compressors.

 

3. Capacitor Start Capacitor Run

As the name says, two capacitors are used in this motor, one is the starting capacitor, and the other is the running capacitor. The starting capacitor has a very high capacitance value, and the running capacitor has a low capacitance value. The starting capacitor is disconnected from the auxiliary winding with the help of a centrifugal switch that is in series with it. The two capacitors are connected parallelly to each other, and they are connected in series with the auxiliary winding. The running capacitor is permanently connected to the circuit. The motor's initial torque and efficiency are high, and you can find this type of induction motor in conveyor belts and pumps.

 

4. Permanent Split Capacitor Induction Motor

In a permanent capacitor induction motor, the auxiliary winding remains in the circuit throughout the motor's running. The same capacitor acts as the starting and running capacitor and has a low capacitance value. This motor has no centrifugal switch to disconnect the auxiliary winding from the circuit. The initial torque achieved in this motor is not as high as the capacitor start induction motor. It is used in applications that require a moderate starting torque of 80% to 100% of the motor's full torque. You can find this motor in heaters, ceiling fans, and exhaust fans.

 

5. Shaded Pole Induction Motor

 

 

The shaded pole induction motor has a different configuration, and there is no auxiliary winding. Instead of auxiliary winding, a shaded ring is used to create the phase difference. The shaded ring and the stator winding are wound on the same poles, but the shaded ring is shaded from the stator winding. The shaded ring is highly inductive; hence when current is passed through the stator winding, an emf is induced in the shaded ring. According to Lenz law, the induced emf in the shaded ring would be opposite in direction and oppose the main flux created by the stator winding. Thus, a phase difference is created between the main flux and the flux created by the shaded ring. This is how a shaded pole induction motor works. The shaded pole induction motor has low starting torque, and you find it in toys, radios, table fans, and other small devices.

It is quite interesting to know about motors and how they function. The invention of the single-phase induction motor and the next invention, the three-phase induction motor, have bought a huge revolution. The application of a single-phase induction motor has a wider scope since we use a single-phase current for home appliances. It can be found in fans, electric shavers, mixers, and many industrial applications.

It is the responsibility of the mechanical engineer and electrical engineer to select the motor for the particular drive based on the power required. The modern electric vehicle we see on roads uses a DC motor for its transmission system. A DC motor has a different configuration with a principle.