The most commonly used motor in the world is the Induction Motor or Asynchronous Motor. It is a Motor that can work without any electrical connection to the Rotor. This post will discuss Induction Motor (Asynchronous Motors), it’s types i.e. single phase, three phase, squirrel case, slip ring etc, Features, How it works, applications, advantages and disadvantages.
What is an Induction Motor (Asynchronous Motor)
An Induction Motor or Asynchronous Motor is the most basic and common type of electric motor which has only Armortisseur winding which means auxiliary winding only on the armature. In an Induction Motor ( or Asynchronous motor), the Stator part of the motor provides electromagnetic field with its winding, to the Rotor part of the motor. This generates an electric current in the Rotor. The electric current produces the torque which results in motion.
Fig. 1 – Introduction to Induction Motor (Asynchronous Motor)
It is referred to as “Asynchronous Motor” as it will always run at a speed less than it’s synchronous speed. Synchronous speed is defined as the speed of the magnetic field of a rotary machine which is again determined by the number of poles and frequency in the machine.
Since in this type of Motor, Rotor gets it flux and rotation by the magnetic field in the Stator, there is a lag between the currents in Stator and Rotor. Due to this, the Rotor never reaches its synchronous speed. Hence the term “Asynchronous Motor”. Fig. 2 shows parts of Asynchronous Motor.
Fig. 2 – Parts of Induction Motor (Asynchronous Motor)
Construction of Induction Motor (Asynchronous Motor)
It consists of mainly two parts namely:
It is the stationary part of an electric motor. This part provides the electromagnetic field required to drive the rotating part of the motor. It is made up of a number of stampings with slots to carry three-phase winding. Each winding is separated by 120 degrees from the other winding.
It is the rotating part of the motor. The more common type of Rotor in Induction Motors (or Asynchronous Motor) is the squirrel cage one. The Rotor is shaped as an anchor with a core which is cylindrical in shape. Around the core are parallel slots which carry the conduction. The core has a bar made of aluminum, copper, or alloy.
Fig. 3 – Basic Rotor and Stator
Types of Induction Motor (Asynchronous Motor)
It is classified into two types:
- Single Phase Induction Motor
- Three Phase Induction Motor
Single Phase Induction Motor
Single Phase Induction Motor is not a self-starting motor. Here motor is connected to a single-phase power supply which carries an AC current to the main winding. Since AC supply is a sinusoidal wave, it produces a pulsating magnetic field in the Stator winding.
Pulsating magnetic fields are two magnetic fields rotating in opposite directions; hence no torque is generated . So after supplying the current, Rotor has to be moved in either direction externally for the motor to start running. Single Phase inductor hence; can have different varieties depending on the device that is used to start the motor and they are:
- Split Phase Motor
- Shaded Pole Motor
- Capacitor Start Motor
- Capacitor Start and Capacitor Run Motor
Fig. 4 – Schematic Diagram of (a) Single Phase (b) Three Phase Induction Motor
Three Phase Asynchronous Motor (Induction Motor)
These are motors which do not require any external device like a capacitor, centrifugal switch or start winding for starting up. The working principle of this motor is based on three single phases which have a phase difference of 120 degrees between them. So the magnetic field which causes rotation will have the same phase difference between them, this will cause the rotor to move without any external torque.
To simplify this further, let us assume the three phases are phase1, phase2 and phase3. So first phase1 gets magnetized and Rotor starts moving in that direction, shortly after, phase2 would be excited and Rotor would then be attracted towards phase2 and then finally phase3. In this manner, the Rotor will continue to rotate.
They are further categorized based on the type of Rotor used:
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor or Wound Rotor Motor
Squirrel Cage Induction Motor
In this type, the Rotor is shaped like a Squirrel Cage hence the name. The Rotor is made of steel with very highly conductive metals like aluminum and copper on its surface. It is very easy to change the speed of this type of Asynchronous Motor by just changing the shape of the bars in the Rotor.
Fig. 5 – Squirrel Cage Induction Motor
Slip Ring Induction Motor or Wound Rotor Motor
This is also known as Phase Wound Induction Motor. Here the Rotor is connected to external resistance via slip rings. The speed of the Rotor is controlled by adjusting the external resistance. Since this motor has more windings than a squirrel cage Induction Motor, it is also named as Wound Rotor Induction Motor.
Fig. 6 – Slip Ring Induction Motor
Features of Induction Motor (Asynchronous Motor)
Following are the features of two different types of Induction Motors.
Features of Single Phase Induction Motor
- Here we will highlight some of the characteristics which are applicable only to single-phase induction motors:
- Single-phase induction motors are not self-starting and use single-phase supply for rotation.
- To reverse the direction of rotation in single-phase motors, it is best to stop the motor and change it else there is a possibility of damage to the motor due to inertia torque which acts against the direction to which you need to change the rotation.
- You would require a capacitor and/or a centrifugal switch to start the motor.
- Starting torque is low in these motors.
- They are mostly used at home or in domestic appliances due to low power factor and efficiency.
Features of Three Phase Asynchronous Motor
Below are listed some of the features of a Three Phase Asynchronous motor which makes it different from a single-phase motor:
- They are self staring motors and do not need special starters.
- There are three Single Phase lines with a phase difference of 120 degrees.
- This has a simpler connection and is more reliable than single-phase induction motors.
- Starting torque is higher in these motors, compared to single-phase motors.
- They are mostly used in factories and industries due to high power factor and efficiency.
How does an Induction Motor (Asynchronous Motor) Work
The phenomenon which makes Induction Motors or Asynchronous Motors work is quite interesting. The DC motors need double excitation to rotate, one to the stator and the other to the rotor. But in these motors, we need to give it only to the stator, which makes this unique. As the name implies, the working principle of this motor is based on induction. Let us take the series of steps that occur in making this motor rotate:
- Supply is given to the Stator windings, there is a flow of current and a magnetic flux is created.
- Winding in the Rotor is arranged in a way that each coil gets short-circuited.
- The Rotor’s short-circuited winding is cut by the Stator’s magnetic flux.
Fig. 7 – Working of Asynchronous Motor
As per Faraday’s Laws of Electromagnetic Induction, a magnetic field interacts with an electric circuit to produce EMF (Electromotive force). So based on this law, current starts flowing in the coils of the rotor.
- The current in Rotor generates another flux.
- Now there are two fluxes, one in Stator and another in the Rotor.
- Rotor flux lags with respect to Stator flux, which would create a torque in the Rotor in the direction of the magnetic field.
Applications of Induction Motors
The applications include:
- They are widely used in Mixers, Toys, fans etc.
- They are also used in Pumps and Compressors.
- Small Asynchronous Motors are used in Electric shavers.
- They are used in Drilling machine, Elevators, Cranes and Crushers.
- They are suitable for Textile mill drives and Oil extracting mills.
Advantages of Induction Motor
Following are some of the advantages of Asynchronous Motors:
- Highly efficient and simple in construction.
- Very rugged and can work in any kind of environment.
- Low maintenance as they do not have many parts like commutators or brushes.
- They can have very high speed without having to worry about being worn out, as they do not have brushes.
- They are easy to operate as there are no electric connectors to the Rotor.
- As they do not have brushes, there is no fear of sparks hence can be used in polluted or explosive environments.
- There is very less speed variation from low load to rated load.
Disadvantages of Induction Motor
The Induction Motors have simple construction which can have a few disadvantages like mentioned below:
- It is difficult to control the speed of an Induction motor hence cannot be used in places that require fine speed control.
- There is a drop in efficiency at low loads.
- They have high input surge currents which gives low voltage at the starting of the motor.
Also See: Youtube Videos on Induction Motors
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