An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor’s rotor can be either wound type or squirrel-cage type.
Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans.
Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives (VFD) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel-cage induction motors are very widely used in both fixed-speed and variable-frequency drive applications. Keep reading this article with PK Halder to understand how induction motor works.
Synchronous speed is the speed of rotation of the magnetic field in a rotary machine, and it depends upon the frequency and number poles of the machine. The induction motor always runs at speed less than its synchronous speed.
The rotating magnetic field produced in the stator will create flux in the rotor, hence causing the rotor to rotate. Due to the lag between the flux current in the rotor and the flux current in the stator, the rotor will never reach its rotating magnetic field speed (i.e. the synchronous speed).
There are basically two types of induction motor. The types of induction motor depend upon the input supply. There are single phase induction motors and three phase induction motors. Single phase induction motors arenot a self-starting motor, and three phase induction motor are a self-starting motor.
Working Principle of Induction Motor
We need to give double excitation to make a DC motor to rotate. In the DC motor, we give one supply to the stator and another to the rotor through brush arrangement. But in induction motor, we give only one supply, so it is interesting to know how an induction motor works.
It is simple, from the name itself we can understand that here, the induction process is involved. When we give the supply to the stator winding, a magnetic flux gets produced in the stator due to the flow of current in the coil. The rotor winding is so arranged that each coil becomes short-circuited.
The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited, according to Faraday’s law of electromagnetic induction, the current will start flowing through the coil of the rotor When the current through the rotor coils flows, another flux gets generated in the rotor. Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the direction of the rotating magnetic field. This is the working principle of both single and three-phase induction motors.
Types of Induction Motors
The types of induction motors can be classified depending on whether they are a single phase or three phase induction motor.
Single Phase Induction Motor
The types of single phase induction motors include:
- Split Phase Induction Motor
- Capacitor Start Induction Motor
- Capacitor Start and Capacitor Run Induction Motor
- Shaded Pole Induction Motor
Three Phase Induction Motor
The types of three phase induction motors include:
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor
We have already mentioned above that the single-phase induction motor is not a self-starting motor, and that the three-phase induction motor is self-starting. So what is a self-starting motor?
When the motor starts running automatically without any external force applied to the machine, then the motor is referred to as ‘self-starting’. For example, we see that when we put on the switch the fan starts to rotate automatically, so it is a self-starting machine.
Point to be noted that fan used in home appliances is a single-phase induction motor which is inherently not self-starting. How? Does a question arise as to how it works? We will discuss it now.
Why is Three Phase Induction Motor Self Starting?
In a three phase system, there are three single phase lines with a 120° phase difference. So the rotating magnetic field has the same phase difference which will make the rotor to move.
If we consider three phases a, b, and c when phase a gets magnetized, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetized and it will attract the rotor, and then phase c. So the rotor will continue to rotate.
Why Single Phase Induction Motor is not Self Starting?
It has only one phase still it makes the rotor to rotate, so it is quite interesting. Before that, we need to know why a single phase induction motor is not a self-starting motor and how we overcome the problem. We know that the AC supply is a sinusoidal wave and it produces a pulsating magnetic field in the uniformly distributed stator winding.
Since we can assume the pulsating magnetic field as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting, and hence the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by an external force, then the motor will start to run. We can solve this problem by making the stator winding into two winding – one is the main winding, and another is auxiliary winding.
We connect one capacitor in series with the auxiliary winding. The capacitor will make a phase difference when current flows through both coils. When there is a phase difference, the rotor will generate a starting torque, and it will start to rotate.
Practically we can see that the fan does not rotate when the capacitor gets disconnected from the motor, but if we rotate with the hand, it will start rotating. That is why we use a capacitor in the single-phase induction motor.
Due to the various advantages of an induction motor, there is a wide range of applications of an induction motor. One of their biggest advantages is their high efficiency – which can go as high as 97%. The main disadvantage of an induction motor is that the speed of the motor varies with the applied load.
The direction of rotation of induction motor can easily be changed by changing the phase sequence of three-phase supply, i.e., if RYB is in a forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor, but in a single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.
Before the development of semiconductor power electronics, it was difficult to vary the frequency, and cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used DC drives or wound rotor motors (WRIM) with slip rings for rotor circuit connection to variable external resistance allowing considerable range of speed control.
However, resistor losses associated with low-speed operation of WRIMs is a major cost disadvantage, especially for constant loads. Large slip ring motor drives, termed slip energy recovery systems, some still in use, recover energy from the rotor circuit, rectify it, and return it to the power system using a VFD.
In many industrial variable-speed applications, DC and WRIM drives are being displaced by VFD-fed cage induction motors. The most common efficient way to control asynchronous motor speed of many loads is with VFDs. Barriers to adoption of VFDs due to cost and reliability considerations have been reduced considerably over the past three decades such that it is estimated that drive technology is adopted in as many as 30–40% of all newly installed motors.
Variable frequency drives implement the scalar or vector control of an induction motor.
With scalar control, only the magnitude and frequency of the supply voltage are controlled without phase control (absent feedback by rotor position). Scalar control is suitable for application where the load is constant.
Vector control allows independent control of the speed and torque of the motor, making it possible to maintain a constant rotation speed at varying load torque. But vector control is more expensive because of the cost of the sensor (not always) and the requirement for a more powerful controller.