Induction Motor Basics

 

Induction motors, also known as asynchronous motors, are the most popularly used motors among all the electric motors in both industrial and domestic applications. Almost 85% of industrial motors in use today are induction motors. Understanding the basics of an induction motor is crucial for engineering students as well as industrial professionals, as these motors are fundamental components of the mechanical and electrical systems.

 

In this tutorial we will explore the basics of induction motors in detail such as what is an induction motor and why is it important to study it? why are induction motors most popularly used in industries? parts of Induction motor and their construction, how an induction motor works? types of induction motors, an introduction to single phase and three phase induction motors and lastly, some specifications and applications of induction motors.

 


What is an Induction Motor?


Induction motors are the AC fed electric motors. The name “induction” derives from their working principle, which is based on Faraday's law of electromagnetic induction, where an electric current is induced in the rotor via the magnetic field generated by the stator. This induced current developed the torque that derives the rotor (also discussed in induction machines).

 

The torque production via electromagnetic induction is only possible at non-synchronous speed (discussed further in this article), hence these motors never run at synchronous speed. Therefore, these motors are commonly known as asynchronous motors. 


The AC supply is given only to the stator winding, unlike synchronous motors, which require AC supply on the stator side and DC supply on the rotor side. Therefore, generally referred to as singly excited motors.


 

Why is it important to study Induction Motor?

 

Studying induction motors is crucial for the comprehensive understanding of modern electrical engineering and technologies, as these motors are the foundational components of electrical as well as mechanical systems. Their widespread use in domestic as well as industrial applications make it essential to study them. 

 

Understanding the concepts of induction motor helps engineers in various aspects such as for gaining practical knowledge for maintenance and troubleshooting, for easily understanding the recent innovation and technological advancement, and for optimizing their performance for various applications etc.



Induction Motor Parts and their Construction


Like any other electric motors, an induction motor consists of two major parts: stator (the annular stationary part) and the rotor (the rotating part) with an intervening air gap, as illustrated in the given figure.


Induction Motor


The stator and rotor are both made up of magnetic material which conducts magnetic flux and allows electromagnetic induction to take place. The detailed construction of both the induction motor stator and rotor is discussed below.

 

 

Induction Motor Stator


Like any other electrical machine, the stator of an induction motor is the annual stationary part on which the stator winding is mounted. The AC supply is given to these windings that generate RMF (Rotating Magnetic Field) in air gap (discussed in the working of induction motors). Not only the windings are mounted on the stator, it also provides the mechanical strength to the motor as well.

 

The stator construction is common for both types of AC electrical machines (synchronous machines and induction machines). It consists of three major parts: the outer frame, the core and the windings


Induction Motor Stator


The outer frame of the induction motor stator is usually made up of cast iron and it provides the mechanical strength to the induction motor.

 

The stator core is made up of steel. Basically, it is a stack of laminated steel conductors. There are n numbers of laminated steel conductors that are stacked together to form the core. You can visualize this with the help of the given figure.



In the stator core slots are provided for the placement of stator winding. Generally, there are three types of slots provided on the stator core that are open slots, closed slots, semi closed slots etc. The choice of the slots depends on the criticality of the application, maintenance requirements and design considerations.

 

In these slots of the stator core, windings are mounted. These windings are typically made up of copper or aluminium and are distributed around the entire core. This winding distribution is done to optimize the distribution of the stator magnetic field around the air gap. 

 

The winding of the stator is wound for the desired number of poles, which determines the speed of the stator rotating magnetic field.  

 

 

Rotor in Induction Motor


The rotor of an induction motor is the rotating part that is positioned within the cavity of the stator with an optimal air gap. It primarily consists of three main parts: shaft, core and either the windings (in case of wound rotor) or bars (in case of squirrel cage rotor).



The rotor shaft is the cylindrical bar on which the rotor core is mounted. It is supported at both ends by the bearings on the end covers. The one end of the rotor shaft is extended out from the end cover to mechanically couple with the load to drive it. 

 

Similar to the stator core, the rotor core is made up of laminated steel conductors that are stacked together to form a cylindrical core. This core is directly fitted onto the rotor shaft. The only difference is the slots are provided on the outer peripheral of the core on which the windings or rotor bars are placed.



Types of Rotor in Induction Motor


Generally, there are two types of rotors used in induction motors: the squirrel cage rotor and the slip ring or wound rotor. 

 

The basic construction of these two rotors is the same, as both contain a shaft and a core as discussed previously. The only difference between these two rotors is the use of winding or conductive bars and their respective supporting components. 

 

Let us explore the constructional features of both rotors separately.



Slip Ring or Wound Rotor


As we discussed above, the basic construction is the same for both types of rotors, as they both contain a shaft and the core. The key difference in this type of rotor is the use of winding. The winding of this type of rotor is made up of insulated copper or aluminium and is wound for the same number of poles as the stator; hence, referred to as wound rotor.

 

This type of rotor is specifically used in large three phase motors where starting torque requirement is high. Consequently, the rotor winding is also three phase and essentially connected in a star configuration, with one terminal of each winding brought out from the motor and connected via slip rings placed on the shaft. These slip rings are tapped by means of copper carbon brushes, through which external resistance can be added, to achieve the excellent starting torque. Due to the use of slip rings, this type of rotor is also known as slip ring rotor.

 

This type of rotor requires more maintenance than the squirrel cage rotor due to the presence of carbon brushes, which wear out over time and need regular inspection and replacement.

 

 


Squirrel Cage Rotor


Unlike slip ring rotors, squirrel cage rotors use conductive bars (copper or aluminium bars). These bars are placed in rotor slots and shorted at both ends by the end rings, as shown in the given figure. These bars are skewed at some angle from the rotor shaft axis to reduce cogging and crawling in the induction motor.

Induction Motor Rotor


The use of conductive bars reduces the maintenance and makes this type of rotor robust. 


This type of rotor is specifically used for small motors where installation of winding is difficult.


As this type of rotor uses conductor bars, we cannot ensure the formation of poles. Instead, this type of rotor automatically creates the images of stator poles and can react to any number of poles.


Air Gap in Induction Motor


As we discussed above, there must be a uniform air gap between stator and rotor. The size of this air gap is an important in determining the performance and efficiency of induction motor 

 

This air gap between the stator and rotor should be as minimal as possible to improve the performance and the power factor of the induction motor.

 


Significance of Air Gap in Induction Motor


This air gap between stator and rotor is necessary to ensure that the rotor can rotate within the cavity of the stator. It should be sufficiently large to prevent collision between stator and rotor during operation, taking tolerances into consideration.

 

However, a larger air gap is not desired because it increases the magnetizing current or reactive power requirement. As a result, the motor will draw a large magnetizing current from the AC supply, causing the reduction of power factor of the motor and the performance as well.

 

Therefore, to reduce the magnetizing current or reactive power requirements this air gap should be kept as minimum as possible.





Working Principle of Induction Motor


As we discussed above in the “definition of induction motor”, an induction motor works on the principle of Faraday's Law on electromagnetic induction, where an electric current is induced in the rotor winding through the magnetic field generated by the stator current, which is quite similar to the transformer. Thus, an induction motor is commonly known as a rotating transformer.

 

Let us understand in detail how an induction motor works?




AC Supply is given to Stator :- When the AC voltage is applied to the stator winding then the current flowing through the stator winding generates a magnetic field in the air gap. This magnetic field  rotates at synchronous speed with respect to the stator and is called the rotating magnetic field (RMF). 

 


Synchronous Speed


The synchronous speed is the speed at which the stator magnetic field rotates in the air gap. This speed depends upon the number of poles on the stator winding and the frequency of the voltage applied to the stator winding. The formula for calculating the synchronous speed is given below.

 

Ns   =    120F/P

 

where,


Ns  =   Synchronous speed

P    =    number of the stator poles 

F    =    frequency of applied voltage.

 



Faraday’s Law of Electromagnetic Induction :- The rotating magnetic field (RMF) produced by the stator current cuts the conductive winding of both the stationary stator and rotor(initially, the rotor is stationary) and induces an emf in both, similar to an ordinary transformer. In this scenario, the stator acts as the primary side of the transformer and rotor acts as the secondary side of the transformer. 

 


Emf Equation of Induction Motor


Let us say the voltage V is applied to the stator winding, having number of turns Ns, winding factor kws and impedance Zs. This voltage derives the current Is in stator winding that establishes a rotating magnetic flux ɸ in the air gap. This magnetic flux cuts the stationary conductor of both rotor and stator and induces emf Es in the stator windings and Er rotor windings having number of turns Nr, winding factor kwr and impedance Zr

 

Es  =  -Ns . dɸ / dt                           ( -ve sign taken to satisfy Lenz’s Law)


ɸ   =   ɸm . sinωt


Es = - kws . Ns . ɸm . ω . cosωt










Es(rms)    =      π √2 .kws . Ns . ɸm. F  


Es(rms)    =      4.44 kws . Ns . ɸm . F      




Similarly,


Er(rms)    =   4.44 kwr . Nr . ɸm . F




Since the rotor winding or bars are short circuited at the both ends, a current is also induced in them, like in the case of a transformer on load. 

 

Now the rotor carries a current, which generates its own magnetic field called rotor RMF (Fr). This field also rotates in the same direction as the stator field at synchronous speed with respect to the stator. This rotor magnetic field Fr opposes the stator magnetic field Fs, causing the reduction of induced voltage in stator winding. Consequently, a reaction current flows in the stator winding from the AC supply to balance the terminal voltage, like an ordinary transformer does on load.

 

As, Es decreases, Is increases, 


V   =   Es   +   Is . Zs

 

After rearranging we get









The increase in stator current Is is due to the increase in rotor (secondary) current Ir. In order to full-fill the load demand rotor draws more current from the stator. This current required by the rotor is referred to the stator side and is called the reaction current as mentioned above.

 


Torque Production :- The reaction current flowing through stator winding produces its own magnetic field that is exactly equal and opposite to the rotor magnetic field. Now the magnetic field rotates in the air gap is the resultant of the stator field Fs and magnetic field produced by the reaction current. 

 

The interaction of this resultant field and rotor field Fr (generated by rotor current Ir), which are stationary with respect to each other, creates a torque tending to move the rotor in the direction of resultant field or the stator field Fs.     

 

The direction of rotation of the rotor is analysed according to Lenz's law in order to oppose the cause i.e. relative speed, the rotor will rotate in the direction of stator RMF.

 

Therefore, an induction motor is a self-starting motor, but this is true only in the case of three phase induction motors. A single-phase induction motor is not the self-starting motor due to its double revolving magnetic field.


Let’s denote the rotor speed is N. It is obvious that N is always less than Ns for induction motors, because, if N = Ns then there is no relative motion (speed) between the rotating magnetic field and the rotor conductors, therefore the induced emf and the rotor current will be zero, hence no torque is developed. And if N > Ns, then the induction motor would act as a generator and generate electricity. (This condition will be achieved by external means like a prime mover.)


That’s why we stated above in the definition of induction motor that the torque production is only possible at non synchronous speed, specifically at a speed less than the synchronous speed.

 

The relative speed between the rotor and the rotating magnetic field is called slip speed and it expressed as 

 

s.Ns  =    Ns – N

 

In the above expression, s.Ns denotes the slip speed, where S represents the slip in the induction motor and Ns represents the synchronous speed.

 

Rearranging the above equation, we get the equation for slip in induction motor.

Induction Motor Working



The equation above shows the formula for slip in induction motor. The slip in induction motor is defined as the per unit speed (with respect to synchronous speed) at which the rotor lags behind the stator field.



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