Induction Motor Basics

Introduction to Induction Motors

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.

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.

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.

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 N_s, winding factor kw_s and impedance Z_s. This voltage derives the current I_s 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 E_s in the stator windings and E_r rotor windings having number of turns N_r, winding factor kw_r and impedance Z_r.

E_s = -N_s . dɸ / dt ( -ve sign taken to satisfy the Lenz’s Law)

ɸ = ɸ_m . sinωt

E_s = - kw_s. N_s . ɸ_m . ω . cosωt

Es_(rms) = π √2 .kw_s. N_s . ɸ_m. F

Es_(rms) = 4.44 kw_s. N_s . ɸ_m .F_s

Similarly,

Er_(rms) = 4.44 kw_r. N_r . ɸ_m .F_r

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. The direction of the flow of induced current is such that it opposes its cause in accordance with Lenz's Law.

Now the rotor carries a current, which generates its own magnetic field called rotor RMF (F_r). This field also rotates in the same direction as the stator field at synchronous speed with respect to the stator. This rotor magnetic field F_r opposes the stator magnetic field F_s, 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, E_s decreases, I_s increases,

V = E_s + I_s. Z_s

After rearranging we get

The increase in stator current I_s is due to the increase in rotor (secondary) current I_r. 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 F_s and magnetic field produced by the reaction current.

The interaction of this resultant field and rotor field F_r (generated by rotor current I_r), 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 F_s.

The direction of rotation of the rotor is analyzed 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.

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.

Types of Induction Motor

On the basis of the nature of electricity supply, induction motors are classified as single-phase induction motor and three-phase induction motor (polyphase induction motor).

Although, the basic principle of working for both the motors is the same, as we discussed above, there are also some differences between the working of these two motors. That’s why we studied these motors separately.

Single Phase Induction Motor

In a single-phase induction motor, the stator is wound for the specific number of poles using a single-phase distributed winding. This type of motor typically used squirrel cage rotors because of their simple construction and low maintenance.

Single phase induction motors are not self-starting motors because the rotating magnetic field generated by single phase winding is the superposition of two rotating magnetic fields (forward field and reverse field), both rotating at synchronous speed. These two components of the magnetic field have the same amplitude and rotate in the opposite direction at synchronous speed. The forward field produces forward torque, causing the motor to tend to rotate in forward direction. Similarly, the backward field produces the backward torque and wants the rotor to rotate in backward direction. However, because the magnitude of both fields is the same, the rotor does not rotate. If an external force is applied in any direction, then the motor continues to rotate in that direction.

So, to start a single-phase induction motor we use different starting methods. On the basis of these starting methods, single phase induction motors can be classified as:

Split phase / Resistance Split Phase Induction Motor

Capacitor Start and Run Induction Motor

Capacitor Start and Capacitor Run / Permanent Capacitor Induction Motor

Shaded Pole Induction Motor

Three phase Induction Motor

In three phase induction motors, the stator is wound with three phase distributed windings for the specific number of poles, connected either in star or in delta configuration. Both types of rotors are used in three phase induction motors, depending on the suitability of the application. A slip ring rotor is used for applications where high starting torque is required, whereas a squirrel cage induction motor is used in such types of applications where starting torque is not the significant concern but the running characteristics of the motor are important.

A three-phase induction motor is a self-starting motor (as we discussed above in the “working of induction motor”). However, it draws a high amount of inrush current at starting due to the relative speed (slip). Therefore, different starting methods are used to limit this inrush current at starting.

Applications of Induction Motors

As we discussed in the introductory part of this article, induction 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. They are widely used in its single-phase form and three phase form due to their simple construction and low maintenance.

The single-phase induction motors are generally used for low power applications and are most popularly used in domestic applications such as fans, washing machines, ACs, refrigerators, air coolers, vacuum cleaners, small farming equipment etc.

Three phase induction motors are generally employed for high power applications and are most popularly used in industrial applications such as cranes, hoist, blowers, manufacturing lines, lathe machines, high power drill machines, etc.

Some Specifications of Induction Motors

Induction motors are singly excited AC fed motors, unlike synchronous motors which require double excitation for their operation.

Induction motors are also known asynchronous motors because they never run at synchronous speed.

As an induction motor establishes a magnetic domain for their operation, for this it draws magnetizing current from the AC mains supply. So, it inherently has a lagging power factor, which is the special concern in the induction motors.