Induction Motor Notes

What is an Induction Motor : Types & Their Working

We know that the main function of a motor is to change the energy from
one form to another like electrical to mechanical. The classification
of motors can be done based on the type of supply like AC motors & DC
motors. Under these AC & DC motors, there are different kinds of motors
comes as induction motor, reluctance motor, DC shunt, PMDC, stepper,
synchronous, etc.

What is an Induction Motor?

The induction motor works on the principle of induction where an

electromagnetic field is induced into the rotor when the rotating magnetic
field of the stator cuts the stationary rotor. Induction machines are by far
the most common type of motor used in industrial, commercial, or
residential settings.
It is a three-phase AC motor. Its characteristic features are:

Induction Motor
 Simple and rugged construction
 Low cost and minimum maintenance
 High dependability and sufficiently high proficiency
 Needs no additional starting motor and necessity not to be
synchronized
What are the Basic Parts of an Induction Motor?

An induction motor has basically two parts: Stator and Rotor.

Stator

The stator is made up of various stampings with slots to carry three-phase

windings. It is wound for a distinct number of poles. The windings are
geometrically divided 120 degrees separated. Two sorts of rotors are used
in Induction motors: Squirrel cage rotor and Wound rotor. Rotor voltage is
induced in the rotor windings rather than being physically connected by
wires.

Rotor

The rotor is the rotating part of the electromagnetic circuit. The most
common type of rotor is the squirrel cage rotor. The rotor comprises a
cylindrical laminated core with axially placed parallel slots for carrying the
conductors. Each slot carries a copper, aluminum, or alloy bar. The rotor
of three-phase induction motors frequently is likewise implied as an
anchor.
Types of Induction Motor

Induction motors are classified into two types namely single phase
induction motor and three-phase induction motor. As their name suggests,
a 1-phase induction motor is connected to a single-phase AC power supply
whereas the 3-phase induction motor can be connected to a three-phase
AC power supply. Again these types of induction motor are classified into
some subcategories. Single-phase is classified into four types whereas 3-
phase is classified into two types.

Types of Induction Motor

Single-phase Induction Motor

The single-phase induction motor is not self-starting. When the motor is

connected to a single-phase power supply, the main winding carries an
alternating current. It is logical that the least expensive, most reduced
upkeep sort engine ought to be utilized most regularly. These are of
different types based on their way of starting since these are not self-
starting. Those are split phase, shaded pole, and capacitor motors. Again
capacitor motors are capacitor start, capacitor run, and permanent
capacitor motors. The permanent capacitor motor is shown below.
In these types of motors, the start winding can have a series capacitor
and/or a centrifugal switch. When the supply voltage is applied, the
current in the main winding lags the supply voltage because of the main
winding impedance. And current in the start winding leads/lags the supply
voltage depending on the starting mechanism impedance.

The angle between the two windings is sufficient to phase difference to

provide a rotating magnitude field to produce a starting torque. At the
point when the motor reaches 70% to 80% of synchronous speed, a
centrifugal switch on the motor shaft opens and disconnects the starting
winding.

Types of Single-Phase Induction Motor

Single-phase induction motor is classified into four types of induction

motors like Split Phase, Capacitor Start, Capacitor Start & Capacitor Run,
and Shaded Pole Induction Motor.

Types of 3 Phase Induction Motor

A 3-phase induction motor includes two essential components namely the

stator & the rotor. In this motor, the stationary part is the stator whereas
the rotating part is the rotor. In this motor, the load is connected to the
shaft. Three-phase armature winding can be wound over the stator. Once
balanced 3- phase current supplies throughout this winding, then a stable
amplitude rotary magnetic field can be formed within the air gap.

This armature winding can be connected toward the three-phase power

supply & carries the load current. This kind of motor is classified into two
types based on its construction like Squirrel Cage Rotor & Wound Rotor

Squirrel Cage Induction Motor

The squirrel cage induction motor construction is extremely simple. In this

motor, the rotor includes a cylindrical core that can be laminated &
includes some slots on the external periphery. These slots are not
comparable & they are twisted by some angles.
These slots assist to stop magnetic locking among the teeth of the stator
& rotor so that smooth operation can be achieved and humming noise can
be reduced. These motors include rotors of bars in place of the rotor
winding where these bars are fabricated with brass, aluminum otherwise
copper.

In this type of motor, the winding of the rotor includes an aluminum bar
otherwise uninsulated copper fixed into semi-closed rotor slots. At both
ends of this motor, these conductors are short-circuited through similar
material’s end ring. As a result, this type of rotor is similar to a squirrel
cage, so this is known as a squirrel cage induction motor.

Wound Rotor or Slip Ring Induction Motor

The slip-ring induction motor is also called a wound rotor motor. In this
motor, the rotor includes a laminated cylindrical core. Similar to the
squirrel cage, there are some slots on the outer periphery. The rotor
winding is placed inside the slots.
In the wound rotor, the insulated windings are wound on top of the rotor
similarly like on the stator. The winding of this rotor can be distributed
uniformly & usually connected in the STAR model. The three terminals of
this STAR connection can be taken out throughout the slip ring. So this is
the reason to call this motor a slip ring induction motor.
Advantages

The motor construction and the way electric power is supplied all give the
induction motor several advantages are shown in the figure below. And
let’s see of them in brief.

Advantages of
Induction Motor
Low Cost: Induction machines are very cheap when compared to
synchronous and DC motors. This is due to the modest design of the
induction motor. Therefore, these motors are overwhelmingly preferred
for fixed speed applications in industrial applications and for commercial
and domestic applications where AC line power can be easily attached.

Low Maintenance Cost: Induction motors are maintenance-free motors

unlike dc motors and synchronous motors. The construction of an
induction motor is very simple and hence maintenance is also easy,
resulting in low maintenance cost.

Ease of Operation: The operation of the induction motor is very simple

because there is no electrical connector to the rotor that supply power
and current are induced by the movement of the transformer performs on
the rotor due to the low resistance of the rotating coils. Induction motors
are self-start motors. This can result in reducing the effort needed for
maintenance.

Speed Variation: The speed variation of the induction motor is nearly

constant. The speed typically varies only by a few percent going from no
load to rated load.

High Starting Torque: The starting torque of the induction motor is very
high which makes the motor useful for operations where the load is
applied before the starting of the motor. 3 phase induction motors will
have self-starting torque, unlike synchronous motors. However, single-
phase induction motors do not have self-starting torque and are made to
rotate using some auxiliaries.
Durability: Another major advantage of an induction motor is that it is
durable. This makes it the ideal machine for many uses. This results in the
motor running for many years with no cost and maintenance.
All these advantages make induction motor to use in many applications
such as industrial, domestic, and in many applications.

Disadvantages

The disadvantages of induction motor include the following.

 This is an invariable speed motor and this motor is not applicable where
uneven speed needs
 This motor speed control is not easy
 This motor includes a high starting inrush current which will cause a
reduction within voltage at the beginning of time.

Applications

The applications of types of induction motor include the following.

 Lifts
 Cranes
 Hoists
 Large capacity exhaust fans
 Driving lathe machines
 Crushers
 Oil extracting mills
 Textile and etc.
Thus, this is all about an overview of types of induction motors. Basically,
these are classified into two major types based on the input supply like
single-phase and three-phase induction motor. Again, these two types of
motors are also sub-divided into different types which are discussed
above. Here, Single phase induction motor is not self-starting whereas a
three-phase induction motor is a self-starting motor. Here is a question for
you, what is the main function of an induction motor?

Calculations
This difference between the speed of the rotor and speed of the rotating
magnetic field in the stator is called slip. It is unitless and is the ratio
between the relative speed of the magnetic field as seen by the rotor the
(slip speed) to the speed of the rotating stator field. Due to this an
induction motor is sometimes referred to as an asynchronous machine.
SLIP
The relationship between the supply frequency, f, the number of poles, p,
and the synchronous speed (speed of rotating field), ns is given by