‘Three Phase Induction Motor’

INTRODUCTION

In the year 1821 British scientist Michael Faraday explained the conversion of electrical energy
into mechanical energy by placing a current carrying conductor in a magnetic field which resulted in the
rotation of the conductor due to torque produced by the mutual action of electrical current and magnetic
field.

Based on his principle the most primitive of machines a DC (Direct Current) machine was designed
by another British scientist William Sturgeon in the year 1832. But his model was overly expensive and
wasn’t used for any practical purpose. Later in the year 1886 the first electrical motor was invented by
scientist Frank Julian Sprague, that was capable of rotating at a constant speed under a varied range of
load, and thus derived motoring action.

Types of three phase induction motor

Classification Based On Principle Of Operation

(a) Synchronous Motors.

Plain

Super

(b) Asynchronous Motors.

Induction Motors:

(a) Squirrel Cage

(b) Slip-Ring (external resistance).

Commutator Motors:

(a) Series

(b) Compensated

(c) Shunt

(d) Repulsion

(e) Repulsion-start induction

(f) Repulsion induction

Classification Based On no of phases:

1. Single Phase

2. Three Phase

Classification Based On Speed Of Operation:

Constant Speed.

Variable Speed.

Adjustable Speed.

Classification Based On Structural Features:

Open

Enclosed

Semi-enclosed

Ventilated

Pipe-ventilated

Riveted frame-eye etc..

General working principle of three phase induction motor

Conversion of electrical power into mechanical power takes place in the rotating part of an
electrical motor. In a DC motor, the electrical power is conducted directly to the armature through brushes
and a commutator, hence, in this sense, a DC motor can be called a conduction motor.

However, in ac motor
The rotor does not receive electric power by conduction but by induction in precisely the same way as the
secondary of a 2-winding transformer receives its power from the primary; that is why such motors are
known as induction motors. An induction motor can be treated as a rotating transformer, i.e., one in
which the primary winding is stationary but the secondary is free to rotate.

Principles of operation of three phase induction motor

Why a three phase Induction motor is self-starting? How does the rotor rotate?

1. When a three phase stator winding having a space displacement of 120° electrical is energized from a 3
   Ø Supply having 120° time displacement a rotating magnetic field is setup in the stator.

2. This rotating magnetic field rotates with synchronous speed Ns =120f/p with respect to
stationary in the air gap

3. This rotating field passes through the air gap and cuts the stationary rotor conductors

4. Due to the relative speed between the rotating flux and the stationary rotor, EMFs are induced in
The rotor conductors

5. If the rotor conductors are short circuited, currents start flowing in the rotor conductor

6. According to Len’s law the direction of the induced current is such that it opposes the cause.

7. Cause is the relative speed between the rotating field and stationary rotor

8. Hence, a rotor has a tendency to reduce the relative speed

9. So rotor begins to move in the direction of rotating field and continues towards synchronous
speed and the machine runs at a speed near but below synchronous speed depending upon load on
shaft

10. As the speed of rotor reaches to synchronous speed (speed of field) relative speed is zero. Hence
no emf, no current and therefore no torque at synchronous speed. Hence rotor never reaches to
synchronous speed.

11. At synchronous speed current is zero in rotor conductor, hence no force acting on rotor conductor
and slip back, somewhat less than synchronous speed.

 Key Terms: of three phase induction motor

Synchronous Speed (Nₛ):

Where:

f= Supply frequency (Hz)

P = Number of poles

Slip (s):

Where Nr= Rotor speed (RPM).
(Typical slip at full load: 2-5%).

 Rotor Frequency (fr)

The frequency of the rotor current depends on slip:

fr==s×f

 Power Flow Equations

Input Power (Pin​):

Pin=3×VL×IL×cos⁡ϕ

VL​ = Line voltage

IL = Line current

cosϕ = Power factor

Rotor Copper Loss (Pcu​):

Pcu=s×Pag​

Pag= Air gap power

Output Power (Pout);

Pout=Pin−(Stator Loss + Rotor Loss + Friction Loss)

Why three phase induction motor not run at synchronous speed?

An induction motor can thus speed up to near synchronous speed but it can never reach
synchronous speed.

three phase induction motor diagram

    ┌──────────────────────────────────────┐
    │          STATOR (Stationary)         │
    │  ┌──────────────────────────────┐     │
    │  │     3-Phase Stator Windings  │◄────┐
    │  │    (R - Y - B Connections)    │     │
    │  └──────────────────────────────┘     │
    │      ▲          ▲           ▲         │
    │     R-phase   Y-phase     B-phase     │
    │                                         │
    │   Laminated Stator Core (Iron)         │
    └────────────────┬──────────────────────┘
                     ▼
            ┌─────────────────┐
            │     AIR GAP     │◄────── Narrow space allowing magnetic field transfer
            └─────────────────┘
                     ▼
            ┌─────────────────┐
            │      ROTOR      │  ◄──── Squirrel Cage or Wound Type
            │ ┌─────────────┐ │
            │ │ Conductor   │ │
            │ │ Bars (Al/Cu)│ │
            │ └─────────────┘ │
            │   Shorted by End Rings         │
            └─────────────────┘
                     ▼
            ┌─────────────────┐
            │  Rotor Shaft    │◄── Connected to Load (e.g., Pump, Fan)
            └─────────────────┘
                     ▼
            ┌─────────────────┐
            │  Bearings       │◄── Allow smooth rotation
            └─────────────────┘
                     ▼
            ┌─────────────────┐
            │  Cooling Fan    │◄── Helps dissipate heat
            └─────────────────┘

Construction of Three Phase Induction

An Induction motor consists of mainly two parts:

(a) Stator
(b) Rotor

Stator:

(i) Stator core is made of laminated steel stampings and has slots and teeth on its inner
periphery to house stator windings. The stampings are 04.to 0.5 mm thick.

(ii) Stator carries a 3-phase winding having space displacement of 120° electrical

(iii) The 3-phase winding is either star or delta connected and is fed from 3-phase supply

(iv) The radial ventilating ducts are provided along the length of the stator core

Rotor

i. The Rotor comprises a cylindrical laminated iron core, with slots on the outer periphery
ii. Like stator, rotor lamination are punched in one piece for small Machine
iii. In a larger machine the lamination are segmented
iv. If there are ventilating ducts on the stator core, an equal number of such ducts is
provided on the rotor core

According to windings rotor are of two types:

a) Squirrel cage rotor
b) Slip ring or wound rotor

Squirrel Cage Rotor

1. This rotor consists of a cylindrical laminated core with parallel slots
2. Rotor slots are usually not quite parallel to the shaft but for reducing the magnetic hum
   and locking tendency rotor slots are slight skew
3. In rotor slots heavy copper, aluminum or alloy bars are housed
4. Rotor bars are permanently short-circuited at the ends. This limits the no external resistance insertion is possible

Advantages of squirrel cage induction rotor

• Its construction is very simple and rugged.
• As there are no brushes and slip ring, these motors requires less maintenance.

Applications:

Squirrel cage induction motor is used in lathes, drilling machine, fan, blower
printing machines etc

Slip Ring or Wound Rotor:

• The rotor is wound for the same number of poles and number of phase as that of stator
• Rotor winding is either star or delta but star connection is preferred
• The three star terminals are connected to three brass slip ring mounted on rotor shaft
• These slip rings are insulated from rotor shaft
• Slip rings connected with brushes and three brushes can further be connected externally to 3 variable rheostats
• This makes possible introduction to additional resistance in the rotor circuit during starting period

Slip ring three phase induction motor

Advantages of slip ring induction motor

Possibility of adding additional resistance to control speed.

Application:

Slip ring induction motor are used where high starting torque is required i.e. in
hoists, cranes, elevator etc.

Difference between Slip Ring and Squirrel Cage Induction Motor

Advantages: three phase induction motor

Thus the three phase induction motor is:

1. Self-starting.
2. Less armature reaction and brush sparking because of the absence of commutators and brushes
3. that may cause sparks.
4. Robust in construction.
5. Economical.
6. Easier to maintain.

Disadvantages: three phase induction motor

1. Its speed cannot be varied without sacrificing some of its efficiency.
2. Just like a dc shunt motor its speed decreases with increase in load.
3. Its starting torque is somewhat inferior to that of a dc shunt motor.

Conclusion

Three-phase induction motors are among the most widely used electrical machines in industrial and commercial applications due to their robust construction, high reliability, low maintenance, and cost-effectiveness. These motors operate on the principle of electromagnetic induction, where a rotating magnetic field generated by the stator induces current in the rotor, producing torque without the need for direct electrical connection to the rotor.