ELECTRICAL MACHINES

Electrical Machines



Table of contents

Introduction3

Discussion3

Electric Motors3

AC Motor4

Theory of Induction motor4

Production of rotating magnetic field5

DC Motor7

Induction motor9

Construction9

Stator9

Rotor10

Squirrel cage rotor10

Phase wound rotor10

Squirrel cage rotor10

Phase wound rotor11

Frame12

Stator and rotor core12

Stator and rotor windings13

Air gap13

Shafts and bearings13

Fans13

Slip rings13

Slip enclosure13

Working15

Advantages of induction motor15

Disadvantages of induction motor15

Starting torque of squirrel cage motor16

Starting torque of slip ring motor16

Slip16

Conclusion17

References18

Referrences for Images19

Electrical Machines

Introduction

With the advent of electrical energy distribution systems and their universal adoption throughout the world for the purpose of lightening and power the field of electric motors especially the AC type has shown a remarkable development in all prospects all over the world. As a result of the technological advancements the engineers throughout the world tried their best of best to come out with the most energy efficient and safe machines particularly, in the context of electric motors.

In this report we will confine our discussion up to electric motors, their construction, operations, applications, performance and emphasize on the Alternating Current type.

Discussion

Electric Motors

An electric motor is a device which transfers electrical energy in to mechanical energy which is either rotational or kinetic energy. There are two major classifications of electric motors on the basis of the type of electric input on which they operate obviously, AC and DC.

AC Motor

Figure 1

Basic Construction of an AC motor

Theory of Induction motor

Above is the basic construction of an AC motor. It consists of a couple of magnets with opposite polarities from each other. The purpose of these magnets is to create a magnetic field in between them whose direction is such that it will remain perpendicular to the coil at every point of rotation. For this purpose the poles of the magnets are made concave to concentrate the magnetic field towards the centre. A rectangular coil of soft iron core wounded with copper wire is placed freely in between the magnets which are connected with the AC terminals through a pair of slip rings known as carbon brushes to prevent wear and tear of the coil and to minimize the frictional effect. The AC motor works on the principle of mutual induction according to which whenever a current carrying coil is placed in a magnetic field it will experience a torque. This torque is actually the result of electrostatic repulsion between the electrons flowing as a current through the coil with negative polarity and the respective attraction and repulsion experienced by them due to the magnetic field. The coil continues to move when a couple of attractive and repulsive force is created due to inertia and as the alternating current changes its direction the net force remain in unidirectional either clockwise or counter clockwise depending on the polarity of the AC connected to the motor's terminal. Since the input current for coil is alternating, the motor will continue to move smoothly at the frequency of input sine wave. It is also termed as synchronous AC motor or Induction motor. This is only the simplified model just for the sake of understanding the basic construction and ...