CHAPTER 6 - AC MOTOR

AC MOTOR INTRODUCTION

     AC motors are used worldwide in many applications to transform electrical energy into mechanical energy. There are many types of AC motors, but three phase AC induction motors, is the most common type of motor used in industrial applications. An AC motor of this type may be part of a pump or fan or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer. The electric motor in its simplest terms is a converter of electrical energy to useful mechanical energy. The electric motor has played a leading role in the high productivity of modern industry, and it is therefore directly responsible for the high standard of living being enjoyed throughout the industrialized world.

     AC motors provide the motive power to lift, shift, pump, drive, blow, drill, and perform a variety of other tasks in industrial, domestic, and commercial applications. The induction motor, the most versatile of the AC motors, has truly emerged as the prime mover in industry, powering machine tools, pumps, fans, compressors, and a variety of industrial equipment.

Fundamentals of three-phase AC motors
     Three-phase AC motors are known as the ‘workhorses of industry’ because of their wide use and acceptance. They are popular because they are low in cost, compact in size, require less maintenance, withstand harsh industrial environments, etc. Three-phase AC motors are a class of motors that convert the three-phase electric power supplied at the input terminals, to mechanical power at the rotating shaft, through the action of a rotating magnetic field, produced by a distributed winding on the stator.

Three-phase AC motors are broadly classified as:
1. Induction motor
2. Synchronous motor
3. Wound rotor induction motor.

1. Induction motor
    As the name implies, no voltage is applied to the rotor. The voltage is applied to the stator winding and when the current flows in the stator winding, a current is induced in the rotor by transformer action. The resulting rotor magnetic field will interact with the stator magnetic field, causing torque to exert on the rotor.

2. Synchronous motor
    Synchronous ac motors are constant-speed electric motors and they operate in synchronism with line frequency. The speed of a synchronous motor is determined by the number of pairs of poles and is always a ratio of the line frequency.

Basic principles of Synchronous Motors:
· The stator is provided with two simple coils, which can be directly connected to the mains.
· The rotor consists of a cylindrical permanent two-pole magnet, which is diametrically magnetized.

3. Wound rotor induction motor
    This motor has a ‘wire wound rotor’ from which three leads are brought out to the slip rings. It is possible to vary the rotor resistance. Introducing different resistances in the rotor circuit through the slip rings does this. The speed and the starting torque will now be variable.

Principle of operation of an induction motor

     An electric motor’s principle of operation is based on the fact that a current-carrying conductor, when placed in a magnetic field, will have a force exerted on the conductor proportional to the current flowing in the conductor and to the strength of the magnetic field. In alternating current induction motors, the windings placed in the laminated stator core produce the magnetic field. The aluminum bars in the laminated rotor core are the current-carrying conductors upon which the force acts. The resultant action is the rotary motion of the rotor and shaft, which can then be coupled to various devices to be driven and produce the output.

References:

http://engg-learning.blogspot.com/2011/03/ac-motor-introduction-ac-motors-are.html 26 April 2014

http://www.johnsonelectric.com/en/resources-for-engineers/motors/basics-of-motors/ac-motors-theory.html 26 April 2014