3 phase induction motor

3 PHASE INDUCTION MOTOR

INTRODUCTION

 An electric motor is a device which converts an electrical energy into a mechanical energy. The motors operating on a.c. supply are called a.c. motor. As a.c. supply is commonly available, the a.c. motors are very popularly used in practice. The a.c. motors are classified as three phase induction motors, single phase induction motor, universal motors, synchronous motors etc. The three phase induction motors are widely used for various industrial application. The important features of three phase induction motors are self starting, higher power factor, good speed regulation and robust construction. This chapter explains the construction, working principle and characteristics of three phase induction motors as well as universal motors. The working of three phase induction motors is based on the principle of rotating magnetic field. 

Motor mainly has two types 

1. Asynchronous Motor

            It is a type of motor where DC supply is needed to supply field current to field windings

2.Induction Motor 

            Where no spare DC excitation needed it supply field current to field windings through electromagnetic induction

Induction motors are widely used in the world but where more torque and constant speed require than these motors are not suitable option 

Induction motor are invented by nikon tesla in 1888 . In induction motor energy is transferred from stator to rotor through electromagnetic induction , Revolving magnetic flux created by stator and that field create EMF in the rotor , So stator Revolving flux and Rotor Emf produce a large torque 

CLASSIFICATION OF INDUCTION MOTOR 

DUE TO PHASE 

1.Single phase Induction motor  

2.Three phase Induction motor

THROUGH CONSTRUCTION 

1.Squirrel Cage Motor

2.Wound Rotor Induction Motor

PRODUCTION OF ROTATING MAGNETIC FIELD 

Suppose we have a three phase motor and three phase supply to this motor so stator will start producing a constant revolving magnetic field as a result same amount EMF will be generate in the rotor 

the resultant torque will always be equal to the magnetizing torque 

 CONSTRUCTION OF INDUCTION MOTOR 

Now a days induction motors are widely used due to its simple construction
Induction motor is same like static transformer, a slight change between them is ,In transformer output is taken from secondary which is in static position where in induction motor Rotor used for that operation,
Their is specific area between stator and rotor known as mechanical clearance its about .04 inches

STATOR 

Stator is the static part of induction motor , The stator of synchronous motor is like induction motor
Induction motor is made of thin laminations which are insulated from each other , mainly they are made of silicon due to which hysteresis losses decrease Their are slots in these windings where motor will receive its windings The stator carry a three phase supply and only 3 phase supply can be fed to it,the number of poles decide the speed of induction motor ,higher the number of poles lower will be speed

stator lamination and slots 

stator poles 

ROTOR

The rotatory part of a motor is known is rotor, its core is made of thin laminations made of copper or aluminium m they are insulated from each other,

types of motor in terms of rotor

Squirrel cage rotor

wire wound motor

Squirrel cage motor

The type of motors are widely used in world , its laminations are made of copper or aluminium, 

 

 

As shown in graph , bars are shorted with each other with the help of end rings,these type of motors are very strong and simple in structure As shown in graph these slots are deviated from straight line to decrease vibrations
the slots between rotor and stator should not be equal otherwise motor will be locked
Reversing the direction of motor
The direction of a motor depends on stator field and stator field depends on phase sequence , so by changing the phase sequence (two phases with each other) the direction of motor will be reversed

Principle of Rotation of rotor

 When three phase supply fed to the stator so stator will produce revolving magnetic flux,this flux will pass through air gap and strike with rotor ,according to Faraday's law emf will be generated in rotor,which is the reason for current flowing in rotor circuit, As per lens law it will oppose its cause so rotor will start rotation in the direction of field ,
The speed of rotor will always be less than stator otherwise no torque will be produce in the circuit

Starting of three phase motor

Need?

As we know that induction motor is same like static transformer, in starting when rotor has not energized ,So when supply given to stator a huge amount of current flow in the circuit that current is 7 to 8 times to the total rated current and that huge current produce 2.5 times to  the rated   torque

 these conditions are not preferable these can be overcome by decreasing the starting voltage,when motor start fully so then rated voltage is given to the motor, this type of method is only valid < 7.5 kw motors ,
Dol method also use for starting the motor , protection is used for dol method discussed in protection system
primary resistance method is also used for starting of motor , in this method resistance os connected in series with  rotor to decrease the torque when motor starts fully then the value of resistance decreases by step ,

Speed Control of induction motor
speed can be controlled by three ways
            by voltage
            by frequency
            by number of poles