Introduction W1

8/12/2019 Introduction W1

1/27

Definition Application of electrical machines Electromagnetism: review Analogies between electric and magnetic

circuits Faradays Law lectromagnetic Force Motor action

Generator action Types and parts of machines

1


2/27

1. Definition

Electric machines are energy conversion devicesthat convert electrical energy to mechanical energyand vice versa through the medium of magneticfield. An electric machine is called a generatorwhenit is used to convert mechanical energy to electrical

energy. On the contrary, when an electric machine isoperated to convert electrical energy to mechanicalenergy, it is called a motor.

2. Application of MachinesElectric motors are used to operate washingmachines, elevators, cranes etc while electricgenerators are used to generate electricity for powergeneration and alternator for charging car battery.

2


3/27

3

Applications of Machines


4/27

4

Applications of Machines


5/27


6/27

6

Electromagnetism

Magnetic fields are the fundamental mechanism by whichenergy is conserved from one form to another in motors,

generators and transformers.

A magnetic field is a condition resulting from electriccharges in motion. For convenience in visualization and

analysis, magnetic fields are represented on diagrams by

close loops. Theses loops, called magnetic flux lines


7/277

Magnetic Circuit

Magnetic circuit showing an arrangement offerromagnetic materials (core) that forms the path and

guide the magnetic flux.

The flux always takes the shortest path


8/278

Megnetomotive force

It is the driving force that causes the magnetic filed to

appear in a magnetic circuit.

Flux Density

Its a measure of the concentration of lines of flux in

a particular section of magnetic circuit

Magnetic Field Intensity

It is the mmf per unit length of the magnetic circuit and it

may vary from point to point throughout the circuit.


9/279

Its a measure of the opposition the magnetic circuit

offers to the flux.

Reluctance

The reluctance of a magnetic circuit is related to itslength, cross sectional area, and permeability.


10/2710

Relative permeability and magnetization curves

Relative permeability is the ratio of the permeabilityof a material to the permeability of free space. It is

very useful for comparing the magnetizability of

different magnetic materials whose relative

permeabilities are known.


11/2711

The ratio =B/H is called magnetic permeability and

has different values for different magnetic materials.


12/2712

Analogies between Electric and

Magnetic Circuits

The relationship between mmf, flux, and reluctance in a

magnetic circuit is an analog of the relationship bwteen emf,

current and resistance respectively, in an electric circuit.


13/27


14/27

14

Magnetic Circuit and its Electrical Analog

A ferromagnetic core is shown in figure below . The depth of

the core is 5 cm. The other dimensions of the core are as

shown in the figure. Find the value of the current that will

produce a flux of 0.005 Wb. With this current, what is the flux

density at the top of the core? What is the flux density at the

right side of the core? Assume that the relative permeability ofthe core is 1000.


15/27


16/27

16


17/27

17

Magnetic Hysteresis and hysteresis losses

Hysteresis means lagging behind. In magnetic hysteresis,

flux density B lags behind the field intensity H

Hysteresis loop shows the characteristics of magneticmaterial. It is obtained by plotting values of flux density

B for periodically reversing field intensity


18/27

Magnetic Field play an important role in

electric machines.

Magnetic fields are produced when current

flows in coils of wire. Magnetic fields have 2 funtions:

- produce torque for motor operation

- generate voltage for generator

operation

18


19/27

dt

Nd

dt

dNe

19

If a conductor is moved by an external force, D in a magneticfield, an electromagnetic force is produced between the two ends

of the conductor. This is the basis for the operation of a

generator.

According to Faradays Law, a magnetic field will induce avoltage, e in a coil. This forms the basis for the operation of a

generator. The voltage, e is given by,

N Number of turns of each coil

Magnetic flux (Weber)


20/27


21/27

21

Michael Faraday showed that passing a current through aconductor freely suspended in a fixed magnetic field creates a

force which causes the conductor to move through the field.

The force created by the current, now known as the Lorentz force,

acts between the current conductor and the magnetic field, or the

magnet creating the field.The magnitude of the force acting on the

conductor is given by:

F = BLI where Fis the force on the conductor, Lis the length of

the conductor and Iis the current flowing through the conductor


22/27

22

Faraday also showed that the converse is true -

moving a conductor through a magnetic field, or

moving the magnetic field relative to the conductor,

causes a current to flow in the conductor.

The magnitude of the EMF generated in this way is

given by:

E = BLvwhere Eis the generator EMF (orback EMFin a

motor) and vis the velocity of the conductor through

the field


23/27

23

In practice, both the motor and the generator effects take place at

the same time.

Passing the current through a conductor in the magnetic field

causes the conductor to move through the field but once the

conductor starts moving it becomes a generator creating a currentthrough the conductor in the opposite direction to the applied

current. Thus the motion of the conductor creates a "back EMF"

which opposes the applied EMF.

Conversely moving the conductor through the field causes a current

to flow through the conductor which in turn creates a force on the

conductor opposing the applied force.


24/27

24


25/27

25


26/27

1. 3 types of rotat ing m achines:

- Synchronous AC Machines

- Asynch ronous Machines or Induc t ionMachines

- DC Mach ines

26


27/27

Introduction W1

Documents

Transcript of Introduction W1