Physics Dc Gen.

8/14/2019 Physics Dc Gen.

1/20

103/23/09

Welcome to

Lectures on D.C.Generators


2/20

203/23/09

DIRECT CURRENT

MACHINESDC machine Construction

The major advantages of DCmachines are: easy speed andtorque regulation.

The stator of the DC motor haspoles, which are excited by DCcurrent to produce magnetic fields.

The rotor has a ring-shapedlaminated iron-core with slots.

Coils with several turns are placed inthe slots. The distance between thetwo legs of the coil is about 180electric degrees.

N S

Stator with

with polesBrush

Rotor

Field


3/20

3

DC Generators.

Basic operation of the generator As the loop rotates, the magnetic

flux through it changes withtime

This induces an emf and acurrent in the external circuit

The ends of the loop areconnected to slip rings thatrotate with the loop

Connections to the externalcircuit are made by stationary

brushes in contact with the sliprings


4/20

4

DC Generators, cont

The output voltage alwayshas the same polarity

The current is apulsatingcurrent

To produce a steady current,many loops andcommutators around the axisof rotation are used

The multiple outputs aresuperimposed and theoutput is almost free offluctuations


5/20

5

DC Generators

If we increase the number

of turns and arrange them

as shown, we can smooth

out the variations in

voltage.

In the old days, autos used

DC generators to recharge

the battery.


6/20

6

DC Generators

Replace the brushes with

slip rings and you get a

DC generator. Notice the

voltage is not steady. The

coil is shown at the top

and bottom of the shaft.


7/20703/23/09

D. C. Generator

Principle of Operation:When a conductor rotates in a magnetic field,

dynamically induced e.m.f is produced in it

according to Faradays law of electromagnetic

induction.


8/20803/23/09

Generated EMF

Let

Ei.epathsparalleltheofoneanyingeneratedEMFEEMFGenerated

ArmatureinpathparallelanyininducedEMFE

rpminrotationArmatureN

ArmatureinPathsParallelofNo.A

polesGeneratorofNo.P

SlotperConductorsofNo.SlotsofNo.

ConductorsArmatureofNo.TotalZ

WbinFlux/Pole

g =

====

===


9/20903/23/09

secN

60dt,revolution1forTime

N/60s/secRevolutionofNo.

WbPd,revolution1intorcut/conducFluxNow,

voltdt

dConductorGenerated/EMFAverage

=

==

=

Hence, according to Faradays Laws of electromagnetic induction,

volt60

PN

60/N

P

volt

dt

dconductorGenerated/EMF

==

=


10/201003/23/09

For Wave wound Generator

No. of Parallel Paths = 2

No. of Conductors (in series) in 1 path = Z/2

volt

120

ZPNvolt

260

PNpathGenerated/EMF ==

Z


11/201103/23/09

For Lap wound Generator

No. of Parallel Paths = P

No. of Conductors (in series) in 1 path = Z/P

volt60

ZNvolt

P60

PNpathGenerated/EMF ==

Z


12/201203/23/09

In General,

Generated EMF

windinglapforP

windingwavefor2Awhere,

volt

A

P

60

ZNEg

==

=


13/201303/23/09

Types of D.C. Generators

According to the way in which their fieldsare excited, generators are classified into :

i) Separately excited D.C. Generator

ii) Self- excited D.C. Generator


14/201403/23/09

i) Separately excited D.C. Generators are those whose field

magnets are energized from an independent source of D.C. Current

Eg

L

O

A

D

+

-

I

Field Winding

R

Brush

Brush

V

If

DropsBrushIREV ag =Ra = Armature Resistance


15/201503/23/09

ii) Self excited D.C. Generators are those whose field

magnets are energized by the currents produced by the generators

themselves.

There are 3 types of self-excited D.C. Generators named according to

the manner in which their field coils are connected to the armature.

These are :

Shunt wound D.C. Generator Series wound D.C. Generator

Compound wound D.C. Generator


16/201603/23/09

i) Shunt wound D.C. Generator

Eg

L

O

A

D

+

-

I

Shunt Field V

Ia

Ish

Rsh

Ra

Rsh Shunt Field Resistance

Ra - Armature Resistance

sha III +=DropsBrushRIEV aag =

shshRIV =


17/20


18/201803/23/09

iii) Compound wound D.C. Generator

Eg

L

OAD

I

Shunt Field

V

Ia

Ish

Series Field

Short Shunt


19/201903/23/09

Eg

L

O

A

D

I

Shunt FieldV

Ia

Ish

Series Field

Long Shunt


20/2020

03/23/09

Portable D. C. Generator
http://upload.wikimedia.org/wikipedia/commons/7/7d/Portable_electrical_generator_angle.jpg

Physics Dc Gen.

Documents