DC MACHINES

BITS PilaniPilani Campus

INTRODUCTION TO MACHINESPART 1 DC MACHINES

EEE DepartmentBITS Pilani, Pilani

BITS Pilani, Pilani Campus

ROTATING MACHINES

Consider a single rectangular conducting loop of length l and width 2r embedded

in a uniform magnetic field having a constant flux density B rotates about the

z-axis with a constant angular velocity w radians per second

Slip Rings

The ends of the loop are connected to slip

rings that rotate with the loop

Slip rings are in contact with fixed carbon

brushes through which an electrical contact

is made

Area of the loop (A) = 2rl


At time t, the angle between the plane of the loop and the y-z axis is wt Flux passing through the loop is

By Faraday’s law, the EMF induced in the carbon brushes –

If the coil has N turns, then the total EMF induced is given by

Side View

Dot Product

The – sign comes automatically in accordance with the Lenz law

ROTATING MACHINES


ROTATING MACHINES – AC GENERATOR


Since the machine produces a sinusoidal voltage, it is called Alternator or AC Generator

Induced voltage depends on –

1) Speed of rotation

2) Flux Density B

3) Number of turns in the coil

4) Area of the coil

Let us replace slip rings with split-rings

Brushes repositioned on either side of the

split ring commutator

Voltage induced across the loop is given by

ROTATING MACHINES

This is the voltage v12 between the 2 halves of the commutator


Side view of a split ring commutator

v

+

_

The voltage across the split ring commutator is given by

This voltage is never negative as it does not change

polarity

This simple machine is called a DC Generator

ROTATING MACHINES


ROTATING MACHINES - DC GENERATOR


DC GENERATOR - CONSTRUCTION

Stator comprising of stationary parts

Magnetic Field is generated when the

Field Windings situated on the poles

are fed with a DC current

More than 1 pair of poles can be used

to have larger B

Rotor comprising of rotating parts

Rotor consists of a cylindrical iron core

that has slots for housing the armature

conductors and a commutator along

with associated brushesShown here are 8 conductors forming 4

coils and a commutator with 4 segments


Conductors 1 and 1’ form coil 1 and so forth

Brushes are depicted as riding on the inside but they

actually make contact with the outside of the commutator

Passing DC current through the field windings produces

Uniform Magnetic Fields in the small air gaps between

the poles and the rotor

When armature windings are connected to the commutator

in an appropriate manner as shown, the individual EMF’s

will not only be rectified but they will be added together

as well

coil-commutator connections

DC GENERATOR

Coils 1 and 2 are in series

and so are coils 3 and 4


DC GENERATOR

EMF induced (e) across a single coil is given as

Voltage across the brushes (v) is given as

When the coils are connected in an appropriate fashion

Small notches in the

waveform result from the

brushes changing from

different segments of the

commutator

In practice many coils are

employed and this

commutator ripple is

quite small


DC GENERATOR – GENERATED VOLTAGE

The generated voltage is given by

where

p -> number of poles

N -> number of armature conductors

a -> number of parallel paths between the brushes

-> flux per pole

n -> Speed of rotation in r.p.m

Ra - Armature Resistance (Resistance of the armature coils,

brushes etc. that complete the circuit)


GENERATOR SYMBOL

-> Field Current𝑖a -> Armature Current

A Generator can be represented in a circuit as :

Induced No load armature EMF ->

constant

𝒗 𝒈=𝑲 ∅ 𝒏Depends on


MAGNETIZATION CURVE OF A DC GENERATOR

Note that some

EMF is induced in

the Generator even

though the field

current is 0. This is

because of the

residual magnetism

of the poles


GENERATOR FIELD EXCITATION

1. SEPARATELY EXCITED GENERATOR

The Field Windings are excited by a separate

source

2. SHUNT CONNECTED GENERATOR

The Field Windings are excited using the

generator’s own induced voltage. This is

done by connecting the Field in parallel with

the Armature



Shunt connected generator is an example of self excited generator

In a shunt connected generator, the Field Winding has many turns and relatively high

field resistance so that the Field current is a small fraction of armature current

Field current can further be controlled by using a Rheostat in series with the field as

shown


3. SERIES CONNECTED GENERATOR


The Field Winding is placed in series with the

Armature

Field current is same as Armature Current and

Field Winding has less turns and less Resistance

This is done to reduce the voltage drop across the

Field Winding even under No Load conditions

We can place a Diverter Resistance in parallel

with the Field Winding as shown to reduce the

voltage drop across the Field Winding



4. COMPOUND CONNECTED GENERATOR

Connecting the Field Winding in

shunt as well as in series with the

Armature as shown

Rheostat and a Diverter Resistance

can be employed as shown



4. COMPOUND CONNECTED GENERATOR

CUMULATIVE COMPOUND DIFFERENTIAL COMPOUND

EMF due to Shunt Field is added to the EMF that is induced by the Series Field

EMF due to Shunt Field is subtracted to the EMF that is induced by the Series Field


GENERATOR BUILDUP

Consider a self excited shunt generator with a control rheostat

When the Switch is open (), some EMF still induced due to

Residual Magnetism

When Switch is closed, this EMF acts on the Field Winding

generating more hence more induced EMF

Repetitive process of

increasing generated

voltage and field current

is known as generator

buildup

Due to Residual Magnetism


EXTERNAL CHARACTERISTICS

Consider a separately excited DC Generator. vL - Load voltageiL - Load currentThe relationship between the load

current and the load voltage is given by

+

-

vL

𝑖 𝐿

𝑣 𝑔

𝑣 𝐿=−𝑅𝑎𝑖 𝐿+𝑣𝑔

Generated EMFArmature Resistance

The plot between load voltage and the

load current is termed as an external

characteristic. We can plot the graph

using the equation above. Theoretically,

it is a straight line but practically a bit

curved.



The external characteristics of a

separately and shunt connected DC

Generator


For a shunt connected generator load current is almost equal to armature current

Due to armature resistance, load voltage will reduce as load current increases

In addition, when the load voltage reduces, the field current decreases thereby

reducing the generated voltage even further


I am unable to understand this. It is a bit complicated


DC MOTORS

DC MACHINES

Engineering

Transcript of DC MACHINES