TOPIC 3

DC MACHINES

DC Machines

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Historical DevelopmentDirect current (DC) motor is one of the first

machines devised to convert electrical power into mechanical power.

Its origin can be traced to the disc type machines inverted by Michael Faraday

Faraday’s primitive design was quickly improved upon and many DC machines were built in the 1880s when DC was principle form of electric power generation

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Historical Development

With the advent of alternating voltage and subsequent invention of induction motor with lower manufacturing costs DC machine became less important.

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In recent years, the use of DC machines has become almost exclusively associated with application where:

the unique characteristic of DC motor of high starting torque justify its cost, or

portable equipment must run from a DC power supply( or battery).

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Historical Development

Historical BackgroundThe ease with which DC motor lends itself to

speed control has long been recognised.

Compatibility with power electronic devises like thyristors (SCR) plus better performance due to availability of new improved material in magnetic circuits and brushes has also revitalised interest in DC machines

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Historical DevelopmentToday conventional DC generators are being

replaced by the solid state rectifiers where ac supply is available.

The same is not true for dc motors because of

Constant mechanical power output or constant torque

High starting torque

Rapid acceleration or deceleration

Responsiveness to feedback signals

Power rating:1W to 10,000 hp

Application of dc motors

steel and aluminum rolling mills,

traction motors in electric vehicles , electric trains and overhead cranes,

control devices and robots,

hand power tools ,

kitchen gadgets like food blenders etc.

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Construction of

DC Machines

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Main Parts

A D.C. machine consists of the three main parts:

Stator

Rotor or Armature

Commutator and brush gear

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DC machine Construction

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Stator

DC machine stator with visible poles.

StatorStator is the stationary (fixed) part of the

machine.

It produces the main magnetic flux.

Stator consists of the following parts:

Yoke Is the outer frame of the stator which is a hollow

cylinder of cast steel or rolled steel.

An even number of pole cores are bolted to the yoke

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StatorYokeYoke serves the following two purposes:

(i) it supports the pole cores and acts as

the protecting cover to the machine

(ii) it forms a part of the magnetic circuit

Pole coresAre made of sheet steel laminations that are

insulated from each other and riveted together

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StatorField WindingsEach pole core has one or

more field windings placed over it to produce a magnetic field.

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The field windings are connected in series with one another such that when the current flows through them , alternate north and south poles are produced in the direction of rotation.

dcmotor 16

A Two Pole DC Motor

dcmotor 17

A Four Pole DC Motor

Rotor or ArmatureThe armature is the

rotating part of the dc machine

It consists of a shaft upon which a laminated cylinder called armature core is mounted.

Armature core has slots on its outer surface which accommodate the armature windings

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Purpose of the Armature

The purpose of the armature is to provide the energy conversion in a DC machine.

armature converts mechanical energy

to electrical energy.

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Armature Winding in a DC Machine

Armature

The conductors are suitably connected to form armature winding.

Two types of winding are used, namely WAVE and LAP.

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Rotor or Armature

Rotor of a dc motor.

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The commutator is a mechanical rectifier which produces

direct current from alternating current generated in the

armature coils.

The commutator which rotates with the armature is

made up of a series of segments which are conductors

separated by thin pieces of an insulator such as mica.

Commutator and Brush gear

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DC Motor Construction

Details of the commutator of a dc motor.

Commutator and brush gear

Each commutator segment is connected to the ends of the armature coil.

Current is collected from or delivered to the armature by means of two or more carbon brushes mounted on the commutator.

Each brush is supported in a metal box called brush holder.

Pressure exerted by the brushes on the commutator can be adjusted and maintained at constant value by a spring.

Commutator and Brush gear

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BrushesThe brushes on a d.c. machine are made up

of carbon.

Carbon has a number of qualities that make it ideal for this purpose:-

It is very soft;

It is a conductor;

It self-lubricates.

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DC Motor Construction

Cutaway view of a dc motor.

EXCITATION

The magnetic flux in a dc machine is produced by field coils carrying current.

The production of magnetic flux in a machine by circulating current in the field winding is calledEXCITATION.

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Types of dc machines

There are two methods of excitation, namely separate excitation and self excitation.

In separate excitation the field coils are energised by separate dc source.

In self excitation the current flowing through the winding is supplied by the machine itself.

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Types of DC Machines

In self excited machine, residual magnetism must be present in the ferromagnetic circuit of the machine in order to start the self-excitation process.

Direct current machines are named according to the connection of the field winding with the armature.

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Types of dc machines

The principle types of DC machines are:

Separately excited DC machine

Shunt-Wound DC machine

Series-Wound DC machine

Compound machine

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Types of DC Machines

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separate excitation series,

shunt compound

Separately excited

Separately excited dc machine

The field coils are energised by a separate dc source.

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Shunt woundShunt wound dc machine the field coils are connected in parallel with

the armature.

Since the shunt field receives the full output voltage of the generator or the supply voltage of a motor, it is generally made of large number of turns of fine wire carrying a small field current

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Series wound

Series wound dc machine

Field coils are connected in series with the armature.

Series winding carries the armature current and consists of few turns of wire of large cross-sectional area.

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Compound wound

Compound wound dc machine

machine having both a shunt winding and series coils.

The shunt winding has many turns of fine wire and the series winding has few turns of large cross-sectional area.

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Compound wound Compound machine may be connected in two

ways:

short shunt compound machine , if the field is connected in parallel with the armature alone

long shunt compound machine , if the shunt field is in parallel with both armature and series field.

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Compound wound

If the magnetic flux produced by the series winding assists(aids) the flux produced by the shunt field winding, the machine is called cumulatively compounded.

If the series flux opposes the shunt field flux, the machine is said to be differentially compounded.

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Equivalent Circuit of a DC Machine

Separately excited dc machine

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Self excited dc machines

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Self excited dc machine

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Machine equations

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RIEV

RIV

DC Generator

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Introduction

The generator is based on the principle of electromagnetic induction discovered in 1831 by Michael Faraday.

Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electric current will flow in the conductor.

So the mechanical energy of the moving wire is converted into the electric energy of the current that flows in the wire as alternating current. 46

Introduction

To change the Simple Generator into a direct-current generator, the current must be made to move in only one direction.

In a dc generator this is achieved by use of mechanical rectifier referred to as COMMUTATOR.

The generated dc is the transferred to the generator terminals through carbon brushes.

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Residual magnetism in self excited DC Generator

The soft iron of the electro-magnets retains a

small amount of magnetism, known as residual

magnetism, even when there is no field current.

This residual magnetism is sufficient to induce

an EMF in the armature of the generator when

it first starts to rotate, which initiates a current

flow from the generator.

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Residual magnetism

Residual magnetism may be lost, due to excess heat, shock or reversal of field current flow.

The residual magnetism can be restored by briefly passing a current through the field.

This is known as field flashing, or flashing

the field.

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Principle of operation of DC Generator

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Convert ___________________ energy to ___________ energy.

Mechanical (motion)

Basic DC Generator

Demo:Spinning a coil of wire in a magnetic field produces _______ current alternating

electrical

Simple DC generator

Let us consider a simple DC generator with following components:

A magnetic field

A single conductor, or loop

A commutator and

Brushes

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Principle of operationThe North and South poles produce a dc

magnetic field.

As the loop is turned in the field the conductor sides cut the magnetic flux lines, which induce an alternating voltage in the conductors

The induced alternating voltage is first changed into unidirectional voltage by the commutator and brushes and then delivered to the load connected across the terminals.

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Commutation

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Commutation is the process of producing aunidirectional or direct current from thealternating current generated in the armaturecoils.

Voltage output using slip rings

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Voltage output using split rings(commutator)

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Simple generator

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Armature ReactionWhen current flows in the armature

conductors it produces a magnetic field surrounding the conductors.

The armature flux reacts with the main flux.

The effect of the armature flux on the` main field flux is called armature reaction.

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Armature Reaction

The armature flux has two effects on the main flux:

(i) It distorts the main flux which causes sparking at the commutator

(ii)It weakens the main flux which reduces the generated voltage

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Effect on flux distribution:magnetic neutral plane shift

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(a)Initially the pole flux isuniformly distributed andthe magnetic neutral planeis vertical

(b) Effect of the air gap onthe flux field causes thedistribution of the flux tobe no longer uniformacross the rotor

Effect on flux distribution: magnetic neutral plane shift

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(c) The fields interact to produce a different flux distribution in the rotor.

(d) The combined flux in the machine has the effect of strengthening or weakening the flux in the pole. Neutral axis is therefore shifted in the direction of motion.

Problems with commutation in real DC machines

One of the problems associated with armature reaction is arcing and sparking at the brushes!

Methods of improving commutationSPARKING can be minimised by:

Use of high resistance brushes which increase the circuit resistance of the coils undergoing commutation

Use of interpoles which are small auxiliary poles placed midway between the main poles. They are connected in series with the armature.

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Interpoles or compoles

Stator of a dc machine showing interpoles

Interpoles

Main poles

Lap and Wave winding

Armature coils are connected to form either LAP or WAVE

windings.

Lap Winding: Ends of the armature coil are connected to adjacent segments on the

commutator so that the total number of parallel paths is equal to total number of the poles ie A=P.

Used in low-voltage, high current machine

Wave Winding: The ends of each armature coil are connected to armature segments

some distance apart, so that only two parallel paths are provided between the positive and negative brushes, ie A=2.

Used in high voltage, low current machines.

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