dc machine - beee

8/12/2019 Dc Machine - Beee

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CONSTRUCTION OF DC MACHINE


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Armature Core or Stack

The armature stack is made up thin magnetic

steel laminations stamped from sheet steel

with a blanking die. Slots are punched in the

lamination with a slot die. Sometimes these

two operations are done as one. The

laminations are welded, riveted, bolted orbonded together.


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Armature Winding

The armature winding is the winding, which

fits in the armature slots and is eventually

connected to the commutator. It either

generates or receives the voltage depending

on whether the unit is a generator or motor.

The armature winding usually consists ofcopper wire, either round or rectangular and

is insulated from the armature stack.


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Field Coils

The field coils are those windings, which are

located on the poles and set up the magnetic

fields in the machine. They also usually consist

of copper wire are insulated from the poles.

The field coils may be either shunt windings

(in parallel with the armature winding) orseries windings (in series with the armature

winding) or a combination of both.


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Yoke

The yoke is a circular steel ring, which

supports the field, poles mechanically and

provides the necessary magnetic path

between the pole. The yoke can be solid or

laminated. In many DC machines, the yoke

also serves as the frame.


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Poles and pole shoe

IF POLE SHOE ENLARGES, LARGER INDUCEDEMF


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Commutator

The commutator is the mechanical rectifier,

which changes the AC voltage of the rotating

conductors to DC voltage. It consists of a

number of segments normally equal to the

number of slots. The segments or commutator

bars are made of silver bearing copper and areseparated from each other by mica insulation.


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Brushes and Brush HoldersBrushes conduct the current from the

commutator to the external circuit. There aremany types of brushes. A brush holder isusually a metal box that is rectangular in shape.The brush holder has a spring that holds thebrush in contact with the commutator. Eachbrush usually has a flexible copper shunt whichextends to the lead wires. Often, the entire

brush assembly is insulated from the frame andis made movable as a unit about thecommutator to allow for adjustment.


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Interpoles

Interpoles are similar to the main field polesand located on the yoke between the main

field poles. They have windings in series with

the armature winding. Interpoles have thefunction of reducing the armature reaction

effect in the commutating zone. They

eliminate the need to shift the brush

assembly.


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Frame, End Bells, Shaft, and Bearings

The frame and end bells are usually steel,

aluminum or magnesium castings used toenclose and support the basic machine parts.

The armature is mounted on a steel shaft,

which is supported between two bearings.The bearings are either sleeve, ball or roller

type. They are normally lubricated by grease

or oil.


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Back End, Front End

The load end of the motor is the Back End.

The opposite load end, most often the

commutator end, is the Front End of themotor.


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Armature Windings


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WINDING TERMINOLOGIES

LAP WINDING

LAP WINDING CONNECTED TO

COMMUTATOR BARS

LAP WINDING


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When the end connections of the coils arebrought to adjacent bars a lap or parallel winding

is formed. In this type winding, there are as manypaths through the armature as there are poles onthe machine. Therefore, to obtain full use of thistype winding, there must be as many brushes as

there are poles, alternate brushes being positiveand negative. The outermost connecting linesrepresent the end connections on the back of thearmature and the inner connecting lines represent

the connections on the front or commutator endof the armature. The lap winding is best suitedfor low voltage, high current ratings because ofthe number of parallel paths.


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LAP WINDING IN CIRCULAR FORMLap Winding,


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Wave Winding

WAVE WINDING


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The WAVE winding is best suited for high

voltage, low current ratings because of the

number of parallel paths.


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E.M.F Equation

Let

= flux/pole in weber

Z = total number of armature conductors

= No.of slots x No.of conductors/slotP = No.of generator poles

A = No.of parallel paths in armature

N = armature rotation in revolutions per minute (r.p.m)

E = e.m.f induced in any parallel path in armature

Generated e.m.f Eg = e.m.f generated in any one of the

parallel paths i.e E.Average e.m.f generated /conductor = d/dt volt (n=1)

Now, flux cut/conductor in one revolution d = P Wb

No.of revolutions/second = N/60

Time for one revolution, dt = 60/Nsecond

Hence, according to Faraday's Laws of Electromagnetic

Induction,


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E.M.F generated/conductor is

For a simplex wave-wound generator

No.of parallel paths = 2=A

No.of conductors (in series) in one path = Z/2

E.M.F. generated/path is

For a simplex lap-wound generator

No.of parallel paths = P=A

No.of conductors (in series) in one path = Z/P

E.M.F.generated/path

In general generated e.m.f where A = 2 - for simplex wave-winding

= P - for simplex lap-winding

=e = Rate of cutting the flux


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A1-A2 ENDS OF ARMATURE

F1-F2 FIELD WINDING

SYMBOL REPRESENTATION OF DC GENERATOR.


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TYPES OF DC GENERATORS


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Flemings Right hand rule


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SEPARATELY EXCITED GENERATOR

The field wdg is supplied from external,

separate dc supply (i.e.) excitation of field wdg

is separate, then the generator is called

separately excited generator.


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SEPARATELY EXCITED GENERATOR

DC SUPPLY

Ia=IL

F1

F2

A1

A2

IF


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Ia= IL

Emf e is not equal to Vt.

IaRais minimum.

Some voltage drop at the contacts of the brush.

So voltage equation is

E=Vt

+Ia

Ra

+Vbrush

E=(PNZ)/(60A)

Vbrushis negligible.


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SELF EXCITED GENERATOR

1. Though Generator does not work, without anycurrent through field wdg, possess somemagnetic flux. This is residual flux and property isresidual magnetism.

2. Voltage building processsmall emf drives smallct through field wdg, further flux producesincreasing field ct and flux. The process iscumulative and continues till generator develops

rated voltage across armature.3. Three typesShunt , series and compound

Generator.


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SHUNT GENERATOR

F1

F2

IaA1

A2

Ish

Vt


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VOLTAGE AND CURRENT RELATIONS

Ia=IL+Ish

Ish=Vt/Rsh

Induced emf still supply IaRavoltage drop and

brush contact drop.

E=Vt+IaRa+Vbrush

E=(PNZ)/(60A)


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Series Generator

IaA1

A2

S1 S2

Vt


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VOLTAGE AND CURRENT RELATIONS

Ia=Ise=IL

E=Vt+IaRa+IaRse+Vbrush

E=Vt+ Ia(Ra+Rse)+Vbrush

E=(PNZ)/(60A)


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Compound Generator

Long shunt Compound Generator

Ia=Ise

Ia=Ish+IL

Ish=Vt/Rsh

Rsh=Resistance of shunt field wdg

E=Vt+IaRa+IaRse+Vbrush

Rse=Resistance of series field wdg.


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Long Shunt Compound Gr

S1

S2

A1

A2

F1

F2

Ish

Ise


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Short shunt Compound Gr

F1

F2

A1

A2

Ia

S1 S2


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Ia=Ise+Ish

Ia=IL+Ish

Ise=IL

Drop across shunt field wdg is drop across armature only not

on Vt. The drop is E-IaRaIsh=(E-IaRa)/Rsh

E=Vt+IaRa+IseRse+Vbrush

Ise=IL

E=Vt+IaRa+ILRse+VbrushE=Vt+IaRa+ILRse

E-IaRa=Vt+ILRse

Ish=(Vt+ILRse)/Rsh


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Cumulative and Differential Compound

Generator

shse

Differential compound Generator


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T=sh+se

sh=Flux produced by shunt winding

se= Flux produced by series , field winding

Depending on the direction of the winding of the poles,

two fluxes produced by shunt and series field may helpor oppose each other.

If the two fluxes help each other, then it is cumulativecompound generator.

If two fluxes oppose each other, then it is differentialcompound generator.

T=sh-se


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Applications of DC generator

1. Shunt generators are extensively used for general light andpower supply, and for charging of batteries, since, in conjunctionwith a field regulator, a constant terminal voltage can bemaintained at all loads.

2. Series generators are mainly used as animation boosters in dc

transmission system, in order to compensate for the drop of voltagedue to the resistance of transmission conductors.

3. compounded generators find use in dc transmission, since it ispossible to keep on a constant voltage at the load end, bygenerating a larger voltage so as to overcome the line drop.

Cumulative compound generator- used for domestic lighting

purposes and to transmit energy over long distances.

differential compound generatorare very rare and used forspecial application like electric arc welding.


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DC Motor

A DC motor is a device which converts

electrical energy into mechanical energy.

D.C. motors are motors that run on Direct

Current from a battery or D.C. power supply.

Direct Current is the term used to describe

electricity at a constant voltage.


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The principle of operation iswhen a current

carrying conductor is placed in a magnetic

field, it experiences mechanical force.

Two fluxes are present.

i) Flux produced by permanent magnet is

called main flux

ii) Flux produced by current carrying conductor


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Principles of Operation

Force in DC Motor


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Magnetic Field in DC Motor


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Torque in DC Motor


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Current Flow in DC Motor


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The magnitude of force experienced byconductor in a motor is given by

Force, F = B lInewton

Where B is the flux density due to flux producedby field winding in weber/m2.

Iis the current in amperes and

lis the length of the coil in meter.

The force, current and the magnetic field are allin different directions.


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Flemings Left hand rule:


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Back EMF in DC motor.


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In generating action, conductor cuts the magnetic flux,emf is induced in conductor.

In DC motor, after motoring action, armature rotatesand armature conductors cut the main flux. So this is

generating action in motor after motoring action.

Induced emf in rotating armature conductors accordingto Faradays law of electromagnetic induction.

Induced emf in armature always act in oppositedirection to supply voltage.

This is Lenz lawdirection of induced emf is always soas to oppose the cause producing it.


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In DC motor, electrical i/p, supply voltage is causefor armature current and in motoring action,induced emf opposes supply voltage.

Emf induced sets up the current through

armature which is in opposite directionsupplyvoltage is forcing through conductor.

Emf always opposes supply voltage - back emf,Eb.

The magnitude is determined byEb= (PNZ)/(60A).


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Symbol of Ebin DC mtotr


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Voltage equation of DC motor

V=Eb+IaRa+Brush drop

Ia=(V-Eb)/Ra

Si ifi f b k f


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Significance of back emf Due to back emf, DC motor is a regulating machine.

Back emf N

When there is load, motor slows down. So the speed of motorreduces, so Ebalso reduces.

The net voltage across armature (V-Eb) increases and motor drawsmore armature current.

Due to increased armature current, force experienced by conductorsand torque on armature increases.

The increase in torque satisfy increase in load.

When load of motor is decreased, speed of motor increases. So Ebincreases.

(V-Eb) cause to reduce, reducing Ia.

The motor speed stops increasing when Ia is enough to produce lesstorque by new load.

So back emf regulates flow of Ia and Ia meets load requirement.

This is the significance of back emf.


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Power Equation of DC motor

V=Eb+IaRaMultiplying by Iaon both sides,

VIa=EbIa+Ia2Ra

VIa=Net electrical power i/p to armature

Ia2Ra=Power loss due to resistance of armature

called armature copper loss.

Difference between Via and Ia2Rais InputLosses givesoutput of armature

EbIais equation of gross mech. Power developed.Power input to armaturearmature copper loss = Gross

mech power developed in armature


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Torque equation of DC motor

The turning or twisting force about an axis iscalled torque.

T= F*R

The wheel rotating at a speed of N rpm,then angular speed is

= (2N)/60 rad/sec.

The work done in one revolution isW= F*Distance travelled in one revolution

=F*2R Joules

P = Power developed = Work

done/

Time

(=F* 2R )/time

for 1 rev.

=(F* 2R )/(60/N)=(F*R)*((2N)/60 )

P=T*watts

T = Torque in Nm

= Angular speed in rad/sec.

Ta-gross torque by armature of motor. It isalso called as armature torque.

The gross mech. Power developed in armature isEbIa.

The speed of motor is N rpm, then

Power in armature = Armature torque

*

EbIa=Ta*((2N)/60 )

Ebin motor is, Eb= (PNZ)/(60A)

(PNZ)/(60A) *Ia =Ta*((2

N)/60 )Ta= (1/2)Ia*(PZ)/A = 0.159

Ia*(PZ)/ANm


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Types of torque in Motor:

There is power loss due tofriction, windage and ironlosses.

The torque which overcomethese losses is called losttorque denoted as T

f.

These losses are also called asstray losses.

The torque at the shaft doesuseful work is called Loadtorque or shaft torque

denoted as Tsh. Ta= Tf+Tsh

Ta = armature torque

Tsh


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No Load condition of Motor

On no load, Tsh= 0 So motor can rotate at a

speed N0rpm on no load.

The motor draws armature

current Ia0. Ia0= (VEb0)/Ra

Eb0back emf on no load to speed N0

TaIa. Flux is present and Iais

present ; Ta0exists on noload (armature torque)

Ta= Tf+Tsh

Tsh= 0

Ta0= Tf

Power developed (Eb0*Ia0) =

Friction , windage and ironlosses.

The stray losses Eb0Ia0isconstant though the load onmotor changes from 0 tofull capacity of motor.

dc machine - beee

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