part 12 emachines EEP 3243

8/9/2019 part 12 emachines EEP 3243

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ELECTRICAL MACHINE

EEP 3243

Lt Cdr Ong Khye Liat RMN

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DC MACHINES


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Overview The steam age signaled the beginning of an industrial revolution.

The advantages of machines and gadgets in helping mass

production.

Thus a search for new sources of energy and novel gadgets

received great attention.

By the end of the 18th century the research on electric charges

received a great boost with the invention of storage batteries.

The moving charges or currents was discovered also associated

with magnetic field like a lodestone.

This led to the invention of an electromagnet and later the forceexerted on a current carrying conductor placed in the magnetic

field was invented.

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Cont. This can be termed as the birth of a motor.

Parallel research was contemporarily being done to invent a source

of energy to recharge the batteries in the form of a d.c. source of

constant amplitude (or d.c. generator).

The research on d.c. motors and d.c. generators proceeded onindependent paths.

The invention of a commutator paved the way for the birth of d.c.

generators and motors.

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Cont. The limitations of the d.c. system however became more and more

apparent as the power demand increased.

The invention of induction machines in the 1880s tilted the scale

in favor of a.c. systems mainly due to the advantage offered by

transformers The d.c. system, however could not be obliterated due to the able

support of batteries. Further, d.c. motors have excellent control

characteristics.

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Lodestone

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Alodestone or

loadstone

is a naturally magnetized piece

ofthe mine

ral

magnetite. They are naturally occurring magnets, that attract pieces of

iron.


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AC Generator

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Cont.

Because of their similar construction, the

fundamental properties of generators and motors

are identical.

If the brushes in AC generator could be switchedfrom one slip ring to the other every time the

polarity was about to change, we would obtain a

constant polarity voltage across the load.

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Cont.

A commutator in its simplest form is composed of a

slip ring that is cut in half, with each segment

insulated from the other as well as from the shaft.

The commutator revolves with the coil and thevoltage between the segments is picked up by 2

stationary brushes.

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Cont.

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Difference Between

AC and DC Generator

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Improving the Waveshape By increasing the number of coils and segments,

we can obtain a DC voltage that is very smooth.

The coils are lodged in a slots of a laminated iron

cylinder, that both constitute the armature.

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Cont. The % ripple is the ratio of RMS value of the AC

component of voltage to the DC component.

(Modern DC generator produce voltage having a

ripple of less than 5%.

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Cont. Actual physical

construction

For reason of symmetry,

the coils are wound so

that 1 coil side is at thebottom of a slot and the

other is at the top.

This armature winding is

called a lap winding.

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Cont. The voltage ea induced in coil

A is exactly the same as thevoltage ec induced in coil C.However , the coil A is movingdownward and coil C ismoving upward. The polarities

of ea and ec, eb and ed areopposite.

This means that ea + eb + ec +ed = 0 at all times

The voltage between the

brushes is equal to (ea + ed) or(eb + ec)

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Induced Voltage

When the armature rotates, the voltage E induced ineach conductor depends upon the flux density

which it cuts.

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Cont. The conductors in slots 1 and 7 are exactly between the

poles, where the flux density is zero so the voltage induced

is zero. On the other hand, the conductors in slots 4 and 10

are directly under the centre of the poles, where the flux

density is greatest.

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Cont. The induced voltage remains essentially constant

as the armature rotates, because the number ofcoils between the brushes is always the same,irrespective of armature position.

Note that the brushes short circuit the coils inwhich the voltage is momentarily zero. They aresaid to be in the neutral position when they are;positioned on the commutator so as to short

circuit those coils in which the induced voltage ismomentary zero.

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Cont.

By shifting the brushes the output voltagedecrease. And in this position, the brushes

continually short circuit generated coils and cause

sparking (poor commutation).

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Neutral Zones

Neutral zones are those places on the surface of

the armature where the flux density is zero.

When the generator operates at no load, the neutral

zones are located exactly between the poles.

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Value of theInduced Voltage

Generated voltage is directly proportional to theflux/pole and the speed of rotation.

Only true if the brushes are on the neutral position.

Equation

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PROBLEM 1

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GeneratorUnderLoad: TheEnergy

ConversionProcess Because the conductors lie in a magnetic field, they

are subjected to a force according to Lorentz's Law.

The individual forces F on the conductors produce

a torque that acts opposite to the direction in whichthe generator is being drive.

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Cont.

To turn the generator, wemust exert a torque on the

shaft to overcome this

opposing electromagnetic

torque which resultingmechanical power is

converted into electrical

power.

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ArmatureReaction Drop The current flowing in the armature

coils also creates a magnetomotiveforce that distorts and weakens the

flux coming from the poles.

The effect of a magnetic field

produced by the armature mmf iscalled armature reaction drop.

The intensity of the armature flux

depends upon its mmf, which in turn

depends upon the current carried by

the armature.

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Cont. The flux in the neutral zone no

longer zero and will induces avoltage in the coils are short

circuited by the brushes.

Sparking may occur and its intensity

depend upon the armature flux andthe load current.

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Cont.

Armature mmf also distortsthe flux produced by the

poles and cause the neutral

zones have shifted in the

direction of rotation. This cause a reduction in the

induced voltage.

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Shifting the Brushes to

Improve Commutation

Due to the shift in the neutral zone, we could movethe brushes to reduce the sparking.

For generators, the brushes are shifted in the

direction of rotation and brushes motors are shiftedagainst the direction of rotation.

After the brushes are shifted the commutation

improves (less sparking).

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Cont.

This procedure is only practical to resolve small DCmachines commutation problem (if the load current

fluctuates).

To counter the effect of armature reaction inmedium and large power DC machines, a set of

commutating poles a placed between the main

poles.

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Cont.

The number of turns on thewindings is designed so that

the poles develop a mmf

equal and opposite to the

mmf of the armature. By nullifying the armature

mmf the flux in the space

between the main poles is

always zero.

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Types ofDC Generator

Separately Excited Generator. A pair of electromagnets (filed

poles) use to replace permanent

magnets.

DC field current is supplied by anindependent source (exciter)

As we increase the load, the

terminal voltage diminished

progressively (10%) because of

the voltage drop across armatureresistance, Ro.

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Cont.

ShuntGenerator (Self exited). Shunt-field winding is connected

in parallel with the armature

terminal, so that the generator

can self excited.

The progressive buildup

continues until Eo reaches a

maximum value determined by

the field resistance and the

degree of saturation.

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Cont. ShuntGenerator (Self exited).

We can determine the no-load value of

Eo if we know the saturation curve of

the generator and the total resistance

Rt of the shunt field circuit.

Critical resistance will be reachedwhere the slope of resistance line is

equal to that of the saturation curve in

its unsaturated region.

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Cont. ShuntGenerator (Self exited).

It is easy to control the induced voltage of a

shunt excited generator by vary the exciting

current.

The terminal voltage of a shunt generator

falls off more sharply (15%) with increasingload than a separately excited generator

because its exciting current falls as the

terminal voltage drops.

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Cont. CompoundGenerator.

This generator was developed to

prevent the terminal voltage of a DC

generator from decreasing with

increasing load.

It is similar to a shunt generator,except that it has additional field coils

connected in series with the armature.

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Cont. Compound Generator.

When no load, the current in the seriescoils is zero and shunt coils carry exciting

current which produces the field flux (just

like standard self excited shunt generator)

When loaded, load current flows through

the series field coils and a mmf developedby these coils acts in the same direction

as the shunt field mmf. So the field flux

under load rises above its original no load

value which raises the value of Eo.

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Cont.

Differential Compound Generator.

The mmf of the series field acts opposite to the shunt

field. So the terminal voltage falls drastically with

increasing load.

Used in DC arc welders, because they tended to limit

the short circuit current and to stabilize the arc during

the welding process.

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Construction ofDC Generator

To appreciate the working and the

characteristics of these machines, it is

necessary to know about the different parts of

the machine - both electrical and non-electrical.

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Cont. The major parts can be

identified as, Body

Poles

Armature

Commutator and brushgear

Commutating poles

Compensating winding

Other mechanical parts

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Body The body constitutes the outer shell within which all

the other parts are housed. This will be closed at both the ends by two end covers

which also support the bearings required to facilitatethe rotation of the rotor and the shaft.

Even though for the generation of an emf in aconductor a relative movement between the field andthe conductor would be enough, due to practicalconsiderations of commutation, a rotating conductorconfiguration is selected for DC. machines. Hence the

shell or frame supports the poles and yoke of themagnetic system.

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Main Poles Solid poles of fabricated steel with separate/integral

pole shoes are fastened to the frame by means of

bolts.

Pole shoes are generally laminated. Sometimes

pole body and pole shoe are formed from the samelaminations.

Riveted through bolts hold the assembly together.

The pole shoes are shaped so as to have a slightly

increased air gap at the tips.

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Inter-poles

These are small additional poles located in betweenthe main poles.

These are also fastened to the yoke by bolts.

These are also called as commutating poles orcompoles.

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Armature The armature is where the moving conductors are

located. The armature is constructed by stacking laminated

sheets of silicon steel. Thickness of theselamination is kept low to reduce eddy current

losses. The core is divided into packets to facilitate

ventilation.

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rmature construction process must ensureprovision of sufficient axial and radial ducts tofacilitate easy removal of heat from the armaturewinding.

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Field Winding As against permanent magnet excited machines the field winding

takes the form of a concentric coil wound around the main poles. These carry the excitation current and produce the main field in

the machine.

The resistance of such winding would be an order of magnitude

larger than the armature winding resistance. The total mmf required is divided equally between north and south

poles as the poles are produced in pairs. The mmf required to be

shared between shunt and series windings are apportioned as per

the design requirements.

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Armature Winding

The armature windings are in general pre-formed, taped andlowered into the open slots on the armature.

In the case of small machines, they can be hand wound. The coilsare prevented from flying out due to the centrifugal forces bymeans of bands of steel wire on the surface of the rotor in smallgroves cut into it.

In the case of large machines slot wedges are additionally used torestrain the coils from flying away.

The end portion of the windings are taped at the free end andbound to the winding carrier ring of the armature at thecommutator end.

The armature must be dynamically balanced to reduce thecentrifugal forces at the operating speeds.

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Compensating Winding

One may find a bar winding housed in the slots

on the pole shoes.

T

his is mostly found in d.c. machines of verylarge rating.

In smaller machines, they may be absent.

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Commutator It consists of copper segments tightly fastened

together with mica/micanite insulating separators on

an insulated base.

The whole commutator forms a rigid and solid

assembly of insulated copper strips and can rotate athigh speeds.

The surface of the commutator is machined and

surface is made concentric with the shaft and thecurrent collecting brushes rest on the same.

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Brush and Brush Holders Brushes rest on the surface of the commutator.

Normally electro-graphite is used as brush material. The actualcomposition of the brush depends on the peripheral speed ofthe commutator and the working voltage. The hardness of thegraphite brush is selected to be lower than that of the

commutator. The brush holders provide slots for the brushes to be placed.The connection from the brush is taken out by means of flexiblepigtail.

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he brushes are kept pressed on the commutator with the helpof springs. This is to ensure proper contact between thebrushes and the commutator even under high speeds ofoperation.

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Cont. Jumping of brushes must be avoided to ensure arc

free current collection and to keep the brush contactdrop low.

Radial positioning of the brushes helps in providingsimilar current collection conditions for both directionof rotation.

For unidirectional drives trailing brush arrangementor reaction arrangement may be used.

Reaction arrangement is preferred as it results in zeroside thrust on brush box and the brush can slidedown or up freely.

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OtherMechanical Parts End covers, fan and shaft bearings form other

important mechanical parts.

End covers are completely solid or have opening

for ventilation. They support the bearings which are

on the shaft. Fans can be external or internal. In most machines

the fan is on the non-commutator end sucking the

air from the commutator end and throwing the same

out.

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Bearings Small machines employ ball bearings at both ends.

For larger machines roller bearings are used

especially at the driving end.

The bearings must be kept in closed housing with

suitable lubricant keeping dust and other foreignmaterials away.

Care must be taken to see that there are no bearing

currents or axial forces on the shaft both of whichdestroy the bearings.

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Cont.

Consequently a four-pole generator could output twice thevoltage of a two-pole generator, a six-pole generator couldoutput three times the voltage of a two-pole. This allows outputvoltage to increase without also increasing the rotational rate.

In a multipole generator, the armature and field magnets aresurrounded by a circular frame or "ring yoke" to which the fieldmagnets are attached. This has the advantages of strength,simplicity, symmetrical appearance, and minimum magneticleakage, since the pole pieces have the least possible surface

and the path of the magnetic flux is shorter than in a two-poledesign.

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12-pole, 72-coil DC Generator

CoilA and C are

momentarily in neutral

zone, B is cutting the

flux coming from thecenter of the poles.

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Cont.

Coil width (coil pitch) is

the coil sides cut the flux

coming from the

adjacent N & S poles.

CoilA sides in slots 1

and 7 and they are in the

neutral zone. Coil B

sides in slot

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Cont. The voltage generated

between brushes x and y isequal to the sum of thevoltages generated by thecoils connected to

commutator segments.

The +ve brush sets areconnected together to formthe +ve terminal and so with

-ve brush sets.

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Torque

Torque is produced when a force exerts a

twisting action on a body, tending to make it

rotate.

Torque is equal to the product of the force times

the perpendicular distance between the axis of

rotation and the point of application of the force.

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Cont. The torque exerted on the pulley by

the tangential force is given by,

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PowerofaMachine

The mechanical power output of a machine dependsupon its rotational speed and the torque it develops.

The power is given by,

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Cont.We can measure the power output of

a

motor by means of a prony brake.

The torque is, T

The power is, P

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PROBLEM 2

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THE END

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Documents

part 12 emachines EEP 3243