electric machines notes

8/11/2019 Electric Machines Notes

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Electric Machine I Lec # 3 and 4Dr. Suhail Aftab Qurashi Syed Abdul Rahman Kashif

The study of DC generator

Mechanical Energy Electrical Energy

Principle:

Faraday’s law of electromagnetic induction.Production of Induced emf:

Emf is induced in a coil when we have, Magnetic flux Flux linkage

Change of flux

Production of magnetic field:

Magnetic field can be produced by,

Permanent magnet Electromagnet

The production of magnetic field by permanent magnet is not a useful way as it is not

flexible, because they cannot control the magnitude of magnetic field. The productionof magnetic field in an electromagnet is flexible as we can change the magnetic field

by the change of current.

The direction of magnetic field can be found by right hand grip rule.The direction of magnetic field can be changed by two ways.

Changing the direction of current Changing the sense of windings

Flux linkage:

Flux linkages means that the magnetic lines of force produced in the field coil must

be cutting the conductor. Flux linkage can be achieved when, The flux linkage can be achieved when the coil lie close to the magnet. The magnetic axis and the coil axis must be coincident.

Change of magnetic flux.

Change of magnetic field can be produced by two ways.

Relative motion between the coil and the magnet:

Change of magnetic flux in a coil can be introduced by the relative motion betweenthe coil and the magnet. This relative motion may be linear or rotary. This type of emf

induced is known as dynamically induced emf

Changing current:

The change of magnetic flux can be introduced in the coil by changing the current in

the field coil. The type of emf induced is known as statically induced emf.

Basics• Generator action: An emf (voltage) is induced in a conductor if it movesthrough a magnetic field.

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• Motor action: A force is induced in a conductor that has a current goingthrough it and placed in a magnetic field• Any DC machine can act either as a generator or as a motor.

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DC Motor Stator Construction:

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Yoke:

The outer frame or yoke serves double purpose: It provides mechanical support for the poles and acts as a protecting cover for the

whole machine It carries the magnetic flux produced by the poles.

In small generators where cheapness rather than weight is the main consideration, yokes

are made of cast iron. But for large machines usually cast steel or rolled steel is

employed. The modern process of forming the yoke consists of rolling a steel slab rounda cylindrical mandrel and the-welding is at the bottom. The feet and the terminal box etc.

are welded to the frame afterwards. Such yokes possess sufficient mechanical strength

and have high permeability.

Pole Cores and Pole Shoes:

The field magnets consist of. pole cores and pole shoes. The pole shoes serve two

purpose they spread out the flux in the air gap and also, being of larger cross-section,

reduce the reluctance of the magnetic path they support the exciting coils (or field coils)

There are two main types of pole construction.

The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened to the pole face by means of counter

sunk screws. In modern design, the complete pole cores and pole shoes are built of thin

laminations of annealed steel which are riveted together under hydraulic pressure.

The thickness of laminations varies from 1 mm to 0.25 mm.The laminated poles may be secured to the yoke in arts of the following two ways:

either the pole is secured to the yoke by means of screws bolted through the yoke

and into the pole body or the holding screws are bolted into a steel bar which passes through the pole across

the plane of laminations.

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Armature Winding:

The armature windings are usually former-wound. These are first wound in the

form of flat rectangular coils and are then pulled into their proper shape in a coil puller.

Various conductors of the coils are insulated from each other. The conductors are placed in the armature which is lined with tough insulating

material. This slot insulation is folded over above the armature conductors placed

in the slot and is secured in place by special hard wooden or fiber wedges.

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Commutator A Commutator is a mechanical rectifier which converts AC to DC.

A Commutator is made of copper segments insulated from each other by mica

sheets. It is mounted over the shaft of the machine.

The ends of the armature windings are connected to the Commutator segments ina proper manner so its purpose is to facilitate the collection of current from

armature winding. It is of cylindrical structure and is built up of wedge-shaped segments of high-

conductivity hard-drawn or drop forged copper. These segments are insulated

from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each

commutator segment is connected to the armature conductor by means of a copperlug or strip (or riser). To prevent them from flying out under the action of

centrifugal forces, the segments have V-grooves, these grooves being insulated by

conical micanite rings.

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Brushes and Bearings

The brushes, whose function is to collect current from commutator, are usually

made of carbon or graphite and are in the shape of a rectangular block. These brushes are housed in brush-holders usually of the box-type variety.

The brush-holder is mounted on spindle and the brushes can slide in therectangular box open at both ends. The brushes are made to bear down on the commutator by a spring whose tension

can be adjusted by changing the position of lever in the notches. A flexible copper pigtail mounted at the top of the brush conveys current from the

brushes to the holder.

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The number of brushes per spindle depends on the magnitude of the current to becollected from the commutator.




Because of their reliability, ball-bearings are frequently employed, though for

heavy duties roller bearings are preferable. The ball and rollers are generally packed in hard oil for quieter operation and for

reduced bearing wear. Sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir

in the bearing bracket.------------------------------------------------------------------------------------------------------

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Armature WindingPole Pitch (Yp):

The distance between two consective opposite poles in term of armature slots is

known as pole pitch. 1 pole pitch is equal to 180 degree electrical. It can also be defined as the no. of armature slots per unit pole.

.

Total SlotsYp

No of poles=

Note:

Emf is only produced in the sides of the coil and a connection serves only to make the

current path complete.

Coil Pitch:

It is also called coil span or back pitch.

It is defined as the distance between the two sides of the same coil in terms ofnumber of armature slots. eg if Yb=5 it means that one side of the coil is in slot 1

and the other side is in the slot no. 6.

Full Pitch Winding:

If the pole span or the coil pitch is equal to the pole pitch then winding is called full

pitched. It means that the coil span is 180 degree electrical. In this case the coil sides lie

under opposite poles hence the induced emfs in them are additive therefore maximumemf is induced in the coil as a whole it being the sum of emfs induced in the two coil

sides eg if there are thirty six slots and four poles then 36/4=9 slots. if no. of slots is 35

then Yb =35/4= 8 because it is customary to drop fractions.

Fractional pitch winding:

If the coil span is less than the pole pitch then the winding is called fractional pith

winding. In this case there is a phase difference between the emfs in the two sides of the

coil hence the total emf around the coil is the vector sum of the emfs in the two coil sidesis less in this case as compared to that in the previous case.

Front Pitch (Yf):

The distance in terms of no. of slots between the finishing end of the first coil and the

starting end of the second coil is called front pitch.

Commutator pitch (Yc):

The number of commutator segments spanned by each coil of the winding is known as

commutator pitch.

Single layer winding:

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In this case only one coil side is placed in each armature slot.




Double layer Winding:

In this case two coils are placed in each armature side.

Starting and Finishing End:

These are the points where terminal of the coil are available.

Types of Winding:

There are two types of windings.

Lap winding and Wave Winding.

Lap Winding:

In lap winding the finishing end of one coil is connected to a Commutator

segment and to the starting end of the adjacent coil situated under the same pole

and so on.

Lap winding is the type of winding in which consective coils overlap partially.

Characteristics of Lap Winding:

.

(Even integer).

No of slots

No of ploes=Yp

1 for progressive winding

Yb = Yp - 1 for retrogressive winding

where Yb is back pitch and always +ve odd integer

Yb Yp= +

1 for progressive winding

Yf = Yp + 1 for retrogressive winding

where Yf is front pitch and always -ve odd integer

Yf Yp= −

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1Yc = ±




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Wave winding:

in wave winding the finishing end of the one coil in n pole is connected to the startingend of the next coil in the n pole and so on.

Consective two coils do not overlap

Characteristics: Average pole pitch

2 =even integer

S yp

p

±= average pole pitch

Yb = yp+1, yp-1, yp

Yf=yp-1, yp+1, yp Both yb and yf are of the same sign 2 yc

Conductor in each parallel path2

z=

In this case, no of parallel paths = no. of carbon brushes

=2 (always)

So it is a high voltage but small current winding.

Comparison:Lap winding Wave winding

Yp= z

p

2 z yp

p

+=

Yb=yp+1 Yb=yp+1, yp-1, yp

Yf=yp-1 Yf=yp-1, yp+1, yp

yb ≠ yf Yb may or may not equal to yf

Yb=+ve, yf=-ve Yb=+ve, yf=-ve

Carbon brushes = poles Carbon brushes=2

Parallel poles = poles Parallel pole =2

Low voltage High voltage

High current Small current

Power for the both lap and wave winding are almost the same, when all the others factorsare the same.

High current and low voltage application electrolysis

welding

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Emf formula:Let

flux

poleφ = in wb.

So flux cut by the conductor in one revolution = 2φ

That is ,d φ = 2φ

Now dt =time in one revolution

If N = speed of armature in r.p.m so60

(sec)dt N

=

For one turn loop, T=1

Faraday’s law of electromagnetic induction is given by,

2

(60 / )

d e

dt N

φ φ = =

2, 2

60

4, 4

60

N e p

N e P

φ

φ

= =

= =

Let ‘p’ be the number of pole in the DC machine

60

P N e

φ =

Here ‘e’gives the emf produced in each turn of the winding.

Now for, lap winding: ( )60

P N Z e

P

φ =

Wave winding: ( )60 2

P N Z e

φ =

Where ‘Z’ is the total number of armature conductors generally we can write,

( )60

P N Z e

A

φ =

Where A = 2 for wave winding

A=no. of poles for lap winding

Types of DC generator:

Generally classified on the bases of their field excitation. On this basis, they are mainly

classified into two types Spectator excited DC generator Self excited DC generator

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Separately excited DC generator:

In the generator, the field winding is excited by an external DC source

a- I

a l

t a

I I

V E R

=

=

Where R a is resistance of the armature winding. Separately excited DC generator is rarelyused in practice.

Self excited DC generator:

In self excited DC generator, no external DC source is needed for field excitation. The

field winding is connected to the armature winding, so the field current is provided by the

emf produced in the armature. Initially when there is no emf in the armature, no fieldcurrent, residual magnetic field is present in the field winding. Self excited DC

generators are commonly used in practice.

Types of self exited DC generators:

Self excited DC generators are often classified on the basis of the field connection witharmature winding.

They are generally of three types. Series type self excited DC generators. Shunt type Self excited DC generators. Compound type Self excited DC generator.

Series type:

In this configuration, the field winding is connected in series with the armature winding

so that the whole armature current flows through the field winding as well as through theload.

In this case, ( )

f l a

t a

I I I

V E I R R

= =

= − + f

Shunt type:

In shunt type generator, the field winding is connected in parallel with the armature

winding so that some of the armature current flows through the field windings and rest

through the load.

In this case,a I

a f c

t a

I I I

V E R

= +

=

Compound type:

In a compound type DC generator, we use the sets of field windings on each pole, one is

the series winding and the other is in parallel with the armature winding.

In this case1 2

f2 2I

a f f I I I

where I

= +

=

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Voltage build up phenomenon in self excited DC

generatorsWe know that, in self excited DC generators, initials 0 f I = and a I is produced due to the

residual magnetism present in the field winding. Now after the production of the armature

current, the field current f I increases, which in return, increases the n I which then

increases f I and so on.

So the emf in self excited DC generators goes on increases until the field winding is

saturated and the further increases in the current f I does not rein force the magnetic field.

At this point (steady state), the voltage becomes constant and maximum (rated voltage).This phenomenon is known as voltage build up phenomenon.

Condition:

Residual magnetism Connect the field winding with the armature winding properly The resistance of the field windings should not be very high, that is, it should be

less than the critical resistance.VI characteristics:

Series type:

In this type of DC generators,

a f l I I = =

f

f

a

t

, R 1.2

R 8

I=0

V

t a a f V E I R I R

usually

when

E

= − −

= Ω

= Ω

=

I

As (load voltage) varies by varying the load, so it is not use for domestic purposes.t V Shunt type:

In this type of DC generator,

f I

a f c

t

f

t a

I I I

V where

a

R

V E I R

= +

=

= −

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Ideally the field winding resistance is infinite all the current pass through the load and if

a R is negligible, the load voltage almost remains constant so it is generally used for

domestic purpose.




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Characteristics of Field Winding:

Series Type:

Rf should be very small. (Rf=1.2 ohm) The wire should be very thick.

Full load current passes through this resistance. Few number of turns are generally sufficient as NI φ = where I is quite large.

Critical Resistance: The maximum total series resistance that is (R L+Rf) with

which the series DC generator would just excite is known as its critical resistance.

Shunt Type:

Rf should be very large. Rf=1K ohm

The wire should be very thin.

It carries a very small amount of current usually in milliampere. It has large number of turns as the current is very small.

Critical resistance: The maximum field resistance with which the DC generatorof shunt type would just excite is known as its critical resistance.

Note:

As a characteristic of field windings for series type and shunt type DC generator are

much different from each other so they are not interchangeable.

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