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Armature core : Armature core is the rotor of a generator. Armature core is cylindrical in shapeon which slots are provided to carry armature windings.

Commutator and brushes : As emf is generated in the armature terminals, it must be taken outto make use of generated emf. But if we can't directly solder wires to Commutator conductors asthey rotate. Thus commutator is connected to the armature conductors and mounted on the sameshaft as that of armature core. Conducting brushes rest on commutator and they slides over whenrotor (hence commutator) rotates. Thus brushes are physically in contact with armatureconductors hence wires can be connected to brushes.

WORKING PRINCIPLE OF A DC GENERATOR :

According to Faraday's law of electromagnetic induction, when a conductor moves in amagnetic field (thereby cutting the magnetic flux lines), a dynamically induced emf is producedin the conductor. The magnitude of generated emf can be given by emf equation of DCgenerator. If a closed path is provided to the moving conductor then generated emf causes acurrent to flow in the circuit.

Thus in DC generators, when armature is rotated with the help of a prime mover and fieldwindings are excited (there may be permanent field magnets also), emf is induced in armatureconductors. This induced emf is taken out via commutator-brush arrangement.

EMF EQUATION OF A DC GENERATOR:

Let = flux/pole in Wb (weber)

Z = total no. of armature conductors

P = no. of generator poles

A = no. of parallel paths in armature N = rotational speed of armature in revolutions per min. (rpm)

E = emf induced in any parallel path in armature

Now,

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Generated e.m.f Eg = e.m.f generated in any one of the parallel paths i.e E.Average e.m.f geneated /conductor = d/dt volt (n=1)

Now, flux cut/conductor in one revolution d = P Wb No.of revolutions/second = N/60Time for one revolution, dt = 60/N second

Hence, according to Faraday's Laws of Electroagnetic Induction,E.M.F generated/conductor is

For a simplex wave-wound generator, No.of parallel paths = 2 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 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

A= P - for simplex lap-winding

CHARACTERISTICS OF DC GENERATOR :

In a separately excited DC generator, the field winding is excited by an externalindependent source. There are generally three most important characteristic of DC generator:

Magnetic or Open Circuit Characteristic of Separately Excited DC Generator

The curve which gives the relation between field current (I f ) and the generated voltage (E 0) in

the armature on no load is called magnetic or open circuit characteristic of a DC generator .The plot of this curve is practically same for all types of generators, whether they are separatelyexcited or self-excited. This curve is also known as no load saturation characteristic curve ofDC generator . Here in this figure below we can see the variation of generated emf on no loadwith field current for different fixed speeds of the armature. For higher value of constant speed,the steepness of the curve is more. When the field current is zero, for the effect residualmagnetism in the poles, there will be a small initial emf (OA) as show in figure.

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Let us consider a separately excited DC generator giving its no load voltage E 0 for a constantfield current. If there is no armature reaction and armature voltage drop in the machine then thevoltage will remain constant. Therefore, if we plot the rated voltage on the Y axis and loadcurrent on the X axis then the curve will be a straight line and parallel to X-axis as shown infigure below. Here, AB line indicating the no load voltage (E 0).

When the generator is loaded then the voltage drops due to two main reasons-

1) Due to armature reaction,

2) Due to ohmic drop ( IaRa ).

Internal or Total Characteristic of Separately Excited DC Generator

The internal characteristic of the separately excited DC generator is obtained bysubtracting the drops due to armature reaction from no load voltage. This curve of actuallygenerated voltage ( Eg ) will be slightly dropping. Here, AC line in the diagram indicatingthe actually generated voltage (E_g ) with respect to load current. This curve is also calledtotal characteristic of separately excited DC generator.

External Characteristic of Separately Excited DC Generator

The external characteristic of the separately excited DC generator is obtained by subtractingthe drops due to ohmic loss ( Ia Ra ) in the armature from generated voltage ( Eg ).

Terminal voltage(V) = Eg - Ia Ra.

This curve gives the relation between the terminal voltage (V) and load current. The externalcharacteristic curve lies below the internal characteristic curve. Here, AD line in the diagram

below is indicating the change in terminal voltage(V) with increasing load current. It can beseen from figure that when load current increases then the terminal voltage decreasesslightly. This decrease in terminal voltage can be maintained easily by increasing the field

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current and thus increasing the generated voltage. Therefore, we can get constant terminalvoltage.

Separately excited DC generators have many advantages over self-excited DC generators.It can operate in stable condition with any field excitation and gives wide range of outputvoltage.

The main disadvantage of these kinds of generators is that it is very expensive of providing a separate excitation source.

CHARACTERISTICS OF SELF EXCITED DC GENERATOR :

In shunt wound DC generators the field windings are connected in parallel with armatureconductors as shown in figure below. In these types of generators the armature current I a dividesin two parts. One part is the shunt field current I sh flows through shunt field winding and theother part is the load current I L goes through the external load.

Three most important characteristic of shunt wound dc generators are discussed below:

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Magnetic or Open Circuit Characteristic of Shunt Wound DC Generator

This curve is drawn between shunt field current(I sh) and the no load voltage (E 0). For a givenexcitation current or field current, the emf generated at no load E 0 varies in proportionally withthe rotational speed of the armature. Here in the diagram the magnetic characteristic curve forvarious speeds are drawn. Due to residual magnetism the curves start from a point A slightly upfrom the origin O. The upper portions of the curves are bend due to saturation. The external loadresistance of the machine needs to be maintained greater than its critical value otherwise themachine will not excite or will stop running if it is already in motion. AB, AC and AD are theslops which give critical resistances at speeds N 1, N 2 and N 3. Here, N 1 > N 2 > N 3.

Critical Load Resistance of Shunt Wound DC Generator

This is the minimum external load resistance which is required to excite the shunt woundgenerator.

Internal Characteristic of Shunt Wound DC Generator

The internal characteristic curve represents the relation between the generated voltage Egand the load current I L. When the generator is loaded then the generated voltage is decreased dueto armature reaction. So, generated voltage will be lower than the emf generated at no load. Herein the figure below AD curve is showing the no load voltage curve and AB is the internalcharacteristic curve.

External Characteristic of Shunt Wound DC Generator

AC curve is showing the external characteristic of the shunt wound DC generator. It isshowing the variation of terminal voltage with the load current. Ohmic drop due to armatureresistance gives lesser terminal voltage the generated voltage. That is why the curve lies belowthe internal characteristic curve.

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The terminal voltage can always be maintained constant by adjusting the of the load terminal.

When the load resistance of a shunt wound DC generator is decreased, then load currentof the generator increased as shown in above figure. But the load current can be increased to acertain limit with (up to point C) the decrease of load resistance. Beyond this point, it shows areversal in the characteristic. Any decrease of load resistance, results in current reduction andconsequently, the external characteristic curve turns back as shown in the dotted line andultimately the terminal voltage becomes zero. Though there is some voltage due to residualmagnetism.

Now, when I L increased, then terminal voltage decreased. After a certain limit, due to heavy loadcurrent and increased ohmic drop, the terminal voltage is reduced drastically. This drasticreduction of terminal voltage across the load, results the drop in the load current although at that

time load is high or load resistance is low.That is why the load resistance of the machine must be maintained properly. The point in whichthe machine gives maximum current output is called breakdown point (point C in the picture).

CHARACTERISTICS OF DC SERIES GENERATOR:

In these types of generators the field windings, armature windings and external load circuit allare connected in series as shown in figure below.

Therefore, the same current flows through armature winding, field winding and the load.

Let, I = Ia = Isc = I L

Here, Ia = armature current

Isc = series field current

IL = load current

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There are generally three most important characteristics of series wound DC generator whichshow the relation between various quantities such as series field current or excitation current,generated voltage, terminal voltage and load current.

Magnetic or Open Circuit Characteristic of Series Wound DC Generator

The curve which shows the relation between no load voltage and the field excitationcurrent is called magnetic or open circuit characteristic curve. As during no load, the loadterminals are open circuited, there will be no field current in the field since, the armature, fieldand load are series connected and these three make a closed loop of circuit. So, this curve can beobtained practically be separating the field winding and exciting the DC generator by an externalsource. Here in the diagram below AB curve is showing the magnetic characteristic of serieswound DC generator. The linearity of the curve will continue till the saturation of the poles.After that there will be no further significant change of terminal voltage of DC generator forincreasing field current. Due to residual magnetism there will be a small initial voltage across thearmature that is why the curve started from a point A which is a little way up to the origin O.

Internal Characteristic of Series Wound DC GeneratorThe internal characteristic curve gives the relation between voltage generated in the

armature and the load current. This curve is obtained by subtracting the drop due to thedemagnetizing effect of armature reaction from the no load voltage. So, the actual generatedvoltage ( Eg) will be less than the no load voltage (E0). That is why the curve is slightlydropping from the open circuit characteristic curve. Here in the diagram below OC curve isshowing the internal characteristic or total characteristic of the series wound DC generator.

External Characteristic of Series Wound DC Generator

The external characteristic curve shows the variation of terminal voltage (V) with the

load current ( I L). Terminal voltage of this type of generator is obtained by subtracting theohomic drop due to armature resistance (Ra) and series field resistance ( Rsc) from the actuallygenerated voltage ( Eg).

Terminal voltage V = Eg - I(Ra + Rsc)

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The external characteristic curve lies below the internal characteristic curve because thevalue of terminal voltage is less than the generated voltage. Here in the figure OD curve isshowing the external characteristic of the series wound DC generator.

It can be observed from the characteristics of series wound DC generator, that with theincrease in load (load is increased when load current increases) the terminal voltage of themachine increases. But after reaching its maximum value it starts to decrease due to excessivedemagnetizing effect of armature reaction. This phenomenon is shown in the figure by the dottedline. Dotted portion of the characteristic gives approximately constant current irrespective of theexternal load resistance. This is because if load is increased, the field current is increased as fieldis series connected with load. Similarly if load is increased, armature current is increased as thearmature is also series connected with load. But due to saturation, there will be no furthersignificance raises of magnetic field strength hence any further increase in induced voltage. Butdue to increased armature current, the effect of armature reaction increases significantly whichcauses significant fall in load voltage. If load voltage falls, the load current is also decreased

proportionally since current is proportional to voltage as per Ohm's law. So, increasing load,tends to increase the load current, but decreasing load voltage, tends to decrease load current.Due these two simultaneous effects, there will be no significant change in load current in dotted

portion of external characteristics of series wound DC generator. That is why series DCgenerator is called constant current DC generator .

CHARACTERISTICS OF DC COMPOUND GENERATOR:

In compound wound DC generators both the field windings are combined (series andshunt). This type of generators can be used as either long shunt or short shunt compound woundgenerators as shown in the diagram below. In both the cases the external characteristic of thegenerator will be nearly same. The compound wound generators may be cumulativelycompounded or differentially compounded. Differentially compound wound generators are very

rarely used. So, here we mainly concentrate upon the characteristic of cumulatively compoundwound generators.

In series wound DC generators, the output voltage is directly proportional with load current andin shunt wound DC generators, output voltage is inversely proportional with load current.

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The electric current in the shunt field winding produces a flux which causes a fall interminal voltage due to armature reaction and ohmic drop in the circuit. But the current in theseries field also produces a flux which opposes the shunt field flux and compensates the drop inthe terminal voltage and try to operate the machine at constant voltage.

The combination of a series generator and a shunt generator gives the characteristic of acumulative compound wound generator.

At no load condition there is no current in the series field because the load terminals areopen circuited. But the shunt field current helps to produce field flux and excite the machine.When the dc generator supplies load, the load current increases and current flows through theseries field. Therefore, series field also provides some field flux and emf is also increased. Thevoltage drop in the shunt machine is therefore compensated by the voltage rise in the seriesmachine.

Characteristics of DC Compound Generator:

For small distance operation the flat compounded generators are generally used because thelength of the feeder is negligible. But to maintain constant voltage over a long period, the overcompounded generators are used. It works as a generator and a booster (boost the terminalvoltage).

External characteristic of DC compound wound generator is drawn between the terminalvoltage and the load current. By adjusting the no. of amp-turns in the series field winding we canget following external characteristics:

If the series turns are so adjusted that with the increase in load current the terminalvoltage also increases, then the generator is called over compounded. The curve AB inthe figure showing this characteristic. When the load current increases then the flux

provides by the series field also increases. It gives the additional generated voltage. If theincrease in generated voltage is greater than the voltage drops due to armature reactionand ohmic drop then, terminal voltage of the generator is increased.

If the series turns are so adjusted that with the increase in load current the terminalvoltage remains constant, then the generator is called flat compounded. The curve AC inthe figure showing this characteristic. When the load current increases then the flux

provides by the series field also increases and gives the additional generated voltage. Ifthe increase in generated voltage is equal to the voltage drops due to armature reactionand ohmic drop then, rated terminal voltage of the generator remains same as no loadvoltage.

If the series field winding has lesser no. of turns then the rated terminal voltage becomesless than the no load voltage, then the generator is called under compounded. Because,the increase in generated voltage is lesser than the voltage drops due to armature reactionand ohmic drop. Curve AD in the figure is showing this characteristic.

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DC MOTOR: A motor is a device which converts an electrical energy into the mechanical energy . The

energy conversion process is exactly opposite to that involved in a d.c. generator. In a generatorthe input mechanical energy is supplied by a prime mover while in a d.c. motor, input electricalenergy is supplied by a d.c. supply. The construction of a d.c. machine is same whether it is amotor or a generator.Principle of Operation of a D.C. Motor

The principle of operation of a d.c. motor can be stated in a single statement as 'when acurrent carrying conductor is placed in a magnetic field' it experiences a mechanical force'. In a

practical d.c. motor, field winding produces a required magnetic field while armature conductors play a role of a current carrying conductors and hence armature conductors experience a force.As a conductors are placed in the slots which are in the periphery, the individual forceexperienced by the conductors acts as a twisting or turning force on the armature which is calleda torque. The torque is the product of force and the radius at which this force acts. So overallarmature experiences a torque and starts rotating. Let us study this motoring action in detail.

Consider a single conductor placed in a magnetic field as shown in the Fig .1(a). Themagnetic field is produced by a permanent magnet but in a practical d.c. motor it is produced bythe field winding when it carries a current.

Fig. 1 Now this conductor is excited by a separate supply so that it carries a current in a particular

direction. Consider that it carries a current away from an observe as shown in the Fig. 1(b). Any

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current carrying conductor produces its own magnetic field around it. hence this conductor also produces its own flux, around. The direction of this flux can be determined by right hand thumbrule. For direction of current considered, the direction of flux around a conductor is clockwise.For simplicity of understanding, the main flux produced by the permanent magnet is not shownin the Fig. 1(b).

Now there are two fluxes present,1. The flux produced by the permanent magnet called flux.2. The flux produced by the current carrying conductor.

There are shown in the Fig.2(a). Form this, it is clear that on one side of the conductor, boththe fluxes are in same direction. In this case, on the left of the conductor there is gathering of theflux lines as two fluxes help each other. As against this, on the right of the conductor, the twofluxes are in opposite direction and hence try to cancel each other. Due to this, the density of theflux lines in this area gets weakened. So on the left, there exists high flux density area while onthe right of the conductor there exists low flux density area as shown in the Fig. 2(b).

Fig. 2 This flux distribution around the conductors acts like a stretched rubber band under tension.

This exerts a mechanical force on the conductor which acts from high flux density area towardslow flux density area. i.e. from left to right for the case considered as shown in the Fig. 2(b).

Fig. 3

Key point : In the practical d.c. motor, the permanent magnet is replaced by a field windingwhich produces the required flux called main flux and all the armature conductors, mounted onthe periphery of the armature drum, get subjected to the mechanical force. Due to this, overallarmature experiences a twisting force called torque and armature of the motor starts rotating.

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1. Direction of Rotation of Motor The magnitude of the force experienced by the conductor in a motor is given by,

F = B l I Newton (N)B = Flux density due to the flux produced by the field winding.l = Active length of the conductor.

I = Magnitude of the current passing through the conductor.The direction of such force i.e. the direction of rotation of a motor can be determined byFleming's left hand. So Fleming's right hand rule is to determine direction of induced e.m.f. i.e.for generating action while Fleming's left hand rule is to determine direction of force experiencedi.e. for motoring action.1.1 Fleming's left hand rule

The rule states that, 'Outstretch the three fingers of the left hand namely the first finger,middle finger and thumb such that they are mutually perpendicular to each other. Now point thefirst finger in the direction of magnetic field and the middle finger in the direction of the currentthen the thumb gives the direction of the force experienced by the conductor'.

The Fleming's left hand rule can be diagramatically shown as in the Fig. 1.

Fig. 1 Apply the rule to crosscheck the direction of force experienced by a single conductor, placed

in the magnetic field, shown in the Fig. 2(a), (b), (c) and (d).

Fig. 2 It can be seen from the Fig. 2 that if the direction of the main field in which current carrying

conductor is placed, is reversed, force experienced by the conductor reverses its direction.Similarly keeping main flux direction unchanged, the direction of current passing through theconductor is reversed. The force experienced by the conductor reverses its direction. However if

both the directions are reversed, the direction of the force experienced remains the same.

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Key point : So in a practical motor, to reverse its direction of rotation, either direction of mainfield produced by the field winding is reversed or direction of the current passing through thearmature is reversed.

The direction of the main field can be reversed by changing the direction of current passingthrough the field winding, which is possible by interchanging the polarities of supply which is

given to the field winding . In short, to have a motoring action two fluxes must exist, theinteraction of which produces a torque.

Significance of Back E.M.F.

It is seen in the generation action, that when a conductor cuts the lines of flux, e.m.f. getsinduced in the conductor. The question is obvious that in a d.c. motor, after a motoring action,armature starts rotating and armature conductors cut the main flux. So there is a generatingaction existing in a motor.

After a motoring action, there exists a generating action. There is an induced e.m.f. in therotating armature conductors according to Faraday's law of electromagnetic induction. This

induced e.m.f. in the armature always acts in the opposite direction of the supply voltage. This isaccording to the Lenz's law which states that the direction of the induced e.m.f. is always so as tooppose the cause producing it. In a d.c. motor, electrical input i.e. the supply voltage is the causeand hence this induced e.m.f. opposes the supply voltage. This e.m.f. tries to set up a currentthrough the armature which is in the opposite direction to that, which supply voltage is forcingthrough the conductor.

So as this e.m.f. always opposes the supply voltage, it is called back e.m.f. and denoted asE b. Though it is obtained as E b, basically it gets generated by the generation action which wehave seen earlier in case of generation. So its magnitude can be determined by the e.m.f.equation which is derived earlier. So,

where all symbols carry the same meaning as seen earlier in case of generators.

Fig. 1

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This e.m.f. is shown schematically in the Fig. 1(a). So if V is supply voltage in volts and R ais the value of the armature resistance, the equivalent electric circuit can be shown as in the Fig.1(b).

TYPES OF D.C. MOTORSSimilar to the d.c. generators, the d.c. motors are classified depending upon the way ofconnecting the field winding with the armature winding. The difference types of d.c. motors are ;1. Shunt motor2. Series motors3. Compound motors

The compound motors are further classified as ;1. Short shunt compound2. Long shunt compound

DC SHUNT MOTOR:

In this type, the field winding is connected across the armature winding and the combination is

connected across the supply, as shown in the Fig. 1.

Fig. 1 D.C. shunt motor

Let R sh be the resistance of shunt field winding.R a be the resistance of armature winding.The value of R a is very small while R sh is quite large. Hence shunt field winding has more

number of turns with less cross-sectional area.1.1 Voltage and Current Relationship

The voltage across armature and field winding is same equal to the supply voltage V.The total current drawn from the supply is denoted as line current I L.

IL = I a + I shIsh = V/R sh

and V = E b + I a R a + V brushV brush is generally neglected.

Now flux produced by the field winding is proportional to the current passing through it i.e.Ish.

Note : As long as supply voltage is constant, which is generally so in practice, the flux producedis constant. Hence d.c. shunt motor is called constant flux motor.

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DC SERIES MOTOR:

In this type of motor, the series field winding is connected in series with the armature and thesupply, as shown in the Fig. 1.

Fig. 1 D.C. series motor

Let R se be the resistance of the series field winding. The value of R se is very small and it ismade of small number of turns having large cross-sectional area.1.1 Voltage and Current Relationship

Let I L be the total current drawn from the supply.So I L = I se = I aand V = E b + Ia Ra + I se R se + V brush

V = E b + Ia (Ra + R se) + V brushSupply voltage has to overcome the drop across series field winding in addition to E b and

drop across armature winding.Note : In series motor, entire armature current is passing through the series field winding. So flux

produced is proportional to the armature current.

for series motorDC COMPOUND MOTOR:

The compound motor consists of part of the field winding connected in series and part ofthe field winding connected in parallel with armature. It is further classified as long shuntcompound and short shunt compound motor.1.1 Long Shunt Compound Motor

In this type, the shunt field winding is connected across the combination of armature and theseries field winding as shown in the Fig. 1.

Fig. 1 Long shunt compound motor

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Let Rse be the resistance of series field and R sh be the resistance of shunt field winding. Thetotal current drawn from supply is I L.

So I L = I se + I sh But I se = Ia

... IL = Ia+ I sh

And I sh = V/R sh And V = E b + Ia R a + Ise R se + V brushBut as I se = Ia ,... V = E b + Ia (R a + R se) + V brush

1.2 Short Shunt Compound MotorIn this type, the shunt field is connected purely in parallel with armature and the series field

is connected in series with this combination shown in the Fig. 2.

Fig. 2 Short shunt compound motor

IL = IseThe entire line current is passing through the series field winding.and I L = Ia + I sh

Now the drop across the shunt field winding is to be calculated from the voltage equation.So V = E b + I se R se + Ia R a + V brush

but I se = I L... V = E b + IL R se + I a R a + V brush... Drop across shunt field winding is,

= V - I L R se = E b + Ia R a + Vbrush

Apart from these two, compound motor can be classified into two more types,i) Cumulatively compound motors and ii) Differential compound motors.Note : If the two field windings are wound in such a manner that the fluxes produced by the twoalways help each other, the motor is called cumulatively compound. If the fluxes produced bythe two field windings are trying to cancel each other i.e. they are in opposite direction, themotor is called differential compound.

A long shunt compound motor can be of cumulative or differential type. Similarly shortshunt compound motor can be cumulative or differential type.

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TORQUE EQUATION OF A D.C. MOTORIt is seen that the turning or twisting force about an axis is called torque. Consider a wheel ofradius R meters acted upon by a circumferential force F newtons as shown in the Fig. 1.

Fig. 1 The wheel is rotating at a speed of N r.p.m. Then angular speed of the wheel is,

= (2N)/60 rad/secSo workdone in one revolution is,

W = F x distance travelled in one revolution= F x 2 R joules

And P = Power developed = Workdone/Time= (F x 2R) / (Time for 1 rev) = (F x 2R) / (60/N) = (F x R) x (2N/60)

...P = T x watts

Where T = Torque in N - m

= Angular speed in rad/sec.Let T a be the gross torque developed by the armature of the motor. It is also called armature

torque. The gross mechanical power developed in the armature is E b Ia, as seen from the powerequation. So if speed of the motor is N r.p.m. then,

Power in armature = Armature torque x

... E b Ia = x (2N/60) but E b in a motor is given by,

E b = (PNZ) / (60A) ... (PNZ / 60A) x Ia = Ta x (2N/60)

This is the torque equation of a d.c. motor.TORQUE SPEED EQUATIONS:

Before analysing the various characteristics of motors, let us revise the torque and speedequations are applied to various types of motors.

... T Ia from torque equation.This is because, 0.159(PZ)/A is a constant for a given motor. Now is the flux produced by the field winding and is proportional to the current passing

through the field winding. Ifield

But for various types of motors, current through the field winding is different. Accordinglytorque equation must be modified.

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For a d.c. shunt motor, I sh is constant as long as supply voltage is constant. Hence falso constant.... T Ia for shunt motors

For a d.c. series motor, I se is same as I a. Hence flux is proportional to the armaturecurrent I a.

..

. T Ia Ia2

for series motors.Similarly as E b = (PNZ)(60A), we can write the speed equation as, E b N

... N E b/ But V = E b + Ia R a neglecting brush drop

... E b = V - I a R a

... Speed equation becomes, N (V-I a R a)/

So for shunt motor as flux is constant,... N V - I a R a

While for series motor, flux is proportional to I a.

FACTORS AFFECTING THE SPEED OF A D.C. MOTORAccording to the speed equation of a d.c. motor we can write,

The factors Z, P, A are constants for a d.c. motor.But as the value of armature resistance R a and series field resistance R se is very small, the

drop I a R a and (R a + R se) is very small compared to applied voltage V. Hence neglecting thesevoltage drops the speed equation can be modified as,

Thus the factors affecting the speed of a d.c. motor are,1. The flux 2. The voltage across the armature3. The applied voltage V

depending upon these factors the various methods of speed control are,1. Changing the flux by controlling the current through the field winding called fluxlmethods.2. Changing the armature path resistance which in turn changes the voltage applied across thearmature called rheostatic control.3. Changing the applied voltage called voltage control method.

D.C. Motor CharacteristicsThe performance of a d.c. motor under various conditions can be judged by the followingcharacteristics

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i) Torque - Armature current characteristics (T Vs I a ) : The graph showing the relationship between the torque and the armature current is called a

torque-armature current characteristic. These are also called electrical characteristics.ii) Speed - Armature current characteristics(N Vs I a ) :

The graph showing the relationship between the speed and armature current characteristic.

iii) Speed - Torque characteristics(N Vs T) : The graph showing the relationship between the speed and the torque of the motor is calledspeed-torque characteristics of the motor. These are also called mechanical characteristic.

The nature of these characteristics can easily be obtained by using speed and torqueequations derived in previous post. These characteristics play a very important role in selecting atype of motor for a particular application.

Characteristics of dc shunt motor:i) Torque - Armature current characteristics

For a d.c. motor T IaFor a constant values of R sh and supply voltage V, I sh is also constant and hence flux is also

constant.... Ta IaThe equation represents a straight line, passing through the origin, as shown in the Fig. 1.

Torque increases linearly with armature current. It is seen earlier that armature current is decided by the load. So as load increases, armature current increases, increasing the torque developedlinearly.

Fig. 1 T Vs I a for shunt motor Now if shaft torque is plotted against armature current, it is known that shaft torque is less

than the armature torque and the difference between the two is loss torque T f as shown. On noload T sh = 0 but armature torque is present which is just enough to overcome stray losses shownas T a0. The current required is I a0 on no load to produce T a0 and hence T sh graph has an intercept

of I a0 on the current axis.To generate high starting torque, this type of motor requires a large value of armature

current at start. This may damage the motor hence d.c. shunt motors can develop moderatestarting torque and hence suitable for such applications where starting torque requirement ismoderate.ii) Speed - Armature current characteristics

From the speed equation we get, N (V- Ia Ra)/

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V- Ia Ra as is constant So as load increases, the armature current increases and hence drop I a Ra also increases.Hence for constant supply voltage, V - I a Ra decreases and hence speed reduces. But as Ra

is very small, for change in I a from no load to full load, drop I a Ra is very small and hence drop inspeed is also not significant from no load to full load.

Fig. 2 N Vs I a for shunt motor

So the characteristics is slightly droping as shown in the Fig. 2.But for all practical purposes these type of motors are considered to be a constant speed

motors.iii) Speed - Torque characteristics

These characteristics can be derived from the above two characteristics. This graph is similarto speed-armature current characteristics as torque is proportional to the armature current. Thiscurve shows that the speed almost remains constant through torque changes from no load to fullload conditions. This is shown in the Fig. 3.

Fig. 3 N Vs T for shunt motor

Characteristics of dc series motor:i) Torque - Armature current characteristics

In case of series motor the series field winding is carrying the entire armature current. Soflux produced is proportional to the armature current.... Ia

Hence T a Ia Ia2

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Thus torque in case of series motor is proportional to the square of the armature current. Thisrelation is parabolic in nature as shown in the Fig. 1.

As load increases, armature current increases and torque produced increases proportional tothe square of the armature current upto a certain limit.

As the entire I a passes through the series field, there is a property of an electromagnet calledsaturation, may occur. Saturation means though the current through the winding increases, theflux produced remains constant. Hence after saturation the characteristics take the place ofstraight line as flux becomes constant, as shown. The difference between T a and T sh is loss torqueTf which is also shown in the Fig. 2.

At start as T a Ia2 , these types of motors can produce high torque for small amount ofarmature current hence the series motors are suitable for the applications which demand highstarting torque.ii) Speed - Armature current characteristics

From the speed equation we get, N (E b/) ) V- Ia R a - Ia R se)/ I a as Ia in case of series motor Now the values of R a and R se are so small that the effect of change in I a on speed overrides

the effect of change in V - I a R a - I a R se on the speed.Hence in the speed equation, E b Vand can be assumed constant. So speed equation reduced

to, N 1/I a

So speed-armature current characteristics is rectangular hyperbola type as shown in the Fig.2.iii) Speed - Torque characteristics

In case of series motors, T Ia2 and N 1/I a Hence we can write, N 1/T

Thus as torque increases when load increases, the speed decreases. On no load, torque is

very less and hence speed increases to dangerously high value. Thus the nature of the speed-torque characteristics is similar to the nature of the speed-armature current characteristics.

The speed-torque characteristics of a series motor is shown in the Fig. 3.

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Why Series Motor is Never Started on No Load?It is seen earlier that motor armature current is decided by the load. On light load or no load, thearmature current drawn by the motor is very small.

In case of a d.c. series motor, Ia andon no load as I a is small hence flux produced is also very small.

According to speed equation, N 1/ as E b is almost constant.So on very light load or no load as flux is very small, the motor tries to run at dangerously

high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and lowtorque condition motor shows a tendency to rotate with dangerously high speed.

This is the reason why series motor should never be started on light loads or no loadconditions. For this reason it is not selected for belt drives as breaking or slipping of belt causesto throw the entire load off on the motor and made to run motor with no load which is dangerous.STARTING METHODS OF A DC MOTORBasic operational voltage equation of a DC motor is given as

E = E b + I aR a and hence I a = (E - E b) / R a Now, when the motor is at rest, obviously, there is no back emf E b, hence armature current will be high at starting.This excessive current will-

1. Blow out the fuses and may damage the armature winding and/or commutator brusharrangement.

2. Produce very high starting torque (as torque is directly proportional to armature current),and this high starting toque will produce huge centrifugal force which may throw off thearmature windings.

Thus to avoid above two drawbacks, starters are used for starting of DC machine.

Starting Methods of a DC MotorThus, to avoid the above dangers while starting a DC motor , it is necessary to limit the startingcurrent. For that purpose, starters are used to start a DC motor. There are various starters like, 3

point starter, 4 point starter, No load release coil starter, thyristor starter etc.The main concept behind every DC motor starter is, adding external resistance to the armaturewinding at starting.

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3 POINT STARTER: The internal wiring of a 3 point starter is as shown in the figure.

When motor is to be started, the lever is turned gradually to the right. When lever touches point1, the field winding gets directly connected across the supply, and the armature winding getsconnected with resistances R1 to R5 in series. Hence at starting full resistance is added in serieswith armature. Then as the lever is moved further, the resistance is gradually is cut out from thearmature circuit. Now, as the lever reaches to position 6, all the resistance is cut out from thearmature circuit and armature gets directly connected across the supply. The electromagnet E (novoltage coil) holds the lever at this position. This electromagnet releases the lever when there isno (or low) supply voltage.

When the motor is overloaded beyond a predefined value, over current release electromagnet Dgets activated, which short circuits electromagnet E , and hence releases the lever and motor isturned off.

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4 POINT STARTER:

The main difference between a 3 point starter and a 4 point starter is that the no voltage coilis not connected in series with field coil. The field gets directly connected to the supply, as thelever moves touching the brass arc. The no voltage coil (or Hold on coil) is connected with acurrent limiting resistance Rh. This arrangement ensures that any change of current in the shuntfield does not affect the current through hold on coil at all. This means that electromagnet pull ofthe hold-on coil will always be sufficient so that the spring does not unnecessarily restore thelever to the off position.

This starter is used where field current is to be adjusted by means of a field rheostat.

2 POINT STARTER : (DC Series motor starter)

Construction of DC series motor starters is very basic as shown in the figure. A startarm is simply moved towards right to start the motor. Thus at first maximum resistance isconnected in series with the armature and then gradually decreased as the start arm movestowards right. The no load release coil holds the start arm to the run position and leaves it at noload.

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SPEED CONTROL METHODS OF DC MOTOR:

We know, back emf of a DC motor E b is the induced emf due to rotation of the armaturein magnetic field. Thus value of the E b can be given by the EMF equation of a DC generator.

E b = PNZ /60A

(where, P= no. of poles, =flux/pole, N=speed in rpm, Z=no. of armature conductors, A=parallel paths)

E b can also be given as,E b = V- I aR a

thus from above equations

N =E

b

60A

/PZ

but, for a DC motor A, P and Z are constant

N K E b/ (where, K=constant)

thus, it shows speed is directly proportional to back emf and inversely proportional to the flux per pole.

SPEED CONTROL METHODS OF DC MOTORSpeed Control of Shunt Motor

1. Flux Control Method

It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing fluxspeed can be increased and vice versa.

To control the flux, a rheostat is added in series with the field winding, as shown in thecircuit diagram. Adding more resistance in series with field winding will increase the speed, as itwill decrease the flux. Field current is relatively small and hence I 2R loss is small, hence thismethod is quiet efficient. Though speed can be increased by reducing flux with this method, it

puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect thecommutation.

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2. Armature Control Method

Speed of the motor is directly proportional to the back emf E b and E b = V- I aR a.That is when supply voltage V and armature resistance R a are kept constant, speed is

directly proportional to armature current I a. Thus if we add resistance in series with armature,Ia decreases and hence speed decreases.Greater the resistance in series with armature, greater thedecrease in speed.

3. Voltage Control Method

a) Multiple voltage control: In this method the, shunt filed is connected to a fixed excitingvoltage, and armature is supplied with different voltages. Voltage across armature is changedwith the help of a suitable switchgear. The speed is approximately proportional to the voltageacross the armature.

b) WARD-LEONARD SYSTEM :

Ward Leonard control system is introduced by Henry Ward Leonard in 1891. Ward Leonardmethod of speed control is used for controlling the speed of a DC motor. It is a basic armaturecontrol method. This control system is consisting of a dc motor M_1 and powered by a DCgenerator G. In this method the speed of the dc motor (M_1) is controlled by applying variablevoltage across its armature. This variable voltage is obtained using a motor-generator set whichconsists of a motor M_2(either ac or dc motor) directly coupled with the generator G. It is a verywidely used method of speed control of DC motor.

Principle of Ward Leonard Method

Basic connection diagram of the Ward Leonard speed control system is shown in the figure below.

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The speed of motor M 1 is to be controlled which is powered by the generator G. Theshunt field of the motor M 1 is connected across the dc supply lines. Now, generator G is driven

by the motor M 2. The speed of the motor M 2 is constant. When the output voltage of thegenerator is fed to the motor M 1 then the motor starts to rotate. When the output voltage of thegenerator varies then the speed of the motor also varies. Now controlling the output voltage ofthe generator the speed of motor can also be controlled. For this purpose of controlling the outputvoltage, a field regulator is connected across the generator with the dc supply lines to control thefield excitation. The direction of rotation of the motor M 1 can be reversed by excitation currentof the generator and it can be done with the help of the reversing switch R.S. But the motor-generator set must run in the same direction.

Advantages of Ward Leonard System

1. It is a very smooth speed control system over a very wide range (from zero to normalspeed of the motor).

2. The speed can be controlled in both the direction of rotation of the motor easily.3. The motor can run with a uniform acceleration.4. Speed regulation of DC motor in this ward Leonard system is very good.

Disadvantages of Ward Leonard System

1. The system is very costly because two extra machines (motor-generator set) are required.2. Overall efficiency of the system is not sufficient especially it is lightly loaded.

Application of Ward Leonard System

This Ward Leonard method of speed control system is used where a very wide and verysensitive speed control is of a DC motor in both the direction of rotation is required. This speedcontrol system is mainly used in colliery winders, cranes, electric excavators, mine hoists,elevators, steel rolling mills and paper machines etc.

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Speed Control Of Series Motor

1. Flux Control Method

a) Field diverter :

A variable resistance is connected parallel to the series field as shown in fig. This variableresistor is called as diverter, as desired amount of current can be diverted through this resistorand hence current through field coil can be decreased. Hence flux can be decreased to desiredamount and speed can be increased.

b) Armature divertor:

Diverter is connected across the armature as in fig .For a given constant load torque, if armature current is reduced then flux must increase. As,

Ta IaThis will result in increase in current taken from the supply and hence flux will increase andsubsequently speed of the motor will decrease.

c) Tapped field control:

As shown in fig field coil is tapped dividing number of turns. Thus we can select different valueof by selecting different number of turns.

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d) Paralleling field coils:

In this method, several speeds can be obtained by regrouping coils as shown in fig

2. Variable Resistance In Series With ArmatureBy introducing resistance in series with armature, voltage across the armature can be

reduced. And hence, speed reduces in proportion with it.

3. Series-Parallel ControlThis system is widely used in electric traction, where two or more mechanically coupled

series motors are employed. For low speeds, motors are joined in series, and for higher speedsmotors are joined in parallel.

When in series, the motors have the same current passing through them, although voltageacross each motor is divided. When in parallel, voltage across each motor is same althoughcurrent gets divided.

REVIEW QUESTIONS

Construction details

1. Explain the constructional details of a DC Generator. Describe its working principle andderive its emf equation. (16) (N/D-12)

2. Explain in detail, the functions of various parts of a DC motor. (16)

Emf equation

1. A six pole lap connected DC generator has a useful flux per pole of 0.045 Wb. If the no-loadvoltage at 400 rpm is 300 V, find the conductors on the armature periphery. (8) (N/D-09)

2. Derive the emf equation from fundamentals and also explain why the lap winding is preferredcompared to wave winding for low voltage high current machines. (8)

3. Derive an expression for emf generated in a DC machine. (8)

Self and separately excited generators

1. A separately excited generator, when running at 1000 rpm supplies 200 A at 125 V. Whatwill be the load current when the speed drops to 800 rpm, if the field current is unchanged?Assume the armature resistance as 0.04 and the brush drop is 2 V.(16) (N/D-09)

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Characteristics of series, shunt and compound generators

1. Explain in detail, the characteristics of a DC generator with neat diagrams. (16) (N/D-09)

2. A DC shunt generator has a terminal voltage of 160 V and a no-load induced emf of 168 V.

The resistances of armature and field are 0.03 and 20 respectively. Find the arcurrent, field current and load current. Neglect armature reaction. (6)

Principle of operation of a DC motor

1. Explain the principle of operation of a DC motor with relevant diagrams. (8) (N/D-11)

Back emf and torque equation

1. Explain the significance of back emf. (6)

2. Prove that the torque developed by a DC motor is proportional to flux and armature current.(8)

Characteristics of series, shunt and compound motors

1. A 25 kW, 250 V, DC shunt generator has an armature and a field resistance of 0.06 and100 respectively. Determine the total armature power developed during operatingcondition. (6) (A/M-10)

2. A 4 pole, 220 V DC shunt motor has 540 lap-wound conductors. It takes 32 A from thesupply mains and develops output power of 5.595 kW. The field winding takes 1 A. Thearmature resistance is 0.9 and the flux per pole is 30 mWb. Calculate the speed in rpm andthe torque developed in Nm. (6) (A/M-10)

3. Explain in detail, the different classifications of DC motors with respect to their equivalentcircuits. (8) (N/D-09)

4. Estimate the percentage reduction in speed of a separately excited DC generator working withconstant excitation of 400 V, bus bar voltage is decreasing as its load varies from 600 kW to400 kW. The machine resistance is 0.02 . Neglect the armature reaction. (8) (A/M-11)

5. The armature winding of a 200 V, 4 pole series motor is lap connected. There are 280 slotswith 4 conductors per slot. The current is 45 A and the flux per pole is 18 mWb. The fieldresistance is 0.3 , armature resistance 0.5 and the total iron and the friction losW. The pulley diameter is 0.41 m. Find the pull in newton at the rim of the pulley. (10)

6. Explain the speed-current, speed-torque and torque-current characteristics of a DC shunt andseries motor. (8)

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Types of starters

1. Draw the schematic diagram of a 3 point starter and explain its working principle. (8)(M/J-12)

2. Explain the operation of a 4 point starter with a neat diagram and also write its advantagesover a three point starter. (10) (A/M-10)

Speed control of DC shunt motors

1. (i) A 4 pole DC motor runs at 600 rpm on full load and takes 25 A , 450 V the armature is lapwound with conductors and flux per pole is given by the equation = (1.7 X 10 -2) x I 0.5 Wbwhere I is the motor current. If the supply voltage and torque are halved, calculate the speedat which the motor will run. Neglect stray losses. (8)

(ii) Explain the various types of speed control techniques of DC Machines. (8) (M/J-12)

2. Explain the procedure to predetermine the efficiencies of a DC Machine as a motor and as a

generator. (8) (N/D-12)3. Explain in detail, the Ward-Leonard system of speed control of a DC motor.4. Explain in detail, the different methods of speed control of a DC motor. (16) (N/D-12)

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UNIT II TRANSFORMER

Introduction Single phase transformer construction and principle of operation EMF equationof transformer-Transformer no load phasor diagram Transformer on load phasor diagram Equivalent circuit of transformer Regulation of transformer Transformer losses andefficiency-All day efficiency auto transformers.

SINGLE PHASE TRANSFORMERWorking Principle of 1-Phase Transformer1. Introduction

The main advantage of alternating currents over direct current is that, the alternating currentscan be easily transferable from low voltage to high voltage or high voltage to low. Alternatingvoltages can be raised or lowered as per requirements in the different stages of electrical networkas generation, transmission, distribution and utilization. This is possible with a static devicecalled transformer. The transformer works on the principle of mutual induction. It transfer anelectric energy from one circuit to other when there is no electrical connection between the tow

circuits. Thus we can define transformer as below :Key point : The transformer is a static piece of apparatus by means of which an electrical poweris transformed from one alternating current circuit to another with the desired change in voltageand current, without any change in the frequency.

The use of transformers in transmission system is shown in the Fig 1.1.

Fig. 1.1 Use of transformer in transmission system

2. Principle of workingThe principle of mutual induction states that when tow coils are inductively coupled and if

current in one coil is changed uniformly then an e.m.f. gets induced in the other coil. This e.m.fcan drive a current, when a closed path is provided to it. The transformer works on the same

principle. In its elementary form, it consists of tow inductive coils which are electricallyseparated but linked through a common magnetic circuit. The two coils have high mutualinductance. The basic transformer is shown in the Fig 1.2.

One of the two coils is connected to source of alternating voltage. This coil in which

electrical energy is fed with the help of source called primary winding (P). The other winding isconnected to load. The electrical energy transformed to this winding is drawn out to the load.

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Fig.1.2 Basic transformer

Fig 1.3 Symbolic representation This winding is called secondary winding (S). The primary winding has N 1number of turns

while the secondary winding has N 2 number of turns. Symbolically the transformer is indicatedas shown in the Fig 1.3.

When primary winding is excited by an alternating voltage, it circulates an alternatingcurrent. This current produces an alternating flux ()which completes its path througmagnetic core as shown dotted in the Fig 1.2. Thus an alternating, flux links with the secondarywinding. As the flux is alternating, according to Faraday's law of an electromagnetic induction,mutually induced e.m.f. gets developed in the secondary winding. If now load is connected to thesecondary winding, this e.m.f. drives a current through it.

Thus through there is no electrical contact between the two windings, an electrical energygets transferred from primary to the secondary.Key point : The frequency of the mutual induced e.m.f. is same as that of the alternating sourcewhich is supplying energy to the primary winding.

3. Can D.C. Supply be used for Transformer? The d.c. supply can not be used for the transformers.The transformer works on the principle of mutual induction, for which current in one coil

must change uniformly. If d.c. supply is given, the current will not change due to constant supplyand transformer will not work.

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Practically winding resistance is very small. For d.c., the inductive reactance X L is zero asd.c. has no frequency. So total impedance of winding is very low for d.c. Thus winding will drawvery high current if d.c. supply is given to it. This may cause the burning of windings due toextra heat generated and may cause permanent damage to the transformer.

There can be saturation of the core due to which transformer draws very large current from

the supply when connected to d.c.Thus d.c. supply should not be connected to the transformers.

CONSTRUCTION OF TRANSFORMER

There are two basic parts of a transformer i) Magnetic Core ii) Winding or Coils.The core of the transformer is either square or rectangular in size. It is further divided into tow

parts. The vertical position on which coils are wound is called limb while the top and bottomhorizontal portion is called yoke of the core. These parts are shown in the Fig.1(a).

Core is made up of lamination. Because of laminated type of construction, eddy currentlosses get minimised. Generally high grade silicon steel laminations (0.3 to 0.5 mm thick) areused. These laminations are insulated from each other by using insulation like varnish. Alllaminations are varnished. Laminations are overlapped so that to avoid the air gap at joints. Forthis generally 'L' shaped or 'I' shaped laminations are used which are shown in the Fig 1(b).

Fig. 1 Construction of transformer

The cross-section of the limb depends on the type of coil to be used either circular orrectangular. The different cross-section of limbs, practically used are shown in the Fig. 2.

Fig. 2 Different cross-sections

Types of WindingsThe coils used are wound on the limbs and are insulated from each other. In the basic

transformer shown in the Fig 1.2 (see post : Working Principle of 1-Phase Transformer ) the two

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windings wound are shown on two different limbs i.e. primary on one limb while secondary onother limb. But due to this leakage flux increases which effects the transformer performance

badly. Similarly it is necessary that the windings should be very closes to each other to have highmutual inductance. To achieve this, the tow windings are split into number of coils and arewound adjacent to each other on the same limb. A very common arrangement is cylindrical coils

as shown in the Fig. 3.

Fig. 3 Cylindrical concentric coils

Such cylindrical coils are used in the core type transformer. Theses coils are mechanicallystrong. These are wound in the helical layers. The different layers are insulated from each other

by paper, cloth or mica. The low voltage winding is placed near the core from ease of insulatingit from the core. The high voltage is placed after it.

The other type of coils which is very commonly used for the shell type of transformer issandwiching coils. Each high voltage portion lies between the two low voltage portion

sandwiching the high voltage portion. Such subdivision of windings into small portion reducesthe leakage flux. Higher the degree of subdivision, smaller is the reactance. The sandwich coil isshown in the Fig. 4. The top and bottom coils are low voltage coils. All the portion are insulatedfrom each other by paper.

Fig. 4 Sandwich coils

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CONSTRUCTION OF SINGLE PHASE TRANSFORMERThe various constructions used for the single phase transformers are,1. Core type 2. shell type and 3. Berry type

1. Core Type Transformer It has a single magnetic circuit. The core rectangular having two limbs. The windingencircles the core. The coils used are of cylindrical type. As mentioned earlier, the coils arewound in helical layers with different layers insulated from each other by paper or mica. Boththe coils are placed on both the limbs. The low voltage coil is placed inside near the core whilehigh voltage coil surrounds the low voltage coil. Core is made up of large number of thinlaminations.

As The windings are uniformly distributed over the two limbs, the natural cooling is moreeffective. The coils can be easily removed by removing the laminations of the top yoke, formaintenance.

The Fig. 1(a) shows the schematic representation of the core type transformer while the Fig

1(b) shows the view of actual construction of the core type transformer.

Fig. 1 Core type transformer 2. Shell Type Transformer

It has a double magnetic circuit. The core has three limbs. Both the windings are placed onthe central limb. The core encircles most part of the windings. The coils used are generallymultilayer disc type or sandwich coils. As mentioned earlier, each high voltage coil is in betweentow low voltage coils and low voltage coils are nearest to top and bottom of the yokes.

The core is laminated. While arranging the laminations of the core, the care is taken that allthe joints at alternate layers are staggered. This is done to avoid narrow air gap at the joint, rightthrough the cross-section of the core. Such joints are called over lapped or imbricated joint.Generally for very high voltage transformers, the shell type construction is preferred. As thewindings are surrounded by the core, the natural cooling does not exist. For removing anywinding for maintenance, large number of laimnations are required to be removed.

The Fig. 2(a) shows the schematic representation while the Fig. 2(b) shows the outawayview of the construction of the shell type transformer.

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Fig 2 Shell type transformer

3. Berry Type Transformer This has distributed magnetic circuit. The number of independent magnetic circuits are more

than 2. Its core construction is like spokes of a wheel. Otherwise it is symmetrical to that of shelltype.

Diagramatically it can be shown as in the Fug. 3.

Fig. 3 Berry type transformer The transformers are generally kept in tightly fitted sheet metal tanks. The tanks are

constructed of specified high quality steel plate cut, formed and welded into the rigid structures.All the joints are painted with a solution of light blue chalk which turns dark in the presence ofoil, disclosing even the minutes leaks. The tanks are filled with the special insulating oil. Theentire transformer assembly is immersed in the oil. Oil serves two functions :i) Keeps the coilcool by circulation and ii) Provides the transformers an additional insulation.

The oil should be absolutely free from alkalies, sulphur and specially from moisture.Presence of very small moisture lowers the dielectric strength of oil, affecting its performance

badly. Hence the tanks are sealed air tight to avoid the contact of oil with atmospheric air andmoisture. In large transformers, the chambers called breather are provided. The breathers preventthe atmospheric moisture to pass on to the oil. The breathers contain the silica gel crystal whichimmediately absorb the atmospheric moisture. Due to long and continuous use, the sludge isformed in the oil which can contaminate the oil. Hence to keep such sludge separate from the oilin main tank, an air tight metal drum is provided, which is placed on the top of tank. This iscalled conservator.

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4. Comparison of Core and Shell Type Transformers

E.M.F EQUATION OF A TRANSFORMERWhen the primary winding is excited by an alternating voltage V 1, it circulates alternating

current, producing an alternating flux . The primary winding has N1number of turns. Thealternating flux linking with the primary winding itself induces an e.m.f in it den1.The flux links with secondary winding through the common magnetic core. It produces inducede.m.f. E 2 in the secondary winding. This is mutually induced e.m.f. Let us derive the equationsfor E 1 and E 2.

The primary winding is excited by purely sinusoidal alternating voltage. Hence the flux produced is als o sinusoidal in nature having maximum value of m as show in the Fig. 1.

Fig. 1 Sinusoidal flux The various quantities which affect the magnitude of the induced e.m.f. are :

= Flux m = Maximum value of flux

N1 = Number of primary winding turns

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N2 = Number of secondary winding turnsf = Frequency of the supply voltageE1 = R.M.S. value of the primary induced e.m.f.E2 = R.M.S. value of the secondary induced e.m.f.

From Faraday's law of electromagnetic induction the voltage e.m.f. induced in each turn is

proportional to the average rate of change of flux.... averagee.m.f. per turn = average rate of change of flux... averagee.m.f. per turn = d/dt

Now d/dt = Change in flux/Time required for change in flux Consider the 1/4 th cycle of the flux as shown in the Fig.1. Complete cycle gets completed in

1/f seconds. In 1/4 th time period, the change in flux is from 0 to m.... d/dt = (m - 0)/(1/4f) as dt for 1/4 th time period is 1/4f seconds

= 4 f m Wb/sec... Average e.m.f. per turn = 4 f m volts

As is sinusoidal, the induced e.m.f. in each turn of both the windings is also sinusoidal innature. For sinusoidal quantity,

From factor = R.M.S. value/Average value = 1.11... R.M.S. value of induced e.m.f. per turn= 1.11 x 4 f m = 4.44 f m

There are number of primary turns hence the R.M.S value of induced e.m.f. of primarydenoted as is E 1,

E1 = N 1 x 4.44 f m voltsWhile as there are number of secondary turns the R.M.S values of induced e.m.f. of

secondary denoted is E 2 is,E2 = N 2 x 4.44 f m volts

The expression of E 1 and E 2 are called e.m.f. equation of a transformer.Thus e.m.f. equations are,

E1 = 4.44 f m

N1

volts ............(1)E2 = 4.44 f m N2 volts .............(2)

Ratios of a Transformer

Consider a transformer shown in Fig.1 indicating various voltages and currents.

Fig. 1 Ratios of transformer

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1. Voltage Ratio We known from the e.m.f. equations of a transformer that

E1 = 4.44 f m N1 and E 2 = 4.44 f m N2Taking ratio of the two equations we

get,

This ratio of secondary induced e.m.f. to primary induced e.m.f. is known as voltagetransformation ratio denoted as K,

Thus,

1. If N 2 > N 1 i.e. K > 1, E 2 > E 1 we get then the transformer is called step-up transformer.2. If N 2 < N 1 i.e. K < 1, we get E 2 < E 1 then the transformer is called step-down transformer.3. If = i.e. K= 1, we get E 2 = E 1 then the transformer is called isolation transformer or 1:1

transformer.2. Concept of Ideal Transformer

A transformer is said to be ideal if it satisfies following properties :i) It has no losses.ii) Its windings have zero resistance.iii) Leakage flux is zero i.e. 100% flux produced by primary links with the secondary.iv) Permeability of core is so high that negligible current is required to establish the flux in it.Key point : For an ideal transformer, the primary applied voltage V 1 is same as the primaryinduced e.m.f. V 2 as there are no voltage drops.

Similarly the secondary induced e.m.f. E 2 is also same as the terminal voltage V 2 across theload. Hence for an ideal transformer we can write,

No transformer is ideal in practice but the value of E 1 is almost equal to V 1 for properlydesigned transformer.

3. Current ratio For an ideal transformer there are no losses. Hence the product of primary voltage V 1 and

primary current I 1, is same as the product of secondary voltage V 2 and the secondary current I 2.So V 1 I1 = input VA and V 2 I2 = output VA

For an ideal transformer,V1 I1 = V 2 I2

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Key point : Hence the currents are in the inverse ratio of the voltage transformation ratio.

4. Voltage ampere rating When electrical power is transferred from primary winding to secondary there are few power

losses in between. These power losses appear in the form of heat which increase the temperatureof the device.Now this temperature must be maintained below certain limiting values as it isalways harmful from insulation point of view. As current is the main cause in producing heat, theoutput maximum rating is generally specified as the product of output voltage and output currenti.e.V 2 I2. This always indicates that when transformer is operated under this specified rating, itstemperature rise will not be excessive. The copper loss (I 2R) in the transformer depends on thecurrent 'I' through the winding while the iron or core loss depends on the voltage 'V' as frequencyof operation is constant. None of these losses depend on the power factor (cos ) oHence losses decide the temperature and hence the rating of the transformer. As losses depend onV and I only, the rating of the transformer is specified as a product of these two parameters VxI.

Key point : Thus the transformer rating is specified as the product of voltage and current calledVA rating.

On both sides, primary and secondary VA rating remains same. This rating is generallyexpresses in KVA (kilo volt amperes rating).

Now V 1 /V2 = I2 /I1 = K... V1 I1 = V 2 I2

If V 1 and V 2 are the terminal voltages of primary and secondary then from specified KVArating we can decide full load currents of primary and secondary, I 1 and I 2. This is the safe

maximum current limit which may carry, keeping temperature rise below its limiting value.

Key point : The full load primary and secondary currents indicate the safe maximum values ofcurrents which transformer windings can carry.

Ideal Transformer on No Load

Consider an ideal transformer on no load as shown in the Fig. 3. The supply voltage is and as it isV1 an no load the secondary current I 2 = 0.

The primary draws a current I 1 which is just necessary to produce flux in the core. As itmagnetising the core, it is called magnetising current denoted as I m. As the transformer is ideal,the winding resistance is zero and it is purely inductive in nature. The magnetising current is I m is

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very small and lags V 1 by 30 o as the winding is purely inductive. This I m produces an alternatingflux which isin phase with I m.

Fig. 1 Ideal transformer on no load

The flux links with both the winding producing the induced e.m.f.s E 1 and E 2 , in the primaryand secondary windings respectively. According to Lenz's law, the induced e.m.f. opposes thecause producing it which is supply voltage V

1. Hence E

1is in antiphase with V

1 but equal in

magnitude. The induced E 2 also opposes V 1 hence in antiphase with V 1 but its magnitude dependson N 2. Thus E 1 and E 2 are in phase.

The phasor diagram for the ideal transformer on no load is shown in the Fig. .2.

Fig. 2 Phasor diagram for ideal transformer on no load It can be seen that flux is reference. Im produces hence in phase with . V1leads I m by

90o as winding is purely inductive so current has to lag voltage by 90 o.E1 and E 2 are in phase and both opposing supply voltage .The power input to the transformer is V 1 I1 cos (V 1 ^ I 1 ) i.e. V 1 Im cos(90

o) i.e. zero. This is because on no load output power is zero and for ideal transformer there are no losses hence input power is also zero. Ideal no load p.f. of transformer is zero lagging.

Practical Transformer on No Load

Actually in practical transformer iron core causes hysteresis and eddy current losses as it issubjected to alternating flux. While designing the transformer the efforts are made to keep theselosses minimum by,

1. Using high grade material as silicon steel to reduce hysteresis loss.2. Manufacturing core in the form of laminations or stacks of thin lamination to reduce eddy

current loss.

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Apart from this there are iron losses in the practical transformer. Practically primary windinghas certain resistance hence there are small primary copper loss present.

Thus the primary current under no load condition has to supply the iron losses i.e. hysteresisloss and eddy current loss and a small amount of primary copper loss. This current is denoted asIo.

Now the no load input current I o has two components :1. A purely reactive component I m called magnetising component of no load current required to produce the flux. This is also called wattless component.

2. An active component I c which supplies total losses under no load condition called powercomponent of no load current. This also called wattful component or core loss component of I o.

Thtotal no load current I o is the vector addition of I m and I c.

In practical transformer, due to winding resistance, no load current I o is no longer at90o with respect to V 1. But it lags V 1 by angle o which is less than 90 o . Thus cos o is called noload power factor of practical transformer.

Fig 1. Practical transformer on no load

The phasor diagram is shown in the Fig. 1. It can be seen that the two components I o are,

This is magnetising component lagging V 1 exactly by 90 o .

This is core loss component which is in phase withV 1.The magnitude of the no load current is given by,

While o = no load primary power factor angleThe total power input on no load is denoted as W o and is given by,

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