DC Machines - Mechanical Engineering · or for feeding the DC Power to the armature of a DC Motor. DC Machines are of three types: 1. Series (A) 2. Shunt (B) 3. Compound (C) DC Series

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DC Machines

Introduction

D. C. machines are seldom used in ordinary applications because all electric supply

companies furnish alternating current However, for special applications such as in steel mills,

mines and electric trains, it is advantageous to convert alternating current into direct current in

order to use D.C. motors. The reason is that speed/torque characteristics of D.C. motors are much

more superior to that of a.c. motors. Therefore, it is not surprising to note that for industrial

drives, D.C. motors are as popular as 3-phase induction motors. Like D.C. generators, D.C. motors

are also of three types viz., series-wound, shunt-wound and compoundwound. The use of a

particular motor depends upon the mechanical load it has to drive.

In DC Machines the field poles are present on the stator called as YOKE. Armature windings

and Commutator are on the Rotor. The Figure gives the cross-sectional view of a 4-pole DC

machine. Brushes press on to the Commutator view for collecting the power from a dc generator or for feeding the DC Power to the armature of a DC Motor. DC Machines are of three types:

1. Series (A)

2. Shunt (B)

3. Compound (C)

DC Series machines has the field winding in series with armature circuit, DC Shunt Machine has field winding across the armature circuit. Where as DC Compound machine has two Field

Windings. One across the armature and the other in series with the armature. DC Machines has

inter poles. Large DC machines have also compensating windings embedded in the pole faces of

the main poles.

DC Machine Posses very versatile

characteristics. DC motor is easily adaptable for

drives requiring wide range speed control and

maintenance. It is highly versatile energy

conservation device. It can meet the demand of

loads requiring high starting torques, high

accelerating and decelerating torques.

In view of these outstanding features,

DC Machines are widely used for the industrial

purpose particularly for tough jobs as are in

steel mills.

There are two types of DC Machines depending

on their field system employed, they are:



HOMO-POLAR Machines HETERO-POLAR Machines

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HOMO-POLAR Machines

These types of machines are used where low Voltage and High Currents are required and

the fields system is unusual as in the Faraday Disc Dynamo, which is an example of this type of

machine.

HETERO-POLAR Machines

These types of machines are most commonly used in Practice. The magnetic poles are mounted

as shown in figure to form alternate south and north poles when traversed along the

circumference of an armature. These machines are used for moderate voltage and high output

power.

Construction of a DC machine Basically the construction of a DC machine includes mainly 7 parts. They are:

i. Magnetic Frame (or) Yoke

ii. Poles

a. Pole Face (or) Pole Core b. Pole Shoe

iii. Pole Windings (or) Field Windings

iv. Armature Core

v. Commutator

vi. Armature Windings

vii. Brushes and Bearings

i. Magnetic Frame (or) Yoke

It is cylindrical in shape and made out of cast steel or cast iron.

It is the outer most part of the DC machine on which the poles

are located on the inner part of the Yoke.

PURPOSE:

It gives mechanical support to the poles situated on the inner

part of the Yoke.

It acts as a protecting layer to the entire DC machine and protects from the atmospheric

ailments.

It acts as Magnetic Flux carriers in the DC machine.

ii. Poles (Pole Core and Pole Shoe)

Pole Shoe:

The field magnet consists of mainly pole shoe and pole

core. The pole shoe serves mainly for two purposes.



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In spreading the magnetic flux in the air gap and being of larger cross-section, reduce

the reluctance of the magnetic path.

It supports the exciting coils (or field coils) shown in figure.

Pole Core:

The pole core itself maybe a solid piece made up of either cast iron or cast steel. But

the pole shoe is laminated.

Construction:

Method: 1

1. In this method the pole core is a simple solid, made out of cast

iron or cast steel. 2. Pole shoe is laminated and is fastened to the pole face by means

of “COUNTER SUNK SCREW” as shown in figure.

Method: 2:

1. In this method both the pole shoe and pole core are laminated

and are riveted permanently under hydraulic pressure.

2. The thickness of each laminated sheet varies from 1 mm to

0.25 mm. All the poles should be fitted on to the Yoke by means of screws inside

the pole such that it should enter the Yoke and gets fixed. This is shown

in the figure.

iii. Pole Windings (or) Field Windings

The field coils or pole coils which consists of

copper wire or strip, are former- wound for the

correct dimension shown in the left figure. Then

the former is removed and wound coil is put into

place over the core as shown in the right figure.

iv. Armature Core

1. It is the main part of the DC machine

which houses or holds the armature coils

or conductors.

2. It is basically cylindrical in shape built by



several circular steel disks.

3. Every disk is punched, the punched

portion is called as “SLOTS” and the un-

punched portion is called as “TEETH”.

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4. The slot must be covered with the insulating material MICA. 5. The laminations are perforated for air duct which permits the axial flow of air through

the armature for cooling purpose.

6. The inner peripheral consists of “KEYWAYS” where as the outer peripheral consists of

“SLOTS and TEETH”.

7. Keyways are used to fix the shaft, which appears as a “DOVE- Tailed” or “Wedge –

Shaped”.

v. Commutator 1. The shape of a Commutator appears to be wedge-shaped and cylindrical structure. 2. The function of the Commutator is to facilitate collection of current from the armature

conductors or coils.

3. It rectifies i.e., it converts the alternating current induced in the armature conductors

into unidirectional current in the external load circuit.

4. The wedge-shaped segments are insulated from each other with the help of thin layers

MICA.

5. Each Commutator segment is connected to the armature conductor by means of a copper

lug or strip (riser).

6. To prevent them from flying out under the action of centrifugal forces, the segments

have V-shaped grooves, these grooves being insulated by conical micanite rings. This is

shown in the figure.

vi. Armature Windings

1. The armature windings are done by

using high grade copper wires or

strips. 2. The armature windings includes any

other factors such as

a. Pole –Pitch

b. Conductor

c. Pitch of a Winding (Y)

d. Back Pitch (YB)

e. Front Pitch (YF)

f. Resultant Pitch (YR)

g. Commutator – Pitch (YG)

It is the distance between the segments to which the two ends of a coil are

connected is defined as Commutator Pitch.

h. Single Layer Winding

i. Two –Layer Winding

j. Multiplex Winding etc…….

vii. Brushes and Bearings



1. The function of Brushes is to collect the current from the Commutator, is usually

made of carbon or graphite and is in the shape of a rectangular block. 2. These brushes are housed in brush-holders usually of the box-type variety.

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3. The brushes are made to bear

down on the commutator by a

spring whose tension can be

adjusted by changing the position

of the lever in the notches. 4. A flexible copper wire is mounted

at the top of the brush conveys

current from the brushes to the

holder.

5. The number of brushes per spindle

depends on the magnitude of the

current to be collected from the Commutator.

6. Because of the reliability, ball-bearings are frequently employed, though for heavy

duties. Roller bearings are most preferred.

7. Sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir

in the bearing bracket. This is shown in the figure.

Operation of a DC Machine as a Generator

Principle:

An Electric Generator is a machine which converts mechanical energy or Power in to

Electrical energy or power. The energy conversion is based on the principle of the production of

dynamically induced emf. Whenever a conductor cuts the magnetic flux dynamically, induced emf

is produced in it according to the Faraday‟s laws of Electromagnetic induction. This emf causes a

current to flow if the conductor circuit is closed.

Hence, two basic essential parts of an electrical generator are

Magnetic Field

Conductor or Conductors which can move

as to cut the flux

Construction of Generators

The generator mainly consists of magnetic

poles which are mounted on the stator and a

armature core in between the magnetic field. The armature conductors are placed within the slots in

the armature core. Initially the armature conductors

are wounded in rectangular shape flat coils. Let us

consider a single turn rectangular copper coil

rotating about its own axis in a magnetic field

provided by either permanent or electromagnets.

The two ends of the coil are joined to two slip rings which are insulated from each other and from the

central shaft as shown in the figure. The collecting

brushes are made up of copper or carbon and are pressed against the split rings. Their function is

to collect the current induced in the coil and to convey it to the external load resistance „R‟. The

rotating coil may be called as „Armature‟ and the magnets as „Field Magnets‟.



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Working (Generator)

Assume the coil to be rotating in clock-wise direction as shown. As the coil assumes

successive position in the field, the flux linked wit it changes. Hence an emf is induces in it which

is proportional to the rate of change of flux linkage

E = NdФ

dt

When the plane coil is at right angle to the lines of flux. Then the flux linked with the coil is

maximum, but rate of change of flux linkage is minimum. As a result, the coil sides do not cut the

flux, rather they move along them parallel. Hence there is no emf induced in the coil. As a result

coil continues rotating further, rate of change of flux linkage increases when it makes an angle of 90°. Here, the coil will be in horizontal, the flux linked with the coil is minimum but rate of change

of flux is maximum. Hence maximum emf is induced in the coil at 90°.

In the next quarter revolution (i.e. from 90° to 180°), flux linked with the coil gradually

increases but the rate of change of flux linkages decreases. Hence the induced emf decreases

gradually till the position, it is reduced to zero. In the first half revolution, emf is induced at the

initial position, maximum when it is at 90° and no emf at 180°. The direction of this induced emf

can be found by applying Fleming‟s right-Hand Rule which gives its direction. Hence the current

flows.

In the next cycle (i.e. from 180° to 360°) the variations in the magnitude of emf are just

like to that of the first half revolution. Therefore for every half revolution, the current gets

reversed in its direction which is known as alternating current. It is later converted in to

unidirectional current (DC) by replacing the slip ring with the split rings. Split rings are made out

of a conducting cylinder cut into two halves or segments insulated from each other by a thin sheet

of mica.

As before the coil ends are joined to

the segments on which rest the copper or

carbon brushes. It is seen that in the first

half revolution current flows i.e. the brush

will be in contact with the segment acts as

a positive end of the supply and other as

negative. In the next half revolution, the

direction of induced current in the coil has

reversed. But at the same time the position

of the segments are also reversed with the

result that the initial brush comes in

contact with the segment which is positive.

Hence the current is purely unidirectional but not

continuous.



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Operation of a DC machine as a motor

Principle:

An electric motor is a machine which converts electrical energy in to mechanical

energy. Its action is based on the principle that when a current carrying conductor is placed in a

magnetic field, it experiences a mechanical force whose direction is given by Fleming‟s Left-Hand

Rule and whose magnitude is given by

Construction of Motors

F = BIL Newtons

Constructionally there is no basic difference

between a DC generator and DC motor. In fact, the

same dc machine can be used interchangeably as a

generator or as a motor. DC motors are also like generators. They were been classified into three,

they are:

1. Series-Wound DC Motors

2. Shunt-Wound DC Motors 3. Compound-Wound Dc Motors

Working of a Dc Motor

Let us consider a part of multipolar DC motor as shown in figure. When the field

magnets are excited by passing current in their field windings and its armature conductors are

supplied with current from the supply mains, they experience a force tending to rotate the

armature. Armature conductors under N-Pole area assumed to carry current downwards (crosses)

and those under S-Pole, to carry current upwards (dots). By applying Fleming‟s Left hand Rule, the

direction of the force on each conductor can be found. It is shown by small arrows placed above

each conductor. It can be seen that each conductor experiences a force „F‟ which tends to rotate

the armature in anti-clockwise direction. These forces collectively produce a driving torque which

sets the armature rotating.

The function of the commutator in the motor is the same as in a generator. By

reversing current in each conductor as it passes from one pole to another, it helps to develop a

continuous and unidirectional torque.

Comparison of Generator and Motor Action



The same DC Machine can be used interchangeably as a generator as well as motor. When

operating as a generator, it is driven by a mechanical machine and it develops voltage which I turn

produces a current flow in an electric circuit. When operating as a motor, it is supplied by electric

current and it acquires torque which in turn produces mechanical rotation.

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Generator Action

Let us consider a part of a generator whose armature is being driven clockwise by its

prime mover as shown in figure. Fig.1 represents the field setup independently by the main poles

and the armature conductors like „A‟ in the shown figure. The resultant field or magnetic lines of

flux may be linked to the rubber bands under tension. Hence, the bet lines of flux set up a

mechanical force on „A‟ much, just like as the bent elastic rubber band of a catapult, Produces a

mechanical force on the piece of a stone. The direction of this force is opposite to that of armature

rotation. Hence, it is known as backward force or magnetic drag on the conductors.

Therefore, against this drag action on all the armature conductors that the prime

mover has to work. The work done in overcoming this opposition is converted into electrical

energy.

E.M.F Equation in a DC Machine

Generator Generated EMF or EMF Equation of a Generator

Generator works on the principle of Faraday‟s laws of Electromagnetic Induction i.e.

when a revolving armature conductors cuts the magnetic lines of flux, a dynamically induced emf

is produced in the armature coils and this emf causes current flow in it. If the rotor circuit is

closed.

We shall now derive an expression for the emf generated in a D.C. generator.

Let

Ø= flux/pole in Wb

Z = total number of armature conductors

P = number of poles

A = number of parallel paths = 2 … for wave winding

= P … for lap winding

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

EG= emf of the generator = emf /parallel path

General EMF Equation – (1)



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For a simple wave-wound generator

No. of parallel paths „A‟ =2

No. of conductors in one path = Z

2

EMF generated per path = ØPN X Z

60 2

= ØPNZ Volt

120

For a simple lap-wound generator

No. of parallel paths „A‟ = P

No. of conductors I one path = Z P

EMF generated per path = ØPN X Z

60 P

= ØNZ Volt

60

If we multiply and divide the above general equation of EG by

„2∏‟ EG = 2∏ X ØPN X Z

2∏ 60 A

By solving, we get

= ωØZ X P Volts ω-omega- in radians 2∏ A

But for a given DC machine Z,P and A are constants. Hence putting Ka = ZP in the general

EMF equation, we get A

EG = KaØN Volt

60

EG = KaØN Volts (where ‟N‟ is in r.p.s)

Motor

When the motor armature rotates, the conductors also rotate and hence cut the flux.

In accordance with the laws of electromagnetic induction, emf is induced in them whose direction,



as found by Fleming‟s right-Hand Rule, is in opposition to the applied voltage. Because of its

opposing direction. It is referred to as counter emf or back emf „Eb‟.

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Eb= ØZN X P Volts

60 A

The equivalent circuit of a motor is as shown below. The rotating armature generates the back emf „Eb‟ is like a battery of emf „Eb‟ put across supply mains of „V‟ volts. This „V‟ has to drive

the current „Ia‟ against the opposition of „Eb‟. the power required to overcome this opposition is

EbIa

The back emf depends on other factors as the armature speed. If the speed of the armature

is high, Eb is large. Hence armature current Ia is small. If the speed is less, then „Eb‟ is less.

Hence more current flows which develops motor torque. So Eb acts like a governor.

Voltage Equation of a Motor

The voltage „V‟ is applied across the motor armature, has to

Overcome the back emf Eb

The armature ohmic drop IaRa

V = Eb + IaRa

This is known as voltage equation of a motor.

Multiplying ob both sides by Ia in the above equation, we get

VIa =EbIa + Ia²Ra

Where VIa – Electrical input to the armature

EbIa – Electrical equivalent of mechanical power developed in the armature

Ia²Ra - Cu Losses in the armature

Hence out of the armature input, some is wasted in I²R loss and rest is converted into

mechanical power within the armature. The motor efficiency is given by the ratio of power

developed by the armature to its input i.e

Eb

V Eb is higher as compared to V, there fore the motor efficiency is higher.

Production of Torque in a DC Machine

The term torque is meant the turning or twisting moment of

a force about an axis. It is measured by the product of the force

and the radius at which this force acts.

Consider a pulley of radius „r‟ meter acted upon by a

circumferential force of „F‟ Newtons which causes it to rotate at

„N‟ rpm as shown in figure.



Then torque τ = F X r (N-m) Work done by this force is one revolution = force X distance

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= F x 2∏r Joule

Power developed = F x 2∏r x N Joules / Second or watt

= (Fr) x 2∏N watt

Now 2∏N = Angular Velocity „ω‟ in radian/second and F x r = torque τ

Therefore power developed = τ x ω watt

Or

P = τω watt

Moreover, if „n‟ is in rpm

Then

ω = 2∏N radians/second

60

Therefore P=2∏N x T

60

P = 2∏ NT = NT 60 9.55

Armature Torque of a Motor

Let „Ta‟ be the torque by the armature of a motor running at „N‟ rps. If „Ta‟ is in N/m,

them power developed

Ta x 2∏N watt ------ (1) We also know that electrical power converted into mechanical power in the armature

EbIa watts -----

(2) Equating (1) and (2), we get

Ta x 2∏N = EbIa ------ (3)

Since Eb = ØZN X P Volts

A

We have, Ta x 2∏N = ØZN X P Ia

A Ta = 1 x ØZ Ia P N-m

A

i.e. Ta = 0.159 ØZ Ia P N-m A

From (3) as shown above

If „N‟ is in rpm, then



Ta =EbIa N-m were „N‟ is in rps 2∏N

Ta = 60 EbIa = 9.55 EbIa N-m

2∏N 2∏N

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Shaft Torque

The whole of the armature Torque, as calculated above, is not available for doing useful

work, because a certain percentage of it is required for supplying iron and frictional losses in the

motor. The torque which is available for doing useful work is known as Shaft Torque (TSh). It is so

called because it is available at the shaft. The motor output is give by

Output in watts N-m „N‟ – in rps

Tsh = 2∏N

= 60 output

2∏ N

The difference (Ta - Tsh) is known as lost torque and is due to ion and friction losses of

the motor.

Note

Ta can also be expressed in kg-m at 1kg=9.81 Nw-m

T= 0.0612 x ØZ Ia P Kg-m A

Since Z,P and A are constant for a particular machine

Therefore Ta ∞ ØIa

1. In case of series motor, field winding carries full current. Hence before saturation „Ø‟ is

directly proportional to the armature current.

Ta ∞ Ia²

2. In case of shunt motor, „Ø‟ is practically constant.

Ta ∞ Ia

Some Definitions of Armature Windings

Coil-Span or Coil Pitch (YS)

It is the distance measured in terms of armature slots (armature conductors) between the

two sides of a coil. It is in fact, the periphery of the armature spanned by the two sides of the coil. Pitch of the winding (Y)

In general, it may be defined as the distance round the armature between the two

successive conductors which are directly connected together or it is the distance between the

bearings of the two consecutive turns.

Y=YB-YF ---- for Lap Winding



Y=YB+YF ---- for Wave winding

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Back Pitch (YB)

The distance measured in terms of the armature conductors, which a coil advances on the

back of the armature is called back pitch and is denoted by YB.

Front Pitch (YF)

The number of armature conductors or elements spanned by a coil on the front

(commutator end of an armature) is called the front pitch and is denoted by YF.

Or

The front pitch may be defines as the distance (in terms of the armature conductors)

between the second conductor of one coil and the first conductor of the next coil which are

connected at the front i.e. commutator end of the armature. Resultant Pitch (YR)

It is defined between the beginning of one coil and the beginning of the next coil to which it

is connected. Commutator Pitch (YC)

It is the distance (measured in commutator bars or segments) between the segments to

which the two ends of a coil are connected.

YC =YB-YF ---- for Lap Winding YC =YB+YF ---- for Wave winding

Armature Windings

Two basic types of windings mostly employed for drum type armature are as follows

Wave-Winding

Lap –Winding

Wave Winding

The most distinguish feature of this winding is that electrically, it

divides the armature conductors in to parallel paths between the +‟ve

and –„ve brushes irrespective of the no. of poles of the machine. As

shown in figure, as the armature current enters the –„ve brush, it finds

2 parallel paths of equal resistances available for going to the =‟ve

brush. Hence it divides into 2 paths, each path consists of Z/2

conductors connected in series (Z-being the total no. of armature

conductors) and each carries of Ia/2. Where Ia is the total

armature current.

For example, in the case of a 4 pole wave wound generator

having 30 armature conductors, each of 2 parallel paths will have 15



conductors.

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The wave winding is one in which the coil end, diverge and go to the segments widely

separated. This is called series winding.

Lap winding

Lap winding is one in which the adjacent ends of the coils are connected

together in closed to each other on the commutator segments. In the lap

windings the armature conductors are divided into many parallel paths as the

no. of poles of the generator. If there are „P‟ poles and „Z‟ armature

conductors, then there will be „P‟ parallel paths, each consisting of Z/P

conductors connected in series between the positive and negative set of

brushes.

From the figure, as the armature current enters the negative brush, it

has four parallel paths available for going to the positive brush. Each path has

Z/4 conductors and carries a current if Ia/4 amps. Advantages

1. The wave windings giving (gives) more emf than the lap winding for the

same no. of poles and armature conductors. 2. In a wave winding equalizing connections are not necessary which are

in lap windings.

3. Lap winding gives more parallel paths, hence its importance lies when large currents are

required.

Armature Windings

Types of DC Windings

Dc armature windings can be divided into two groups mainly depending upon the way in which the coil ends are connected to the commutator segments. They are

1. Lap Windings

2. Wave windings

In the lap windings, the two ends of any coil are taken to

the adjacent commutator segments and the coil ends are bent

inwards as shown below. Where as in the wave wingding, the coil

ends are bent in opposite directions and taken to the commutator

segments some distance apart as shown.

A Simplex lap winding can be progressive or retrogressive. In general, lap windings, the coil

ends are connected to the adjacent commutator segments. In progressive winding, the start and

finish of the coils ends are connected to the respective commutator segments in an ascending

order and in the retrogressive winding the ends are connected to the connected segments in the

descending order.

In a wave winding, the conductors in each parallel path are distributed symmetrically over

all the poles of the machine. They are used for high voltage and low current equipment.



Difference between Lap and Wave Windings

Lap Windings Wave Windings

1. Number of parallel paths is equal

to the number of poles. 1. Number of parallel paths are always 2.

2. The number of brush positions on the commutator equals to

the number of poles.

2. A minimum of two brush positions are required irrespective of the number of

poles.

3. The two ends of an armature coil are connected to the two

adjacent commutator

segments.

3. Two ends of an armature coil are connected to the two commutator

segments.

4. The winding forms a continuous

closed circuit. These are used for

supplying low voltages and high

current loads.

4. The winding forms a continuous closed

Circuit. These are used for supplying

High voltages and low currents.

DC Machines - Mechanical Engineering · or for feeding the DC Power to the armature of a DC Motor. DC Machines are of three types: 1. Series (A) 2. Shunt (B) 3. Compound (C) DC Series

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