d.c.machines-introduction

D.C.MACHINES

Electrical Generator, Construction and Working: Electrical Generator : An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a WIND TURBINE. , a hand crank, or any other source of mechanical energy. In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. . He also built the first electromagnetic generator called the 'Faraday disc', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current. The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets. The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux. Simple loop Generator construction : To see the construction and working of a simple loop generator Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional (observe in fig). Generator working : In figure see the case when the coil is rotating in anticlock-wise direction with out commutator. As the coil assumes successive positions in the field, the flux linked with it changes.Hence, an e.m.f is induced in it which is proportional to the rate of change of flux linkages (e=-N dΦ/dt).

http://powerelectrical.blogspot.com/2007/03/electrical-generatorgenerator.html

D.C.MACHINES

When the plane of the coil is at right angles to lines of flux then flux linked with the coil is maximum but rate of change of flux linkages is minimum. It is so because in this position, the coil sides do not cut or shear the flux, rather they slide along them i.e they move parallel to them.Hence,there is no induced e.m.f in the coil.Generaly this no e.m.f is taken as the starting position i.e zero degrees position.The angle of rotation or time wil be measured from this position. As the coil continues rotating further, the rate of change of flux linkages (and hence induced e.m.f in it ) increases till the coil rotates 90° from its startinig position. Here the coil plane is vertical (see in fig) i.e parallel to the lines of flux.As seen, the flux linked with the coil is minimum but rate of change of flux linkages is maximum. Hence , maximum e.m.f is induced in the coil when in this position. In the next quarter revolution i.e from 90° to 180°,the flux linked with the coil gradually increases but the rate of change of flux linkages decreases.Hence,induced e.m.f decreases gradually till it becomes zero. So,we find that in the first half revolution of the coil, no e.m.f is induced in it at 0°, maximum when the coil is at 90° position anno e.m.f when coil is at 180°.The direction of this induced e.m.f can be found by applying Fleming's Right hand rule. In the next half revolution i.e from 180° to 360°, the variations in the magnitude of e.m.f are similar to those in the first half revolution.Its value is maximum when coil is at 270° and minimum when the coil is at 360° position.But it wil be found that th direction of induced current is reverse of the previous direction of flow. Therefore,we find that the current which we obtain from such a simple generator reverses its direction after every half revolution.Such a current undergoing periodic reversals is known as alternating current.It should be noted that alterating current not only reverses its direction, it does not even keep its magnitude constant while flowing in any one direction.The two half- cycles may be called positive and negative half-cycles respectively. Now see when the coil is rotating with commutator.In this case the slip rings are replaced by split rings.The split rings are made out of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulating material (you can see in fig). As before, the coil ends are joined to these segments on which rest the carbon or copper brushes. In case of split rings, the positions of the segments of split rings have also reversed when the current induced in the coil reverses i.e when the curent direction reverses the brushes also comes in contact with reverse segments as that of positive half-cycle.Hence, this current is unidirectional.It should be noted that the position of the brushes is so arranged that the change over of segments from one brush to other takes place when the plane of the rotating coil is at right angles to the plane of the lines of flux.It is so because in that position, the induced e.m.f in the coil is zero.You can observe this in two cases by pausing the waveform.

D.C.MACHINES

Another important point is that now the current induced in the coil is alternating as before.It is only due to the rectifying action of the splitunidirectional in the external circuit. To better understand the operation of generator Fleming's Right hand Rule Fleming's right hand rule (for generators) shows the direction of conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually at right angles, as shown in the diagram .

• The Thumb represents the direction of Motion of the conductor• The First finger represents the direction of the Field• The Second finger represents the

classical direction, from positive to negative).

GENERATOR EQUIVALENT CIRCUIT AND MAIN PARTS:

Another important point is that now the current induced in the coil is alternating as before.It is only due to the rectifying action of the split-rings (also called commutator) that it becomes

in the external circuit.

To better understand the operation of generator

Fleming's right hand rule (for generators) shows the direction of induced current conductor moves in a magnetic field.

ld with the thumb, first finger and second finger mutually at right angles, as

Thumb represents the direction of Motion of the conductor. First finger represents the direction of the Field. Second finger represents the direction of the induced or generated Current

classical direction, from positive to negative).


Another important point is that now the current induced in the coil is alternating as before.It is rings (also called commutator) that it becomes

flow when a

ld with the thumb, first finger and second finger mutually at right angles, as

direction of the induced or generated Current (in the


D.C.MACHINES

Equivalent circuit :

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure:

• Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.

• Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.

• Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.

• Measure the load resistance RL, if you don't already know it. • Calculate the generator's AC internal resistance RGAC from the formula

Note 1:The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal. Note 2: If the generator is an AC type (distinctly not a dynamo), use an AC voltmeter for the voltage measurements.

Maximum power : The maximum power theorem applies to generators as it does to any source of electrical energy. This theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. However, under this condition the power transfer efficiency is only 50%, which means that half the power generated is wasted as heat and Lorentz force or back emf inside the generator. For this reason, practical generators are not usually designed to operate at maximum power output, but at a lower power output where efficiency is greater.

D.C.MACHINES

Terminology: The parts of a dynamo or related electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This causes great confusion when working with compound machines such as a brushless alternator or when conversing with people who are used to working on a machine that is configured differently than the machines that the speaker is used to.

Mechanical

• Rotor: The rotating part of an alternator, generator, dynamo or motor. • Stator: The stationary part of an alternator, generator, dynamo or motor.

Electrical

• Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on ei

• Field: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a permanent magnet.

Main Parts of Generator : Actual generator consists of the following essential parts1. Magnetic frame or Yoke 2. PoleCore 6. Armature Windings or Conductors

The parts of a dynamo or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term.

es great confusion when working with compound machines such as a brushless alternator or when conversing with people who are used to working on a machine that is configured differently than the machines that the speaker is used to.

otating part of an alternator, generator, dynamo or motor. : The stationary part of an alternator, generator, dynamo or motor.

producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator.

: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a

Main Parts of Generator : generator consists of the following essential parts

. Pole-cores and Pole-Shoes 3.Field Poles 4. Field Coils . Armature Windings or Conductors 7.Commutator 8. Brushes and Bearings

can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term.

es great confusion when working with compound machines such as a brushless alternator or when conversing with people who are used to working on a machine that is

producing component of an alternator, generator, dynamo or

: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a

. Field Coils 5. Armature . Brushes and Bearings

D.C.MACHINES

In the above figure,views A through E, shows the component parts of dc generators.

Yoke Yoke is a outer frame. It serves two purposes. (i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and (ii) It carries the magnetic flux produced by the poles.

In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed.The modern process of forming the yoke consists of rolling a steel slab round a cylndrical mandrel and then welding it at the bottom. The feet and the terminal box etc, are welded to the frame afterwards.Such yokes possess sufficient mechanical strength and have high permeability.

Pole Cores and Pole Shoes The field magnet consist of pole ores and pole shoes. The pole shoes have two purposes (i) they spread out the flux in the air gap and also, being larger cross section,reduce the reluctance of the magnetic path (ii) they support the exciting coils (field coils)

Field Poles The pole cores can be made from solid steel castings or from laminations. At the air gap, the pole usually fans out into what is known as a pole head or pole shoe. This is done to reduce the reluctance of the air gap. Normally the field coils are formed and placed on the pole cores and then the whole assembly is mounted to the yoke.

Field Coils The field coils are those windings, which are located on the poles and set up the magnetic fields in the machine. They also usually consist of copper wire are insulated from the poles. The field

D.C.MACHINES

coils may be either shunt windings (in parallel with the armaseries with the armature winding) or a combination of both.

Armature Core or Stack The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a blanking die. Slots are punched in the lamare done as one. The laminations are welded, riveted, bolted or bonded together.

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets.In addition to this its most important function is to provide a path of low reluctance to the flux through the armature from a Ndrum shaped and is built up of usually circular sheet stee discs or laminations approxthick.It is keyed to the shaft.

Armature Windings The armature windings are usually formerrectangular coils and are then pulled into their proper shape in a coil puller.Various conductors of the coil are insulated rom each other.The conductors are placed in the armature slots which are lined with tough insulating material.This slot insulation is folded over above the armature conductors placed in the slot and is secured in place by special hard

Commutator A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator.It converts the alternating current induced in the armature conductors into

unidirectional current in the external load circuit.

It typically consists of a set of copper contacts, fixed around the circumference of the rotating part of the machine (the rotor), and a set of springpart of the machine (the stator) that complete the

coils may be either shunt windings (in parallel with the armature winding) or series windings (in series with the armature winding) or a combination of both.

The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a blanking die. Slots are punched in the lamination with a slot die. Sometimes these two operations are done as one. The laminations are welded, riveted, bolted or bonded together.

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic ts.In addition to this its most important function is to provide a path of

low reluctance to the flux through the armature from a N-pole to a S-pole.It is cylindrical or drum shaped and is built up of usually circular sheet stee discs or laminations approx

The armature windings are usually former-wound. These are first wound in the form of a flat rectangular coils and are then pulled into their proper shape in a coil puller.Various conductors of

e coil are insulated rom each other.The conductors are placed in the armature slots which are lined with tough insulating material.This slot insulation is folded over above the armature conductors placed in the slot and is secured in place by special hard wood or fibre wedges.

A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator.It converts the alternating current induced in the armature conductors into

the external load circuit.

It typically consists of a set of copper contacts, fixed around the circumference of the rotating ), and a set of spring-loaded carbon brushes fixed to the stationary ) that complete the electrical circuit from the rotor's windings to

ture winding) or series windings (in

The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a ination with a slot die. Sometimes these two operations

It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic ts.In addition to this its most important function is to provide a path of

pole.It is cylindrical or drum shaped and is built up of usually circular sheet stee discs or laminations approximtely 5mm

wound. These are first wound in the form of a flat rectangular coils and are then pulled into their proper shape in a coil puller.Various conductors of

e coil are insulated rom each other.The conductors are placed in the armature slots which are lined with tough insulating material.This slot insulation is folded over above the armature

or fibre wedges.

A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator.It converts the alternating current induced in the armature conductors into

It typically consists of a set of copper contacts, fixed around the circumference of the rotating loaded carbon brushes fixed to the stationary

from the rotor's windings to

D.C.MACHINES

the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster and are designed to be replaced easily without dismantling the machine. The copper contacts are usually inaccessible and, on small motors, are not designed to be repaired. On large motors the commutator may be re-surfaced with abrasives. Each segment of thefrom the adjacent segments; a large motor may contain hundreds of segments.

Brushes and Bearings The brushes whose function is to collect currnet from commutator,are usually made of carbon or graphite and are in the shape of a rectusually of the box type variety.

Because of their reliability, ball-bearings are frequently employed,though for heavy duties,roller bearings are preferable.The ball and rollers are generally packed iand for reduced bearing wear,sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir in the bearing bracket.

Armature and its Windings:

Armature : Gramme -Ring armature The old Gramme-Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and thgoes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil.

View B shows a composite view of a Grammephysical relationship of the coils and commutator locations.

The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux.

the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster nd are designed to be replaced easily without dismantling the machine. The copper contacts are

usually inaccessible and, on small motors, are not designed to be repaired. On large motors the surfaced with abrasives. Each segment of the commutator is insulated

from the adjacent segments; a large motor may contain hundreds of segments.

The brushes whose function is to collect currnet from commutator,are usually made of carbon or graphite and are in the shape of a rectangular block. These brushes are housed in brush

bearings are frequently employed,though for heavy duties,roller bearings are preferable.The ball and rollers are generally packed in hard oil for quieter operation and for reduced bearing wear,sleeve bearings are used which are lubricated by ring oilers fed from oil reservoir in the bearing bracket.

Armature and its Windings:-

Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B. One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil.

w B shows a composite view of a Gramme-ring armature. It illustrates more graphically the physical relationship of the coils and commutator locations.

ring armature are placed on an iron ring. A disadvantage of this that the windings located on the inner side of the iron ring cut few lines of flux.

the outside of the machine. Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster nd are designed to be replaced easily without dismantling the machine. The copper contacts are

usually inaccessible and, on small motors, are not designed to be repaired. On large motors the commutator is insulated

The brushes whose function is to collect currnet from commutator,are usually made of carbon or angular block. These brushes are housed in brush-holders

bearings are frequently employed,though for heavy duties,roller n hard oil for quieter operation

and for reduced bearing wear,sleeve bearings are used which are lubricated by ring oilers fed

Ring armature,now obselete is shown in figure view A. Each coil is connected to two commutator segments as shown. One end of coil 1 goes to segment A, and the other end

e other end of coil 2 goes to segment B. The rest of the coils are connected in a like manner, in series, around the armature. To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil

ring armature. It illustrates more graphically the

ring armature are placed on an iron ring. A disadvantage of this that the windings located on the inner side of the iron ring cut few lines of flux.

D.C.MACHINES

Therefore, they have little, if any, voltage induced in them. For this reason, the Grammearmature is not widely used.

Drum-type armature : A drum-type armature is shown in figure.The armature windings are placed in slots cut in a drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armatureis greater than in the Gramme-ring. You can see that the drumefficient than the Gramme-ring. This accounts for the almost universal use of the drumarmature in modern dc generators.

Armature Windings : Drum-type armatures are wound with either of two types of windings Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arrangedprogressively or retrogressively and connected in simplex,duplex and triplex.The following rules,however,apply to both types of the windings: (i)The front and back pitch are each approximately equal to the polefull-pitched.This results in increased e.m.f round the coils.For special purposes,fractionalwindings are deliberately used. (ii)Both pitches should be odd, otherwise it would be difficult to place the coils properly on the armature.For example if YB and YF werelie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie inthe lower half of some other slot.(iii) The number of commutator segments is equa to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.(iv) The winding must close upon itself i.e if we start from agiveto another,then all conuctors should be traversed and we should reach the same point again without a break or discontinuty in betwen. Lap Winding : View A This type of winding is used in dc generators designed for highwindings are connected to provide several parallel paths for current in the armature. For this reason, lap-wound armatures used in dc generators require several pairs

Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme

own in figure.The armature windings are placed in slots cut in a shaped iron core. Each winding completely surrounds the core so that the entire length of

the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armaturering. You can see that the drum-type armature is much more

ring. This accounts for the almost universal use of the drumdc generators.

es are wound with either of two types of windings - the Lap Winding or the Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arrangedprogressively or retrogressively and connected in simplex,duplex and triplex.The following rules,however,apply to both types of the windings:

The front and back pitch are each approximately equal to the pole-pitch i.e windings should be his results in increased e.m.f round the coils.For special purposes,fractional

Both pitches should be odd, otherwise it would be difficult to place the coils properly on the

armature.For example if YB and YF were both even,then all the coil sides and conductors would lie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie inthe lower half of some other slot.

The number of commutator segments is equa to the number of slots or coils because the front ends of conductors are joined to the segments in pairs.

The winding must close upon itself i.e if we start from agiven point and move from one coil to another,then all conuctors should be traversed and we should reach the same point again without a break or discontinuty in betwen.

This type of winding is used in dc generators designed for high-current applications.windings are connected to provide several parallel paths for current in the armature. For this

wound armatures used in dc generators require several pairs of poles and brushes.

Therefore, they have little, if any, voltage induced in them. For this reason, the Gramme-ring

own in figure.The armature windings are placed in slots cut in a shaped iron core. Each winding completely surrounds the core so that the entire length of

the conductor cuts the main magnetic field. Therefore, the total voltage induced in the armature type armature is much more

ring. This accounts for the almost universal use of the drum-type

the Lap Winding or the Wave Winding. The difference beween the two is merely due to the different arrangement of the end connections at the front or commutator end of armature.Each winding can be arranged progressively or retrogressively and connected in simplex,duplex and triplex.The following

pitch i.e windings should be his results in increased e.m.f round the coils.For special purposes,fractional-pitched

Both pitches should be odd, otherwise it would be difficult to place the coils properly on the both even,then all the coil sides and conductors would

lie either in the upper half of slots or in the lower half.Hence, it would become impossible for one side of the coil to lie in the upper half of one slot and the other side of the same coil to lie in

The number of commutator segments is equa to the number of slots or coils because the

n point and move from one coil to another,then all conuctors should be traversed and we should reach the same point again

applications.The windings are connected to provide several parallel paths for current in the armature. For this

of poles and brushes.

D.C.MACHINES

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been connected.This type of winding derivesucceding coils.Following points regarding simplex lap winding should be noted:

1. The back and front pitches are odd and of opposite sign.But they can't be equal. They differ by 2 or some multiple th

2. Both YB and YF shpuld be nearly equal to a pole pitch.3. The average pitch YA = (Y4. Commutator pitch YC = ±1.5. Resultant pitch YR is even, being the arithmetical difference of two odd numbers i.e Y

YB - YF. 6. The number of slots for a 2

commutator segments is also the same.7. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the

winding and 'P' the number of poles.Taking the where m=1 fo simplex lap and m =2 for duplex winding etc.

• If YB > YF i.e YB = YF + 2, then we get a progressive or rightwinding which progresses in the clockwise direction as seen from the comutatthis case YC = +1.

• If YB < size="1">F i.e YB = YF one which advances in the antiside.In this case YC = -1.

• Hence, it is obvious that for

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been connected.This type of winding derives its name from the fact it doubles or laps back with its succeding coils.Following points regarding simplex lap winding should be noted:

The back and front pitches are odd and of opposite sign.But they can't be equal. They differ by 2 or some multiple thereof.

shpuld be nearly equal to a pole pitch. = (YB + YF)/2.It equals pole pitch = Z/P. = ±1.

is even, being the arithmetical difference of two odd numbers i.e Y

number of slots for a 2-layer winding is equal to the number of coils.The number of commutator segments is also the same. The number of parallel paths in the armature = mP where 'm' is the multiplicity of the winding and 'P' the number of poles.Taking the first condition, we have Ywhere m=1 fo simplex lap and m =2 for duplex winding etc.

+ 2, then we get a progressive or right-handed winding i.e a winding which progresses in the clockwise direction as seen from the comutat

< size="1">F i.e YB = YF - 2,then we get a retrogressive or left-handed winding i.e one which advances in the anti-clockwise direction when seen from the commutator

Hence, it is obvious that for

In lap winding, the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil situated under the same pole an so on,till all the coils have been

s its name from the fact it doubles or laps back with its succeding coils.Following points regarding simplex lap winding should be noted:

The back and front pitches are odd and of opposite sign.But they can't be equal. They

is even, being the arithmetical difference of two odd numbers i.e YR =

layer winding is equal to the number of coils.The number of

The number of parallel paths in the armature = mP where 'm' is the multiplicity of the first condition, we have YB = YF ± 2m

handed winding i.e a winding which progresses in the clockwise direction as seen from the comutator end.In

handed winding i.e clockwise direction when seen from the commutator

D.C.MACHINES

The figures below shows the simplex lap winding in circular form and in

Wave Winding View B, shows a wave winding on a drum

The figures below shows the simplex lap winding in circular form and in development

, shows a wave winding on a drum-type armature. This type of winding is used in dc

development form.

type armature. This type of winding is used in dc

D.C.MACHINES

generators employed in high-voltage applications. Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles. This configuration allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation.

When the end connections of the coils are spread apart as shown in Figure a wave or s

eries winding is formed. In a wave winding there are only two paths regardless of the number of poles. Therefore, this type winding requires only two brushes but can use as many brushes as poles. Because the winding progresses in one direction round the armature in a series of 'waves' it is know as wave winding.If, after passing once round the armature,the winding falls in a slot to the left of its starting point then winding is said to be retrogressive.If, however, it falls one slot to the right, then it is progressive.

The figures below shows the simplex wave winding in circular form and in development form.

D.C.MACHINES

Points to note in case of Wave winding :

1. Both pitches YB and YF are odd and of the same sign.2. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2,

in which case, they are respectively one more or one less than the average pitch.3. Resultant pitch YR = YF + Y4. Commutator pitch, YC = Y

bars ± 1 ) / No.of pair of poles.5. The average pitch which must be

commutator bars ± 1)/No.of pair of poles.6. The number of coils i.e N7. It is obvious from 5 that for a wave winding, the number of armature conductors with 2

either added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.

8. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Commutation in a D.C.Generator:

Commutation Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. More simply stated,commutation is the mechDC machine, as shown in Figure

Points to note in case of Wave winding :

are odd and of the same sign. Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2,

which case, they are respectively one more or one less than the average pitch.+ YB.

= YA (in lap winding YC = ±1 ). Also YC = (No.of commutator bars ± 1 ) / No.of pair of poles. The average pitch which must be an integer is given by YA = (Z ± 2)/P = (No.of commutator bars ± 1)/No.of pair of poles. The number of coils i.e NC can be found from the relation NC = (PYA ± 2)/2.It is obvious from 5 that for a wave winding, the number of armature conductors with 2

er added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding.The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Commutation in a D.C.Generator:-

Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. More simply stated,commutation is the mechanical conversion from AC to DC at the brushes of a DC machine, as shown in Figure

Back and front pitches are nearly equal to the pole pitch and may be equal or differ by 2, which case, they are respectively one more or one less than the average pitch.

= (No.of commutator

= (Z ± 2)/P = (No.of

± 2)/2. It is obvious from 5 that for a wave winding, the number of armature conductors with 2

er added or subtracted must be a multiple of the number of poles of the generator.This restriction eliminates many even numbers which are unsuitable for this winding. The number of armature parallel paths = 2m where 'm' is the multiplicity of the winding.

Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. More

anical conversion from AC to DC at the brushes of a

D.C.MACHINES

voltage output is taken from an armature that has an ac voltage induced in it. You should remember from our discussion of the elememechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the voltage polarity in the armature loop reverses. A dc voltage is applied to the load because the outpasses under a brush. The segments are insulated from each other. In figure , commutation occurs simultaneously in the two coils that are briefly shortby the brushes. Coil B is shortshort-circuited by the positive brush. The brushes are positioned on the commutator so that each coil is short-circuited as it moves through its own electrical neutral plane. As you have seen previously, there is no voltage generated in the coil at that time. Therefore, no sparking can occur between the commutator and the brush. Sparking between the brushes and the commutator is an indication of improper commutation. Improper brush placement is the m

cause of improper commutation. entire commutation is done by the commutator in D.C Generators. Now we will see how the commutator performs commutation.

Commutator Action The commutator converts the AC voltage generated in the rotating loop int

Commutation is the process by which a dc voltage output is taken from an armature that has an ac voltage induced in it. You should remember from our discussion of the elementary dc generator that the commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the voltage polarity in the armature loop reverses. A dc voltage is applied to the load because the output connections are reversed as each commutator segment passes under a brush. The segments are insulated from each other.

In figure , commutation occurs simultaneously in the two coils that are briefly shortby the brushes. Coil B is short-circuited by the negative brush. Coil Y, the opposite coil, is

circuited by the positive brush. The brushes are positioned on the commutator so that circuited as it moves through its own electrical neutral plane. As you have

usly, there is no voltage generated in the coil at that time. Therefore, no sparking can occur between the commutator and the brush. Sparking between the brushes and the commutator is an indication of improper commutation. Improper brush placement is the m

cause of improper commutation. entire commutation is done by the commutator in D.C Generators. Now we will see how the commutator performs commutation.

The commutator converts the AC voltage generated in the rotating loop int

Commutation is the process by which a dc voltage output is taken from an armature that has an ac voltage induced in it. You should

ntary dc generator that the commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the voltage polarity in the armature loop reverses. A dc voltage is

put connections are reversed as each commutator segment

In figure , commutation occurs simultaneously in the two coils that are briefly short-circuited ited by the negative brush. Coil Y, the opposite coil, is

circuited by the positive brush. The brushes are positioned on the commutator so that circuited as it moves through its own electrical neutral plane. As you have

usly, there is no voltage generated in the coil at that time. Therefore, no sparking can occur between the commutator and the brush. Sparking between the brushes and the commutator is an indication of improper commutation. Improper brush placement is the main

This entire commutation is done by the commutator in D.C Generators. Now we will see how the

D.C.MACHINES

of connecting the brushes to the rotating loop. The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact with one of the ends of the looimpractical. Instead, the brushes are connected to the ends of the loop through the commutator.

In a simple one-loop generator, the commutator is made up of two semicylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material, as shown in Figure . Each half of the commutator segments is permanently attached to one end of the rotating loop, and the commutator rotates with the loop. The brushes, usually macarbon, rest against the commutator and slide along the commutator as it rotates. This is the means by which the brushes make contact with each end of the loop.

Each brush slides along one half of the commutator and then along the other half. The brushes are positioned on opposite sides of the commutator; they will pass from one commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. Every time the ends of treverse polarity, the brushes switch from one commutator segment to the next. This means that one brush is always positive with respect to another. The voltage between the brushes fluctuates in amplitude (size or magnitude) between zero and some maalways of the same polarity (Figure). In this manner, commutation is accomplished in a DC generator.

o a DC voltage. It also serves as a means of connecting the brushes to the rotating loop. The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact with one of the ends of the loop. Since the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the

loop generator, the commutator is made up of two semicylindrical pieces of a h conducting material, usually copper, separated by an insulating material, as shown in

Figure . Each half of the commutator segments is permanently attached to one end of the rotating loop, and the commutator rotates with the loop. The brushes, usually macarbon, rest against the commutator and slide along the commutator as it rotates. This is the means by which the brushes make contact with each end of the loop.

Each brush slides along one half of the commutator and then along the other half. The shes are positioned on opposite sides of the commutator; they will pass from one

commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. Every time the ends of treverse polarity, the brushes switch from one commutator segment to the next. This means that one brush is always positive with respect to another. The voltage between the brushes fluctuates in amplitude (size or magnitude) between zero and some maximum value, but is always of the same polarity (Figure). In this manner, commutation is accomplished in a DC

a DC voltage. It also serves as a means of connecting the brushes to the rotating loop. The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact

p. Since the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the

loop generator, the commutator is made up of two semicylindrical pieces of a h conducting material, usually copper, separated by an insulating material, as shown in

Figure . Each half of the commutator segments is permanently attached to one end of the rotating loop, and the commutator rotates with the loop. The brushes, usually made of carbon, rest against the commutator and slide along the commutator as it rotates. This is the

Each brush slides along one half of the commutator and then along the other half. The shes are positioned on opposite sides of the commutator; they will pass from one

commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. Every time the ends of the loop reverse polarity, the brushes switch from one commutator segment to the next. This means that one brush is always positive with respect to another. The voltage between the brushes

ximum value, but is always of the same polarity (Figure). In this manner, commutation is accomplished in a DC

D.C.MACHINES

One important point to note is that, as the brushes pass from one segment to the other, there is an instant when the brushes contact both segments at the same time. The induced voltage at this point is zero. If the induced voltage at this point were not zero, extremely high currents would be produced due to the brushes shorting the ends of the loop together. The point at which the brushes contact both commutator segments, when the induced voltage is zero, is called the "neutral plane."

D.C.MACHINES

E.M.F Equation of Generator:- Let Φ = flux/pole in weber Z = total number of armture conductors = No.of slots x No.of conductors/slot P = No.of generator poles A = No.of parallel paths in armature N = armature rotation in revolutions per minute (r.p.m) E = e.m.f induced in any parallel path in armature Generated e.m.f Eg = e.m.f generated in any one of the parallel paths i.e E. Average e.m.f geneated /conductor = dΦ/dt volt (n=1) Now, flux cut/conductor in one revolution dΦ = ΦP Wb No.of revolutions/second = N/60 Time 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

D.C.MACHINES

where A = 2 - for simplex wave= P - for simplex lap-winding Terminal Voltage DC generator output voltage is dependent on three factors : (1) the

number ofseries in the armature, (2) armatustrength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature can not be changed in a normally operating generator, and it is usually impractical to change the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generato DC Generator Ratings A DC generator contains four ratings.Voltage: Voltage rating of a machine is based on the insulation type and design of the machine. Current: The current rating is based on the size of the conductor and the

for simplex wave-winding

winding

DC generator output voltage is dependent on three factors : (1) the

conductor loops in series in the armature, (2) armature speed, and (3) magnetic field strength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature can not be changed in a normally operating generator, and it is usually

hange the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generato

A DC generator contains four ratings. Voltage rating of a machine is based on the insulation type and

The current rating is based on the size of the conductor and the

DC generator output voltage is dependent on three factors : (1) the

conductor loops in re speed, and (3) magnetic field

strength. In order to change the generator output, one of these three factors must be varied. The number of conductors in the armature can not be changed in a normally operating generator, and it is usually

hange the speed at which the armature rotates. The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generator.

Voltage rating of a machine is based on the insulation type and

The current rating is based on the size of the conductor and the

D.C.MACHINES

amount of heat that can be dissipated in the generator. Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors,bearings , and other components of the generator. Speed: Speed rating, at the upper limit, is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current (as speed increases, a higher field current is necessary to produce the same voltage).

Classification of Generators; (Series,Shunt,Compound):-

Generators are usually classified according to the way in which their fields are excited.The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products.Generators may be divided in to (a) Separately-excited generators and (b) Self-excited generators. (a) Separately-excited generators are those whoe field magnets are energised from an independent external source of DC current. (b) Self-excited generators are those whose field magnets are energused by the current produced by the generators themselves.Due to residual magnetism, there is always present someflux in the poles.When the armature is rotated, some e.m.f and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux. Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections — SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Compound-wound generators are further classified as cumulative-compound and differential-compound. Series-wound generator In the series-wound generator, shown in figure, the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit

D.C.MACHINES

A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire. The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice. Shunt wound In this field winding is connected in parallel with the armature conductors and have the full voltage of the generator applied across them.The field coils consist of many turns of small wire. They are connected in parallel with the load. In other words, they are connected across the output

voltage of the armature. Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator.

D.C.MACHINES

In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR). In a series-wound generator, output voltage varies directly with load current. In the shunt-wound generator, output voltage varies inversely with load current. A combination of the two types can overcome the disadvantages of both. This combination of windings is called the compound-wound dc generator. Compound-wound generator : Compound-wound generators have a series-field winding in addition to a shu

nt-field winding, as shown in figure. The shunt and series windings are wound on the same pole pieces. They can be either short-shunt or long-shunt as shown in figures. In a comound generator, the shunt field is stronger than the series field.When series field aids the shunt field, generator is said to be commutatively-compounded.On the other hand if series field opposes the shunt field,the generator is said to be differentially compounded. In the compound-wound generator when load current increases, the armature voltage decreases just as in the shunt-wound generator. This causes the voltage applied to the shunt-field winding

to decrease, which results in a decrease in the magnetic field. This same increase in load current, since it flows through the series winding, causes an

D.C.MACHINES

increase in the magnetic field produced by that winding. By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant. This is shown in figure, which shows the voltage characteristics of the series-, shunt-, and compound-wound generators. As you can see, by proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. Actual curves are

seldom, if ever, as perfect as shown.

Generator Losses:-

Generator Losses In dc generators

, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the armature, are considered as losses and may be defined as follows: 1. Copper loss in the winding 2. Magnetic Losses 3. Mechanical Losses Copper loss The power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.

loss is the Copper loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire.Copper loss is again divided as (i) Armature copper loss

http://powerelectrical.blogspot.com/2007/03/generator-losses-copperhysteresis-eddy.html

http://powerelectrical.blogspot.com/2007/03/generator-losses-copperhysteresis-eddy.html

D.C.MACHINES

= Armature copper loss. Where Ra =resistance of armature and interpoles and series field winding etc. This loss is about 30 to 40% of full -load losses.

(ii) Field copper loss : It is the loss in series or shunt field of generator. is the field copper loss in case of series generators, where Rse is the resistance of the series field widing.

is the field copper loss in case of shunt generators. This loss is about 20 to 30% of F.L losses. (iii) The loss due to brsh contact resistance.It is usually inluded in the armture copper loss. Magnetic Losses (also known as iron or core losses) (i) Hysteresis loss (Wh) Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.

(ii) Eddy Current Loss (We) The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss. Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.

D.C.MACHINES

As you can see, eddy current losses are kept low when the core material is made up of many thin sheets of metal.Laminations in a small generator armature may be as thin as 1/64 inch. The laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small. Most generators use armatures with laminated cores to reduce eddy current losses.

These magnetic losses are practically constant for shunt and compound-wound generators, because in their case, field current is constant. Mechanical or Rotational Losses There are no sources in the current document.i These consist of (i) friction loss at bearings and comutator. (ii) air-friction or windage loss of rotating armature These are about 10 to 20% of F.L losses. Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush tension. A smooth and clean commutator also aids in the reduction of brush friction.

Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also known as rotational losses for obvious reasons. As said above, field Cu loss is constant for shunt and compound generators.Hence, stray losses and shunt Cu loss are constant in their case.These losses are together known as standing or constant losses Wc.

D.C.MACHINES

Hence, for shunt and compound generators, Total loss = armature copper loss + Wc Armature Cu loss is known as variable loss because it varies with the load current. Total loss = Variable loss + constant losses Wc

Source: Internet

d.c.machines-introduction

Documents