DC Generator Suleiman

Lecture 1 DC GENERATOR

1.0 Objective

General Learning Objective – GLO Describe the operational problems and corrective actions for DC generators Specific Learning Objectives – SLO •State that no two generator sets have identical responses to changes in load. • Describe the effect of running two dc generators in parallel without an equalizer connection. • Describe how to remedy reversal of polarity in dc generators. • Describe the correct way to run compound generators in parallel • Explain the two main causes of unsatisfactory load sharing

1.2 Overview • Most of the power generator types that are in use nowadays are AC types. • However, there several circumstances that favour DC generators over AC generators. • The system of the DC generators are designed to provide high currents at minimum voltage requirement for the charging of battery and operation of direct current loads. • DC generators does not need power supplies or battery chargers. • In addition to that, DC generators has no need for a transfer switch because transfer switch only reduces the reliability of the system. DC generators have lowers the system‟s the overall costs in major power applications.

Advantage of DC Generators over AC Generators • One of the most notable advantages is the efficiency of its fuel consumption. • Most of the DC generators that are available on the market these days have features that lets you choose the engine speed of the generator unit (this feature does not affect the regulation of the generator unit‟s voltage. • It enables the battery to adjust in order to meet the requirements for battery loading and charging.

1.3 Introduction ELECTRICAL SYSTEM & ROTATING ELECTRICAL MACHINES

Types of Electric Current • Direct current (d.c.) - unidirectional • alternating (a.c.) - varying magnitudes with time

Electrical system

Electrical Units Current Potential Difference & emf Resistance Power

Magnetism & Faraday‟s Laws Michael Faraday One of the most prominent scientists of the nineteenth century, Michael Faraday made significant contributions to both physics and chemistry. He discovered the phenomenon known as electromagnetic induction by observing that a current flows in a wire that is moved through a magnetic field. His discovery of electromagnetic induction contributed to the development of Maxwell‟s equations, and led to the invention of the electric generator. Culver Pictures, Inc. "Michael Faraday," Microsoft® Encarta® 97 Encyclopedia. © 1993-1996 Microsoft Corporation. All rights reserved.

Torque As the Rotor Turns

Generator Principle

Basic “practical” Alternator • Use electro-magnet (can be controlled) current to excite magnet much less than output current. • therefor rotate magnet - the required sliprings & brushes then have to handle a much lower current.

How Does a Generator Work • A simple AC generator consists of two parts: – Rotor • Turned by the driver and consists of a coil of wires wound around a magnetic material to produce a rotating magnetic field

– Stator or Armature • Rotating magnetic field induces a voltage in this coil of wires

Simplified 3-phase Generator •3Ø generators have 3 windings in the stator that are spaced 120° apart to produce 3 independent outputs. Voltage

Phase Phase Phase A B C

Time

Frequency of Generated Voltage The frequency of the generated emf (voltage) is dependent on two factors:  The number of magnetic poles on the rotor  The speed of the rotor (rpm)

such that,

Frequency 

pairs of poles  rpm 60

If we consider a 4 pole generator, then to produce an emf with a frequency to match the grid system in Malasia (ie 50 Hz), then from the above formula, the speed of the rotor must be:

Frequency  60 Pairs of Poles 50   60  1500 rpm 2

Speed 

•Adding additional magnets or “poles” will change the frequency for a given speed.

Generator pole 4-pole Generator - salient pole type Cylindrical Rotor type

Generator Stator - being built Frame

Laminated core

Windings •Individual strips of copper •Each strip is wrapped with insulation •Multiple strips formed into a bar shape •Hollow strips used in water cooled designs

Generator System Gas turbine

5 4 3

1 5

4

Gas turbine

3

2

1

Rectifies

PMG

Rotating elements

Field winding

Output winding

Rotor (permanent magnet)

Field winding

The grid

Output winding

Stator winding

Main AC generator

Static elements

Rotating rectifier assembly

Exciter

Automatic voltage regulator

Pilot exciter

Basic schematic showing the rotating and static elements 02/03/00

011061 PPT

ACG Main Parts

ACG Main parts

ACG Main Parts

ACG Main part

ACG Main Part

ACG Main Part

ACG Main Part

ACG Main Part

ACG Main parts

DCG Main parts

ACG Main Part

ACG Main Parts

DCG Main Part

ACG & DCG Main part

DCG Main Part

DCG Main Parts

ACG Main parts

Prime Mover Main Part

Prime Mover Main Part

Prime Mover Main Part

Prime Mover Main Part

1.4 Construction of DC Machines

INTRODUCTION . . . . . Like other electrical machines, synchronous machines may be operated as motors or generators. A synchronous machine operated as a generator is called “alternator.” CONSTRUCTIONAL DETAILS The basic components of a synchronous machine are the stator, which houses the armature conductors, and a rotor, which provides the necessary field. The number of poles on a synchronous machine depends upon the speed of rotation and the frequency we wish to produce. A revolving-field synchronous machine has a stationary armature called stator. An exciter is used to give DC supply to the field system.

STATOR The stator is identical to that of a three-phase induction motor. The stator, also known as the armature, is made of good quality steel, laminated to minimize eddy current losses. The stator laminations are held together by a stator frame. Slots are cut to place the armature/stator winding, which is wound for three phases. Slot Hollow Cylindrical core Internal circumference External circumference

ROTOR There are two types of rotors: Salient pole rotor and non-salient pole (round/cylindrical) rotor. SALIENT-POLE ROTORS These rotors are generally used in low speed generators driven by hydraulic (water) turbines. The rotor is characterized by large number of poles and large diameter is required to provide the necessary space for the poles. The coils are connected in series so that the adjacent poles have opposite polarities. In addition to the dc field winding, often a squirrelcage winding known as damper winding embedded in the pole-faces is added (to dampen the oscillation of the rotor).

ROUND ROTOR OR CYLINDRICAL ROTOR It is well known that high-speed steam turbines are smaller and more efficient than low-speed turbines. However, to generate the required frequency, not less than two poles are used (minimum number is two). The rotor is a long, solid steel cylinder, which contains a series of longitudinal slots milled out of the cylindrical core. Field coils, firmly wedged into the slots and retained by high-strength end-rings serve to create the pole. The high speed of rotation impose an upper limit on the diameter of the rotor. The cylindrical construction offers the following benefits:  It results in a quite operation at high speed.  It provides better balance than the salient-pole rotor.  It reduces the windage loss.

General Arrangement of a Four Pole DC Machine • Fix parts consist of 4 steel cores C (Pole Cores – attached to a steel ring R, called the yoke). • Pole Core with pole tips to support the windings & increase the cross sectional area and thus reduce the reluctance of the air gap. • Each pole carries a winding F so connected as to excite the poles alternately N & S. • Armature core „A‟ – consist of steel laminations (lamination to reduce eddy-current losses). • The periphery of laminations provided with slots to provide mechanical security to the armature windings and to provide a shorter air-gap for the magnetic flux to cross between the pole face and the armature „teeth‟. • The conductors (armature windings) are insulated from each other.

General Arrangement of a four pole DC Machine •Magnetic flux which emerges from N1 divides. Half going towards S1 and half towards S2. •If armature revolves clockwise, and moving under the N poles Applying Fleming‟s Right Hand rule : Emf generated in the conductors is going inward & if moving under the S poles, emf is coming outwards. •The emf generated in a conductor remains constant while it is moving under a pole face, and then decreases rapidly to zero when the conductor is midway between the pole tips of adjacent poles.

Basic part of a DC Machine consist of Stationary and Rotating components :

Stationary part - Stator : • • •

Yoke – A steel ring where magnetic poles are attached. Around magnetic poles are field windings. Field windings - Many turns of conductors wound around the pole core. Current passing through the conductors creates electromagnet.

Components

Commutator

Armature lamination with tapered slots Armature of Dc generator showing commutator, stacked laminations, slots and shaft

Cross section of slot containing 4 conductors

Rotating Part – Armature & Commutator: • Armature – mounted in bearings housed in the stator. • Armature consist of : • Core – Made of laminated cylinder of iron or steel with teeth cut onto the lamination to house the armature winding • Armature Winding – Single or multi loop conductor • Commutator reverses the current flowing in the armature coils as the armature rotates. • Commutator convert the alternating current (AC) generated in the armature coils into DC. • A commutator consists of opposite pairs of conductors, usually tapered copper segments insulated from one another by mica sheets, and contact to an external circuit is provided by carbon or metal brushes. Commutator is mounted on the shaft.

Overview of DC Generator Components

Overview of DC Generator Components

Armature Windings • In real generators the armature coils are wound onto the armature using a variety of methods. • Drum winding involves a number of coils separately wound round the armature and connected in series. In the figure below an armature is wound such that at every instant as it rotates two equal e.m.f's in parallel exist across the brushes

Armature Reaction • The field current in a generator produces mmf, which results in the filed flux in accordance with the magentization curve. • When the machine is driven by a prime mover, an emf is induced in the armature. • Armature reaction - The armature mmf distorts the flux density distribution (Cross magnetizing effect) and also produces demagnetization effect. • Armature reaction causes poor commutation leading to sparking, especially when the armature current changes rapidly.

Compensating Windings & Interpoles • Compensating windings to overcome the difficulty cause by armature reactions. • Conductors embedded in slots in the field pole faces. • Connected in series with the armature, but carry current in opposite direction so as to cancel the armature reaction flux. • In addition, the voltage in the coils undergoing commutation can be cancelled by providing interpoles. • Interpoles- placed midway between the main poles and their windings are connected in series with the armature.

Interpoles & Armature Reaction

1.5 Operation DC machine

PRINCIPLE OF OPERATION A STATIONARY-FIELD SYNCHRONOUS MACHINE Here the field system is stationary (stator) & the armature system is rotating(rotor). The poles present in the field system create a dc field which passes through the air-gap and is cut by the revolving armature. The armature is driven by a prime mover (mechanical power input). The armature has a three-phase winding whose terminals are connected to three slip-rings mounted on the shaft. A set of brushes, sliding on the slip-rings, enables us to connect the armature to an external three-phase load or source. As the armature rotates, a three-phase voltage is induced (Faraday‟s law of electromagnetic induction) in the rotor windings, whose value depends upon the speed of rotation and the dc exciting current in the stationary poles. NS  P f  The frequency of the voltage is given by 120 Stationary field machines are used when the power output of the generator is < 5 kVA. However, for greater capacities, it is cheaper, safer and more practical to employ the revolving dc field synchronous machine.

DC Generator Basic Operating Principle •

• •

Input to an electrical machine is mechanical energy and output is electrical energy (Voltage appearing at the output electrical terminal) A generator converts mechanical energy to electrical energy by virtue of magnetic induction. Whenever a conductor is moved within a magnetic field in such a way that it cuts magnetic lines of flux, an electromotive force is generated in the conductor. The amount of generated voltage depends on or proportional to: – 1. the strength of the magnetic field, – 2. the speed at which the conductor moves, – 3. the length of the conductor.

• • •

That is, E = Blv [B= flux density (weber/m^2), l= length of the conductor, v= velocity of the conductor]

Fleming‟s Right Hand Rule As the Basis For DC Generator

Basic Of DC Generator • The following series of diagrams shows successive positions of the steadily rotating loop. N and S represent poles of a horse shoe magnet. • At the instant represent by position 1 & 3 below, no emf is produced because the wires are moving parallel to the field and are not cutting the lines of forces (the black half of the loop is at the top in the vertical position )

Position 1

Position 3

• As the loop moves from position 1 to position 2, lines of force are cut at an increasing rate, even though the rotation rate is steady. • At position 2, the sides of the loop are cutting lines of force at the maximum rate. • The induced current in the loop is a flow of electrons directed toward the brush on the right and away from the brush on the left. • This forcing of electrons from the rotating coil toward the right hand brush gives this brush a negative charge. • The removal of electrons from the left hand brush gives it a positive charge. • In the stationary wiring of the external circuit, electrons flow from the negative brush to the positive brush

Position 2

Position 4

•Between position 3 & 4, increasing voltage and current rate are produced just as between 1 and 2, But The black side of the loop, previously moving downwards through the field, is now moving upwards, thus, the current direction in the black side is reversed (compared diagram for position 2 & 4).

Position 3

• However, the black side of the loop is now taking electrons from the left brush, forcing them around through the white side of the loop towards the right hand brush, thus, the charges on each brush remain the same as before. Position 4

• In the rotating loop itself, the generated current alternates in direction. • As the commutator rotates, the black half segment does the following : • In position 2, the black segment is negative and supplies electrons to the right hand brush. • In position 4, the black segment is positive but at the instant it become positive, it pivots away from the right hand segment and contacts the left hand brush again. • Similar, to the white segments also. As a result, the right hand brush always supply with electrons (Thus -ve), left hand brush always losing electrons. (+ve)

Position 2

Position 4

Slip rings

Pole

Fan

DC excitation winding Rotor of a four-pole salient pole generator.

Metal frame

Laminated iron core with slots

Insulated copper bars are placed in the slots to form the three-phase winding

Details of a stator (generator)

Generator

Exciter View of a two-pole round rotor generator and exciter.

Operation •DC of DC Generator is generated in rotating coils surrounded by a stationary field magnet. •What happens when one loop of wire is rotated in the field between the N and S poles ? •In order to produce direct (one way) current in the outside circuit served by the generator, the ends of the loop are fastened to semicircular metal strips, insulated from each other, that rotate with the loop. •Commutator - two half circle segments form the part of this generator. •Commutation - The Process of transferring current from one connection to another within an electric circuit, either by mechanical switching or electronic switching •The stationary brushes adjusted to bridge the gap in the slip rings of the commutator at the instant when the emf induced tin the coil has zero value and due to reverse.

Operation of DC Generator • • •

Consist of 2 active conductors. (AB & CD). Connected in series by connection BC. Front connections connected to slip rings.

Operation : 1. As one conductor AB moves down thru the field, the other CD moves up and induced emf. 2. Emf = on conductor AB, A is positive relative to B & C is positive relative to D. (Fleming‟s right hand rule check on conductor AB). 3. The induced current, as shown by arrows, from terminal Y to terminal X thru external circuit. 4. After the coil has rotated half revolution, conductor DC begins to move downwards & conductor AB upwards. 5. The polarity induced is now in reverse to that of the first half revolution. 6. On Conductor CD, D becomes positive relative to C and B becomes positive relative to A. 7. Thus, Terminal X now becomes positive and Y becomes Negative. 8. Alternating Emf is generated.

Basic Of DC Generator 360o

Sinusoidal waveform of emf generated.

How Does Commutator Function? Brushes on magnetic Neutral Axis

X

D

S

N

A Y

Posn 2 + X

A

S

N

D Y

Posn 4 +

How Does Commutator Function? • Posn 2 : Negative End „D‟ of Conductor CD is connected to the Positive Brush “Y” & & the positive end „A‟ of AB is connected to the negative brush X. • For Posn 4, When the emf has reverse in the conductors of the coil, end „D‟ which is now positve is connected to the negative brush X and end „A‟ of AB is now negative and connected to the positive of brush “Y” . • Brush polarity is decided by the direction of current flow in the external circuit. • Thus current flows from “Y” to “X” . (+ve brush to –ve Brush). • Continuity of flow will go to „A‟ to „B‟ in the conductor of the armature. • Thus, the commutator picks up the negative part of the emf and inverse it to the positive side of the emf. • The waveform of the emf thus becomes unidirectional at the terminal but pulsating

Conversion of AC to DC • The AC is converted to DC because the brushes always side on the segment attached to the loop that is passing through the magnetic field in the same direction. • The brushes change from one segment to the other at the point where the loop is moving parallel to the magnetic flux lines. • At this point, no voltage is being induced into the loop of wire (Neutral plane of Generator)

Operation of DC Generator

• If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. • To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution. • This reversal is accomplished by means of a commutator, a split metal ring mounted on the shaft of the armature. • The two halves of the commutator ring are insulated from each other and serve as the terminals of the armature coil. • Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. • As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. • Thus there is a flow of unidirectional current in the outside circuit to which the generator is connected. • DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage.

Commutation • Commutation - The positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. • Commutation - Mechanical conversion from AC to DC at the brushes of a DC machine. • The commutator converts the AC voltage generated in the rotating loop into a DC voltage • 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 commutator.

Commutation • 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. • 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 means by which the brushes make contact with each end of the loop.

Commutation • 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 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 fluctuates in amplitude (size or magnitude) between zero and some maximum value, but is always of the same polarity. • In this manner, commutation is accomplished in a DC generator.

Commutation in DC Generator

Commutation • 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

Operation of DC Generator

• Modern DC generators use drum armatures that usually consist of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. • In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. • A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant. • Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. • Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.

STATOR / ARMATURE WINDING The armature of most synchronous generators are wound with three distinct and independent windings to generate three-phase power. Each winding is said to represent one phase of a three-phase generator. The threewindings are exactly alike in shape and form but are displaced from each other by exactly 120 deg. electrical. The three-phase windings may be connected to form either a star (Y) or a delta connection. If S is the no. of slots in the armature, P is the no. of poles, and q is the no. of phases, then the no. of coils per pole per phase (n, which is called the phase group and must be an integer), is given by n

S Pq

The number of slots (coils) per pole is called the pole span. Each coil in a phase group can be wound as a full-pitch coil or fractional-pitch coil. Each full-pitch coil in the armature can be made to span 180 deg. electrical.

STATOR / ARMATURE WINDING . . . . . Coil span =6

One side of the coil

Full-pitch coil

Other side of the coil

Pole span = 6 slots 1

2

3

4

5

6

7

8

9

10 11 12

Coil Coil span =5

One side of the coil

Fractional-pitch coil

Other side of the coil

Pole span = 6 slots 1

Coil

2

3

4

5 6

7

8

9

10

11

12

Variations in EMF generated • Pole pitch – the distance between the centres of adjacent poles. • If the induced EMF at this point were not zero, due to the shorting of the ends of the loop together, spark would be produced. • O – Conductor moving between the pole tips of S2 & N1 • CD- Emf generated while conductor moving under pole face of N1. (+ve). • E – Conductor is midway between the pole tips of N1 & S1. • EFGH – Represents the variation of emf while the conductor is moving through the next pole pitch.

Waveform of emf generated in a conductor while the conductor is moving through two pole pitches.

•Commutator segments slides on the fixed brushes. •Each brush links the conductors when the conductor is moving in specific range on the magnetic field. •Therefore each brush only conducts the current of the same polarity, either +ve or –ve resulting in unidirectional current in the load circuit. •One important point to note is that, as the brushes pass from one segment other segment, at an instant when the brushes contact both segments at the same time, the position of the moving coil should be such that the EMF at the instant is zero.

1.6 EMF Equation

EMF EQUATION When the generator is driven by the prime mover, a revolving field is produced by the rotor field winding. If a positive voltage is induced in a conductor when the N pole sweeps across it, a similar negative voltage is induced when the S pole speeds by. The frequency of the ac voltage induced in a conductor depends upon the number of pole-pairs and the speed of rotation of the conductor (rps) relative to the field. If n is the speed of the machine in rpm and p is the number of poles, then the frequency of the voltage generated is: p n pn f  .  2 60 120

f = frequency of the voltage induced in Hz p/2 = total number of pole pairs n/60 = speed of the rotor in rps.

EMF EQUATION . . . . . The average induced voltage in each phase winding as the rotor sweeps by is given by:  N = number of turns in the phase winding E ave  N t  = change of flux in a given time, t. The flux changes by m in one-half of the pole pitch(=90deg.) (pole pitch is the distance between the North pole and the South pole of a pole-pair and is equal to 180 deg.). The time taken to travel one-half of the pole pitch is ¼ of the cycle of the ac waveform.   Eave  N m  N m  4 fNm .....V 1 1 1 Therefore, 4

.T

. 4 f

For sinusoidal voltage , (Form factor, 1.11 = Erms / Eave.) Erms = 1.11 Eave = 4.44 f N m where m is the maximum flux per pole in Wb.

1.7 Application of DC

EXCITATION The field excitation of a large synchronous machine is an important part of its overall design. The field must ensure not only a satisfactory ac voltage level, but also respond to sudden load and prime mover speed changes in order to maintain system stability. Quickness of the response is one of the important features of the field excitation. In order to attain it, the field of the synchronous machine is excited by 2 dc generators: a main exciter and a pilot exciter. The main exciter feeds exciting current to field of the alternator (via brushes & slip ring). It is regulated by control signals that vary the current Ic, produced by the pilot exciter. The power rating of the exciter depends upon the capacity of the synchronous machine. Pilot exciter feeds power to the field winding of the main exciter. This cascade arrangement is used to get quick response. To avoid problems due to carbon brushes of exciters, brushless excitation systems are being used recently.

BRUSHLESS EXCITATION Owing to brush wear and carbon dust, it is always required to clean, repair and replace brushes, slip rings and commutator on conventional dc excitation systems. To eliminate this problem, brushless excitation systems have been developed. Such a system consists of a threephase stationary field generator whose ac output is rectified by a group of rectifiers. The dc output from the rectifiers is fed directly into the field of the synchronous machine. The armature of the ac exciter and the rectifiers are mounted on the main shaft and turn together with the synchronous generator.

We continue with lecture #2

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• 2.1 DC Generator characteristics

Lecture 2 • Generator classification and connection • Load effects on generator

Voltage Characteristic • •

1. 2.

Voltage produced in the rotating armature is proportional to the speed of rotation (rpm) and the flux per pole Φ. However, the voltage available at the generator terminals is less, due to : Internal IR drop in the armature circuit. Effects of Armature Reaction.

Thus : VL = (E S– E O) – IaRa

• •

At no load, the terminal voltage VL is the internal voltage E0. `

Magnetization Curve 1. With Zero field current, there is some voltage generated due to residual magnetism. 2. There is a linear range of voltages that are proportional to field current 3. There is a saturation region, where little increase in voltage results from increasing excitation.

Voltage Drop Due to Armature Reaction

Armature Windings

A 4-pole machine with and eleven slop armature core with eleven coils A-H. These coils may each have a number of turns A number of methods are available for connecting the coils to the commuter segments. These can be broadly divided into two generic types as shown in the figure below. Lap windings and wave windings.

Armature Windings- difference between lap and wave winding Armature windings depend wires are joined to the commutator. These are called: 1. In a wave wound armature, the conductor are divided into two parallel paths. Each path supplying half the total current output. Produce high voltage, low current outputs. 2. In a lap wound generator, there are as many parallel paths in the armature as the number of poles of the generator. The total current output divides equally between them. Produce high current, low voltage output. E

ZN P

( ) volts 60 A   flux per pole in webers Z  Total number of armature coductors P  No. of generator poles A  No. of parallel paths in armature N  rpm of the armature

Lap & Wave Winding

•In lap winding the two ends of any coil are taken to adjacent segments of the commuter and in wave windings the ends are taken to spaced segments. •A lap winding system has the same number of parallel paths as there are poles. •The wave winding system has only 2 paths in parallel.

•

• Figure 5.5 a. Armature of Fig 5.4 in the process of being wound, coil-forming machine gives the coils the desired shape. b One of the 81 coils ready to be placed in the slots c Connecting the coil ends to the commutator bars. d. Commutator connections ready for brazing (H Roberge)

Field Winding connection DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. When the field winding is connected in parallel with the armature, the machine is said to be shunt wound. If the field winding is connected in series with the armature, the machine is said to be series wound. A compound wound machine has a combination of series and shunt windings.

Series Wound • A series-wound generator has its field in series with the armature. • For series wound fields the field coils consists of a few turns of heavy gauge wire which carry the full supply current. • When current flows in the external circuit the e.m.f across the field is practically zero. • A small current in the external circuit results in a small magnetic field and hence a small e.m.f . Larger currents result in larger e.m.f.s but a maximum is attained when the core is magnetically saturated. • With higher currents the e.m.f of the generator drops as a consequence of the increasing p.d. across the internal resistance of the windings. • If the generator was short circuited the generator e.m.f. would fall to zero. • The series-wound generator is used principally to supply a constant current at variable voltage.

Shunt Wound • A shunt-wound generator has the field connected in parallel with the armature Shunt wound field windings consist of many turns of wire the ends of which are connected to the brushes in parallel with the external circuit. • As the external current increases the p.d. across the internal resistance of the generator increases thus decreasing the terminal p.d. • If E is the generated e.m.f and Ia and Ra are the armature current and resistance and V is the terminal p.d. • V = E - IaRa • In normal operation the armature resistance Ra is small and the terminal voltage remains reasonably constant. • There is a tendency of the terminal voltage to fall off with increasing current. This can be offset by having a shunt wound generator with a few turns of series winding of thick wire. This arrangement, called a compound wound machine, produces a virtually constant supply voltage.

Compound DC Generator • Compound-wound generators have part of their fields in series and part in parallel. • Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under varying electrical loads. • Compound generator is the most widely used DC generator. • The speed of a compound generator affects is generating characterisitic. • Therefore, the compounding can be varied by adjusting the engine governor for higher or lower speed and then adjusting the shunt field for the proper no load voltage. • The range of the shunt field rheostat and the engine characteristic usually limit the amount of speed variation that may be obtained for this purpose. • Compound generators can be connected either cumulatively or differentially.

Various Type of Compounding • Compound - DC Generator with both Shunt & Series Winding. Compound generators inherently have a smaller voltage droop than a shunt generator. • Cumulatively compound – The fluxes of the two coil aid each other. • Differentially compound – The fluxes of the two coils oppose each other. • Overcompounded – Full load voltage is greater than no load voltage due to number of turns of series winding. • Flat Compounded – No load voltage is the same as full load voltage due to number of turns of series winding.

Terminal Voltage Vs Load Current

Terminal Voltage, VT

Over Compound

Level Compound

Under Compound

Differential Compound

Load Current, IL

Characteristics • Thus, different type of winding will behaves differently when a load is applied. • The behaviour of a DC machine under various conditions is shown by means of graphs, called characteristic curves or just characteristics. There are 2 types of characteristics, namely: 1) Generated voltage/field current characteristic called the open-circuit characteristic. 2) Terminal voltage/load current characteristic called the load characteristic.

or

Classification according to method of field excitation

• DC generators are classified according to the method of their field excitation. These groupings are: • A separately-excited generator is used only in special cases, such as when a wide variation in terminal voltage is required, or when exact control of the field current is necessary. Its disadvantage is requirement for separate source of direct current.

• When a load is connected across the armature terminals, a load current Ia will flow. The terminal voltage V will fall from its opencircuit emf E, due to a volt drop caused by current flowing through the armature resistance, Ra. Thus,

2.2 DC Generator classification

SELF EXCITED GENERATOR • These types are the one where the field winding receives its supply from machine‟s own armature. • These are sub-divided into: - shunt, -series and -compound wound generators.

Generator Characteristic of Separated Excited • The effect of varying the load current has on the terminal voltage. • Relation between the generated emf Ea and the terminal Voltage, V : V = Ea – RaIa. • Terminal Voltage, V decreases slightly with increase in the load current (Due to RaIa drop). • Internal generated voltage is independent of Ia , Ea assume to be constant if saturation is neglected and hence the terminal voltage should be a straight line (for a separately excited generator). • But due to armature reaction, the terminal voltage drop is slightly greater than RaIa . An increase in Ia causes an increase in armature reaction. • Increase in armature reaction causes flux weakening. • Flux weakening causes a decrease in Ea

Generator Arrangement of Separated Excited A separately-excited generator is used only in special cases, such as when a wide variation in terminal voltage is required, or when exact control of the field current is necessary. Its disadvantage is requirement for separate source of direct current.

SHUNT WOUND GENERATOR •

This is when the field winding is connected in parallel with the armature. The field winding has high resistance, thus requires small current from the armature current. Terminal voltage, V = E – Ia.Ra or Generated emf, E = V + Ia.Ra Ia = If + I, where Ia = armature current If = field current (V/Rf) I = load current

Characteristics •

•

• •

•

The generated emf E is proportional to ω, thus at constant speed, ω = 2πn, E  . Also the flux is proportional to field current If until magnetic saturation of generator‟s iron circuit occurs. The open circuit characteristic is shown below: As the load current increases, armature current will increases, hence armature volt drop, Ia.Ra increases. The emf E is larger than V i.e. V = E – Ia.Ra. Since E is constant, V decreases with increasing load. In practice, the voltage drop about 10% between noload and full-load. The shunt-wound generator is the type mostly used in practice, but the load current must be limited to below maximum value, to avoid excessive terminal voltage fluctuation. Typical applications are battery charger and motor car generators.

Generator Characteristic of Shunt Excited • The amount of field current in the machine depends on its terminal voltage. • Terminal voltage will decrease with an increase in load because the armature RaIa drop and the armature reaction. • When V decreases, the field current of the machine also decrease. • Hence flux decreases and decrease Ea, thereby, causing terminal voltage to drop still further. • If the load is increased excessively, up to terminal voltage is short circuited (V = 0), the field current (If = V / Rf ) is zero and the field will collapse. • No emf is generated except for a small voltage (Er) due to residual flux, resulting in a small circulating current (Er / Ra ). • Generator is self protected against short circuit at its terminals.

SERIES WOUND GENERATOR •

This is where the field winding is connected in series with the armature Characteristic • The emf E is proportional to ω, and at constant speed, ω (=2πn) is constant. Thus E is proportional to . For current magnitude below magnetic saturation of the yoke, poles, air gaps and armature core, the flux is proportional to the current, hence E  I. • For higher current value, the generated emf is approx constant. The values of field resistance and armature resistance are small, thus terminal voltage V is nearly equal to E. • In a series-wound generator, field winding is in series with armature and not possible to have any field current when terminals are open circuited, thus not possible to obtain an open circuit characteristic. • Series-wound generators are rarely used in practice, but can be used as a „booster‟ on DC transmission lines.

Generator Characteristic of Series Excited • • SEPARATELY EXCITED GENERATORS • Field winding is connected to the source of supply, other than from armature. • While being operated at no load, develops a small terminal voltage proportional to the residual flux. • As the load increases, the field current rises, so Ea goes up more rapidly. • The (Ra + Rs)Ia drop goes up too, but at first, the increase in Ea goes up more rapidly than the (Ra + Rs)Ia drop rises, so V increases. • After a while, the machine approaches saturation, and Ea becomes almost constant. • At that point, the resistive drop is the predominant effect, and V starts to fall. • Generator has a steep voltage characteristic.

Generator Arrangement of Series Excited

COMPOUND WOUND GENERATOR •

Two methods of connection are used, both having a mixture of shunt and series windings, designed to combine the advantages of each. • The left hand side shows long-shunt compound generator while the right hand one represent a short-shunt compound generator. • The latter is the most generally used form of DC generator.

Characteristics •

•

•

• •

The magnetic flux produced by the series and shunt fields are additive. Included in this group are overcompounded, level-compounded and undercompounded machines. The degree of compounding obtained depending on the number of turns of wire on the series winding. A large number of series winding turns results in an over-compounded characteristic, in which the fullload terminal voltage exceeds the no-load voltage. A level-compound machine gives a full-load terminal voltage which is equal to the no-load voltage. An under-compounded machine gives a full-load terminal voltage which is less than the no-load voltage. However, even this latter characteristic is little better than the shunt generator alone. Compound-wound generators are used in electric arc welding, with lighting sets and with marine equipment.

ARMATURE REACTION •

Armature reaction is the effect that the magnetic field produced by the armature current has on the magnetic field produced by the field system. • In a generator, armature reaction results in a reduced output voltage, and in a motor, armature reaction results in increased speed. • Magnetic field in a generator has a straight, uniform pattern. • .

But the current generated in the armature causes another magnetic field.

• Both magnetic fields combine (main field and armature field) making the total magnetic field take the direction shown below. • The distortion or bending of the magnetic field of the generator, caused by the magnetic field of the current in the armature is called armature reaction. • To overcome the effect of armature reaction is to fit compensating windings, located in slots in the pole face.

EFFECT OF ARMATURE REACTION If the distortion is not corrected, when the armature is producing current the actual field in the generator is twisted. Twisted field can effect such as: •The bunching of the lines at the corners of the field poles cause an irregularly in the voltage output, more importantly, the field iron is not used efficiently, and the total flux is less, making the average voltage output low. •The twisted field changes the timing of the current reversals in the armature coil.

MAGNITUDE OF VOLTAGE GENERATED •

•

• •

• • •

There is no difference of construction between a DC motor and a DC generator. In fact, the only difference is the generated emf in the motor is less than the terminal voltage, whereas in generator, the generated emf is greater than terminal voltage. The relationship between the current, the emf for generator may be expressed thus if E is the emf generated in armature, V, the terminal voltage , Ra, the resistance of armature circuit and Ia the armature current, then when D is operating as a generator, E = V + Ia.Ra When the machine is operating as a motor, the emf E, is less than the applied voltage V, and the direction of the current Ia is the reverse of that when the machine is acting as a generator, hence, E = V – Ia.Ra Thus, V = E + Ia.Ra Since the emf generated in the armature of a motor is in opposition to the applied voltage, it is sometimes referred to as a back emf.

2.3 Loading effects

EFFECT OF LOADING A DC GENERATOR •

When a DC generator is under load, some fundamental flux and current relationship take place that are directly related to the mechanical-electrical energy conversion process. Consider for example, a 2 pole generator that is driven counterclockwise while delivering current I to the load. • The current delivered by the generator also flows though all armature conductors. If we look inside the machine, we would discover that current always flows in the same direction in those conductors that are momentarily under a N pole. The same is true for conductors that are momentarily under an S pole. However, the currents under the N pole flow in the opposite direction to those under an S pole. • The conductors lie in a magnetic field, they are subjected to a force. If we examine the direction of current flow and the direction of flux, we find that the individual force, F, on the conductors all act clockwise.

• In effect, they produce a torque that acts opposite to the direction in which the generator is being driven. • To keep generator going, we must exert a torque on a shaft to overcome this opposing electromagnetic torque. • The resulting mechanical power is converted into electrical power, which is delivered to the generator load. • That is how the energy conversion process takes place.

OVERLOAD LOAD CHARACTERISITC CURVES FOR DC GENERATOR

Synchroscope

Generator “Synchroscope” - local panel

Current & voltage - Inductive load

Generator supplying Inductive or Capacitive load

Parallel Running of DC Generator • Power plants will sometimes be found to have several small generators rather than large single units capable of taking care of the maximum peak loads. • The several units can then be operated singly or in various parallel combinations on the basis of the actual load demand, resulting in efficiency, continuity of service, and additions to the plant capacity as the power plant load increases. • However, parallel combinations are not practical if the generators are subject to speed fluctuations. • Continuity of service is obviously impossible if a power plant constitutes a single unit because a breakdown of the prime mover or the generator would require complete shutdown of the entire station. • If, however, there are several generators in parallel and one breaks down, it can be repaired with care, not in a rush, provided that other machines are available to maintain service.

Response to change of Load While on Parallel Running • DC generators are subject to abrupt changes in speed which heretofore prevented a parallel connection, because when the speeds of the generators connected in parallel changed in relation to each other the faster one would tend to motorize the slower one.

Interconnected DC Generators • In order to have two or more DC generators operate in parallel, it is necessary to adjust their field excitations. • Field excitations to be adjusted so that their open circuit voltages are nearly equal. • The armatures should then be connected to the bus bar so that like terminals are connected together. • When the voltage of the incoming generator is equal to the busbar voltage, no current flows in the line, and the incoming generator is said to be „floating‟ on the line. • By controlling the field excitation, the division of load between the generators can be controlled in any desired manner.

Control of load sharing between 2 shunt generators - Bus Bar

+ Bus Bar

Rheostat +

A

- +

Shunt Field

B

-

Field Regulator

Control of load sharing between 2 shunt generators Bus Voltage

IL1

IL2 c‟

b‟

b

E‟t

Excitation increased

a

Et

Machine B Initial (Floating)

Machine A Initial

Excitation decreased

I‟L2

I‟L1

b‟‟b = c‟c = IL Load on Machine B

IL 0

Load on Machine A

Interconnected DC Generators • Machine A feeding a current IL to the bus at a terminal voltage Et, and machine B is „floating‟. • Point „a‟ indicates the operating point of machine A. It is required to transfer a certain amount of load to machine B. • To achieve this, the excitation of machine B is increased suitably. • Points b and b‟ are new operating points for machines A and B respectively so that b‟b = total load current IL, the bus voltage increasing to E‟t. • The load on machine A is now IL and that on machine B is IL2. In order to maintain the bus voltage to its original value Et, the excitation of machine A must now be decreased suitably so that its load characteristic shift downwards. • The new operating point for machines A & B are respectively c and c‟, load shared by them being I‟L1 and I‟L2, so that I‟L1 and I‟L2 = IL

• Parallel operation of compound generators is not possible without the use of special connection. • If the shunt field excitation of one generator increases slightly, the armature current of this machine increases, thereby increasing the series field flux and in turn the generated voltage and again inturn, increases the armature current. • Hence, a slight variation in the shunt field excitation of compound generator results in a large variation in the loads shared. • To prevent such instability in load division, the series field winding are connected in parallel, at one end by an equalizer bus and the other end thru the main bus. • If now the armature current of one generator increases, it divides equally into all the series fields, and thus the series field excitation of all machines in parallel increases. • As such, a change in load current in one machine affects all machines in parallel and avoids the cumulative effect which leads to instability.

Two generator sets may not have identical responses to changes in Load • Due to following reasons:

– 1. Even if the generators are identical in design and rating there are bound to be differences in magnetic and electrical properties.

– 2. No two engines have identical governor characteristics or speed of response to load-changes.

• Discrepancies in the shape of the voltage curve so that the amount of hump in the curves is widely different.

voltage

Two main causes of unsatisfactory load sharing

Rated volt

• Wide differences in the voltage drop across the series field plus the connection to the busbars. * Hump- Increase in voltage at intermediate loads on a flat compounded machine. It is about 2-3% of rated voltage.

Full load

Ampere

Control of DC Generators • Generally, a DC generator is controlled by a variable resistance (Rheostat). • After the generator is brought up to proper speed by the prime mover, The Rheostat may be manually or automatically controlled., • The adjustment of the rheostat controls the amount of the exciter current fed to the field coils. • Metering requires the use of a DC voltmeter and ammeter of appropriate ranges in the generator output circuit. • Match set of shunt wound or compound wound generators with series field equalizer connections are used for parallel operation • Precautions must be observed when connecting the machine to the buses.

Compound Generators in parallel. • Undercompounded generators connected in parallel will operate satisfactorily. • But over compounded generators will not operate satisfactorily in parallel unless their series field are also parallel. • This is done by bringing their negative connections of each generator to a common point . • The conductor or bus use to connect this to the brushes is called equalizer. • When the equalizer is used, a stabilizing action takes place. If generator A takes more than its proper share of the load, the increased current will flow through the series field of Generator A, but some of it will also flow thru the equalizer and thru the series field of the Generator B. • Thus both generator are affected in the similar manner and neither machine takes the entire load.

Procedure For DC Generator parallel operation • Make a visual inspection to ensure that all repairs have been completed, that the unit is free of tools or other debris. • Ensure that the disconnect links are in place. • Start prime mover • Close field switch • Raise voltage by decreasing field rheostat resistance or, in auto, increasing the voltage regulator set point to a few volts above line voltage. • Check line to ground voltages to detect any grounds on oncoming generator. • Close circuit breaker, generator should pick up some load. • Balance loads appropriately with field rheostat, or in Auto , voltage regulator adjusting pots (potentiometer)

Parallel Operation of DC Generator • When the load on the station increases beyond the capacity of Generator A, it is essential to connect Generator B to operate in parallel with Generator A in order to share the total load on the station. Method : • Bring up the speed of the prime mover of Generator B (Incoming Generator). • The incoming generator field circuit switch is closed, as a result, the generator will build up its voltage. • Close the Circuit Breaker. • Adjust the excitation of the Generator B so that it generates voltage equal that of the bus bar voltage.

• Polarities of the generator B should be the same as those on the bus bar. • Now, the main switch is closed, thus putting Generator B in parallel with Generator A. However, Generator B is still running idle (Not Supplying any load). • Adjust the field Rheostat of Generator A&B simultaneously. • The field current of Generator A should be reduced slowly while that of Generator B is increased. • While shifting the load, care should be taken that the incoming generator is not overloaded. • Incase Generator A is to be shutdown, the whole load can be shifted onto Generator B provided it has a capacity to supply that load without overloading.

Overcompounded DC Generator in Parallel with Equalizer - Bus Line + Bus Line Equalizer

Rheostat

Rheostat + -

+ A Shunt Field

Series Field

-

B

Compound Generator & Equalizer Connection • When two compound wound generators are operated in parallel, it has been customary to use an equalizer connection between the two armatures to insure that the current does not reverse its direction in one of the series fields. • If the equalizer connection, which is a very low resistance conductor, is not used, the two machines may not operate satisfactorily in parallel. • This equalizer connection, however, is only effective for very small changes of outputs and therefore is impractical to use with DC diesel generators, which are subject to large changes in output. • When an output change occurs which is too large to be accommodated by the standard equalizer connection instability will result because any tendency on the part of one generator to assume more than its proper share of the total load will cause it to take on still more load. • In the meantime the second generator continues to drop its load until it is running without load. It is, in fact, even possible for one of the machines to carry the entire load and, in addition, drive the other generator as a motor.

Effect of running two dc generators in parallel without an equalizer connection. • In response to a change in load, engine governors will act causing a change of speed of engines. • The higher speed engine will start taking more load than the other generator, which will increase its series field strength causing it to grab further load. • The process will continue until the current in the second generator reverses. • As a result polarity of the series field of this generator is changed. The generator rather becomes a motor taking a huge current from the other generator. • Until the circuit is very quickly interrupted serious damage will occur.

The correct way to run compound generators in parallel

• Correct way to run DC generators in parallel is to connect an equalizer connection. In any case an equalizer will prevent the change of polarity in the series field.

How To Prevent Reversal of Polarity of DC Generator • For Parallel running – Connect to Equalizer Connection to prevent a change of polarities in the series field. • Ensure the governor function of both the generators operate satisfactory to the change of speed. • Ensure switch gears to the equalizing connection are properly maintained. • Calibrate the instruments (DC voltmeter & Ammeter) on the switchboards at appropriate maintenance interval

Remedial after reversal of polarity in dc generators • Lift all the brushes • Close the circuit breaker • Shunt field will be magnetized with correct polarity.

Basic Difference Between a DC and AC generator

Example 5-1 •

The armature of a permanent-magnet dc generator has a resistance of 1 W and generates a voltage of 50 V when the speed is 500 r/min. If the armature is connected to a source of 150 V, calculate the following: a. The starting current b. The counter-emf when the motor runs at 1000 r/min. At 1460 r/min. c. The armature current at 1000 r/min. At 1460 r/min.

Example a. At the moment of start-up, the armature is stationary, so Eo = 0 V (Fig. 5.3a). The starting current is limited only by the armature resistance: • / = Es/R = 150 V/l W = 150 A

b. Because the generator voltage is 50 V at 500 r/min, the cemf of the motor will be 100 V at 1000 r/min and 146 V at 1460 r/min. c. The net voltage in the armature circuit at 1000 r/min is • Es - Eo = 150 - 100 = 50 V • • •

•

•

The corresponding armature current is I = (Es - Eo)/R = 50/1 = 50 A (Fig.5.3b) speed @1460 r/min, the cemf will be 146 V, almost equal to the source voltage. Under these conditions, the armature current is only / = (Es - Eo)/R = (150 - 146)/1 = 4A

. Figure 5.3 See Example 5.1