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Transcript of Energy Conversion
Energy Conversion
Magnetism and Electromagnetism
Centuries ago, it was discovered that certain types of mineral rock possessed unusual properties of attraction to the metal iron. One particular mineral, called lodestone, or magnetite, is found mentioned in very old historical records (about 2500 years ago in Europe, and much earlier in the Far East) as a subject of curiosity. Later, it was employed in the aid of navigation, as it was found that a piece of this unusual rock would tend to orient itself in a north-south direction if left free to rotate (suspended on a string or on a float in water). A scientific study undertaken in 1269 by Peter Peregrinus revealed that steel could be similarly "charged" with this unusual property after being rubbed against one of the "poles" of a piece of lodestone.
Unlike electric charges (such as those observed when amber is rubbed against cloth), magnetic objects possessed two poles of opposite effect, denoted "north" and "south" after their self-orientation to the earth.
Magnetomotive Force or MMFMagnetomotive force is a force that sets up or tends to set
up magnetic flux in a circuit by passing an electric current through a number of turns of a wire.
Where:F – mmf(ampere-turn)N – number of turnsI – current carried(ampere)
In cgs unit,
Example• A solenoid has 250 turns. What is the
magnetomotive force when the current is 0.12 A?
• The coil in a magnetic contactor requires a 0.5 A to provide a magnetizing force of 500AT. How many turns are necessary?
Magnetic Flux(Φ)Magnetic flux is the number of magnetic
lines of forces in a magnetic field.
maxwell – unit of magnetic flux equal to one line of force.weber – SI unit of magnetic flux equal to lines or maxwell
Flux Density (β) It is given by the flux passing per unit area
through a plane at right angles to the flux. It is usually designated by β and is measured in .
Tesla – SI unit of magnetic flux density equal to webers per square meter.
Gauss – cgs unit of magnetic flux density equal to maxwells per square centimeter.
ExampleHow many magnetic lines of force will pass
a 5 sq. cm perpendicular area on a magnetic field having a flux density of 2000 gauss?
The phenomena of magnetism and electromagnetism are dependent upon a certain property of the medium called its permeability. Every medium is supposed to possess two permeabilities: (i)absolute permeability (μ) and (ii) relative permeability (μr). For measuring relative permeability, vacuum or free space is chosen as the reference medium. It is allotted an absolute permeability of μ0 = 4π × 10−7 henry/metre. Obviously, relative permeability of vacuum with reference to itself is unity. Hence, for free space, absolute permeability μ0 = 4π × 10−7 H/m relative permeability μr = 1.
Permeabilitythe ability of a material to conduct
magnetic flux through it.
Magnetic Field Strength (H)Magnetic field strength at any point within a
magnetic field is numerically equally to the force experienced by a N-pole of one weber placed at that point. Hence, unit of H is N/Wb.
In cgs, Oersted – gilbert per centimeter
Infinitely long Straight Wire
ExampleIf a current 5A flows through a long wire of
radius 0.004 meter, what is the magnetic field intensity produced 0.02 meter away from the surface?
Reluctance- is the property of a material that opposes
flux flow. It is equal to the ratio of the magnetomotive force in a magnetic circuit to the magnetic flux through any cross section of the magnetic circuit.
Magnetic Circuit- a closed path in which magnetic inductin
of flux flows
Reluctance in Series
Reluctance in Series
Electromagentism(page 279 theraja)
DC Generators
Introduction
• Electrical GeneratorA machine that converts mechanical energy into
electrical energy. The energy conversion is based on the principle of the of Faraday’s first law of Electromagnetic Introduction, which states that whenever a conductor cuts magnetic lines of flux, an emf is developed in the conductor
Operating Principle
• Generating an AC Voltage
• The difference between AC and DC generators:– AC generators use slip rings– DC generators use commutators– Otherwise, the machine constructions are essentially the
same.
Essential Parts
• Magnetic frame or Yoke• Pole-cores and Pole-shoes• Pole coils or Field coils• Armature core• Armature windings or Conductors• Commutator• Brushes and Bearings
• YokeThe outer frame or yoke serves double purpose:
— It provides mechanical support for the poles and acts as a protecting cover for the whole machine
— It carries the magnetic flux produced by the poles
• Pole cores and Pole shoesThe pole shoes serve two purposes:
— they spread out the flux in the air gap and reduces the reluctance of the magnetic path
— support the exciting coils
• Pole coilsThe field coils or pole coils are former wound for the
correct dimension. Then the former is removed and wound coil is put into place over the core.
When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conductors.
• Armature CoreIt houses the armature conductors or coils and causes
them to rotate and hence cut the magnetic flux of the field magnets. Its most important function is to provide a path of very low reluctance to the flux through the armature from a N-pole to a S-pole.
• Armature WindingsThe armature windings are usually former wound.
These are first wound in the form of flat rectangular coils and are then pulled into their proper shape in a coil puller. The conductors are placed in the armature slots whish are lined
with tough insulating material.
• CommutatorThe function of the commutator is to facilitate
collection of current from the armature conductors.
• Brushes and BearingsThe brushes whose function is to collect current from
commutator, are usually made of carbon or graphite and are in the shape of a rectangular block.
The Armature Winding
• Lap windingForms a loop as it expands around the armature core.
windingiveretrogress
windingeprogressiv
on so andduplex for 2 simplex,for factor(1ty multiplici m
pitchfront Y
pitchback Y
where
2
f
b
mYY fb
• Wave windingForms a wave as it expands around the armature core.
windingiveretrogress
windingeprogressiv
on so andduplex for 2 simplex,for factor(1ty multiplici m
pitchfront Y
pitchback Y
poles ofnumber P
elements windingofnumber total Z
pitch average Y
where
2
2
f
b
fb YY
YP
mZY
• Total number of elements or conductors
• Number of armature current paths(a)
• Coil pitch
poleperslots
slotsinspancoilY
mamPa
slotsofnumbertotalslotelementsZ
s
wavelap
2
) )(/(
• ExampleIn a lap winding the front pitch is 17 and the back is
19. What is the average pitch?
Solution
182
17192
ave
ave
fb
ave
Y
Y
YYY
Generated EMF of a DC Generator
constantality proportion k
pathscurrent armature ofnumber a
)pole(weberper flux
conductors ofnumber total Z
pm)rotation(r core armature of speed N
poles ofnumber P
emf(volt) generated E
60
where
kNEa
PNZE
• ExampleA 4 pole dc generator with simplex wave winding has
72 slots. The fluxper pole is 2.88 x 106. What is the speed of the prime mover when the open circuit voltage of the generator is 120 volts?
Solution
rpmN
N
)s per slot conductor(assume a
PNZE
868
10)1)(2(60
)1088.2)(722)((4120
2
1060
86
8
Types of DC Generator According to Excitation
• Separately ExcitedThe field windings of the generator is excited from a
separate source usually a battery
ag
aaL
La
L
LL
EIP
RIVE
II
V
PI
• ExampleA DC generator has no-load output voltage of 120
volts. Its armature resistance is 0.95 ohm and its field coils are separately energized. If the load is rated 2000 W at 115 V. Neglecting the effect of armature reaction, what power could be delivered to the load?
Solution
WP
P
RIP
AI
I
RR
EI
R
R
P
VR
L
L
LLL
L
L
La
L
L
L
L
LL
29.1667
)6.6(894.15
894.156.695.0
120
6.62000
115
2
2
2
2
• Self – ExcitedThe field windings of the generator is supplied or
excited from its own generated emf.– Shunt Generator– Series Generator– Compound Generator• Long Shunt• Short Shunt
• Shunt generator
aaL
ag
shLa
L
LL
sh
Lsh
RIVE
EIP
III
V
PI
R
VI
• ExampleA shunt generator delivers 450A at 230V and the
resistance of the shunt field and armature are 50Ω and 0.03Ω respectively. Calculate the generated emf.
Solution
AI
I
III
AI
AI
I
R
VI
a
a
shLa
L
sh
sh
sh
Lsh
6.454
6.4450
450
6.450
230
VE
E
RIVE
g
g
aag
64.243
)03.0)(6.454(230
• Series Generator
ag
seaaL
La
L
LL
EIP
RRIVE
II
V
PI
)(
• ExampleA DC series generator is supplying a current of 10A to a
load through a feeder of total resistance, 1.5Ω. The generated voltage is 550 volts. The armature and series field resistances are respectively. Determine the voltage:a. at the terminals of the generatorb. at the feeder
Solution
VV
V
RIVV
VV
V
RRIEV
L
L
feederaTL
T
T
seaaT
524
)5.1(10539
539
)6.05.0(10550
)(
• Long Shunt Generator
)( seaaL
ag
shLa
L
LL
sh
Lsh
RRIVE
EIP
III
V
PI
R
VI
• ExampleA long shunt comound generator delivers a load
current of 50A at 500V and has armature, series field and shunt field resistances of 0.05Ω, 0.03Ω and 250Ω respectively. Calculate the generated voltage and the armature current. Allow 1V per brush for contact drop.
Solution
AI
I
III
AI
AI
I
R
VI
a
a
shLa
L
sh
sh
sh
Lsh
52
250
50
2250
500
VE
E
dropbrushRRIVE
g
g
seaag
16.506
2)05.003.0)(52(500
)(
• Short Shunt Generator
seLLsh
seLaaL
ag
shLa
L
LL
sh
Lsh
RIVV
RIRIVE
EIP
III
V
PI
R
VI
• ExampleA short shunt compound generator delivers a load
current of 30A at 220V, and has armature, series-field and shunt-field resistances of 0.05Ω, 0.30Ω and 200Ω respectively. Calculate the induced emf and the armature current. Allow 1.0V per brush for contact drop.
Solution
AI
I
III
AI
I
R
VI
VV
V
RIVV
a
a
shLa
sh
sh
sh
shsh
sh
sh
seLLsh
145.31
145.130
145.1200
229
229
)3.0(30220
VE
E
dropbrushRIRIVE
g
g
seLaag
56.232
2)3.0)(30()05.0)(145.31(220
• Losses and Efficiency– Armature I2R loss
I2R loss in the armature windings
– Shunt field I2R loss I2R loss in the shunt field windings
shshsh
aaa
RIP
RIP
2
2
– Series field I2R loss I2R loss in the series field windings
– Friction and Windage loss The mechanical power required to drive the unexcited machine at normal speed
– Core loss or Iron loss Loss due to the magnetic field at no-load rated voltage corrected for effect of IR drop
a. Eddy current lossb. Hysteresis loss
sesese RIP 2
Power Stages
• Mechanical efficiency
motordrivingofoutput
IE ag
m
• Electrical efficiency
• Overall or Commercial Efficiency
ag
e IE
VI
generatedwattstotal
circuitloadinavailablewatts
suppliedpowermechanical
circuitloadinavailablewattsc
• ExampleThe field circuit of a 200kW, 230V shunt generator is
8A when running full load at rated terminal voltage. If the combined brush and armature resistance is 0.03ohm, solve for the electrical efficiency of the generator.
Solution
kWP
P
RIP
I
I
III
AI
I
V
PI
a
a
aaa
a
a
shLa
L
L
L
LL
1.23
)03.0(56.877
56.877
856.869
56.869230
200000
2
2
%91.88
%10084.11.23200
200
%100
84.1
)230(8
shaL
L
sh
sh
shshsh
PPP
P
kWP
P
VIP
• Voltage RegulationPercent voltage regulation is the percentage rise in the
terminal voltage of a generator when its load is removed.
voltageterminalloadfullV
voltageterminalloadnoV
V
VVVR
FL
NL
FL
FLNL
where
%100%
Parallel Operation of DC Generators
In order to increase the capacity of a system serving different loads, a number of generators are connected in parallel
Basic requirements1. The same external characteristics2. Terminal polarity must be the same3. Terminal voltage must be equal in magnitude
• ExampleTwo identical 600kW, 230V dc generators are
operating in parallel and take equal shares of an 800kW, 250V busload. The voltage regulation of each machine is 5%. If one of the generators is accidentally tripped off from the line, what is the voltage of the remaining busload?
Solution
AII
II
AI
I
V
PI
AII
II
V
PII
LL
LL
LOAD
LOAD
L
LOADLOAD
FLFL
FLFL
FL
FLFLFL
16002
3200
3200250
800000
69.2608230
600000
21
21
21
21
21
machine. trippeditsby offgiven load the todue is therefore
busload in the change The line. thefrom trippedhas 2gen Assume :NOTE
1
1
11
84.226
)230(05.0
)69.2608(
)(%
)(
)(%
VI
VI
VVR
IVI
I
VVR
I
V
FL
FL
FL
L
VV
V
VVV
VV
V
II
LNEW
LNEW
LOLDLNEW
BUS
95.242
05.7250
05.7
160084.2261
DC Motors
Introduction
• Electric motorA machine that converts electrical energy into
mechanical energy
Direct current motors are seldom used in ordinary industrial applications because all electric utilities supply AC. However, for special applications such as in steel mills, mines, and electric trains, it is sometimes advantageous to transform the AC into DC in order to use DC motors. The reason is that the torque-speed characteristics of DC motors can be varied over a wide range while retaining high efficiency.
Speed Characteristics of a DC Motor
)(
Z
a
)(
60
voltemfcounteroremfbackE
poleperflux
conductorsofnumber
polesofnumberP
pathscurrentarmatureofnumber
rpmspeedN
wherePZ
EaN
b
b
Motor Torque
By the term torque is meant the turning or twisting moment of a force about an axis. It is measured by the product of the force and the radius at which this force acts.
T=F x r
• Torque in the Armature
)(
)(
)(
)(
283.6
weberpoleperflux
conductorsofnumberZ
polesofnumberP
pathsarmatureofnumbera
rpmrotationarmatreofspeedN
amperecurrentarmatureI
rnewtonmetedevelopedtorqueT
wherea
IPZT
a
a
• Shaft torqueThe torque which is available for doing useful
work is known as shaft torque Tsh. It is so called because it is available at the shaft.
N-mN
output.=Tsh
559
• Example
A DC motor takes an armature current of 110A at 480V. The armature circuit resistance is 0.2Ω. The machine has 6 poles and the armature is lap-connected with 864 conductors. The flux per pole is 0.05Wb. Calculate (i)the speed and (ii)the gross torque developed by the armature.
Solution
rpmN
N
PZ
EaN
VE
E
RIVE
b
b
b
aasb
636
)05.0)(864)(6(
)458)(6(60
)(60
458
)2.0(110480
mNT
T
a
IPZT
a
a
aa
3.7566
)110)(05.0)(864)(6(159.0
159.0
Speed of a Motor
• For series motor
• For shunt motor
2
1
1
2
1
2
Φ
Φ
E
E
N
N
b
b
1
2
1
2
b
b
E
E
N
N
Types of DC Motor
• Shunt MotorThe armature and the field coils are connected in
parallel
sh
ssh
sham
aasb
R
VI
III
RIVE
• ExampleA shunt motor is taking 72A at 120V while developing
an output of 10bhp. Armature resistance is 0.05ohm. Shunt field resistance is 60 ohms. Determine the counter emf.
Solution
AI
I
III
AI
I
R
VI
a
a
shma
sh
sh
sh
ssh
70
272
260
120
VE
E
RIVE
b
b
aasb
5.116
)05.0(70120
• Series Motor
)( seaasb
am
RRIVE
II
• Long Shunt Compound Motor
)( seaasb
sh
ssh
sham
RRIVE
R
VI
III
• ExampleA long shunt compound motor draws a line current of
42A from a 230V dc source. The armature resistance is 0.1 ohm while the series and shunt field resistances are 0.2 ohm and 50 ohm respectively. If the iron and friction losses amount to 500W, determine the overall efficiency of the machine.
Solution
WP
P
RIP
AI
I
III
AI
I
R
VI
a
a
aaa
a
a
shma
sh
sh
sh
ssh
876.139
1.0)4.37(
4.37
6.442
6.450
230
2
2
WP
P
PPPPP
WP
P
VIP
WP
P
RIP
losses
losses
frictionironshsealosses
sh
sh
shshsh
se
se
sease
628.1977
5001058752.279876.139
1058
230)6.4(
752.279
2.0)4.37(
&
2
2
%53.799660
628.19779660
9660
)42(230
input
lossesinput
input
output
input
input
msinput
P
PP
P
P
WP
P
IVP
• Short Shunt Compound Motor
)semaasb
sh
semssh
sham
RIRIVE
R
RIVI
III
ExampleA 220V short shunt compound motor has an armature
resistance of 0.4Ω, a shunt field resistance of 110Ω and a series field resistance of 0.6Ω. If this motor draws an armature current of 50A at rated load, determine horsepower developed in the armature.
Solution
VE
E
RIRIVE
AI
II
III
II
II
R
RIVI
b
b
aasemsb
m
mm
sham
msh
msh
sh
semssh
97.168
)4.050()6.0718.51(220
718.51
)005454.02(50
005454.02110
)6.0(220
hpP
P
IEP
d
d
abd
325.11746
)50)(97.168(746
Speed Regulation
Speed regulation is the percentage rise in speed when the mechanical load of the motor is removed.
voltageterminalloadfullN
voltageterminalloadnoN
N
NNNR
FL
NL
FL
FLNL
where
%100%
Power Developed in the Armature
)(
)(
)(
voltemfcounteroremfbackE
amperecurrentarmatureI
wattarmaturetheindevelopedpowerP
where
IEP
b
a
d
abd
Losses and Efficiency
• Armature loss
• Shunt field loss
• Series field loss
aaa R=IP 2
shshsh R=IP 2
sesese R=IP 2
• Efficiency
input
lossesinput
input
ouput
P
PP
P
P
Power Stages
ExampleA 100 volt shunt motor is developing 6 hp while
operating at an overall efficiency of 86%. The armature and shunt field resistances are 0.06 and 50Ω respectively. Determine stray power losses.
Solution
AI
I
V
PI
WP
P
PP
WP
hp
WhpP
m
m
s
inm
in
in
outin
out
out
04.52100
65.5204
65.520486.0
4476
4476
)1
746(6
WP
RIP
AI
I
III
AI
I
R
VI
a
aaa
a
a
shma
sh
sh
sh
shsh
24.150
)06.0(04.50
04.50
204.52
250
100
22
WP
P
PPPPP
WP
IVP
stray
stray
strayshaoutin
sh
shssh
41.378
20024.150447665.5204
200
)2)(100(
Speed Control of Shunt Motors
• Variation of Flux or Flux Control MethodBy decreasing the flux, the speed can be increased and
vice versa. The flux of a d.c motor can be changed by changing Ish with help of a shunt field rheostat.
• Armature or Rheostatic Control Method
This method is used when speeds below the no load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or resistance in series with the armature circuit.
• Voltage Control Method– Multiple Voltage Control
In this method, the shunt field of the motor is connected permanently to a fixed exciting voltage but the armature is supplied with different voltages by connecting it across one of the several different voltages by means of suitable switchgear. The armature speed will be approximately proportional to these different voltages. The intermediate speeds can be obtained by adjusting the shunt field regulator.
– Ward-Leonard System This system is used where an unusually wide and
very sensitive speed control is required as for colliery winders, electric excavators, elevators and the main drives in steel mills and blooming and paper mills.
Speed control of Series motors
• Flux control methodVariation in the flux of series motor can be brought
about in any one of the following ways.– Field Divertors
The series winding are shunted by a variable resistance known as field divertors.
– Armature Divertor A divertor across the armature can be used for
giving speeds lower than the normal speed.
– Trapped Field Control Field This method is often used in electric traction. The
number of series field turns in the circuit can be changed at will. With full field, the motor runs at its minimum speed which can be raised in steps by cutting out some of the series turns.
– Paralleling Field Coils In this method several speeds can be obtained by
regrouping the field coils
• Variable Resistance in Series with MotorBy increasing the resistance in series with the
armature the voltage applied across the armature terminals can be decreased.