CHAP6 Electromagnetism - taiwan921.lib.ntu.edu.t · 2 導入...
Transcript of CHAP6 Electromagnetism - taiwan921.lib.ntu.edu.t · 2 導入...
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CHAP6 CHAP6 ElectromagnetismElectromagnetism
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Hall effect sensorsHall effect sensorsmagnetic magnetic reed switchesreed switchesMRIMRI
Edwin H. Hall 1879NPmajority carrierconcentrationmobilityelectric probe
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Basic Properties of Basic Properties of MagnetismMagnetism
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magnetmagnet
magnetizationmagnetizationnatural magnetnatural magnetartificial magnetartificial magnet
permanent permanent magnetmagnettemporary magnettemporary magnet
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1/21/2
magnetic fieldmagnetic field
magnetic lines of forcemagnetic lines of force
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1/21/2
NNSSNN
lines of forceflux lines
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1/21/2
magnetic polemagnetic poleSSNN
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2/22/2
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pole strengthpole strength
221
rmmkF
mm11mm22weberweberwbwb permeabilitypermeability
41k
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rr
r2IH
Magnetic flux lines surrounding a current-carrying conductor
magnetizing forcemagnetizing forceHH
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1/21/2
ElectromagnetElectromagnet
retentivityretentivityRetentivityRetentivity
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2/22/2
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Direction of Flux vs.Direction of Flux vs.
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AmpereAmperes rights right--hand hand rulerule
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Exercise 2
For the electromagnet in Fig. 6.58, determine the direction of IFor the electromagnet in Fig. 6.58, determine the direction of Ineeded to establish the flux pattern, and label the induced nortneeded to establish the flux pattern, and label the induced north and h and south poles.south poles.
N S
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ApplicationApplication
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Magnetic RelayMagnetic Relay
Coil Coil normally closednormally closedNCNCnormally openednormally openedNONOCCAANONONCNCCCBB
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Circuit BreakerCircuit Breaker
movable armmovable arm
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Magnetically Controlled LockMagnetically Controlled Lock
corecorecorecore
corecore
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Electric BellElectric Bell
CoilCoil
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Magnetic CircuitMagnetic Circuit
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ReluctanceReluctance
reluctancereluctanceairairwoodwoodglassglass
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PermeabilityPermeability
PermeabilityPermeabilityflux lineflux line
Relative permeabilityRelative permeability100100
current
length
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Magnetic FluxMagnetic Flux
magnetic fluxmagnetic fluxelectromagnetic coreelectromagnetic corereluctancereluctance
magnetic fluxmagnetic flux
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Magnetomotive ForceMagnetomotive Force
Magnetomotive force Magnetomotive force mmfmmf
magnetic magnetic circuitcircuitfluxflux
fluxflux
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vs. vs.
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Exercise 4
a.a. If the length of a magnetic core is increased for the same If the length of a magnetic core is increased for the same magnetomotive force, what will happen to the magnitude of the magnetomotive force, what will happen to the magnitude of the resulting flux?resulting flux?
b.b. If the area of a magnetic core is doubled and the length reducedIf the area of a magnetic core is doubled and the length reduced to to oneone--third, what will be the effect on the resulting flux if the third, what will be the effect on the resulting flux if the magnetomotive force is held constant?magnetomotive force is held constant?
AA
A 1 1/36
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Flux densityFlux densityCGSCGScentimetercentimeter--gramgram--secondsecondGuassGuassMKSMKSmetermeter--kilogramkilogram--secondsecondteslateslaweberweber
1 Tesla=10,000 Guass1 Tesla=10,000 Guass
11,000mG
Wb = 108
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EXAMPLE 6.1EXAMPLE 6.10.4T
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Gauss MeterGauss Meter
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HH
magnetizing force Hmagnetizing force Hmagnetomotive forcemagnetomotive forcemmfmmf
A
NINI
AB
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B vs.B vs.H H 1/41/4BrBrthe retentivity levelthe retentivity levelcoilcoilcorecore
PermeabilityPermeability
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B vs. H B vs. H 2/42/4
NN HHOBOB
BBBBrHcBBrHcH=0H=0residual magnetismresidual magnetismBrBr
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B vs. H B vs. H 3/43/4
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B vs. H B vs. H 4/44/4
magnetization curvemagnetization curveHysteresis loopHysteresis loopBBHHBB--HH
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Normal Magnetization CurveNormal Magnetization CurveBBHHB=B=HH
Cast steelCast steel
BBHHRRBBHH
PermeabilityPermeabilityFig. 6.15
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BB--H MeterH Meter
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EXAMPLE 6.2EXAMPLE 6.2
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EXAMPLE 6.2EXAMPLE 6.2
1.46T
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Descriptive Example Descriptive Example 1/31/3
331010--44 WbWb
AmpereAmperes Circuital Laws Circuital Lawmagnetic circuitmagnetic circuitelectric circuitelectric circuitKVL KVL AmpereAmperes s Circuital LawCircuital Lawclosed closed pathpathmmfmmf
0HNI HNI
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Descriptive Example Descriptive Example 2/32/3
BH
Fig.6.15Fig.6.15BBHHH = 770 At/mH = 770 At/mII
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Descriptive Example Descriptive Example 3/33/3
rr
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Exercise 5
a.a. For the system in Fig. 6.17, determine the current I if the areaFor the system in Fig. 6.17, determine the current I if the area is is doubled.doubled.
b.b. Is the resulting current in part (a) half of that obtained in thIs the resulting current in part (a) half of that obtained in the e descriptive example? Why not ?descriptive example? Why not ?
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Exercise 5
AA661010--44 mm22
Fig.6.15Fig.6.15BBHHH = 275 At/mH = 275 At/m
T5.0m106Wb103
AB 24
4
mA110t200
)m08.0)(m/At275(NHIHNI
A35.7%
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Exercise 6
a. For the system in Fig. 6.17, determine the resulting flux if thecurrent is reduced to 200 mA.
b. Find the relative permeability of the core.
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II200mA200mA
m/At500NIHHNI
Fig.6.15Fig.6.15HHBBB = 0.77TB = 0.77T
Wb1031.2AB 4
5.1225
mAWb1054.1
HB
0r
3
Exercise 6
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EXAMPLE 6.3EXAMPLE 6.3
corecoreHNI AmpereAmperes Circuital Laws Circuital Lawclosed pathclosed pathmmfmmf
0HNI
Find Find fluxflux
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EXAMPLE 6.3EXAMPLE 6.3
Fig.6.15Fig.6.15HHBB
HB
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Exercise 7
For the magnetic system in Fig. 6.59, determine:a. The magnetomotive force.b. The magnetizing force applied to the core.c. The flux density.d. The flux in the core.
Wb104BA.d
T1)m/At400)(Am/Wb104)(2000(HHB.c
m/At400m2.0
At80H.b
At80)mA400)(t200(NI.a
4
70r
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Exercise 8Exercise 8
Determine the current I necessary to establish the flux indicated in Fig. 6.60.
T7.0m102Wb104.1
AB 24
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Fig.6.15Fig.6.15BBHHH =400 At/mH =400 At/m
A6.1NHI
HNI
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Exercise 9
Determine the current I1 necessary to establish a net flux = 5 10-4 Wb in the transformer in Fig. 6.61.
T25.1m104Wb105
AB 24
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Fig.6.15Fig.6.15BBHHH =1500 At/mH =1500 At/m
A375.0I)m15.0)(m/At1500(A2)t75(I)t200(
HININ
1
1
2211
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Air Gap EncounteredAir Gap EncounteredAir gapBH
GapGapBBCoreCoreBBHH
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EXAMPLE 6.4EXAMPLE 6.4
AmpereAmperes Circuital Laws Circuital Lawclosed pathclosed pathmmfmmf
KVLKVL
0HHNI gapgapcorecore
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EXAMPLE 6.4EXAMPLE 6.4 CoreCoreAir gapAir gapBBB B corecore=B =B air gapair gap
Fig.6.15BH
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EXAMPLE 6.4EXAMPLE 6.4
mmf 6:1
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Exercise 10
Repeat Problem 7 if an air gap of 0.01 in. is cut through the coRepeat Problem 7 if an air gap of 0.01 in. is cut through the core.re.
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Exercise 11
Repeat Problem 8 if an air gap of 250Repeat Problem 8 if an air gap of 250m is cut through the core.m is cut through the core.
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gc24
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BBT7.0m102Wb104.1
AB
Fig.6.15Fig.6.15BBHHH =400 At/mH =400 At/m
A386.4t50At3.219I
At3.219)B1096.7()m2.0)(m/At400(NI
m2.0m750,199m250m2.0HHNI
gg5
c
ggcc
Exercise 11
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Exercise 12
Determine the current required to establish a flux of 5 Determine the current required to establish a flux of 5 1010--44 Wb in Wb in the core of the transformer in Fig. 6.18.the core of the transformer in Fig. 6.18.
T0.1m105Wb105
AB 24
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Fig.6.15Fig.6.15BBHHH =780 At/mH =780 At/m
A56.1NHI
HNI
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Exercise 13
If the air gap in Fig. 6.19 is doubled (1/16 in.), will the current required to establish the same flux increase by a factor of 2 also? Determine the resulting current and comment on the results.
Air gapAir gapfluxfluxB=1.33TB=1.33T
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Exercise 13
m109.15in161
T33.1BBB
4g
gc
Fig.6.15Fig.6.15BBHHHHcc =1750 At/m=1750 At/m
A44.22A56.4I)m109.15)(m/At33.11096.7()m08.0)(m/At1750(I)t400(
HHNIm/At1059.10B1096.7H
45
ggcc
5g
5g
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Exercise 14Exercise 14
Find the magnetic flux established in the series magnetic circuit in Fig. 6.62.
m5027.0r2
Fig.6.15Fig.6.15HHBBB =0.7TB =0.7T
m/At88.397NIH
HNI
Wb103.6BA 3
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D'arsonval movementD'arsonval movement 1/2
D'arsonval movementD'arsonval movementdcdcacac
movablemovablestationary partsstationary partsair gapair gap
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D'arsonval movementD'arsonval movement 2/2
movable coremovable core1k1k1mA1mAmovable coremovable coremovable coremovable corenameplatenameplatefull scalefull scalemovable coremovable core
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AmmeterAmmeter
1A1AmovementmovementRRshuntshuntRRshuntshunt
movementmovement1mA1mA
1A1ARsRs
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VoltmeterVoltmeter
movementmovement1V1V100V100VmovementmovementRRseriesseriesmovementmovement1V1V
RRseriesseries
movementmovement1V1V1k1k
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Exercise 15Exercise 15
Using a 50Using a 50--A A 20,00020,000 momovement, design: vement, design: a. A 10a. A 10--A ammeter. A ammeter.
Rshunt ~0.1
b. A 10b. A 10--V voltmeter.V voltmeter.
Rseries~180k
1.0A10
)k20)(A50(IVRshunt
k180
A50)k20)(A50(V10
IVR
RS
RSseries
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TransformerTransformer
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TransformerTransformer 1/21/2
TransformerTransformer
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Transformer Transformer 2/22/2
primary windingprimary windingsecondary windingsecondary winding
corecore--type transformertype transformershellshell--type transformertype transformer
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Application of TransformerApplication of Transformer
impedance matchingimpedance matching
blockingblocking
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Factors makeFactors make
stray capacitancestray capacitance
reversing reversing magnetic fieldmagnetic fieldhysteresis losshysteresis loss
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Transformer Ratio Transformer Ratio 1/21/2
NNPPNNSS
)tcos(E)t(E PP
dtdN)tcos(E)t(E PPP )tsin(
NE)t(
P
P
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Transformer Ratio Transformer Ratio 2/22/2
)tcos(ENN)tsin(
dtd
NEN
dtdN)t(E P
P
S
P
PSSS
S
P
S
P
EE
NNa a: Transformer ratioa: Transformer ratio
a1stepstep--down transformerdown transformer
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Current RatioCurrent Ratio
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Voltage RatioVoltage Ratio
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Example 6.5
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Example 6.5
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Impedance RatioImpedance Ratio
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Impedance RatioImpedance Ratio
ZZPP is the impedance seen at the is the impedance seen at the primary of a transformer.primary of a transformer.primary primary primary primary
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Example 6.6
impedance impedance matchingmatching
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Example 6.6
88
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Example 6.6
loadloadpowerpowerZZLLprimaryprimaryZZPPRRThThZZPP=800=800
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Example 6.6
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Exercise 16Exercise 16
A transformer with a turns ratio a = NA transformer with a turns ratio a = Npp/N/Nss = 12 has a load of 2.2 = 12 has a load of 2.2 kk applied to the secondary. If 120 V is applied to the primary, applied to the secondary. If 120 V is applied to the primary, determine:determine:
a.a. The reflected impedance at the primary. The reflected impedance at the primary. ZP = a2 ZL = 316.84kb.b. The primary and secondary currents. The primary and secondary currents. IP = EP/ZP = 0.379 mA IS =
a IP = 4.55 mAc.c. The load voltage. The load voltage. VS = ISRL = 10.01 Vd.d. The power to the load. The power to the load. P = IS2RL = 45.55 mWe.e. TheThe powerpower supplied by source. supplied by source. P = 45.55 mW
real
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Exercise 17Exercise 17
For a power transformer, Ep = 120 V, ES = 6000 V, and Ip = 20A:a. Determine the secondary current IS. IP/IS = ES/EP = 50 IS =0.4Ab. Calculate the turns ratio a. a= NP/NS = 1/50c. Is it a step-down or a step-up transformer? Step-up transformerd. If NS = 100 turns, Np = ? NP=NS a = 2 turns
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Exercise 18Exercise 18
An inductive load ZAn inductive load ZLL = 4= 4 + j4 + j4 is applied to a 120 V /6 V is applied to a 120 V /6 V filament transformer.filament transformer.
a.a. What is the magnitude of the secondary current?What is the magnitude of the secondary current?b.b. What is the power delivered to the load?What is the power delivered to the load?c.c. What is the primary current?What is the primary current?
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Exercise 18Secondaryinductive load ZL = 4 + j 4= 5.66 45EPP = 120 V120 VVV SS= E= ESS = 6 V= 6 V
A06.1ZVI
L
SS
b.b.real powerreal power
a. Secondary current
c. c. primary currentprimary current
W494.44IP 2S
mA53NNII
P
SSP
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Exercise 19Exercise 19
A purely capacitive load ZA purely capacitive load ZLL = = --j2j2 is applied to a 120 V /6 V is applied to a 120 V /6 V filament transformer with a 12 VA apparent power rating.filament transformer with a 12 VA apparent power rating.
a.a. Determine the magnitude of the secondary current.Determine the magnitude of the secondary current.b.b. Calculate the power delivered to the primary and secondary.Calculate the power delivered to the primary and secondary.c.c. Do you expect the transformer to heat up if operating under thesDo you expect the transformer to heat up if operating under these e
conditions?conditions?
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Exercise 19Secondarycapacitive load ZL = -j 2 EPP = 120 V120 VVV SS= E= ESS = 6 V= 6 V
A0.3ZVI
L
SS
b.b. delivered to the primary and secondarydelivered to the primary and secondary
a.
c. Apparent powerc. Apparent power12 VA12 VAsecondarysecondary2A2A3A3Aheat upheat up
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Exercise 20Exercise 20
If E = 120 V, RIf E = 120 V, RTH TH = 0.5 k= 0.5 k, and a =5 in the network in Fig. 6.25, and a =5 in the network in Fig. 6.25, , determine the load value for max power to the load.determine the load value for max power to the load.
Determine the power to the load under these conditions. Determine the power to the load under these conditions.
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Exercise 20a.max. power transferprimaryimpedance0.5 kZP = 0.5 kZP REFLECTED IMPEDANCE
L2
P ZaZ 20ZR LL
b.primary
W2.7RIP
mA600IaImA120ZR
V120I
L2
SL
PSPTH
P