chapter 14 electromagnetism
TRANSCRIPT
-
7/25/2019 Chapter 14 Electromagnetism
1/38
ElectromagnetismElectromagnetism
Topics Covered in Chapter 14
14-1: Ampere-turns of Magnetomotive Force (mmf)
14-2: Field ntensit! (H)
14-": B-H Magneti#ation Curve
14-4: Magnetic $!steresis
14-%: Magnetic Field around an &lectric Current
Chapter
14
2007 The McGraw-Hill Companies, Inc. All rights reserve.
-
7/25/2019 Chapter 14 Electromagnetism
2/38
Topics Covered in Chapter 14Topics Covered in Chapter 14
14-': Magnetic olarit! of a Coil
14-: Motor Action *et+een T+o Magnetic Fields
14-,: nduced Current
14-: .enerating an nduced /oltage 14-10: ela!s
McGraw-Hill 2007 The McGraw-Hill Companies, Inc. All rights reserve.
-
7/25/2019 Chapter 14 Electromagnetism
3/38
14-1: Ampere-turns of14-1: Ampere-turns of
Magnetomotive Force (mmf)Magnetomotive Force (mmf)
The strength of a coils magnetic field is proportional to
the amount of current flo+ing through the coil and the
num*er of turns per given length of coil3 Ampere-turns I5 N mmf
Iis the amount of current flo+ing through Nturns of
+ire3
This formula specifies the amount of magneti#ing forceor magnetic potential (mmf)3
-
7/25/2019 Chapter 14 Electromagnetism
4/38
14-1: Ampere-turns of14-1: Ampere-turns of
Magnetomotive Force (mmf)Magnetomotive Force (mmf)
The 6 a**reviation for
ampere-turn is A 7 t3
The cgs unit of mmf is thegilbert8 a**reviated .*3
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-1: T+o e;amples of eual ampere-
turns for the same mmf3 (a) < is 2 5 % 103
(b) INis 1 5 10 103
-
7/25/2019 Chapter 14 Electromagnetism
5/38
14-: Field !ntensit" (14-: Field !ntensit" (HH))
The length of a coil influences the intensit! of a
magnetic field3 ntensit! is different from mmf3
&uation: H mmf=length
>nits: A7t=m
ampere-turns per meter
His the m?s unit
The cgs unit for His the oersted (@e)8 +hich euals 1
gil*ert per centimeter3 6horter magnetic circuits produce a greater field
intensit!
-
7/25/2019 Chapter 14 Electromagnetism
6/38
14-: Field !ntensit" (14-: Field !ntensit" (HH))
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-2: elation *et+een ampere-turns of mmf and the resultant field intensit! Hfor different
cores3
-
7/25/2019 Chapter 14 Electromagnetism
7/38
14-: Field !ntensit" (14-: Field !ntensit" (HH))
ermea*ilit! ()
Permeabilityis a measure of the a*ilit! to concentrate
magnetic fields3 Materials +ith high permea*ilit! can
concentrate flu;8 and produce large values of flu;densit! Bfor a specified H3
The amount of flu; produced *! Hdepends on the
material in the field3
These factors are reflected in the formulas: B 5 H
= B= H
The unit is teslas per ampere-turn per meter: A 7 t=m
T
-
7/25/2019 Chapter 14 Electromagnetism
8/38
14-: Field !ntensit" (14-: Field !ntensit" (HH))
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
-
7/25/2019 Chapter 14 Electromagnetism
9/38
14-#:14-#: B-HB-H Magneti$ation CurveMagneti$ation Curve
The B-Hmagneti#ation
curve sho+s ho+
much flu; densit! B
results from increasing
field intensit! H3 Saturationis the effect
of little change in flu;
densit! +hen the field
intensit! increases3
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-": B-Hmagneti#ation curve for
soft iron3
-
7/25/2019 Chapter 14 Electromagnetism
10/38
14-4: Magnetic %"steresis14-4: Magnetic %"steresis
Hysteresis refers to a situation +here the magnetic flu;
lags the increases or decreases in magneti#ing force3
Hysteresis lossis energ! +asted in the form of heat
+hen alternating current reverses rapidl! and moleculardipoles lag the magneti#ing force3
For steel and other hard magnetic materials8 hysteresis
lossesare much higher than in soft magnetic materials
li?e iron3
-
7/25/2019 Chapter 14 Electromagnetism
11/38
14-4: Magnetic %"steresis
$!steresis oop
B1is due to retentivity,
+hich is the flu; densit!
remaining after the
magneti#ing force is
reduced to #ero3
-
7/25/2019 Chapter 14 Electromagnetism
12/38
14-4: Magnetic %"steresis
Demagneti#ation (Also Called Degaussing) To demagneti#e a magnetic material completel!8 the
retentivit! B1must *e reduced to #ero3
The practical +a! to do so is to magneti#e anddemagneti#e the material +ith a decreasing h!steresisloop:
A magnetic field is produced *! alternating current3
The magnetic field and the magnetic material aremoved a+a! from each other8 or the currentamplitude is reduced3
The h!steresis loop then *ecomes smaller andsmaller until it effectivel! collapses3
-
7/25/2019 Chapter 14 Electromagnetism
13/38
14-4: Magnetic %"steresis
Demagneti#ation (Also Called Degaussing)
This method of demagneti#ation is called degaussing3
Applications of degaussing include:
Metal electrodes in a color picture tu*e &rasing the recorded signal on magnetic tape3
-
7/25/2019 Chapter 14 Electromagnetism
14/38
14-&: Magnetic Field around an14-&: Magnetic Field around an
Electric CurrentElectric Current
6traight Conductor
A straight conductorcan *e a short *ut continuous
length of conducting +ire +ith no *ends3
A magnetic field is produced *! the flo+ of currentthrough a straight conductor3
The magnetic field around a straight conductor is
circular and perpendicular to the a;is of the conductor3
The polarit! of the circular field is countercloc?+ise+hen vie+ed along the conductor in the direction of
electron flo+3
These reuirements appl! to an! charge in motion3
-
7/25/2019 Chapter 14 Electromagnetism
15/38
14-&: Magnetic Field around an14-&: Magnetic Field around an
Electric CurrentElectric Current
Cloc?+ise and Countercloc?+ise Fields
The left-handrule for conductors:
.rasp the conductor +ith !our left hand so the thum*
points in the direction of electron flo+3 Eour fingers+ill encircle the conductor in the same direction as
the circular magnetic field lines3
-
7/25/2019 Chapter 14 Electromagnetism
16/38
14-': Magnetic olarit" of a14-': Magnetic olarit" of a
CoilCoil Bending a straight conductor into the form of a loop
produces t+o effects:
The magnetic field lines are more dense inside the
loop3All the lines inside the loop aid in the same direction3
This ma?es the loop field the same as a *ar magnet8
+ith opposite poles at opposite faces of the loop3
-
7/25/2019 Chapter 14 Electromagnetism
17/38
14-': Magnetic olarit" of a14-': Magnetic olarit" of a
CoilCoil
Magnetic olarit!
A coil of +ire conductor +ith more than one turn is
called a solenoid3
To determine magnetic polarit! for a solenoid8 grasp theelectromagnet +ith the left hand3 hen the fingers of
the left hand curl around the turns of an electromagnet
in the direction of electron flo+8 the thum* points to the
north pole3 The magnetic polarit! depends on the direction of
current flo+ and the direction of +inding3 The current is
determined *! the connections to the voltage source:
flo+ runs from negative to positive3
-
7/25/2019 Chapter 14 Electromagnetism
18/38
14-': Magnetic olarit" of a14-': Magnetic olarit" of a
CoilCoil
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-10: eft-hand rule for north pole of a coil +ith currentI3 The Iis electron flo+3
-
7/25/2019 Chapter 14 Electromagnetism
19/38
14-: Motor Action *et+een T+o14-: Motor Action *et+een T+o
Magnetic FieldsMagnetic Fields
Motor actionis the result of t+o magnetic fields
interacting +ith one another3
The fields can attract or repel3
Motion is produced from a stronger field to+ard a
+ea?er field3
-
7/25/2019 Chapter 14 Electromagnetism
20/38
14-: Motor Action *et+een T+o14-: Motor Action *et+een T+o
Magnetic FieldsMagnetic Fields
Current in a conductor has its o+n magnetic field3
hen placed in the magnetic field of a separate source8
the t+o can produce motor action3
The conductor must *e perpendicular to the field3 t
must also *e in the same plane3
-
7/25/2019 Chapter 14 Electromagnetism
21/38
14-: Motor Action *et+een T+o14-: Motor Action *et+een T+o
Magnetic FieldsMagnetic Fields
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-1": Motor action of current in a straight conductor +hen it is in an e;ternal magnetic
field3 The HIis the circular field of the current The HMindicates field lines *et+een the north and
south poles of the e;ternal magnet3
-
7/25/2019 Chapter 14 Electromagnetism
22/38
14-: Motor Action *et+een T+o14-: Motor Action *et+een T+o
Magnetic FieldsMagnetic Fields
Torque is the effect of a force producing rotation3
Torue is produced +hen opposing magnetic fields in a
loop produce an up+ard force on one side of the loopand do+n+ard force on the other3
Torue is the *asis of all electric motors3
Torue is proportional to current8 so the amount of
rotation indicates ho+ much current flo+s through the
coil3
-
7/25/2019 Chapter 14 Electromagnetism
23/38
14-,: !nduced Current14-,: !nduced Current
.enerator Action
hen a moving conductor cuts across flu; lines8 a
voltage is induced3
The amount of induced voltage is proportional to: The conductor velocit!
The amount of flu;
The num*er of turns of +ire
The polarit! of induced voltage is determined *! en#s
la+ (discussed in section 14-)3
-
7/25/2019 Chapter 14 Electromagnetism
24/38
14-,: !nduced Current14-,: !nduced Current
en#s la+ states that the direction of an induced
current must *e such that its o+n magnetic field +ill
oppose the action that produced the induced current3
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-1%: nduced current produced *! magnetic flu; cutting across turns of +ire in a coil3
Direction of Ihere is for electron flo+3
The directionof the induced currentis
determined by the left-hand rule for electron
flow. If the fingers coil around the direction of
electron shown, under and over the winding, the
thumb will oint to the left for the north ole.
-
7/25/2019 Chapter 14 Electromagnetism
25/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
The motion of flu; across a conductor in an open circuit
forces free electrons to move3
6ince the ends are open8 electrons accumulate at them8
creating a potential difference3 The potential difference is an electromotive force
!emf",generated *! the +or? of cutting across the flu;3
nduced emf increases +ith the num*er of turns in a
coil3 The polarit! of the induced voltage follo+s from the
direction of induced current3
-
7/25/2019 Chapter 14 Electromagnetism
26/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
#araday$sa+ of nduced /oltage
The amount of voltage induced *! flu; cutting the turns
of a coil is determined *! three factors:
The amount of flu; More voltage is generated *! a stronger magnet3
The num*er of turns ncreasing the turns generates more voltage3
The time rate of cutting3 ess voltage is generated +hen the conductor moves slo+l!3
&ither the flu; or the conductor can move3
-
7/25/2019 Chapter 14 Electromagnetism
27/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
Farada!s a+ of nduced /oltage
The amount of induced voltage can *e calculated *!
Farada!s la+:
N num*er of turns
d/dt ho+ fast the flu; cuts across the conductor3
vind Nd(+e*ers)
dt(seconds)
-
7/25/2019 Chapter 14 Electromagnetism
28/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-1: /oltage induced across coil cut *! magnetic flu;3 (a) Motion of flu; generating voltage
across coil3 (b) nduced voltage acts in series +ith coil3 (c) nduced voltage is a source that can
produce current in an e;ternal load resistor RLconnected across the coil3
-
7/25/2019 Chapter 14 Electromagnetism
29/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
Consider a magnetic flu; cutting a
conductor that is not in a closed
circuit8 as sho+n in Figure 14-1'3The motionof flu; across the
conductor forces free electrons to
move3The potential difference is an
electromotive force (emf) generated
and onl! present +hile the motion of
flu; is cutting across the conductor3
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
-
7/25/2019 Chapter 14 Electromagnetism
30/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3
Fig3 14-1,: .raphs of induced voltage produced *! magnetic flu; changes in a coil3 (a) inear
increase of flu; 3 (b) Constant rate of change for d/dtat 2 *=s3 (c) Constant induced voltage
of '00 /8 for a coil +ith "00 turns3
-
7/25/2019 Chapter 14 Electromagnetism
31/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
Farada!s a+ of nduced /oltage
The amount of induced voltage is calculated *!
Farada!s la+:
The induced voltage is directl! proportional to the
num*er of turns times d/dt.
To generate more voltage:
ncrease the num*er of turns
ncrease the flu;
Decrease the time
vind=N
d(Wb)
dt(seconds)
-
7/25/2019 Chapter 14 Electromagnetism
32/38
14-: .enerating an14-: .enerating an
!nduced /oltage!nduced /oltage
olarit! of nduced /oltage
The polarit! of induced voltage is determined *! en#s
la+3
An induced voltage has the polarit! that opposes thechange causing the induction3
A*solute polarit! depends upon three points:
hether the flu; is increasing or decreasingG
The method of +indingG
hich end of the coil is the reference3
-
7/25/2019 Chapter 14 Electromagnetism
33/38
14-10: ela"s14-10: ela"s
A relayis an electromechanical device that operates on
the *asis of electromagnetic induction3
t uses an electromagnet to open or close one or more
sets of contacts3 ela!s8 li?e s+itches8 have poles and thro+s3
ela!s can s+itch or control high po+er loads +ith a
lo+ amount of input po+er3
n remote-control applications8 rela!s can control highpo+er loads long distances a+a! more efficientl! than
can mechanical s+itches3
-
7/25/2019 Chapter 14 Electromagnetism
34/38
14-10: ela"s14-10: ela"s
The s+itching contacts of a rela! ma! *e:
-
7/25/2019 Chapter 14 Electromagnetism
35/38
14-10: ela"s14-10: ela"s
An 6DT rela! has *oth
-
7/25/2019 Chapter 14 Electromagnetism
36/38
14-10: ela"s14-10: ela"s
ela! 6pecifications:
The follo+ing are a rela!s most important ratings:
ic?up voltage The minimum amount of rela! coil voltagenecessar! to energi#e or operate the rela!3
ic?up current The minimum amount of rela! coil currentnecessar! to energi#e or operate the rela!3
$olding current The minimum amount of current reuired to ?eep
a rela! energi#ed or operating (less than thepic?up current)3
Dropout voltage The ma;imum rela! coil voltage at +hich the rela!is no longer energi#ed3
-
7/25/2019 Chapter 14 Electromagnetism
37/38
14-10: ela"s14-10: ela"s
ela! 6pecifications:
mportant ratings8 cont3
Contact voltage
rating
The ma;imum voltage the rela! contacts are
capa*le of s+itching safel!3
Contact currentrating
The ma;imum current the rela! contacts arecapa*le of s+itching safel!3
Contact voltage drop The voltage drop across the closed contacts of arela! +hen operating3
nsulation resistance The resistance measured across the rela!contacts in the open position3
-
7/25/2019 Chapter 14 Electromagnetism
38/38
14-10: ela"s14-10: ela"s
The pic?up current is greater than the hold current *ecause of the air gap3
ess current is no+
reuired to
overcome the spring
and hold the rela!closed3
There is continuit! *et+een the main contact and the