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At medium and low frequencies, inductorsare often manufactured using one of a rangeof ferrite pot cores.
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The core material of theseinductors is commonly available in several andthe complete pot core assembly comprises amatched pair of core halves, a single-sectionbobbin, a pair of retaining clips and a coreadjuster. Eectively, the coil winding is totallyenclosed in a high permeability ferrite pot. Typicalvalues of inductance for these componentsrange between !! # and !!m# with atypical saturation $u% density of &'! mT.(nductance values of iron cored inductors arevery much dependent upon the applied )* andtend to fall rapidly as the value of applied )*increases and saturation is approached. +a%imumcurrent ratings for larger inductors arerelated to operating temperatures and should bede-rated when high ambient temperatures aree%pected. here reliability is important, inductorsshould be operated at well below their nominal ma%imum current ratings.
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inally,
ferrite
a high permeability nonconductivemagnetic material/ is often used asthe core material for inductors used in highfrequency0lters and as broadband transformersat frequencies of up to 1! +#2. At these frequencies,inductors can be reali2ed very easilyusing these cores with just a few turns of wireigure '.3/4)* 5E6E7T879+8T87
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An e.m.f. will be induced across the ends of aconductor when there is relative motion betweenit and a magnetic ﬁeld. The induced voltage willtake its greatest value when moving at right angleto the magnetic ﬁeld lines and its least value(i.e. zero!) when moving along the direction ofthe ﬁeld lines
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cutting at rightangles through the lines of magnetic $u% seeigure '.'/ results in a generated e.m.f. andthe magnitude of which will be given by:
E
;
Blv
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(f the 0eld is cut at an angle,
θ
rather thanat right angles/, the generated e.m.f. will begiven by:
E
;
Blv
sin
θ
where
θ
is the angle between the direction of motion of the conductor and the 0eld lines.
AC GENERATOR
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In a simple AC generator a loop of wire rotatesinside the magnetic ﬁeld produced by two oppositemagnetic poles. Contact is made to the loopas it rotates by means of slip rings and brushes.DC GENERATOR
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A simpleDCgenerator uses an arrangement similarto that used for an AC generator but with theslip rings and brushes replaced by a commutatorthat reverses the current produced by thegenerator every 180
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The denser the 0eldthe greater the output voltage will be)* +8T87A simple )* motor consists of a very similararrangement to that of the )* generator that wemet earlier. A loop of wire that is free to rotateis placed inside a permanent magnetic 0eld seeigure '.&3/. hen a )* current is applied tothe loop of wire, two equal and opposite forcesare set up which act on the conductor in thedirections indicated in igure '.&3. The direction of the forces acting on each armof the conductor can be established by againusing the right-hand grip rule and leming<sleft-hand rule. 6ow because the conductors areequidistant from their pivot point and the forcesacting on them are
equal and opposite
, theyform a
couple
. The
moment
of this couple isequal to the magnitude of a single force multipliedby the distance between them and this
Figure 5.124
Torque on a current-carrying loopsuspended within a magnetic ﬁeld.
moment is =nown as
torque
,
T
. 6ow,
T
;
Fd
where
T
is the torque in 6ewton-metres, 6m/,
F
is the force in 6/ and
d
is the distance in m/.e already =now that the magnitude of theforce
F
is given by
F
;
BIl
> therefore, the torqueproduced by the current carrying thus the torquee%pression can be written:
T
;
BIld
where
T
is the torque in 6m/,
B
is the $u%density in T/,
I
is the current in A/,
l
is thelength of conductor in m/ and
d
is the distancein m/.(n a practical situation the conductor wouldbe wound to form a coil. (f the coil has
N
turnsand each loop of the coil has a length,
l
, then
the torque produced will be given by:
T
;
BlINd
?ou can more easily remember this as@B(6)C4/
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(n real motors, this rotating coil is =nown asthe
armature
and consists of many hundreds of turns of conducting wire. This arrangement isneeded in order to ma%imi2e the force imposedon the conductor by introducing the
longest possible
conductor into the magnetic 0eld.
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6ow returning to the simple motor illustratedin igure '.&', we =now that when currentis supplied to the armature
rotor
/ a torque isproduced. (n order to produce continuousrotary motion, this torque turning moment/must always act in the same direction. Therefore, the current in each of the armatureconductors must be reversed as the conductorpasses between the north and south magnetic0eld poles. The
commutator
acts li=e a rotatingswitch, reversing the current in each armatureconductor at the appropriate time to achieve thiscontinuous rotary motion. ithout the presenceof a commutator in a)*motor, only a half-turnof movement is possible
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The torque produced by a DC motor is directlyproportional to the product of the current ﬂowingin the rotating armature winding.
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In order to avoid the need for a large permanentmagnet, a separate ﬁeld winding can be used ina DC machine (i.e. a motor or generator). Thisﬁeld winding is energized with DC. In the case ofa DC generator, this current can be derived fromthe output of the generator (in which case it isreferred to as
self-excited
) or it can be energizedfrom a separate DC supply
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8ne other application of the motor principleis used in simple analogue measuring instruments.Dome meters, including multimeters usedto measure current, voltage and resistance, operateon the principle of a coil rotating in amagnetic 0eldDE7(ED +8T87(n the series-wound )* motor the 0eld windingis connected in series with the armature andthe full armature current $ows through the 0eldwinding see igure '.1!/. This arrangementresults in a )* motor that produces a large
starting torque at slow speeds. This type of motor is ideal for applications where a heavyload is applied from rest. The disadvantage of this type of motor is that on light loads themotor speed may become e%cessively high. orthis reason this type of motor should not beused in situations where the load may be accidentallyremovedD#6T 86) +8T87(n the shunt-wound )* motor the 0eld windingis connected in parallel with the armatureand thus the supply current is dividedbetween the armature and the 0eld winding This arrangement results in a )*motor that runs at a reasonably constant speedover a wide variation of load but does not performwell when heavily loadedDE7(ED D#6T *8+F86) +8T87 The compound-wound )* motor has bothseries and shunt 0eld windings see igure'.13/ and is therefore able to combine some of the advantages of each type of motor
Starter-generator
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Dtarter-generators eliminate the need for separatestarter motors and )* generators. Theyusually have separate 0eld windings one for thestarter motor and one for the generator/ togetherwith a common armature winding.
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hen usedfor starting, the starter-generator is connectedas a series-wound )* motor capable of producinga very high starting torque.
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whenused as a generator the connections are changedso that the unit operates as shunt-wound generatorproducing reasonably constant current overa wide range of speed.
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(n the start condition, the low-resistancestarter 0eld and common armature windingsof the starter-generator are connected in seriesacross the)*supply via a set of contactors. Thisarrangement ensures that a torque is producedthat is suGcient to start an aircraft<s turbineengine.
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hen the engine reaches self-sustaining speed,the current is bro=en through the 0rst set of contactors and a second set of contactors operate,removing the e%ternal )* power supplyfrom the starter-generator and reconnecting thearrangement so that the armature voltage generatedis fed to the higher-resistance shunt 0eldand the aircraft<s main voltage regulator.

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