Unit:2 Inductors & Transformers:
Inductors: - Definition, symbol, Inductance, Inductive reactance (XL ), Energy stored in an inductor QFactor. Types of Inductors: - Air core, Iron core and ferrite core inductors. Use of Inductors: - Filter chokes
AFC & RFC and Variable inductor.
Transformers: - Principle and construction of transformer, Specification of transformer. Types of
Transformer: - Step-up, step-down transformer Relays: - Principle, construction and working of electromagnetic
relays Types of Switches: (Expiation using Symbols)
INDUCTOR
Inductor: - It is an electromagnetic component consisting of a conducting wire wound on a core or former
in cylindrical or spiral form to provide a specific amount of inductance in the circuit.
Inductance:- It is the property of a coil, which opposes charges in current by means of energy stored in the
form of magnetic field. ( A current carrying conductor or a coil has magnetic field is associated with it. The
strength of the magnetic field is directly proportional to the amount of the current in the coil. The change in flux
induces a voltage or emf in the coil. The polarity of induced voltage is such that it opposes the change in current
through the coil. The magnitude of this voltage is directly proportional to the rate of change current i.e v α
dI/dt= L dI/dt. L is constant of proportionality known as inductance of the coil.)
It is measured in Henry (H). A coil has an inductance of one henry if the emf induced in it is one volt when
the current is changing at the rate of one ampere per second.
The inductance of a coil is given by,
Where µo & µr are the absolute and relative permeability of the core material, A crosssectional area of the core, N is the number of turns of the coil and L is the length of the core.
Inductive Reactance (XL):- The opposition presented by a inductor to the flow of complex current (A.C.
Current) through it is called inductive reactance.
The opposition produced by resistor to the flow of electric current is termed as resistance. Resistance
dissipates energy in the form of heat while, reactance diminishes current by setting up an counter e.m.f. and
therefore controls the source of A.C. e.m.f. without wastage of electrical energy.
The inductive reactance is given by,
XL = WL = 2 πƒL Ώ
If f =0, then XL =0, i.e. Inductor offers no reactance to DC current.
Energy stored in Inductor. Capacity for doing work is called Energy. According to Faraday’s law, change in
current through inductor produces back e.m.f. e, i.e. e = L di/dt.
Therefore work done per second = e i = L.i.di/dt
Therefore total work done when current increases from zero to its maximum value I is,
W = L.i.di/dt . dt =LI2 /2 =Energy stored in Joules.
Uses of inductors :- 1) In tuning circuits 2) In filter circuits 3) In Transformers
Quality factor of an Inductor: - Ability of a inductor to store the energy as compared to the dissipation of
energy in it is called Q factor or quality factor or figure of merit.
Where R is the dc resistance of the coil.
Types of Inductors :1) Air core 2) Iron core 3) Ferrite core & 4) Choking Inductors
1)Air core Inductor:- This inductor is made of coils of wire wound on a frame made of simple cardboard.
Fig shows a symbol & shape of an air core inductor. The air core inductor has a
very low value of inductance. They are suitable for radio frequency (RF)
applications.
2) Iron core Inductors :- Iron is a ferromagnetic material. When the space inside the former (plastic) of
coil is filled with solid iron or laminated iron core, the
magnetic flux lines produced by a.c. flowing in a coil find a
much easier path through the iron and do not tend to leak
the outside of this coil. Instead the lines of flux produced by
a.c. complete their path through the iron. In addition when
the current flows in a coil, the iron becomes magnetized and
adds its own flux lines to those produced by a.c. Hence total
flux linked with coil increases.
Hence the inductance increases. To avoid eddy current losses, the iron is laminated in the form I & E as
shown in fig. above. They are insulated from each by varnish.
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The iron – cored inductors are also commonly known as choke.
Use:- (1) Used as A.F. and R.F. choke (2) Used as Filter chokes
3) Ferrite Core Inductors: - This inductor is made of coils of wire wound on a ferrite
core. A ferrite is a magnetic material consisting of iron, cobalt or nickel embedded in an
insulator binder. The ferrite core has a very low eddy current loss. Therefore ferrite core
inductors are used at high frequencies.
Variable inductor: These inductors are similar to the fixed ferrite core inductors. But the core is
adjustable. The variable inductors are used in tuning & filter circuits. Fig shows a symbol
for a variable core inductor. In case of large inductance coils, a variable inductor is made
of a long coil wound on a ferrite core provided with a slider contact. The slider contact
can be used to vary the inductance of a coil.
Chokes: In many applications, the inductors are used to minimize ac, while
permitting flow of direct current. In such applications, the inductor is called a choke. The choke is an inductance
in series with an external resistance toprevent to ac signal voltage from developing any appreciable output
across resistance at the frequency of the source.
Audio frequent choke: Since audio frequencies are not much higher than the ripple frequency of the rectifier
supply, the lower inductance filter choke can be used as AFC. AFCs can be used when it is desired to prevent
AF current from flowing one part of the circuit to another.
Radio frequency choke: These are used to block RF current flowing from one part of the circuit to another.
As the frequency increases into radio frequency range, the inductance needed for choking or blocking purpose
become smaller.
Transformer:
Transformer is a device which converts alternating current at high voltage into low voltage and vice versa.
The transformer works on the principal of mutual inductance i.e changing current in one coil can induces an
e.m.f. in the second coil. Transformers can be used
i. to step-up or step-down the a.c voltage.
ii. to transform the load resistance to effect matching for maximum power transfer
iii. for isolation purposes between two different circuits.
iv. Auto transformers are used for variable voltage Operation
Principle of operation: When two coils are placed closed to each other, the
expanding & collapsing magnetic flux of one coil links with the other & induced emf
is produced in the other coil. Then these two coils are said to have mutual inductance
M.
In fig (a) the coil L1 is connected to a generator that produces varying ac current
in the turns. The winding L2 is not connected to L1 but the turns are linked by
magnetic field. A varying current in L1 therefore induces voltage across L2. When
the induced voltage produces current in L2, its varying magnetic field induces
voltage in L1. The two coils L1 & L2 have mutual inductance because current in one
induce voltage in the other.
Construction: Consider the core type transformer shown in fig(b). It
consists of two highly inductive coils which are electrically separate but
magnetically linked through an iron core of low reluctance. The two coils
posses’ high mutual inductance.
If one coil is connected to an ac source, an alternating flux sets up in
the laminated core, most of which is linked with the other coil. Hence
mutually induced voltage is produced in the second coil. If the second coil
is closed, a current flow in it & electrical energy is transferred from the
first coil to the second coil. The first coil in which electric energy is fed is
called primary winding & the other from which energy is drawn out is
called secondary windings.
Turn Ratio :- The ratio of the number of turns in the secondary ( Ns ) to the number of turns in the primary
( Np ) is called the turn radio of the transformer.
Ns
Turns ratio = -------Np
Voltage ratio:- The ratio of voltage across secondary[Vs] to the voltage across primary [Vp] is called as
voltage ratio.
Vs
Ns
Voltage ratio = ------- = -------
2
Ns
Np
Step-up Transformer:- If Ns > Np, the secondary voltage is higher than primary voltage .These type of
transformer is called Step- up Transformer. Thus for step up Transformer
T.R > 1
Step-down Transformer: - If Ns < Np, the secondary Voltage is less than the primary voltage. Such
transformer is called Step – down Transformer. Thus for step-down transformer, TR < 1.
Current Ratio :- With zero losses, assumed for the transformer, The power in the secondary always equals
the power in the primary. i.e. Power in the secondary = Power in the primary. i.e.
Ip
Vs
Ns
Current ratio = ----- = ----- = ------Is
Vp
Np
The current ratio is the inverse of the voltage ratio i.e. Voltage step-up in the secondary means current stepdown, and vice versa.
Transformer Efficiency:-Efficiency is defined as the ratio of power out to the power in.
i.e. efficiency = Pout / Pin
For ideal transformer Efficiency is = 1 & For practical transformer Efficiency is < 1
Specifications of transformer:i.
Power ratings KVA or VA
ii.
Rated value of voltage of primary & secondary
iii.
Rated frequency
iv.
Tapping if require for tapped transformer
v.
Temp rise
vi.
Load loss at rated current at 3480K
vii.
Efficiency
viii.
Regulation at full load at unity factor & 0.8 power factor lagging.
Relays: A relay is a device, which functions as an electrically operated switch. In response to an electrical
signal, known as control signal, the relay opens or closes its switch contacts in some prearranged & fixed
combination. The contacts may be in the same circuit as the operating signal or in another circuit or a
combination circuit. The relays are widely used in industry as control device. Main applications include control
of high power load circuits, low voltage control of remote equipments & isolation of control circuits from load
circuits.
Classification: electromagnetic relay, crystal can relay, dry reed relay, mercury wetted relay, solid state
relay & time delay relay.
Electromagnetic relay: An electromagnetic relay consists of an iron core & its surroundings coil of wire,
iron yoke, armature & a set of contacts. Arrangement is as shown in fig.
The iron yoke provides a low reluctance path for the magnetic flux. It is
shaped in such a way that a magnetic circuit can be closed by the armature & set
of contacts. The armature is a movable piece of iron. It is hinged to the yoke &
held by a spring in such a way that there is an air gap in the magnetic circuit.
There are three contacts in an electromagnetic relay namely normally closed
(NC)or break contact, normally open (NO) or make contact & a movable contact.
The NC & NO contacts are placed on a mounting bracket, while the movable
contact is placed on an armature.
When a voltage is applied at the coil input of relay, it magnetizes the yoke.
This causes the armature to move towards the yoke. As a result of this, the
armature opens the NC contact & closes the NO contact. When the voltage is
removed from the coil input, a spring returns the armature to its original position.
This operation takes place in few milliseconds.
The electromagnetic relays are available separately for dc & ac operations. The dc electromagnetic relays
are faster than ac relays.
Switch: An electrical switch is a mechanical device usually used to open(disconnect) or to close (connect)
an electrical circuit. It is used in series with the load & source to switch the power ON & OFF to the load. When
switch is in its ON position, the current flows through it, because the resistance of the switch contacts is very
small. However, when it is in its OFF position, the current does not flow through it because of the high
resistance between the switch contacts.
Types: There are number of types of switches. The switches may be classified according to the mode of
operation, construction or function such as manual switches, precision switches, etc. We consider manual
switches.
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1) Single Pole Single Throw (SPST) switch: This switch has only one lever or arm for making contact
known as pole as shown in fig. It can connect or disconnect only a single circuit. Its
current rating is from 0.5A to 6A and voltage from 6V to 30V. It is primarily used as
an ON-OFF switch in small electrical appliances. e.g. a switch on a electric switch
board.
2) Single Pole Double Throw (SPDT) switch: This type of switch has two ON
positions 1 & 2 & one OFF position as shown in fig. Slide type SPDT available in
1A, 4A for a voltage range of 6V and 30V. it has only one lever that can be moved
into either of the two ON positions. This switch can connect either of the two circuits.
3) Double Pole Single Throw (DPST) switch: This switch has only one ON
position. But it makes two contacts (poles) simultaneously as shown in fig. This
switch can be used as a SPST switch.
4) Double Pole Single Throw (DPST) switch: The switch has two ON positions
& makes two contacts simultaneously as shown in fig. This switch can be used as two
DPST switches together.
5) Toggle switch: This switch is similar to a SPST witch. It has been
specifically designed with very low contact resistance for ON OFF applications in
electronic circuits.
6) Dual in line package(DIP) switch: This switch contains a number of SPST
switches as shown in fig. It is a miniature switch which is designed for installation on
PCB’s. The DIP switch is used for indidual circuit switching or for encoading a
binary code in computer circuitary.
7) Rotary switch: This switch is used to direct small signal currents from one
circuit to another by changing the connections, when the switch knob is turned. Used
in TV, radio, multi-meter etc.
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