electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,
Speed Control of Series Motors
1.
Flux Control Method:
:
Variations in the flux of a series motor can be brought about in any one of the
following ways:
(a) Field
Diverters:
The series winding are shunted by a variable resistance known as
field diverter. Any desired amount of current can be passed through the
diverter by adjusting its resistance. Hence the flux can be decreased and
consequently, the speed of the motor increased.
(b) Armature Diverter:
A diverter across the
armature can be used for giving speeds lower than the normal speed. For a given
constant load torque, if Ia is reduced due to armature diverter, the must
increase (∵TaIa ) This results in an increase in current taken from the supply(which
increases the flux and a fall in speed (N I/ )). The variation in speed can
be controlled by varying the diverter resistance.
(c) Trapped Field Control Field: This method is often
used in electric traction. The number of series filed turns in the circuit can
be changed. With full field, the motor runs at its minimum speed which can be
raised in steps by cutting out some of the series turns.
(d) Paralleling
Field coils: this method used for fan motors, several speeds can be obtained by
regrouping the field coils. It is seen that for a4-pole motor, three speeds can
be obtained easily.
Variable Resistance in Series with Motor
By increasing the resistance in series with the armature
the voltage applied across the armature terminals can be decreased. With
reduced voltage across the armature, the speed is reduced. However, it will be
noted that since full motor current passes through this resistance, there is a
considerable loss of power in it.
electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,
To explain the theory and the underlying principle behind
the functioning of an LED
The first known report of a light-emitting solid-state
diode was made in 1907 by
the British experimenter H. J. Round. In the mid 1920s,
Russian Oleg Vladimirovich Losev independently created the first LED, although
his research was ignored at that time.
• In 1955, Rubin Braunstein of the Radio Corporation of
America reported on infrared emission from gallium arsenide (GaAs) and other
semiconductor alloys.
• Experimenters at Texas Instruments, Bob Biard and Gary
Pittman, found in 1961 that gallium arsenide gave off infrared radiation when
electric current was applied. Biard & Pittman received the patent for the
infrared light-emitting diode.
• In 1962, Nick Holonyak Jr., of the General Electric
Company and later with the University of Illinois at Urbana-Champaign,
developed the first practical visible spectrum LED. He is seen as the
"father of the light-emitting diode".
• In 1972, M. George Craford, Holonyak's former graduate
student, invented the first yellow LED and 10x brighter red and red-orange
LEDs.
• Shuji Nakamura
of Nichia Corporation of Japan demonstrated the first high brightness blue LED
based on In GaN. The 2006 Millennium Technology Prize was awarded to Nakamura
for his invention.
THEORY
A Light emitting diode (LED) is essentially a pn junction
diode. When carriers are injected across a forward-biased junction, it emits
incoherent light. Most of the commercial LEDs are realized using a highly doped
n and a p Junction.
To understand the principle, let’s consider an unbiased
pn+ junction band . The depletion region
extends mainly into the p-side. There is a potential barrier from Ec on the
n-side to the Ec on the p-side, called the built-in voltage, V0. This potential
barrier prevents the excess free electrons on the n+ side from diffusing into
the p side.
When a Voltage V is applied across the junction, the
built-in potential is reduced from V0 to V0 – V. This allows the electrons from
the n+ side to get injected into the p-side. Since electrons are the minority
carriers in the p-side, this process is called minority carrier injection. But
the hole injection from the p side to n+ side is very less and so the current is
primarily due to the flow of electrons into the p-side. Appendix 1) results in
spontaneous emission of photons (light). This effect is called injection electroluminescence.
These photons should be allowed to escape from the device without being
reabsorbed.
The recombination can be classified into the following
two kinds
• Direct recombination
• Indirect recombination
Direct Recombination:
In direct band gap materials, the minimum energy of the
conduction band lies directly above the maximum energy of the valence band in
momentum space energy
. In this material, free electrons at the bottom of the
conduction band can recombine directly with free holes at the top of the
valence band, as the momentum of the two particles is the same. This transition
from conduction band to valence band involves photon emission (takes care of the
principle of energy conservation). This is known as direct recombination.
Direct recombination occurs spontaneously. GaAs is an example of a direct
band-gap material.
Indirect
Recombination
In the indirect band gap materials, the minimum energy in
the conduction band is shifted by a k-vector relative to the valence band. The
k-vector difference represents a difference in momentum. Due to this difference
in momentum, the probability of direct electronhole recombination is less. In
these materials, additional dopants(impurities) are added which form very
shallow donor states. These donor states capture the free electrons locally;
provides the necessary momentum shift for recombination. These donor states
serve as the recombination centers. This is called Indirect (non-radiative)
Recombination.
E-k plot of an indirect band gap material and an example of
how Nitrogen serves as a recombination center in GaAsP. In this case it creates
a donor state, when SiC is doped with Al, it recombination takes place through
an acceptor level. The indirect recombination should satisfy both conservation
energy, and momentum. Thus besides a photon emission, phononemission or
absorption has to take place. GaP is an example of an indirect band-gap
material.
The wavelength of the light emitted, and hence the color,
depends on the band gap energy of the materials forming the p-n junction. The
emitted photon energy is approximately equal to the band gap energy of the semiconductor.
The following equation relates the wavelength and the energy band gap.
LED Materials
An important class of commercial LEDs that cover the visible
spectrum are the III-V(see Appendix 5). ternary alloys based on alloying GaAs
and GaP which are denoted by GaAs1- yPy. InGaAlP is an example of a quarternary (four
element) III-V alloy with a direct band gap. The LEDs realized using two
differently doped semiconductors that are the same material is called a homo junction.
When they are realized using different band gap materials they are called a
hetero structure device hetero structure LED is brighter than a homo Junction
LED.
LED
Structure
The LED structure plays a crucial role in emitting light
from the LED surface. The LEDs are structured to ensure most of the
recombinations takes place on the surface by the following two ways.
• By increasing the doping concentration of the
substrate, so that additional free minority charge carriers electrons move to
the top, recombine and emit light at the surface.
• By increasing the diffusion length L = √ Dτ, where D is
the diffusion coefficient and τ is the carrier life time. But when increased
beyond a critical length there is a chance of re-absorption of the photons into
the device. The LED has to be structured so that the photons generated from the
device are emitted without being reabsorbed. One solution is to make the p
layer on the top thin, enough to create a depletion layer. Following picture
shows the layered structure.
There aredifferent ways to structure the dome for efficient emitting LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,
A vacuum
circuit breaker is such kind of circuit breaker where the arc quenching takes
place in vacuum. The technology is suitable for mainly medium voltage
application. For higher voltage vacuum technology has been developed but not
commercially viable. The operation of opening and closing of current carrying
contacts and associated arc interruption take place in a vacuum chamber in the
breaker which is called vacuum interrupter. The vacuum interrupter consists of
a steel arc chamber in the centre symmetrically arranged ceramic insulators.
The vacuum pressure inside a vacuum interrupter is normally maintained at 10 -
6 bar.
The
material used for current carrying contacts plays an important role in the
performance of the vacuum circuit breaker. CuCr is the most ideal material to
make VCB contacts. Vacuum interrupter technology was first introduced in the
year of 1960. But still it is a developing technology. As time goes on, the
size of the vacuum interrupter is being reducing from its early 1960’s size due
to different technical developments in this field of engineering. The contact
geometry is also improving with time, from butt contact of early days it
gradually changes to spiral shape, cup shape and axial magnetic field contact.
The vacuum circuit breaker is today recognized as most reliable current
interruption technology for medium voltage switchgear. It requires minimum
maintenance compared to other circuit breaker technologies.
Advantages
of Vacuum Circuit Breaker or VCB
Service life of
vacuum circuit breaker is much longer than other types of circuit breakers.
There is no chance of fire hazard as oil circuit breaker. It is much
environment friendly than SF6 Circuit breaker. Beside of that contraction of
VCB is much user friendly. Replacement of vacuum interrupter (VI) is much
convenient
Operation
of Vacuum Circuit Breaker
The
main aim of any circuit breaker is to quench arc during current zero crossing,
by establishing high dielectric strength in between the contacts so that
reestablishment of arc after current zero becomes impossible. The dielectric
strength of vacuum is eight times greater than that of air and four times
greater than that of SF6 gas. This high dielectric strength makes it possible
to quench a vacuum arc within very small contact gap. For short contact gap,
low contact mass and no compression of medium the drive energy required in
vacuum circuit breaker is minimum. When two face to face contact areas are just
being separated to each other, they do not be separated instantly, contact area
on the contact face is being reduced and ultimately comes to a point and then
they are finally de-touched. Although this happens in a fraction of micro
second but it is the fact. At this instant of de-touching of contacts in a
vacuum, the current through the contacts concentrated on that last contact
point on the contact surface and makes a hot spot. As it is vacuum, the metal
on the contact surface is easily vaporized due to that hot spot and create a
conducting media for arc path. Then the arc will be initiated and continued
until the next current zero.
electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,
MCB
Nowadays we use more commonly miniature circuit breaker
or MCB in low voltage electrical network instead of fuse.
The MCB has some advantages compared to fuse.
1.It
automatically switches off the electrical circuit during abnormal condition of
the network means in over load condition as well as faulty condition. The fuse
does not sense but miniature circuit breaker does it in more reliable way. MCB
is much more sensitive to over current than fuse.
2.Another
advantage is, as the switch operating knob comes at its off position during
tripping, the faulty zone of the electrical circuit can easily be identified.
But in case of fuse, fuse wire should be checked by opening fuse grip or cutout
from fuse base, for confirming the blow of fuse wire.
3.Quick
restoration of supply can not be possible in case of fuse as because fuses have
to be rewirable or replaced for restoring the supply. But in the case of MCB,
quick restoration is possible by just switching on operation.
4.Handling
MCB is more electrically safe than fuse.
Because
of to many advantages of MCB over fuse units, in modern low voltage electrical
network, miniature circuit breaker is mostly used instead of backdated fuse
unit.Only
one disadvantage of MCB over fuse is that this system is more costlier than
fuse unit system.
Working Principle Miniature Circuit Breaker
There are two arrangement of operation of
miniature circuit breaker. One due to thermal effect of over current and other
due to electromagnetic effect of over current. The thermal operation of
miniature circuit breaker is achieved with a bimetallic strip whenever
continuous over current flows through MCB, the bimetallic strip is heated and
deflects by bending. This deflection of bimetallic strip releases mechanical
latch. As this mechanical latch is attached with operating mechanism, it causes
to open the miniature circuit breaker contacts. But during short circuit
condition, sudden rising of current, causes electromechanical displacement of
plunger associated with tripping coil or solenoid of MCB. The plunger strikes
the trip lever causing immediate release of latchmechanism consequently open the circuit breaker
contacts. This was a simple explanation of miniature circuit breaker working
principle.
Miniature
Circuit Breaker Construction
The trip unit is the main part, responsible for
proper working of miniature circuit breaker. Two main types of trip mechanism
are provided in MCB. A bimetal provides protectionagainst over load current and an electromagnet
provides protection against short-circuit current.
Operation of Miniature Circuit Breaker
There are three mechanisms provided in a single miniature
circuit breaker to make it switched off. If we carefully observe the picture
beside, we will find there are mainly one bi - metallic strip, one trip coil
and one hand operated on - off lever. Electric current carrying path of a
miniature circuit breaker shown in the picture is like follows. First left hand
side power terminal - then bimetallic strip - then current coil or trip coil -
then moving contact - then fixed contact and - lastly right had side power
terminal. All are arranged in series.
If circuit is overloaded for long time, the bi - metallic strip becomes over heated and deformed. This deformation of bi metallic strip causes, displacement of latch point. The moving contact of the MCB is so arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB. The current coil or trip coil is placed such a manner, that during short circuit fault the mmf of that coil causes its plunger to hit the same latch point and make the latch to be displaced. Hence the MCB will open in same manner. Again when operating lever of the miniature circuit breaker is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in same manner. So, whatever may be the operating mechanism, that means, may be due to deformation of bi - metallic strip, due to increased mmf of trip coil or may due to manual operation, actually the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact. When the the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters into arc splitters and is finally quenched. When we switch on an MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.
