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FAQ:
Why star delta starter is preferred with induction motor?
Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage
drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which
reduces voltage 3 times, that is why current also reduces up to 3 times and
hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor
winding
State the difference between generator and alternator
Generator and alternator are two devices, which converts mechanical energy
into electrical energy. Both have the same principle of electromagnetic
induction, the only difference is that their construction. Generator persists
stationary magnetic field and rotating conductor which rolls on the armature
with slip rings and brushes riding against each other, hence it converts the
induced emf into dc current for external load whereas an alternator has a
stationary armature and rotating magnetic field for high voltages but for low
voltage output rotating armature and stationary magnetic field is used.
Why AC systems are preferred over DC systems?
Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission
and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much
lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead
of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system,
as the sine wave current will naturally tend to zero at some point making the
current easier to interrupt.
Why back emf used for a dc motor? highlight its significance.
The induced emf developed when the rotating conductors of the armature
between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the
current flowing through the conductor, when the armature rotates, is called back
emf. Its value depends upon the speed of rotation of the armature conductors. In
starting, the value of back emf is zero.
Explain the application of storage batteries.
Storage batteries are used for various purposes, some of the applications are
mentioned below:
• For the operation of protective devices and for emergency lighting at
generating stations and substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad
casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity
supplies are not possible
Why do the bulbs in my exterior fixture burn out so often?
This is usually caused by several factors.
1. Use of non-brand named bulbs.
2. Larger wattage bulbs, which cause excessive heat build-up shorting the life of
the bulb.
3. Power Surges.
Can I dim fluorescent lights?
Yes. Dimming fluorescent requires not only a special dimmer, but also special
fixtures. You cannot place a typical incandescent dimmer on existing fluorescent.
BASIC DATA ABOUT ATOM :
Mass of electron = 9.11 x 10-31 kg
Mass of proton = 1.67 x 10-27 kg
Mass of neutron = mass of proton
Mass of electron = 1/1840 mass of proton
Diameter of nucleus is of the order of 10-14 m
Diameter of orbits = 104 times dia of molecule
Diameter of electron = 10-15 m
Charge on electron = - 1.602 x 10-19 coulomb
Charge on proton = + 1.602 x 10-19 coulomb.
UNIT OF CURRENT :
The charge on an electron is measured in terms of coulomb. The unit of current
is coulomb per second and is called ampere.Thus
I (Ampere) = coulomb/second = ∆ q / ∆ t
One coulomb is equivalent to the charge of 6.28 x 1018 electrons.
1 emu of current = 3 x 1010 esu of current.
ELECTROMOTIVE FORCE :
Electromotive force or potential of a body is the work done in joules to bring a
unit electric charge from infinity to the body. It is expressed in terms of volts (V).
The potential difference is defined as that which causes current to flow in the
closed circuit.
RESISTANCE :
Resistance is the property of a substance due to which it opposes the flow of
electrons (i.e., electric current) through it. The unit of resistance is ohm (Ω).
Metals, acids and salt solutions are good conductors of electricity. Silver, copper
and aluminium offer least resistance to flow of current and are called very good
conductor of electricity. The electrons while flowing through the molecules or
the atoms of the conductor, collide with other atoms and electrons, thereby
producing heat.
Some substances offer relatively greater difficulty or hindrance to the passage of
these electrons. Such substances are called poor conductors or insulators of
electricity. Some of the insulators are glass, bakelite, mica, rubber, polyvinyl
chloride (P.V.C.), dry wood, etc.
The resistance of a conductor depends on:
1. Length of conductor- it varies directly with the length
2. Cross-sectional area of the conductor - it varies inversely with the crosssectional area
3. Its resistivity i.e. the nature of composition, etc., of the material of which the
conductor is made up
4. Temperature of the conductor - it almost varies directly with the temperature.
Thus R, the resistance of a conductor is given by
R=ρl/A
where
ρ = specific resistance or resistivity of the material,
l = length of the conductors,
A = cross-sectional area of conductor.
Ohm's Law :
If the temperature and other conditions remain constant, the current through a
conductor is proportional to the applied potential difference and it remains
constant. Thus
Current = Applied Voltage / Resistance of the circuit
Resistance = Applied voltage / Current in the circuit
Potential across resistance = Current x Resistance.
Conditions for Ohm's Law :
1. Ohm's law can be applied either to the entire circuit or a part of a circuit.
2. When ohm's law is applied to a part circuit, part resistance and the potential
across the part resistance should be used.
3. The Ohm's law can be applied to DC as well as AC circuits. However, in case of
AC circuits impedance Z, is used in place of resistance. Thus
I = E / Z = Applied voltage / Impedance in the circuit
Conductance (G ) :
Conductance is the reciprocal of ( R ) and is measure of the ease with which the
current will flow through a substance. Thus
G= 1 / R
The unit of conductance is mho.
ELECTRICAL POWER :
Electrical power is expressed in terms of watts (W) and is given by
W= E x I = I2 x R = E2 / R
Power is also expressed in terms of
kW ( kilowatt ) ( =1000 W ) or
MW ( megawatt ) which is 1000 kW or 1000,000 W.
Electrical Energy :
Electrical energy is expressed in terms of kilowatt hours (kWh). Thus
1 kWh = 1 kW x 1 hour = 1000 watt-hours = 1000 x 60 x 60 watt-sec.
RESISTANCE COMBINATIONS :
Resistances in series :
When resistances are connected in series, same current flows through all
resistances, and overall resistance R, is given by
R = R1 + R2 + R3
Also,
V = V1 + V2 + V3 = IR1+ IR2 + IR3 .