If one lamp connects between
two phases it will glow or not?
If the voltage between the two phase is
equal to the lamp voltage then the lamp will glow. When
the voltage difference is big it will damage the lamp and when
the difference is smaller the lamp will glow depending on the type of
lamp.
How do you select a cable
size (Cu & Al) for a particular load?
At first calculate the electrical current of the
load, after that derate the electrical current considering derating
factor (depending on site condition and laying of cable) after choose the cable
size from cable catalog considering derating electrical current.After that
measure the length of cable required from supply point of load to load point.
Calculate the voltage drop which will max 3% (resistance and
reactance of cable found from cable catalog of selecting cable)
if voltage drop>3% then choose next higher size of cable.
What are HRC fuses and where
it is used?
HRC stand for "high rupturing capacity"
fuse and it is used in distribution system for electrical transformers.
Which power plant has high load factor?
All base load power plants have a high load factor.
If we use high efficiency power plants to supply the base load,we can reduce
the cost of generation. Hydel power plants have a higher efficiency than thermal
& nuclear power plants.
Mention the methods for
starting an induction motor?
The different methods of starting an induction motor
DOL: direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter
What is
the difference between earth resistance and earth electrode
resistance?
Only one of the terminals is evident in the earth
resistance. In order to find the second terminal we should recourse to its
definition: Earth Resistance is the resistance existing between the
electrically accessible part of a buried electrode and another point of the
earth, which is far away.
The resistance of the electrode has the following components:
(A) the resistance of the metal and that of the connection to it.
(B) the contact resistance of the surrounding earth to the electrode.
What is use of lockout relay
in ht voltage?
A lock-out relay is generally placed in line before
or after the e-stop switch so the power can be shut off at one central
location. This relay is powered by the same electrical source as the control
power and is operated by a key lock switch. The relay itself may have up to 24 contact
points within the unit itself. This allows the control power for multiple
machines to be locked out by the turn of a single key switch.
What is the power factor of
an alternator at no load?
At no load Synchronous Impedance of the alternator
is responsible for creating angle difference. So it should be zero lagging
like inductor.
Why most of analog o/p
devices having o/p range 4 to 20 mA and not 0 to 20 mA ?
4-20 mA is a standard range used to indicate
measured values for any process. The reason that 4mA is chosen instead of 0 mA
is for fail safe operation .For example- a pressure instrument gives
output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale.
Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA.
So if range is 0-20 mA then we can differentiate whether it is due to broken
wire or due to 0 psi.
Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?
Since two bulbs are in series they will get equal
amount of electrical current but as the supply voltage is constant across
the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across
40W is more (V=IR) so 40W bulb will glow brighter.
What is meant by knee point voltage?
Knee point voltage is calculated for electrical
Current transformers and is very important factor to choose a CT. It is
the voltage at which a CT gets saturated.(CT-current transformer).
What is reverse power relay?
Reverse Power flow relay are used in generating
stations's protection. A generating stations is supposed to fed power to the
grid and in case generating units are off,there is no generation in the plant
then plant may take power from grid. To stop the flow of power from grid to
generator we use reverse power relay.
What will happen if DC supply
is given on the primary of a transformer?
Mainly transformer has high inductance and low
resistance.In case of DC supply there is no inductance ,only resistance will
act in the electrical circuit. So high electrical current will flow
through primary side of the transformer.So for this reason coil and insulation
will burn out.
What is the difference
between isolators and electrical circuit breakers?
Isolators are mainly for switching purpose under
normal conditions but they cannot operate in fault conditions .Actually they
used for isolating the CBs for maintenance. Whereas CB
gets activated under fault conditions according to the fault
detected.Bus bar is nothing but a junction where the power is getting
distributed for independent loads.
What are the advantage of
free wheeling diode in a Full Wave rectifier?
It reduces the harmonics and it also reduces
sparking and arching across the mechanical switch so that it reduces the
voltage spike seen in a inductive load
What are Motor Generator Sets
and explain the different ways the motor generator set can be used ?
Motor Generator Sets are a combination of an
electrical generator and an engine mounted together to form a single piece of
equipment. Motor generator set is also referred to as a gen set, or more
commonly, a generator, The motor generator set can used in the following
different ways:
1.Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonically-related frequency
What is power quality meter ?
Power Quality meters are common in many industrial
environment. Small units are now available for home use as well. They give
operators the ability to monitor the both perturbations on the power supply, as
well as power used within a building, or by a single machine or appliance. In
some situations, equipment function and operation is monitored and
controlled from a remote location where communication is via modem, or
high-speed communication lines.So we can understand the importance of power
measurement through power quality meters.
What is the different between
digital phase converter and ordinary phase converter?
Digital phase converter are a recent development in
phase converter technology that utilizes proprietary software in a powerful
microprocessor to control solid state power switching components. This
microprocessor, called a digital signal processor (DSP), monitors the phase conversion
process, continually adjusting the input and output modules of the converter to
maintain perfectly balanced three-phase power under all load conditions.
Explain
the operation of variable frequency transformer?
A variable frequency transformer is used
to transmit electricity between two asynchronous alternating current domains.
A variable frequency transformer is a doubly-fed electric machine
resembling a vertical shaft hydroelectric generator with a three-phase wound
rotor, connected by slip rings to one external ac power circuit. A
direct-current torque motor is mounted on the same shaft. Changing the
direction of torque applied to the shaft changes the direction of power flow;
with no applied torque, the shaft rotates due to the difference in
frequency between the networks connected to the rotor and
stator. The variable frequency transformer behaves as a continuously
adjustable phase-shifting transformer. It allows control of the power flow
between two networks .
What is the main use of
rotary phase converter ?
Rotary phase converter will be converting single
phase power into true balanced 3 phase power,so it is often called as single
phase to three phase converter .Often the advantages of 3 phase motors, and
other 3 phase equipment, make it worthwhile to convert single phase to 3 phase
so that small and large consumers need not want to pay for the extra cost of a
3 phase service but may still wish to use 3 phase equipment.
Which type of oil is used as
a transformer oil?
Transformer oil, or insulating oil, is usually a
highly-refined mineral oil that is stable at high temperatures and has
excellent electrical insulating properties. It is used in oil filled
transformers, some types of high voltage capacitors, fluorescent lamp ballasts,
and some types of high voltage switches and circuit breakers. Its
functions are to insulate, suppress corona and arcing, and to serve as a
coolant.
Well into the 1970s, poly chlorinated biphenyls (PCB)s were often used as a
dielectric fluid since they are not flammable. They are toxic, and under
incomplete combustion, can form highly toxic products such as furan. Starting
in the early 1970s, concerns about the toxicity of PCBs have led to their
banning in many countries.
Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used,
where the added expense of a fire-resistant liquid offsets additional building
cost for a transformer vault. Combustion-resistant vegetable oil-based
dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8)
esters are also becoming increasingly common as alternatives to naphthenic
mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and
have a lower volatility and higher flash points than mineral oil.
If we give 2334 A, 540V on
Primary side of 1.125 MVA step up transformer, then what will be the Secondary
Current, If Secondary Voltage=11 KV?
As we know the Voltage & current relation
for transformer-V1/V2 = I2/I1
We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So,I2 = 114.5 Amps
what are the points to be
consider for MCB (miniature circuit breaker selection?I(L)*1.25=I(MAX)
maximum current. MCB specification are done on maximum current flow in circuit.
what is the full form of KVAR?
We know there are three types of power in
Electrical as Active, apparent & reactive. So KVAR is stand for ``Kilo
Volt Amps with Reactive component.
What is excitation?
Excitation is applying an external voltage
to DC shunt coil in DC motors.
In three pin plug why earth pin diameter is higher than other two pin? what its
purpose ?
Because Current flow in the conductor is inversely
proportional to the conductor diameter. So if any short circuits occur in the
system first high currents bypassed in the Earthling terminal.( R=Pl/a area of
the conductor increases resistance value decreases)
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