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Q.1 What does 10V AC mean? Is it the RMS
voltage or Peak voltage or Average voltage?
Answer. In
general, AC voltages and currents are mentioned in RMS values only. It is
sensible to compare with steady DC voltages and currents.
For example 230V AC supply means, 230Vrms AC Supply.
For example 230V AC supply means, 230Vrms AC Supply.
Q.2 What does the Voltmeter in AC mode show? Is it
RMS value or peak value?
Answer.
Multimeter in AC mode shows RMS value of the voltage or current. Also when it
is DC mode it will show the RMS value only.
Q.3 Can fuses with an AC voltage rating be used in a DC
applications?
Answer: Fuses
must be rated for the voltage AC or DC in which they will be used. Generally,
fuses have a DC voltage rating that is half of the maximum AC voltage rating.
Q.4 Why in a three pin plug the earth pin is
thicker and longer than the other pins?
Answer: It
depends upon R=ρL/A where area (A) is inversely proportional to resistance (R),
so if area (A) increases, R decreases & if R is less the leakage
current will take low resistance path so the earth pin should be thicker. It is
longer because the The First to make the connection and last to disconnect
should be earth Pin. This assures Safety for the person who uses the electrical
instrument.
Q.5 Why humming sound occurred in HT transmission line?
Answer: This
sound is coming due to ionization (breakdown of air into charged particles) of
air around transmission conductor. This effect is called as Corona effect, and
it is considered as power loss.
Q.6 What the main difference between UPS & inverter?
Answer:
Uninterrupted power supply is mainly use for short time. Means according to ups
VA it gives backup. Ups is also two types: on line and offline. Online ups
having high volt and amp for long time backup with high dc voltage. But ups
start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc
and 120amp to 180amp battery with long time backup
Q.7 Why the capacitors work on ac only?
Answer:
Generally capacitor gives infinite resistance to dc components (i.e., block the
dc components). It allows the ac components to pass through.
Q.8 Why We use of Stones/Gravel in electrical
Switch Yard
Answer:
Reducing Step and Touch potentials during Short Circuit Faults Eliminates the
growth of weeds and small plants in the yard Improves yard working condition
Protects from fire which cause due to oil spillage from transformer and also
protects from wild habitat.
Q:What
are the operation carried out in Thermal power station?
A: The water is obtained in the boiler and the coal is burnt
so that steam is obtained this steam is allowed to hit the turbine, the turbine
which is coupled with the generator generates the electricity
Q: What is the diff. btwn. Electronic regulator and ordinary
rheostat regulator for fans?
A: The difference between
the electronic and ordinary regulator is the fact that in electronic reg. power
losses tend to be less because as we minimize the speed the electronic reg.
give the power necessary for that particular speed but in case of ordinary
rheostat type reg. the power wastage is same for every speed and no power is
saved. In electronic regulator triac is employed for speed control. by varying
the firing angle speed is controlled but in rheostatic control resistance is
decreased by steps to achievespeed control.
Q: What is 2 phase motor?
A: A two phase motor is often a motor with the the starting
winding and the running winding have a phase split. e. g; ac servo motor. where
the auxiliary winding and the control winding have a phase split of 90 degree.
Q:What does quality factor depend on in resonance?
A:Quality
factor q depends on frequency and bandwidth.
Q:What are the types of power in electrical power?
A: There are normally three types of power are counted in
electrical power. They are,
· Apparent
power
· Active power
· Reactive
power
Q:What are the advantages of VSCF wind electrical system?
A:Advantages of VSCF wind
electrical system are:
• No complex
pitch changing mechanism is needed.
• Aero turbine
always keeps going at maximum efficiency point.
• Extra energy
in the high wind speed region of the speed – duration curve can be extracted
• Significant
reduction in aerodynamic stresses, which are associated with constant – speed
operation.
Q:What is slip in an induction motor?
A:Slip can be defined as the distinction between the flux speed
(Ns) and the rotor speed (N). Speed of the rotor of an induction motor is
always less than its synchronous speed. It is usually expressed as a percentage
of synchronous speed (Ns) and represented by the symbol ‘S’.
Q:What
is inrush current?
A:Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.
A:Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.
Q:In a Tap changing transformer where is the tap connected, is
it connected in the primary side or secondary side?
A:Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.
Q:Why transformer ratings are in kva?
A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.
A:Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.
Q:Why transformer ratings are in kva?
A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.
Q: What is difference between fuse and breaker?
A: Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.
Q:What is the difference between delta-delta, delta-star transformer?
A:Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.
A: Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.
Q:What is the difference between delta-delta, delta-star transformer?
A:Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.
Q:
Capacitor is load free component but why ampere meter shows current when
capacitor bank breaker close?
A: As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.
A: As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.
Q:How to determine alternating current frequency?
A:Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are “n” pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.
A:Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are “n” pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.
Q:Why electricity in India is in the multiples of 11 like 11kv,
22kv, 33kv ?
A:Transformer Induced voltage equation contains 4.44 factor.
E=4.44*f*T*phi
E -Induced emf per phase
T -number of turns
f -frequency
phi -maximum flux per pole
A:Transformer Induced voltage equation contains 4.44 factor.
E=4.44*f*T*phi
E -Induced emf per phase
T -number of turns
f -frequency
phi -maximum flux per pole
From the
equation we see that E is proportional to 4.4 and it is in turn multiple of
11.So always transmission voltage is multiple of 11
ElecticalQuestion 1: What is
Electrical machine?
Electrical
machine is an electro mechanical device which converts input motive power (
prime mover like turbine in case of generator or electrical supply in case of
dc motor) to output motive power (Electrical power in case of the generator or
mechanical output in case of motor). This uses the principle of electro
magentic induction principle.
Electrical Question 2: What is the advantage of DC motors over
AC motors?
With the advert
of the ac power generation and transmission over long distances with less
losses using electrical transformers, dc machines were out ruled by the ac
machines in industrial and home applications. However dc motors still exist in
the industries because of some special properties of the dc machines where ac
drives have failed to attain.
DC motors have some special properties such as:
- High
Starting Torque
- Excellent
speed control capability
For traction
applications in electrical trains and cranes motor drives require to have high
starting torque. DC series motor has very good starting torque capabilities
compared to induction motors and synchronous motors. So still in present
traction applications DC series motors are used In some applications such as
paper mills and roll mills speed control of the motor drive should be accurate
else the process will be failure. In such applications of finer speed control
DC shunt machines are widely used.
Electrical Question 3: What is
the use of Commutator in DC machine?
Commutator is
the part in DC machine that makes the difference from the ac machine. In DC
machine the current flow in the armature is ac. This current is made uni
directional with the help of commutator. In ac machines slip rings are used to
collect the armature current from the armature and supply to the load. In DC
machines this slip rings are replaced by the split rings. Split rings consists
of 2 conductor materials separated by insulating material.
In first half cycle the induced current will be taken by the split rings or
commutator which is in one position.During the other half cycle when the
induced current in the armature changes the split rings position also changes
results in the delivering the unidirectional current to the external load.
Electrical Question 4: Explain essential parts of DC machine?
Essential parts
of the dc machine are:
- Field
System
- Armature
- Commutator
- Brushes
- Brearings
and Shaft
Electrical Question 5: Why field poles are laminated in DC
machine ?
Field poles of
a dc machine is laminated in order to reduce the eddy currents. Eddy currents
are the circulating currents that are induced in the iron by the alternating
magnetic field. These eddy currents produced will create a flux which opposes
the main flux. So this eddy current production is considered as loss in the dc
machine.Hence in order to reduce this eddy current losses thin laminations are
considered and insulating coating is provided in between the laminations.
Electrical Questions 6: Do commutator convert ac to dc or dc to ac ?
Commutator in a
dc machine converts both ac to dc current and dc to ac current. Consider the dc
generator the current produced in the armature due to the electro magnetic
induction is ac. However commutator converts the bi directional current to uni
directional current and supplies the current the external load circuit.
In DC motor the
voltage we apply at the terminals is dc. This dc current is converted to ac and
supply to the armature circuit so as to produce the rotational torque. Hence
Commutator converts both ac to dc variables and dc to ac variables.
Electrical Question 7: Why armature teeth of DC machine is
skewed ?
The air gap
length changes between the slot and teeth of the armature when the armature is
rotating under constant magnetic field. This results in the production of small
variation in the torque hence this results can be observed as small vibrations
in the frame of the machine. So in order to avoid this vibration effect armature
teeth is skewed.
Q. What are the different types of
transmission lines based on capacity?
A.
1. Short
transmission lines (up to 50 Km) (<20 KV)
2. Medium transmission lines (50-150 Km) (>20 KV & <100 KV)
3. Long transmission lines (>150 Km) (>100 KV)
2. Medium transmission lines (50-150 Km) (>20 KV & <100 KV)
3. Long transmission lines (>150 Km) (>100 KV)
Q. What is per unit value?
A. Per unit of
any quantity is defined as the ratio actual to base value expressed in decimal.
Q. What are the advantages of per unit
system?
A.
1. The pu impedances of machines of same type and widely different ratings lie
within a narrow range.
2. Per unit values referred to either side of the transformer remains same. Pu impedance of a 3 phase transformer is independent of type of winding
3. Computational efforts are reduced by a great extent.
4. The chance of confusion between line and phase quantities in a three phase balanced system is greatly reduced.
5. Manufactures usually provide impedance values in pu.
2. Per unit values referred to either side of the transformer remains same. Pu impedance of a 3 phase transformer is independent of type of winding
3. Computational efforts are reduced by a great extent.
4. The chance of confusion between line and phase quantities in a three phase balanced system is greatly reduced.
5. Manufactures usually provide impedance values in pu.
Q. What is the need for base values?
A. The various
components of a power system may operate at different voltage and power levels.
So, it will be convenient if voltage, power, current and impedance ratings of
these components are expressed with reference to a common value called base
value.
Q. What is a one line / Single line diagram?
A. A Single or
One line diagram is a diagrammatic representation of a power system, in which
the components are represented by their standard symbols and interconnection
between them is shown by a straight line.
Q. What is an impedance diagram and what are the
approximations made while drawing it?
A. The
impedance diagram is the equivalent circuit of a power system in which various
components are represented by their approximate or simplified equivalent
circuits.
Approximations:
1. The neutral impedance is neglected.
2. All static loads are represented as impedances.
3. Transmission line is represented by its equivalent.
Approximations:
1. The neutral impedance is neglected.
2. All static loads are represented as impedances.
3. Transmission line is represented by its equivalent.
Q. What is a reactance diagram?
A. The
reactance diagram is drawn omitting all static loads, neutral impedance,
resistances, magnetizing components of each transformer and the capacitance of
the transmission line from the impedance diagram.
Q. What are positive sequence components?
A. The positive
sequence components of a three phase unbalanced vectors consists of three
Vectors of equal magnitude, displaced from each other by 120 degrees in phase
and having the Same phase sequence as the original vectors.
Q. What are negative sequence components?
A. The negative
sequence components of a three phase unbalanced vectors consists of three
Vectors of equal magnitude displaced from each other by 120 degree in phase and
having the phase sequence opposite to that of the original vectors.
Q. What are zero sequence components?
A. The zero
sequence components of a three phase unbalanced vectors consists of 3 vectors
of equal magnitude and with zero phase displacement from each other.
Q. When do you call a fault symmetrical and unsymmetrical?
A. A fault is
called symmetrical fault if the fault current is equal in all the phases. The
fault is unsymmetrical fault if the fault current is not equal in all the
phases.
Q. What is a PQ bus?
A. A bus is
called PQ bus when real and reactive components of power are specified for the
bus. In a load bus the voltage is allowed to vary within permissible limits.
Q. What is swing bus?
A. A bus is
called swing bus when the magnitude and phase of the bus voltage are specified
for it. The swing bus is the reference bus for load flow solution and it is
required for accounting line losses. Usually one of the generator bus is
selected as swing bus.
Q. What is the need for slack bus?
A. The slack
bus is needed to account for transmission line losses. In a power system the
total power generated will be equal to sum of power consumed by loads and
losses. In a power system only the generated power and load power are specified
for buses. The slack bus is assumed to generate the power required for losses.
Since the losses are unknown the real and reactive power are not specified for
slack bus.
Q. What is a bus admittance matrix?
A. The matrix
consisting of the self and mutual admittances of the network of a power System
is called bus admittance matrix.
Q. What is a bus impedance matrix?
A. The matrix
consisting of driving point impedances and transfer impedances of the Network
of a power system is called bus impedance matrix.
Q. What are ACSR conductors and their advantages?
A. ACSR
conductors are composite stranded conductors. The outer strands have high
conductivity and low tensile strength (Aluminum). The central strands have low
conductivity and high tensile strength (steel). With ACSR conductors skin
affect can be minimized.
Q. What is Ferranti effect?
A. When the
transmission line is operating at no load or light load condition, the
receiving end voltage is more than the sending voltage. This phenomena is
called Ferranti effect. It is prominent is long transmission lines.
Q. What are the major components of a distribution system?
A. Feeder: A
line or conductor that connects the major station to the distributor is known
as feeder.
Distributor: A line or conductor to which various consumers are connected through service mains is known as distributor.
Service Main: A line which connects the consumer to the distributor is known as service main.
Distributor: A line or conductor to which various consumers are connected through service mains is known as distributor.
Service Main: A line which connects the consumer to the distributor is known as service main.