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Hello
Engineers.
Today we are
sharing alternator interview questions with answer.
Q. 1. What are
the two types of turbo-alternators ?
Ans. Vertical and horizontal.
Ans. Vertical and horizontal.
Q.
2. How do you compare the two ?
Ans. Vertical type requires less floor space and while step bearing is necessary to carry the weight of the moving element, there is very little friction in the main bearings. The horizontal type requires no step bearing, but occupies more space.
Ans. Vertical type requires less floor space and while step bearing is necessary to carry the weight of the moving element, there is very little friction in the main bearings. The horizontal type requires no step bearing, but occupies more space.
Q. 3. What is
step bearing ?
Ans. It consists of two cylindrical cast iron plates which bear upon each other and have a central recess between them. Suitable oil is pumped into this recess under considerable pressure.
Ans. It consists of two cylindrical cast iron plates which bear upon each other and have a central recess between them. Suitable oil is pumped into this recess under considerable pressure.
Q. 4. What is
direct-connected alternator ?
Ans. One in which the alternator and engine are directly connected. In other words, there is no intermediate gearing such as belt, chain etc. between the driving engine and alternator.
Ans. One in which the alternator and engine are directly connected. In other words, there is no intermediate gearing such as belt, chain etc. between the driving engine and alternator.
Q. 5. What is
the difference between direct-connected and direct-coupled units ?
Ans. In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling.
Ans. In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling.
Q.
6. Can a d.c. generator be converted into an alternator ? If yes then how ?
Ans. Yes. A DC generator can be converted into an alternator. By providing two collector rings on one end of the armature and connecting these two rings to two points in the armature winding 180° apart.
Ans. Yes. A DC generator can be converted into an alternator. By providing two collector rings on one end of the armature and connecting these two rings to two points in the armature winding 180° apart.
Q. 8. Would
this arrangement result in a desirable alternator ?
Ans. No
Ans. No
Q. 9. How is a
direct-connected exciter arranged in an alternator ?
Ans. The armature of the exciter is mounted on the shaft of the alternator close to the spider hub. In some cases, it is mounted at a distance sufficient to permit a pedestal and bearing to be placed between the exciter and the hub.
Ans. The armature of the exciter is mounted on the shaft of the alternator close to the spider hub. In some cases, it is mounted at a distance sufficient to permit a pedestal and bearing to be placed between the exciter and the hub.
Q. 10. Any
advantage of a direct-connected exciter ?
Ans. Yes, economy of space.
Ans. Yes, economy of space.
Q. 11. Any
disadvantage ?
Ans. The exciter has to run at the same speed as the alternator which is slower than desirable. Hence, it must be larger for a given output than the gear-driven type, because it can be run at high speed and so made proportionately smaller.
Ans. The exciter has to run at the same speed as the alternator which is slower than desirable. Hence, it must be larger for a given output than the gear-driven type, because it can be run at high speed and so made proportionately smaller.
Q. What do you mean by zone of protection?
A. The protection is organized to provide a
ring of security around each and every element of power system. If there is any
fault associated within the ring the relays associated with it must trip all
the allied circuit breakers so as to remove any faulty element from rest of the
power system. This ring of security is called ‘Zone of protection’.
Q. What are internal and external faults?
A. Faults that occur within the zone are
termed as Internal Faults and that occur outside the zone of protection are
termed as external faults (can also be called as through faults).
Q. What do you mean by reach and reach point
of the relay?
A. The farthest point from the relay which is
still inside the zone of protection is called reach point and the distance
between relay and reach point is called reach.
Q. Why does the adjacent zones of protection
overlap?
A. If the
adjacent zones do not overlap there will be some portion of the power system
which is left out unprotected.
Q. How do you select the pickup value of a relay?
A. The relay
should allow normal load as well as a certain degree of overload to be
supplied. So the pickup value of the relay should be more than maximum load. At
the same time, the relay should be sensitive enough to respond to the smallest
fault. So, pickup value should be less than the smallest fault current.
So
Q. How to set the operating time of the relay?
A. Naturally
when a fault occurs it is sensed by both the primary and backup protection. As
the operating time of primary relay is less than that of the backup relay, it
will operate first. There will be an overshoot time (due to moment of inertia
of moving relay system). Consider a primary relay A having operating time of
0.1 sec. Assume circuit breaker operating time as 0.5 sec and overshoot time as
0.4 sec. Now the operating time of backup relay B will be 0.1+0.5+0.4= 1 sec.
Q. Explain the operating principle of DTOC and IDMT relays?
A. A Definite
time over current relay (DTOC) can be adjusted to trip out at a definite amount
of time, after it picks up. It has a time adjustment setting and pickup
setting.
The operating
principle of IDMT relay is based on the idea that more severe a fault is, the
faster it should be cleared to avoid damage to apparatus.
Q. When will you go for IDMT and DTOC relays?
A. Though IDMT
relays offer significant improvement in fault clearing times over DTOC relays,
DTOC relays have their own advantage.
Consider a
system where .
Since the fault
current as a function of fault location is proportional to , it would remain
more or less constant throughout the length of feeder. Therefore, the IDMT
relay will not be of much help. So DTOC relays can be used for lines which are
short in length.
Q. When are the Directional relays used?
A. Directional
relays are used mainly in single end fed system of parallel feeders, ring main
feeder system etc.
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 different between resistance grounding system
and resistance earthing system?
A. Resistance
grounding system means connecting the neutral point of the load to the ground
to carry the residual current in case of unbalanced conditions through the
neutral to the ground whereas resistance earthing system is done in an electric
equipment in order to protect he equipment in occurrence of fault in the
system.
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 is a load curve?
A. The curve
showing the variation of load on the power station with respect to (w.r.t) time
is known as a load curve.
Q. What is the difference between base load and peak load?
A. Base
load: The unvarying load which occurs almost the whole day on the station
is known as base load.
Peak load: The various peak demands of load over and above the base load of the station is known as peak load.
Peak load: The various peak demands of load over and above the base load of the station is known as peak load.
Q. What is power factor?
A. The cosine
of angle between voltage and current in an a.c. circuit is known as power
factor.
Q. What are the disadvantages of low power factor and how
to improve it?
A. Disadvantages: Large KVA rating of equipment,
Greater conductor size, High copper losses, Poor voltage regulation
The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load.
The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load.
Q. Why is a.c transmission preferred over d.c
transmission?
A. 1. The power can be generated at
high voltages.
2. The maintenance of a.c. sub-stations is easy and cheaper.
3. The a.c. voltage can be stepped up or stepped down by transformers with ease and efficiency.Although a.c transmission is preferred due to the above reasons d.c transmission is superior (2 conductors instead of 3, no inductance, capacitance, no corona, no skin effect etc).
2. The maintenance of a.c. sub-stations is easy and cheaper.
3. The a.c. voltage can be stepped up or stepped down by transformers with ease and efficiency.Although a.c transmission is preferred due to the above reasons d.c transmission is superior (2 conductors instead of 3, no inductance, capacitance, no corona, no skin effect etc).
Q. Why is electric power transmitted at
higher voltages?
A. Higher the transmission voltage
lesser will be the volume of conductor required.
Efficiency will be high at higher voltages.
Percentage line drop will be less.
Efficiency will be high at higher voltages.
Percentage line drop will be less.
Q. What are the different types of insulators
in Overhead lines?
A. Pin type insulators, Suspension type insulators,
Strain insulators, Shackle insulators.
Q. What is string efficiency?
A. The ratio of
voltage across the whole string to the product of number of discs and the
voltage across the disc nearest to the conductor is known as string efficiency.
Q. What are the different methods used to improve string
efficiency?
A. String
efficiency can be improved by using longer cross arms, by grading the
insulators and by using guard rings.
Q. What is corona?
A. The
phenomenon of violet glow, hissing noise and production of ozone gas in an
overhead transmission line is known as corona.
Q. What are the advantages and disadvantages of corona?
A. Advantages:
1. Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
2. Corona reduces the effects of transients produced by surges.Disadvantages:
1. Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
2. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
3. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
1. Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
2. Corona reduces the effects of transients produced by surges.Disadvantages:
1. Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
2. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
3. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
Q. How to reduce the corona effect?
A. Corona
effect can be reduced by
1. Increasing the conductor size
2. Increasing the conductor spacing
1. Increasing the conductor size
2. Increasing the conductor spacing
Q. What is sag? Explain its importance?
A. The
difference in level between points of supports and the lowest point on the
conductor is called sag.
If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension. In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag.
If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension. In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag.
Q. What is skin effect?
A. The tendency
of alternating current to concentrate near the surface of a conductor is known
as skin effect.
Q. Why is the need for transposition of
conductors?
A. When the
3-phase line conductors are unsymmetrically spaced, the flux linkages and
inductance of each phase are not the same. A different inductance in each
phase results in unequal voltage drops in the three phases even if the currents
in the conductors are balanced. Therefore, the voltage at the receiving
end will not be the same for all phases. In order that voltage drops are
equal in all conductors, we generally interchange the positions of the
conductors at regular intervals along the line so that each conductor occupies
the original position of every other conductor over an equal distance.
This is called transposition.
Q:Why
we use ac system in India why not dc ?
A:Firstly, the output of power stations comes from a rotary turbine, which by it’s nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. thirdly the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivilant of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
A:Firstly, the output of power stations comes from a rotary turbine, which by it’s nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. thirdly the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivilant of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
Q:Which type of motor is used in trains, what is the rating of
supply used explain Working principal?
A:Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.
A:Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.
Q:Battery banks are in connected in series or parallel and why?
A:Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)
A:Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)
Query:What
is electrical diversity factor in electrical installations?
Resolution :Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.
Resolution :Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.
Query:Why field rheostat is kept in minimum position while
armature rheostat at maximum position?
Resolution :In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting torque.
Resolution :In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting torque.
Query:Why
computer humming sound occurred in HT transmission line?
Resolution :This computer humming 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.
Resolution :This computer humming 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.
Query:What is rated speed?
Resolution :At the time of motor taking normal current (rated current)the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.
Resolution :At the time of motor taking normal current (rated current)the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.
Query:What is different between resistance grounding system and
resistance earthing system?
Resolution :Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.
Resolution :Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.
Query:Why should be the frequency 50 Hz 60Hz only why not others
like 45, 95 56 or anything , why should we maintain the frequency constant if
so why it is only 50 Hz 60Hz?
Resolution :We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are are made to operate at these frequency.
Resolution :We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are are made to operate at these frequency.