Function and Classification Of Substation

Function and Classification Of Substation


Here, In this article, the main function of substation and classification of substation is given below, which is more helpful in field work.

Function and Classification Of Substation



Functions Of Substation


The main functions of substations are to receive energy transmitted at high voltage from the generating stations, reduce the voltage to a value appropriate for local distribution and provide facilities for switching.


Classification Substation


According to service

∆Transformer substations

∆Industrial substations

∆Switching substations

∆Power factor correction

∆Frequency changer substations

∆Converting substations.


According to design


1.Indoor type substations

2.Outdoor substations

         (i)Pole mounted substations

         (ii)Foundation mounted substations.

Equipment for Substations and Switchgear Installations


The main equipment for substations and switchgear is mentioned below.


✓Main bus-bars

✓Insulators

✓Isolators

✓Circuit breakers

✓Load-interrupter switches

✓Fuses

✓Power transformers

✓Current and potential transformers

✓Indicating and metering instruments

✓Protective relays

✓Carrier-current equipment

✓Control cables.

Construction of Single Phase Induction Motor | Electrical Machine Theory

Construction of Single Phase Induction Motor | Electrical Machine Theory


The theory of single phase induction motor and Construction of single phase induction motor is shown in this article, which is more helpful to strong your knowledge on Electrical Machine.


Construction of Single Phase Induction Motor | Electrical Machine Theory


Single phase induction motors are in very wide use in industry,especially in fractional horse-power field.

They are extensively used for electric drive for low power constant speed apparatus such as machine tools, domestic apparatus and agricultural machinery in circumstances where a three phase supply is not readily available.

There is a large demand for single-phase induction motors in sizes ranging from a fraction of horse-power up to about 5 H.P.Though these machines are useful for small outputs. 

they are not used for large powers as they suffer from many disadvantages and are never used in cases where three-phase machines can be adopted.

The main disadvantages of single-phase induction motors are:


✓Their output is only 50%of the three-phase motor,for a given frame size and temperature
rise.

✓They have lower power factor.

✓Lower-efficiency.

✓These motors do not have inherent starting torque.

✓More expensive than three-phase motors of the same output.

The magnetic field produced by the stator coils is pulsating, through varying sinusoidally
with time.

Ferrari pointed out that such a field can be resolved into two equal fields but rotating in opposite directions with equal angular velocities.

The maximum value of each component is equal to half the maximum of the pulsating field.

Split-phase Motors


Since the single-phase induction is not self-starting, means must be provided to create an initial torque,But the initial torque is only possible if a rotating flux is created in the stator.

It is known that a rotating flux is produced when there is a difference of 90 between
the currents of two stationary coils.

If the stator possesses two fluxes hauing a large phase difference the result is a rotating flux.

Phase Angle Control | Principle Of Phase Angle Control

Phase Angle Control  |  Principle Of Phase Angle Control


In integral cycle control method, the output voltage can e controlled by opening and closing
the switch for one or several half cycles of the ac input voltage.

Phase Angle Control  |  Principle Of Phase Angle Control


Whereas in phase controlled switching, the output voltage is controlled by opening and
closing the switch between a cycle.


 i.e., switching device is so operated that the source and load gets connected for a part of each half cycle of the input voltage.



The principle of operation of phase control can be explained by considering the circuit
diagram as shown in  below figure

Principle of  phase angle Control


Thyristor T is forward biased during positive half cycle, s turned ON at firing angle alpha and will remain ON till wt=π  for resistive load.


During the period of thyristor load voltage becomes Vm sin (alpha)  and load current becomes Vm sin (alpha) / R  as shown in below Figure.

Waveforms of single phase half wave ac voltage controller



During negative half cycle,  from wt = π to 2π,  thyristor gets turned OFF and diode
D becomes forward based.


As a result diode D starts conducting during negative half cycle. It can be noted that only
positive cycle can be controlled whereas negative half cycle cannot.


Therefore, this circuit configuration is referred as single phase half wave voltage controller or 1 phase unidirectional voltage controller.


The undesirable de component is introduced in the load and supply circuits because the
positive and negative cycles are not identical for voltage and current waveforms as shown
in upper figure.

Advantages of phase angle control

Here only one scr is required so control is very simple.


Disadvantages of Phase Angle Control


The output voltage waveform is not fully controlled.


Supply current, output current and voltage are in dc component.


Application of Phase Angle Control


Used in lighting and heating application.




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Basic Electrical Engineering Viva Questions | Basic Electrical Engineering Viva Questions

Basic Electrical Engineering Viva Questions | Basic Electrical Engineering Viva Questions

Basic Electrical Engineering Viva Questions | Basic Electrical Engineering Viva Questions




1. The circle diagram for an induction motor cannot be used to determine

(a) efficiency

(b) power factor

(c) frequency

(d) output


Ans: a






2. Blocked rotor test on induction motors is used to find out

(a) leakage reactance

(b) power factor on short circuit

(c) short-circuit current under rated voltage

(d) all of the above


Ans: d






3. Lubricant used for ball bearing is usually

(a) graphite

(b) grease

(c) mineral oil

(d) molasses


Ans: b






4. An induction motor can run at synchronous speed when

(a) it is run on load

(b) it is run in reverse direction

(c) it is run on voltage higher than the rated voltage

(d) e.m.f. is injected in the rotor circuit


Ans: d






5. Which motor is preferred for use in mines where explosive gases exist ?

(a) Air motor

(b) Induction motor

(c) D.C. shunt motor

(d) Synchronous motor


Ans: a




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Electrical Circuit Questions | Electrical Engineering Quiz Questions

Electrical Circuit Questions | Electrical Engineering Quiz Questions

Electrical Circuit Questions | Electrical Engineering Quiz Questions





1. In a three-phase induction motor

(a) iron losses in stator will be negligible as compared to that in rotor

(6) iron losses in motor will be negligible as compared to that in rotor

(c) iron losses in stator will be less than that in rotor

(d) iron losses in stator will be more than that in rotor


Ans: d






2. In case of 3-phase induction motors, plugging means

(a) pulling the motor directly on line without a starter

(b) locking of rotor due to harmonics

(c) starting the motor on load which is more than the rated load

(d) interchanging two supply phases for quick stopping

Ans: d





3. Which is of the following data is required to draw the circle diagram for an induction motor ?

(a) Block rotor test only

(b) No load test only

(c) Block rotor test and no-load test

(d) Block rotor test, no-load test and stator resistance test


Ans: d





4. In three-phase induction motors sometimes copper bars are placed deep in the rotor to

(a) improve starting torque

(b) reduce copper losses

(c) improve efficiency

(d) improve power factor


Ans: a





5. In a three-phase induction motor

(a) power factor at starting is high as compared to that while running

(b) power factor at starting is low as compared to that while running

(c) power factor at starting in the same as that while running


Ans: b



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Electrical Questions And Answers | Electrical Questions Asked In Interview

Electrical Questions And Answers | Electrical Questions Asked In Interview

Electrical Questions And Answers | Electrical Questions Asked In Interview




1. The repulsion-start induction-run motor is used because of

(a) good power factor

(b) high efficiency

(c) minimum cost

(d) high starting torque


Ans: d





2. In case of a shaded pole motor the direction of rotation of the motor is

(a) from main pole to shaded pole

(b) from shaded pole to main pole

(c) either of the above depending on voltage

(d) either of the above depending on power factor


Ans: a





3. In case of high speed universal motor which of the following needs more attention ?

(a) End play

(b) Air gap

(c) Insulation in rotor

(d) Balancing of rotor


Ans: d





4. The wattage rating for a ceiling fan motor will be in the range

(a) 200 to 250 W

(b) 250 to 500 W

(c) 50 to 150 W

(d) 10 to 20 W


Ans: c





5. The wattage of motor for driving domestic sewing machine will be around

(a) 100 to 150 W

(b) 40 to 75 W

(c) 10 to 30 W

(d) 5 to 10 W


Ans: a




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Electrical Engineering Viva Questions | Electrical Engineering Technical Questions




1. If the centrifugal switch of a two-value capacitor motor using two capacitors fails to open then

(a) motor will not come up to speed

(b) motor will not carry the load

(c) current drawn by the motor will be excessively high

(d) electrolytic capacitor will, in all probability, suffer break down


Ans: d





2. In a universal motor, the most common cause of brush sparking is

(a) open armature winding

(b) shorted armature winding

(c) shorted field winding

(d) high commutator mica

(e) all of the above


Ans: e





3. If starting winding of a single-phase induction motor is left in the circuit, it will

(a) run faster

(b) spark at light loads

(c) draw excessive current and overheat

(d) run slower


Ans: c





4. Most of the fractional horsepower motors have either

(a) hard and annealed bearings

(b) ball or roller bearings

(c) soft and porous bearings

(d) plain or sleeve bearings


Ans: d





5. Which of the following statements regarding reluctance-start motor is incorrect ?

(a) It is similar to reluctance motor

(b) It is basically an induction motor and not a synchronous one

(c) So far as its basic working principle is concerned, it is similar to shaded pole motor

(d) the air-gap between rotor and salient poles is non- uniform


Ans: a



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Question And Answer of Testing Of DC Machine | Electrical Machine Questions

Question And Answer of Testing Of DC Machine |  Electrical Machine Questions

Question And Answers related to Testing if DC Machine is important for GATE examination and also in compititive exams. All the important and basic questions are covered.

Question And Answer of Testing Of DC Machine |  Electrical Machine Questions


Q.1

One D.C.motor drives another D.C.
motor.The second D.C.motor when
excited and driven

(a)runs as a generator

(b)does not run as a generator

(c)also runs as a motor

(d)comes to stop after sometime

Ans.

(a)runs as a generator



Q.2

Which of the following D.C.motors has
the least drop in speed between no-load
and nominal load?

(a)Series motor without commutating
poles

(b)Series motor with commutating
poles.

(c)Shunt motor with commutating
pole

(d)Compound motor without com-
mutating poles

Ans.

(c)Shunt motor with commutating
pole



Q.3

The speed of a series motor at no load
is

(a)infinity

(b)4000 r.p.m.

(c)2000 r.p.m.

(d)1000 r.p.m.

(e)none of the above

Ans.

(a)infinity



Q.4

In a D.C.motor if the back e.m.f.is
absent

(a)motor will burn

(b)motor will not run at all

(c)motor will run at very slow speed

(d)motor will run at very high speed

Ans.

(a)motor will burn



Q.5

A series motor is started without load.
The effect is that

(a)the back e.m.f.decreases

(b)the torque increases rapidly

(c)the speed increases rapidly

(d)the current drawn increases rapidly

Ans.

(c)the speed increases rapidly



Q.6

What will happen if supply terminals
of D.C.shunt motor are interchanged?

(a)The direction of rotation will
reverse

(b)Motor will stop

(c)Motor will run at speed lower than
the normal speed in the same direc-
tion

(d) Motor will run at its normal speed
in the same direction as it was run-
ning

Ans.

(d) Motor will run at its normal speed
in the same direction as it was run-
ning



Q.7

When the electric train is moving down
a hill the D.C.motor act as

(a)D.C.series generator

(b)D.C.shunt generator

(c)D.C.shunt motor

(d)D.C.series motor

Ans.

(a)D.C.series generator



Q.8

Which of the following methods is most
economical for finding the no-load los-
ses of a large D.C..shunt motor?

(a)Retardation test

(b)Swinburne's test

(c)Hopkinson's test

(d)none of the above

Ans.

(b)Swinburne's test



Q.9

Which of the following statement is in-
correct?If a starter is not used with
large D.C.motor,it will draw a starting
current which

(a)will produce very low starting
torque

(b)will produce excessive line voltage
drop

(c)will damage the commatator

(d)is many times its full-load current

Ans.

(a)will produce very low starting
torque



Q.10

The rated speed of a given D.C.shunt
motor is 900 r.p.m.To run this machine
at 1000 r.p.m.,which of the following
speed control scheme will be used?

(a)Ward-Leonard control

(b)Armature current resistance control

(c)Field resistance control

(d)None of the above

Ans.

(c)Field resistance control



Q.11

In electric motors carbon brushes are
used to

(a)provide a path for flow of current

(b)prevent sparking during commuta-
tion

(c)prevent overheating of armature
windings

(d)brush off carbon deposits on the
commutator

Ans.

(a)provide a path for flow of current



Q.12

What is the effect produced by the
electric current in an electric motor?

(a)Heating effect only

(b)Magnetic effect only

(c)Heating as well as magnetic effect

(d)Heating as well as chemical effect

Ans.

(c)Heating as well as magnetic effect



Q.13

In a motor,energy conversion would
not have been possible but for the

(a)production of opposing back e.m.f.
Eb in the armature

(b)input energy from supply

(c)use of commutator

(d)application of Fleming's left-hand
rule

Ans.

(a)production of opposing back e.m.f.
Eb in the armature



Q.14

In a D.C.motor,unidirectional torque
is produced with the help of

(a)end plates

(b)bushes

(c)commutator

(d)both(b)and(c)

Ans.

(d)both(b)and(c)



Q.15

In a D.C.motor,the ratio Eo/Va indi-
cates

(a)running torque of the motor

(b)starting torque of the motor

(c)efficiency of the motor

(d)speed regulation of the motor

Ans.

(c)efficiency of the motor


Testing Of DC Machine | Electrical Machine Theory

Testing Of DC Machine |  Electrical Machine Theory


Testing Of DC Machine is first and foremost section after it manufacturers, So testing Of Dc Machine is necessary to learn for all electrical engineer who is more interested in Electrical Machine. 

Testing Of DC Machine |  Electrical Machine Theory 


For testing Of DC Machine various tests are required which all are described below


Testing Of DC Machine


The following important performance tests are conducted on D.C.machines:

1.The magnetisation or open circuit test

2.The load characteristics

3.The determination of efficiency curve

4.The temperature rise test.


The method for determining efficiency can be divided into following three methods:


(1)Direct method

The direct tests can be used only on small machines.


If brake is to be applied to a series motor,the brake must be tight before the motor is
started,otherwise the armature may get damaged and fly to pieces.

(2)Indirect method

The simplest of the indirect tests is Swinburne test.This method may be applied to series motors,because the speed of a series motor being very high at no-load.


it is not possible to run a series motor on no-load.


(3) Regenerative method

Hopkinson's test or Back to back test, It is a regenerative test.


The power taken from the supply is that required to overcome the losses only.


(4) Retardation test

This method is applicable to shunt motors and generators and is used for finding the stray losses.


(5) Field's test

This test is applicable to two similar series motors.




What is Insulating Material? | Electrical Safety Theory

What is Insulating Material? |  Electrical Safety Theory


Insulation is more important part of wire or cable which is protect cable outer surface from environmental impurities and which is provide mechanical strength of cable

What is Insulating Material? |  Electrical Safety Theory


Electrical insulating materials' are defined as materials which offer a very large resistance to flow of current, and for that reason they are used to keep the current in its proper path along the conductor.


Thermoplastic materials'are those which soften on the application of heat, with or
without pressure but they require cooling to set them to shape.


Thermosetting materials'are those plastics which require heat and pressure to
mould them into shape.


Insulating varnishes'are generally classified according to composition as“oil varnishes” and “spirit varnishes".


The electrical insulating films comprise a group of thin flexible insulating materials made from various polymers.


The function of insulating liquids is to provide electrical insulation and heat transfer.


Transformer oil serves two purposes:

(1)It transfers heat by convection from winding and core to the cooling surfaces.

(2)It maintains the insulation of the windings.
Presence of even a trace of water in transformer oil reduces its insulation strength
considerably.


'Sludge' formation produces the following effects:

(1)Rate of heat transfer is reduced.

(2)Ducts are clogged.

(3)Increase in operating temperature results.

The various tests which are carried out on transformer oil are given below:

(1)Moisture test

(2)Acidity test

(3)Sludge resistance test

(4)Electric strength test

If the gas is used in applications under high pressure'which may cause liquification, chemical instability and corrosion problem may be encountered.


Therefore insulating gases are normally used under conditions such that the liquification of gas does not occur.


Insulation resistance is the resistance between two conductors(or systems of conductors) usually separated by insulating materials.


It is the total resistance in respect of two
parallel paths,one through the body and other over the surface of the body.


The potential gradient at which breakddwn occurs is termed as dielectric strength.


Dielectric constant(Permittivity)is the ratio of the electric flux density in the material to that produced in free space by the same electric force.