Classification And Working Principle of Alternator

Classification And Working Principle of Alternator 


Working principle and classification of Alternator is provided in this article, this theory is very basic and useful to learn operation of alternator and also electrical machine.

Classification And Working Principle of Alternator 



A machine for generating alternating currents is referred to as an alternator.

Alternators, according to their construction, are divided into the following two classifica-
tions:

1.Revoluing-armature type

2.Revolving-field type.

1.Revolving-armature type alternator


It has stationary field poles and revoluing armature.

It is usually of relatively small kVA capacity and low-voltage rating.

It resembles a D.C. generator in general appearance except that it has slop-rings instead of a commutator.

The field excitation must be direct current and therefore, must be supplied from an external direct current source.

2.Revolving-field type alternator:


It has a stationary armature or stator,inside of which the field poles rotate.

Most alternators are of the revolving-field type, in which the revolving-field structure or rotor has slip rings and brushes to supply the excitation current from an outside D.C.source.

The armature coils are placed in slots in a laminated core called the 'stator'which is made
up of thin steel punchings or laminations securely clamped and held in place in the steel
frame of the generator.

Usually the field voltage is between 100 and 250 volts and the amount of power delivered to the field circuit is relatively small.

The following are the principal aduantages of the revolving-field type alternators:

✓The armature windings are more easily braced to prevent deformation under the mechanical stresses due to short-circuit currents and centrifugal forces.

✓The armature(stator)winding must be insulated for a high voltage, while the voltage of field circuit is low(100 to 250 volts). It is much easier to insulate the high-voltage winding when it is mounted on the stationary structure.

✓Only a small amount of power at low voltage is handled by the slip ring contacts.

✓It is easier to build and properly balance high-speed rotors when they carry the field...
structure.

✓The armature winding is cooled more readily because the stator core can be made large
enough and with many air passages or cooling ducts for forced air circulation.

Operating Principle 


When the rotor rotates, the stator conductors
(being stationary)are cut by the magnetic flux,hence they have induced e.m.f.produced in them.

Application Of DC Generators | Electrical Machine Theory

Application Of DC Generators |  Electrical Machine Theory


Application of DC Generators theory is more helpful to grow knowledge in Electrical Machine Theory, also helpful for clear the practical fundamentals of electrical machine.

Application Of DC Generators |  Electrical Machine Theory


There are many types of electrical generators according to it's application, all the application according to it's type are included below.

Separately Excited Generators:


(i)The separately excited generators are usually more expensive than self-excited generators as they require a separate source of supply.Consequently they are generally used where self-excited are relatively unsatisfactor.

 These are used in Ward Leonard systems of speed control,because self-excitation would be unsuitable at lower voltages.


(ii)These generators are also used where quick and requisite response to control is important
since separate excitation gives a quicker and more precise response to the changes in the resistance of the field circuit.


Shunt Generators:

(i)These generators are used to advantage,in conjunction with automatic regulators,as
exciters for supplying the current required to excite the fields of A.C.generators.The regulator controls


the voltage of the exciter by cutting in and out some of the resistance of the shunt-field rheostat, thereby holding the voltage at whatever value is demanded by operating conditions.


This is one of the most important applications of shunt generators.


(ii)Shunt generators are used to charge batteries.


Series Generators

The field of application of series generator is limited.These are used for the following purpose:


(i)Series are lighting

(ii)Series incandescent lighting

(iii)As a series booster for increasing the voltage across the feeder carrying current
furnished by some other sources.

(iv)Special purposes such as supplying the field current for regenerative braking of D.C.
locomotives.


Compound Generators

The compound generator is used for more than any other type.


(i)It may.be built and adjusted automatically to supply an approximately constant voltage at the point of use,throughout the entire range of load.This is very great advantage.

It is possible to provide a constant supply voltage at the end of a long feeder by the simple expedient of over compounding the generator, because the resistance drop in the line is compensated for by the rising characteristic of the generator.

When the point of utilisation is near the generator, a flat-compound machine may be used.


(ii)Differentially compounded generator.finds the result of application as an arch welding
generator where the generator is practically short-circuited every time the electrode touches the metal plates to be welded.


(iii) Compound generators are used to supply power to:


✓ railway circuits
✓ motors of electrified steam rail-roads,
✓ industrial motors in many fields of industry,
✓ incandescent lamps
✓ elevator motors etc.


Single Phase Motors | MCQs | Electrical Machine Questions

Single Phase Motors | MCQs | Electrical Machine Questions


MCQs related to single phase motor is provided in this post which is more useful for preparation of gate examination and all type of electrical compititive exams.

Single Phase Motors | MCQs | Electrical Machine Questions



Q.1


A schrage motor can run on

(a)zero slip

(b)negative slip

(c)positive slip 

(d)all of the above

Ans.

(d)all of the above



Q.2

A universal motor can run on

(a)A.C.only

(b)D.C.only

(c)either A.C.or D.C.

(d)none of the above

Ans.

(c)either A.C.or D.C.



Q.3

Which of the following single-phase
motors is suitable for timing and con-
trol purposes?

(a)Reluctance motor

(b)Series motor

(c)Repulsion motor

(d)Universal motor

Ans.

(a)Reluctance motor



Q.4

Single phase induction motor usually
operates on

(a)0.6 power factor lagging

(b)0.8 power factor lagging

(c)0.8 power factor leading

(d)unity power factor

Ans.

(a)0.6 power factor lagging



Q.5

In split-phase motor auxiliary winding
is of

(a)thick wire placed at the bottom of
the slots

(b)thick wire placed at the top of the
slots

(c)thin wire placed at the top of the
slots

(d)thin wire placed at the bottom of the
slots

Ans.

(c)thin wire placed at the top of the
slots



Q.6

Which of the following motors will
operate at high power factor?

(a)Shaped pole motor

(b)Split phase motor

(c)Capacitor start motor

(d)Capacitor run motor

Ans.

(d)Capacitor run motor





Q.7

In a two value capacitor motor,the
capacitor used for running purposes is

(a)air capacitor

(b)paper spaced oil-filled type

(c)ceramic type

(d)a.c.electrolytic type

Ans.

(b)paper spaced oil-filled type



Q.8

Which of the following motors can be
run on A.C. as well as D.C.supply?

(a)Universal motor

(b)Repulsion motor

(c)Synchronous motor

(d)Reluctance motor

Ans.

(a)Universal motor



Q.9

In A.C.series motor compensating
winding is employed to

(a)reduce the effects of armature reac-
tion

(b)increase the torque

(c) reduce sparking at the brushes

(d)none of the above

Ans.

(c) reduce sparking at the brushes



Q.10

Which of the following single-phase in-
duction motors is generally used in
time phonographs?

(a)Resistance start

(b)Capacitor start capacitor run

(c)Shaded pole

(d)Universal

Ans.

(c)Shaded pole



Q.11

Which of the following motors has
highest starting torque?

(a)Repulsion motor

(b)Shaped pole motor

(c)Capacitor-start motor

(d)Split-phase motor

Ans.

(c)Capacitor-start motor



Q.12

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)high starting torque



Q.13

In case of a shaded pole motor the direc-
tion 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)from main pole to shaded pole



Q.14

In case of high speed universal motor
which of the following needs more at-
tention?

(a)End play 

(b)Air gap

(c)Insulation in rotor

(d)-Balancing of rotor

Ans.

(d)Balancing of rotor



Q.15

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

(a)200 to 250W 

(b)250 to 500W

(c)50 to 150W 

(d)10 to 20W

Ans.

(c)50 to 150W 


Q.16

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

(a)100 to 150W 

(b)40to75W

(c)10 to 30W

(d)5 to 10W.

Ans.

(a)100 to 150W

DC Generators | MCQs | Electrical Machine Questions

DC Generators | MCQs  |  Electrical Machine Questions


We provide mcq type Questions And Answers which is more important for all electrical engineer for preparation of gate examination and all electrical compititive exam.

DC Generators | MCQs  |  Electrical Machine Questions


Question And Answer (DC Generators)


Q.1

The purpose of providing dummy coils
in a generator is

(a)to enhance flux density

(b)to amplify voltage

(c)to provide mechanical balance for
the rotor

(d)to reduce eddy currents

Ans.

(c)to provide mechanical balance for
the rotor


Q.2

In a D.C.generator,the armature reac-
tion results in

(a)demagnetisation of the centres of
poles

(b)magnetisation of interpoles

(c)demagnetisation of the leading pole
tip and magnetisation of the trail-
ing pole tip

(d)magnetisation of the leading tip and
demagnetisation of the trailing pole
tip

Ans.

(c)demagnetisation of the leading pole
tip and magnetisation of the trail-
ing pole tip



Q.3

In a D.C.generator in case the brushes
are moved so as to bring them in mag-
netic neutral axis,then,there will be

(a)demagnetisation

(b)cross-magnetisation

(c)cross-magnetisation as well as
demagnetisation

(d)none of the above

Ans.

(c)cross-magnetisation as well as
demagnetisation



Q.4
The polarity of a D.C.geperator can be
reversed by

(a)reversing the field current

(b)increasing field current

(c)reversing field current as well as
direction of rotation

(d)any of the above

Ans.

(a)reversing the field current



Q.5

In D.C.generators,the brushes on com-
mutator remain in contact with conduc-
tors which

(a)lie under south pole

(b)lie under north pole

(c)lie under interpolar region

(d)are farthest from the poles

Ans.

(c)lie under interpolar region



Q.6

In a lap wound D.C.generator having
p as number of poles and Z as number
of conductors,the maximum number of
equilizer rings will be

(a)Z

(b)p

(c)2Z / P

(d)2P / Z

Ans.

(c)2Z / P



Q.7

If brushes of a D.C.generator are
moved in order to bring these brushes
in magnetic neutral axis,there will be

(a)demagnetisation only

(b)cross magnetisation as wellas mag-
netisation

(c)crossmagnetisation as well as
demagnetising

(d)cross magnetisation only

Ans.

(c)crossmagnetisation as well as
demagnetising



Q.8

Armature reaction of an unsaturated
D.C.machine is

(a) crossmagnetising

(b)demagnetising

(c)magnetising

(d)none of above

Ans.

(a) crossmagnetising



Q.9

D.C.generators.are connected to the
busbars or disconnected from them
only under the floating condition

(a)to avoid sudden loading of the
primemover

(b)to avoid mechanicaljerk to the shaft

(c)to avoid burning of switch contacts

(d)all above

Ans.

(d)all above



Q.10

Eddy currents are induced in the pole
shoes of a D.C.machine due to

(a)oscillating magnetic field

(b)pulsating magnetic flux

(c)relative rotation between field and
armature

(d)all above

Ans.

(c)relative rotation between field and
armature



Q.11

In a D.C.machine,short-circuited field
coil will result in

(a)odour of burning insulation

(b)unbalanced magnetic pull produc-
ing vibrations

(c)reduction of generated voltage for
which excitation has to be increased
to maintain the voltage

(d)all above

Ans.

(d)all above



Q.12

Equilizer rings are required in case
armature is

(a)wave wound

(b)lap wound

(c)delta wound

(d)duplex wound

Ans.

(b)lap wound



Q.13

Welding generator will have

(a)lap winding

(b)wave winding

(c)delta winding

(d)duplex wave winding

Ans.

(a)lap winding



Q.14

In case of D.C.machine winding,num-
ber of commutator segments is equal to

(a) number of armature coils

(b)number of armature coil sides

(c)number of armature conductors

(d)number of armature turns

Ans.

(a) number of armature coils



Q.15

For a D.C.machines laboratory follow-
ing type of D.C.supply will be suitable

(a)rotary converter

(b)mercury are rectifier

(c)induction motor D.C.generator set

(d)synchronous motor D.C.generator
set

Ans.

(c)induction motor D.C.generator set


Sychronous Motor Questions and Answers

Sychronous Motor Questions and Answers 


MCQ type Questions And answers of sychronous motor is provided in this post which is more helpful in preparation of gate examination and also for competitive examination.

Sychronous Motor Questions and Answers 


Question And Answer (Sychronous Motor)




Q.1

Hunting in a synchronous motor takes
place

(a)when supply voltage fluctuates

(b)when load varies

(c)when power factor is unity

(d)motor is under loaded

Ans.

(b)when load varies



Q.2

When load on an over-excited or under-
excited synchronous motor is increased,
rate of change of its armature current
as compared with that of power factor
is

(a)more

(b)less

(c)equal

(d)twice

Ans.

(b)less



Q.3

The rotor copper losses,in a synchro-
nous motor are met by

(a)d.c.source 

(b)armature input

(c)motor input 

(d)supply lines

Ans.

(a)d.c.source 



Q.4

The maximum power developed in a
synchronous motor occurs at a coupling
angle of

(a)30°

(b)60°

(c)90°

(d)180°

Ans.

(c)90°



Q.5

When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor

(a)remains same as the original value

(b)decreases to half the original value

(c)tends to becomes zero

(d)increases to two times the original
value

Ans.

(d)increases to two times the original
value



Q.6

In which of the following motors the
stator and rotor magnetic field rotate
at the same speed?

(a)Universal motor

(b)Synchronous motor

(c)Induction motor

(d)Reluctance motor

Ans.

(b)Synchronous motor



Q.7

Synchronsizing powerofa synchronous
machine is

(a)direcly proportional to the synchro-
nous reactance

(b)inversely proportional to the syn-
chronous reactance

(c)equal to the synchronous reactance

(d)none of the above

Ans.

(b)inversely proportional to the syn-
chronous reactance



Q.8

Synchronous motors are

(a)not-self starting

(b)self-starting

(c)essentially self-starting

(d)none of the above

Ans.

(a)not-self starting



Q.9

The standard full-load power factor ratings for synchronous motors are

(a)zero or 0.8 leading

(b)unity or 0.8 lagging

(c)unity or 0.8 leading

(d)unity or zero

Ans.

(c)unity or 0.8 leading



Q.10

A synchronous motor running with normal excitation adjusts to load increases essentially by increase in

(a)back e.m.f.

(b)armature current

(c)power factor 

(d)torque angle

Ans.

(b)armature current



Q.11

A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because

(a)synchronous motor has no slip

(b) stator supply is not required to produce magnetic field

(c)mechanical load on the rotor remains constant

(d)synchronous motor has large airgap

Ans.

(b) stator supply is not required to produce magnetic field



Q.12

A synchronous motor working at leading
power factor can be used as

(a)voltage booster

(b)phase advancer

(c)noise generator

(d)mechanical synchronizer

Ans.

(b)phase advancer



Q.13

Slip rings are usually made of

(a)carbon or graphite

(b) brass or steel

(c)silver or gold

(d)copper or aluminium

Ans.

(b) brass or steel



Q.14

An over-excited synchronous motor is used for

(a)fluctuating loads

(b)variable speed loads

(c)low torque loads

(d)power factor corrections

Ans.

(d)power factor corrections



Q.15

When the voltage applied to a synchronous motor is increased, which of the following will reduce?

(a)Stator flux 

(b)Pull in torque

(c)Both(a)and(b)

(d)None of the above

Ans.

(d)None of the above

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Also Read Electrical Engineering Interview Questions:

Q.1 If the potential at point A is higher that at point B ( Va > Vb ) then the potential difference will be _________?

A. Negative

B. Positive

C. Zero

D. 6 volt

Ans.

B. Positive


Q.2 Electromotive force is measured in _________?

A. Volt

B. Volt . Meter

C. Volt / Meter

D. Volt / coulomb

Ans.

A. Volt


Q.3 How many electrons are carried in 1 coulomb charge?

A. 6.25 × (10) 12

B. 6.25 × (10) 15

C. 6.25 × (10) 18

D. 6.25 × (10) 21

Ans.

C. 6.25 × (10) 18


Q.4 What is the unit of resistivity?

A. Ohm

B. Mho

C. Ohm . Meter

D. Ohm/ Meter

Ans.

C. Ohm . Meter


Q.5 What is the resistivity of copper at 20 ° C ?

A. 1.72 × (10) -8

B. 2.82 × (10) -8

C. 1

D. 0

Ans.

A. 1.72 × (10) -8


Q.6 What is the actual equation of ohm's law ?

A. V= I R

B. I = V / R

C. VI = R

D. V / I = R

Ans.

D. V / I = R



Q.7 What is the equation of capacitor in series connection?

A. C = C1 +C2 + C3 + .....+ Cn

B. C = --C1 --C2 --C3 --......-- Cn

C. 1/C = 1/ C1 + 1/ C2 + 1/C3 +.....+ 1/ Cn

D. C = -C1 + C2 - C3 +....- Cn

Ans.

C. 1/C = 1/ C1 + 1/ C2 + 1/ C3 +....+ 1/ Cn


Q.8 Find the passive Element....

A. Current source

B. Resistor

C. Voltage source

D. Thyristor

Ans.

B. Resistor


Q.9 Which one is not Type of current?

A. Direct current

B. Sinosuidal current

C. Fluctuating current

D. Alternating current

Ans.

B. Sinosuidal current


Q.10 What is the unit of apparent power?

A. VA

B. VAR

C. KVR

D. Watt

Ans.

A. VA

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Also Read Electrical Engineering Interview Questions:

Q.1 What is the formula of power factor?

A. P.f = R / I

B. P.f = R / V

C. P.f = R / Z

D. P.f = V / I

Ans.

C. P.f = R / Z


Q.2 What is the resonance condition?

A. XL = - Xc

B. XL = Xc

C. - Xl = Xc

D. XL + Xc = 0

Ans. 

B. XL = Xc


Q.3 What is the unit of conductance?

A. Ohm B. Mho

C. Ohm . m D. Ohm / meter

Ans.

B. Mho


Q.4 What is the power factor at resonance condition?

A. One B. Two

C. Three D. Zero

Ans.

A. One ( unity power factor)


Q.5 Who is the first inventor of electricity?

A. Thomas Elwa Edison

B. Charles coulomb

C. Benjamin Franklin

D. J. J. Thomson

Ans. 

C. Benjamin Franklin


Q.6 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


Q.7 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


Q.8 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


Q.9 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

Q.10 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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Also Read Electrical Engineering Interview Questions:

Q.1

While conducting short-circuit test on a transformer the following side is short circuited

(A) High voltage side

(B) Low voltage side

(C) Primary side

(D) Secondary side

Ans.
B. Low voltage coil



Q.2

Which of the following is not a routine test on transformers?

(A) Core insulation voltage test

(B) Impedance test

(C) Radio interference test

(D) Polarity test
Ans.
C. Radio interference test



Q.3

In a certain loaded transformer, the secondary voltage is one-fourth the primary voltage. The secondary current is

(A) one-fourth the primary current

(B) four times the primary current

(C) equal to the primary current

(D) one-fourth the primary current and equal to the primary current

Ans.
(B) four times the primary current


Q.4

The rating of transformer may be expressed in ____________.

(A) kVAR

(B) kW

(C) Horse power

(D) kVA
Ans.
(D) kVA


Q.5

The primary winding of a power transformer should always be

(A) open

(B) shorted

(C) switched

(D) fused
Ans.
(D) Fused


Q.6

A transformer is used for

(A) changes ac to dc

(B) changes dc to ac

(C) steps up or down dc voltages

(D) steps up or down ac voltages

Ans.
(D) step up or down ac voltages


Q.7

What is the working principle of transformer?

(A) Thermal

(B) Electromagnetic induction

(C) Electromagnetic radiation

(D) Thermal leakage

Ans.
(B) Electromagnetic radiation


Q.8

Which one term is not related to Transformer?

(A) core

(B) yoke

(C) pole

(D) winding

Ans.
(C) pole


Q.9

Which type of winding is used in shall type of transformer?

(A) sandwich winding

(B) circular winding

(C) rectangular winding

(D) square winding

Ans.
(A) sandwich winding


Q.10

How many magnetic circuit has a core type transformer?

(A) one

(B) two

(C) three

(D) Four

Ans.
(A) One



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