Nature of Electricity

Nature of Electricity

 

Electricity is the most common form of energy. Electricity is used for various applications such as lighting, transportation, cooking, communication, production of various goods in factories and much more. None of us exactly know that what is electricity. The concept of electricity and theories behind it, can be developed by observing its different behaviors. For observing nature of electricity, it is necessary to study the structure of matters. Every substance in this universe is made up of extremely small particles known as molecules. The molecule is the smallest particle of a substance into which all the identities of that substance are present. The molecules are made up of further smaller particles known as atoms. An atom is the smallest particle of an element that can exist.

 

There are two types of substances. The substance, that's molecules are made of similar atoms is known as an element. The matter whose molecules consisting dissimilar atoms, is called a compound. The concept of electricity can be achieved from the atomic structures of substances.

Structure of Atom

An atom consists of one central nucleus. The nucleus is made up of positive protons and charge less neutrons. This nucleus is surrounded by numbers of orbital electrons. Each electron has a negative charge of - 1.602 × 10 ^{- 19}Coulomb and each proton in the nucleus has a positive charge of + 1.602 × 10  ^- 19 Coulomb. Because of the opposite charge there is some attraction force between the nucleus and orbiting electrons. Electrons have relatively negligible mass compared to the mass of the nucleus. The mass of each proton and neutrons is 1840 times the mass of an electron. As the modulus value of each electron and each proton are same, the number of electrons is equal to the number protons in an electrically neutral atom. An atom becomes positively charged ion when it loses electrons and similarly an atom becomes negative ion when it gains electrons. 

Atoms may have loosely bonded electrons in their outermost orbits. These electrons require a very small amount of energy to detach themselves from their parent atoms. These electrons are referred as free electrons which move randomly inside the substance and transferred from one atom to another. Any piece of substances which as a whole contains an unequal number of electrons and protons is referred as electrically charged. When there is more number of electrons compared to its protons, the substance is said to be negatively charged and when there is more number of protons compared to electrons, the substance is said to be positively charged.

The basic nature of electricity is, whenever a negatively charged body is connected to a positively charged body by means of a conductor, the excess electrons of negative body starts flowing towards the positive body to compensate the lack of electrons in that positive body.  
Hope you got the very basic concept of electricity from the above explanation. There are some materials which have plenty of free electrons at normal room temperature. Very well known examples of this type of materials are, silver, copper, aluminium, zinc etc. The movement of these free electrons can easily be directed to a particular direction if the electrical potential difference is applied across the piece of these materials. Because of plenty of free electrons these materials have good electrical conductivity. These materials are referred as good conductor. The drift of electrons in a conductor in one direction is known as the current. Actually electrons flow from lower potential (-Ve) to higher potential (+Ve) but the general conventional direction of current has been considered as the highest potential point to lower potential point, so the conventional direction of current has been just opposite of the direction of flow of electrons. In non-metallic materials, such as glass, mica, slate, porcelain, the outermost orbit is completed and there is almost no chance of loosing electrons from its outermost shell. Hence there is hardly any free electron present in this type of material.
Hence, these materials cannot conduct electricity in other words electrical conductivity of these materials is very poor. Such material are known as non - conductor or electrical insulator. The nature of electricity is to flow through a conductor while an electrical potential difference applied across it, but not to flow through insulator even high electrical potential difference applied across them.

 

 

What Is Electrical Engineering

What Is Electrical Engineering

Electrical engineering is one of the newer branches of engineering, and dates back to the late 19th century. It is the branch of engineering that deals with the technology of electricity. Electrical engineers work on a wide range of components, devices and systems, from tiny microchips to huge power station generators.

Early experiments with electricity included primitive batteries and static charges. However, the actual design, construction and manufacturing of useful devices and systems began with the implementation of Michael Faraday's Law of Induction, which essentially states that the voltage in a circuit is proportional to the rate of change in the magnetic field through the circuit. This law applies to the basic principles of the electric generator, the electric motor and the transformer. The advent of the modern age is marked by the introduction of electricity to homes, businesses and industry, all of which were made possible by electrical engineers.

Some of the most prominent pioneers in electrical engineering include Thomas Edison (electric light bulb), George Westinghouse (alternating current), Nikola Tesla (induction motor), Guglielmo Marconi (radio) and Philo T. Farnsworth (television). These innovators turned ideas and concepts about electricity into practical devices and systems that ushered in the modern age.

Since its early beginnings, the field of electrical engineering has grown and branched out into a number of specialized categories, including power generation and transmission systems, motors, batteries and control systems. Electrical engineering also includes electronics, which has itself branched into an even greater number of subcategories, such as radio frequency (RF) systems, telecommunications, remote sensing, signal processing, digital circuits, instrumentation, audio, video and optoelectronics.

The field of electronics was born with the invention of the thermionic valve diode vacuum tube in 1904 by John Ambrose Fleming. The vacuum tube basically acts as a current amplifier by outputting a multiple of its input current. It was the foundation of all electronics, including radios, television and radar, until the mid-20th century. It was largely supplanted by the transistor, which was developed in 1947 at AT&T's Bell Laboratories by William Shockley, John Bardeen and Walter Brattain, for which they received the 1956 Nobel Prize in physics.

What does an electrical engineer do?

"Electrical engineers design, develop, test and supervise the manufacturing of electrical equipment, such as electric motors, radar and navigation systems, communications systems and power generation equipment, states the U.S. Bureau of Labor Statistics. "Electronics engineers design and develop electronic equipment, such as broadcast and communications systems — from portable music players to global positioning systems (GPS)."

If it's a practical, real-world device that produces, conducts or uses electricity, in all likelihood, it was designed by an electrical engineer. Additionally, engineers may conduct or write the specifications for destructive or nondestructive testing of the performance, reliability and long-term durability of devices and components.

Today’s electrical engineers design electrical devices and systems using basic components such as conductors, coils, magnets, batteries, switches, resistors, capacitors, inductors, diodes and transistors. Nearly all electrical and electronic devices, from the generators at an electric power plant to the microprocessors in your phone, use these few basic components.

Critical skills needed in electrical engineering include an in-depth understanding of electrical and electronic theory, mathematics and materials. This knowledge allows engineers to design circuits to perform specific functions and meet requirements for safety, reliability and energy efficiency, and to predict how they will behave, before a hardware design is implemented. Sometimes, though, circuits are constructed on "breadboards," or prototype circuit boards made on computer numeric controlled (CNC) machines for testing before they are put into production.

Electrical engineers are increasingly relying on computer-aided design (CAD) systems to create schematics and lay out circuits. They also use computers to simulate how electrical devices and systems will function. Computer simulations can be used to model a national power grid or a microprocessor; therefore, proficiency with computers is essential for electrical engineers. In addition to speeding up the process of drafting schematics, printed circuit board (PCB) layouts and blueprints for electrical and electronic devices, CAD systems allow for quick and easy modifications of designs and rapid prototyping using CNC machines. A comprehensive list of necessary skills and abilities for electrical and electronics engineers can be found at MyMajors.com.

Electrical engineering jobs and salaries

Electrical and electronics engineers work primarily in research and development industries, engineering services firms, manufacturing and the federal government, according to the BLS. They generally work indoors, in offices, but they may have to visit sites to observe a problem or a piece of complex equipment, the BLS says.

Manufacturing industries that employ electrical engineers include automotive, marine, railroad, aerospace, defense, consumer electronics, commercial construction, lighting, computers and components, telecommunications and traffic control. Government institutions that employ electrical engineers include transportation department's, national laboratories and the military.

Most electrical engineering jobs require at least a bachelor's degree in engineering. Many employers, particularly those that offer engineering consulting services, also require state certification as a Professional Engineer. Additionally, many employers require certification from the Institute of Electrical and Electronics Engineers (IEEE) or the Institution of Engineering and Technology (IET). A master's degree is often required for promotion to management, and ongoing education and training are needed to keep up with advances in technology, testing equipment, computer hardware and software, and government regulations.

As of July 2014, the salary range for a newly graduated electrical engineer with a bachelor's degree is $55,570 to $73,908, according to Salary.com. The range for a mid-level engineer with a master's degree and five to 10 years of experience is $$74,007 to $108,640, and the range for a senior engineer with a master's or doctorate and more than 15 years of experience is $97,434 to $138,296. Many experienced engineers with advanced degrees are promoted to management positions or start their own businesses where they can earn even more.

The future of electrical engineering

Employment of electrical and electronics engineers is projected to grow by 4 percent between now and 2022, because of these professionals' "versatility in developing and applying emerging technologies," the BLS says.

The applications for these emerging technologies include studying red electrical flashes, called sprites, which hover above some thunderstorms. Victor Pasko, an electrical engineer at Penn State, and his colleagues have developed a model for how the strange lightning evolves and disappears.

Another electrical engineer, Andrea Alù, of the University of Texas at Austin, is studying sound waves and has developed a one-way sound machine. "I can listen to you, but you cannot detect me back; you cannot hear my presence," Alù told LiveScience in a 2014 article.

And Michel Maharbiz, an electrical engineer at the University of California, Berkeley, is exploring ways to communicate with the brain wirelessly.

The BLS states, "The rapid pace of technological innovation and development will likely drive demand for electrical and electronics engineers in research and development, an area in which engineering expertise will be needed to develop distribution systems related to new technologies."

 

 

Basic Electrical Engineering Test -1

 [1] Electrostatics is a branch of electricity concerned with
(a) Energy flowing across a gap between conductors
(b) Charges at rest
(c) Charges in motion
(d) Energy in the form of charges
 

[2] Four 2 μF capacitors are connected in series. The equivalent capacitance is
(a) 8 μF
(b) 0.5 μF
(c) 2 μF
(d) 6 μF
 

[3] State which of the following is false.
The capacitance of a capacitor
(a) Is proportional to the cross-sectional area of the plates
(b) Is proportional to the distance between the plates
(c) Depends on the number of plates
(d) Is proportional to the relative permittivity of the dielectric
 

[4] The capacitance of a capacitor is the ratio
(a) Charge to potential difference between plates
(b) Potential difference between plates to plate spacing
(c) Potential difference between plates to thickness of dielectric
(d) Potential difference between plates to charge
 

[5] Which of the following statement is false?
(a) An air capacitor is normally a variable type
(b) A paper capacitor generally has a shorter service life than most other types of
capacitor
(c) An electrolytic capacitor must be used only on a.c. supplies
(d) Plastic capacitors generally operate satisfactorily under conditions of high temperature
 

[6] The potential difference across a 10 μF capacitor to charge it with 10mC is
(a) 10V
(b) 1 kV
(c) 1V
(d) 10V
 

[7] The energy stodeeppink in a 10μF capacitor when charged to 500V is
(a) 1.25 mJ
(b) 0.025 μJ
(c) 1.25 J
(d) 1.25 C
 

[8] The capacitance of a variable air capacitor is at maximum when
(a) The movable plates half overlap the fixed plates
(b) The movable plates are most widely separated from the fixed plates
(c) Both sets of plates are exactly meshed
(d) The movable plates are closer to one side of the fixed plate than to the other
 

[9] The unit of magnetic flux density is the:
(a) Weber
(b) Weber per metre
(c) Ampere per metre
(d) Tesla
 

[10] The charge on a 10 pF capacitor when the voltage applied to it is 10 kV is
(a) 100 μC
(b) 0.1 C
(c) 0.1 μC
(d) 0.01 μC
 

[11] Four 2 μF capacitors are connected in parallel. The equivalent capacitance is
(a) 8 μF
(b) 0.5 μF
(c) 2 μF
(d) 6 μF
 

[12] In a series a.c. circuit the voltage across a pure inductance is 12V and the voltage across a pure resistance is 5V. The supply voltage is
(a) 13V
(b) 17V
(c) 7V
(d) 2.4V
 

[13] Inductive reactance results in a current that
(a) Leads the voltage by 90deg
(b) Is in phase with the voltage
(c) Leads the voltage by π rad
(d) Lags the voltage by π/2 rad
 

[14] A 10Ω resistor is connected in parallel with a 15 Ω resistor and the combination in series with a 12 Ω resistor. The equivalent resistance of the circuit is:
(a) 37Ω
(b) 18 Ω
(c) 27 Ω
(d) 4 Ω
 

[15] The equivalent resistance when a resistor of (1/3)Ω is connected in parallel with a (1/4)Ω resistance is:
(a) 1/7 Ω
(b) 7Ω
(c) 1/12 Ω
(d) 3/4 Ω

Transformer mcq part -7

1. Which of the following loss in a transformer is zero even at full load ?
(a) Core loss
(b) Friction loss
(c) Eddy current loss
(d) Hysteresis loss

Ans: b

2. Which of the following is the most likely source of harmonics in a transformer ?
(a) poor insulation
(b) Overload
(c) loose connections
(d) Core saturation

Ans: d

3. If a transformer is continuously operated the maximum temperature rise will occur in
(a) core
(b) windings
(c) tank
(d) any of the above

Ans: b

4. The hum in a transformer is mainly attributed to
(a) load changes
(b) oil in the transformer
(c) magnetostriction
(d) mechanical vibrations

Ans: c
 

5. The maximum load that a power transformer can carry is limited by its
(a) temperature rise
(b) dielectric strength of oil
(c) voltage ratio
(d) copper loss

Ans: c

6. The efficiency of a transformer, under heavy loads, is comparatively low because
(a) copper loss becomes high in proportion to the output
(b) iron loss is increased considerably
(c) voltage drop both in primary and secondary becomes large
(d) secondary output is much less as compared to primary input

Ans: a

7. An open-circuit test on a transformer is conducted primarily to measure
(a) insulation resistance
(b) copper loss
(c) core loss
(d) total loss
(e) efficiency
(f) none of the above

Ans: c

8. A no-load test is performed on a transformer to determine
(a) core loss
(b) copper loss
(c) efficiency
(d) magnetising current
(e) magnetising current and loss

Ans: e

9. The voltage transformation ratio of a transformer is equal to the ratio of
(a) primary turns to secondary turns
(b) secondary current to primary current
(c) secondary induced e.m.f. to primary induced e.m.f.
(d) secondary terminal voltage to primary applied voltage

Ans: c

10. Part of the transformer which is most subject to damage from overheating is
(a) iron core
(b) copper winding
(c) winding insulation
(d) frame or case
(e) transformer tank

Ans: c

11. If a transformer is switched on to a voltage more than the rated voltage
(a) its power factor will deteriorate
(b) its power factor will increase
(c) its power factor will remain unaffected
(d) its power factor will be zero

Ans: a

12. Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is
(a) approximately equal to one
(b) less than one
(c) great than one
(d) none of the above

Ans: a

13. Minimum voltage regulation occurs when the power factor of the load is
(a) unity
(b) lagging
(c) leading
(d) zero

Ans: c

14. In a step-down transformer, there is a change of 15 A in the load current. This results in change of supply current of
(a) less than 15 A
(b) more than 15 A
(c) 15 A
(d) none of the above

Ans: a

15. The efficiencies of transformers compared with that of electric motors of the same power are
(a) about the same
(6) much smaller
(c) much higher
(d) somewhat smaller
(e) none of the above

Ans: c

Transformer mcq part - 6

1. A shell-type transformer has
(a) high eddy current losses
(b) reduced magnetic leakage
(c) negligibly hysteresis losses
(d) none of the above

Ans: b

2. A transformer can have regulation closer to zero
(a) on full-load
(b) on overload
(c) on leading power factor
(d) on zero power factor

Ans: c

3. A transformer transforms
(a) voltage
(b) current
(c) current and voltage
(d) power

Ans: d

4. Which of the following is not the standard voltage for power supply in India ?
(a) llkV
(b) 33kV
(c) 66 kV
(d) 122 kV

Ans: d

5. Reduction in core losses and increase in permeability are obtained with transformer employing
(a) core built-up of laminations of cold rolled grain oriented steel
(b) core built-up of laminations of hot rolled sheet
(c) either of the above
(d) none of the above

Ans: a

6. In a power or distribution transformer about 10 per cent end turns are heavily insulated
(a) to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns
(b) to absorb the line surge voltage and save the winding of transformer from damage
(c) to reflect the line surge and save the winding of a transformer from damage
(d) none of the above

Ans: a

7. For given applied voltage, with the increase in frequency of the applied voltage
(a) eddy current loss will decrease
(b) eddy current loss will increase
(c) eddy current loss will remain unchanged
(d) none of the above

Ans: c

8. Losses which occur in rotating electric machines and do not occur in trans formers are
(a) friction and windage losses
(b) magnetic losses
(c) hysteresis and eddy current losses
(d) copper losses

Ans: a

9. In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are
(a) hysteresis and eddy current losses
(b) friction and windage losses
(c) copper losses
(d) none of the above

Ans: a

10. Which of the following statements regarding an idel single-phase transformer having a turn ratio of 1 : 2 and drawing a current of 10 A from 200 V A.C. supply is incorrect ?
(a) Its secondary current is 5 A
(b) Its secondary voltage is 400 V
(c) Its rating is 2 kVA
(d) Its secondary current is 20 A
(e) It is a step-up transformer

Ans: d

11 The secondary of a current transformer is always short-circuited under operating conditions because it
(a) avoids core saturation and high voltage induction
(b) is safe to human beings
(c) protects the primary circuit
(d) none of the above

Ans: a

12. In a transformer the resistance between its primary and secondary should be
(a) zero
(b) 10 Q
(c) 1000 Q
(d) infinity

Ans: d

13. A good voltage regulation of a transformer means
(a) output voltage fluctuation from no load to full load is least
(b) output voltage fluctuation with power factor is least
(c) difference between primary and secondary voltage is least
(d) difference between primary and secondary voltage is maximum

Ans: a

14. For a transformer, operating at constant load current, maximum efficiency will occur at
(a) 0.8 leading power factor
(b) 0.8 lagging power factor
(c) zero power factor
(d) unity power factor

Ans: d

15. Which of the following protection is normally not provided on small distribution transformers ?
(a) Overfluxing protection
(b) Buchholz relay
(c) Overcurrent protection
(d) All of the above

Ans: b

16. Which of the following acts as a protection against high voltage surges due to lightning and switching ?
(a) Horn gaps
(b) Thermal overload relays
(c) Breather
(d) Conservator

Ans: a

17. The efficiency of two identical transformers under load conditions can be determined by
(a) short-circuit test
(b) back-to-back test
(c) open circuit test
(d) any of the above

Ans: b

18. Which of the following insulating materials can withstand the highest temperature safely ?
(a) Cellulose
(b) Asbestos
(c) Mica
(d) Glass fibre

Ans: c

19. Which of the following parts of a transformer is visible from outside ?
(a) Bushings
(b) Core
(c) Primary winding
(d) Secondary winding

Ans: a

20. The noise produced by a transformer is termed as
(a) zoom
(b) hum
(c) ringing
(d) buzz

Ans: b

Transformer mcq part -5

1. The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and
night is taken care of by which part of transformer
(a) Conservator
(b) Breather
(c) Bushings
(d) Buchholz relay

Ans: a

2. An ideal transformer is one which has
(a) no losses and magnetic leakage
(b) interleaved primary and secondary windings
(c) a common core for its primary and secondary windings
(d) core of stainless steel and winding of pure copper metal
(e) none of the above

Ans: a

3. When a given transformer is run at its rated voltage but reduced frequency, its
(a) flux density remains unaffected
(b) iron losses are reduced
(c) core flux density is reduced
(d) core flux density is increased

Ans: d

4. In an actual transformer the iron loss remains practically constant from noload to fullload because
(a) value of transformation ratio remains constant
(b) permeability of transformer core remains constant
(c) core flux remains practically constant
(d) primary voltage remains constant
(c) secondary voltage remains constant

Ans: c

5. An ideal transformer will have maximum efficiency at a load such that
(a) copper loss = iron loss
(b) copper loss < iron loss
(c) copper loss > iron loss
(d) none of the above

Ans: a

6. If the supply frequency to the transformer is increased,"the iron loss will
(a) not change
(b) decrease
(c) increase
(d) any of the above

Ans: c

7. Negative voltage regulation is indicative that the load is
(a) capacitive only
(b) inductive only
(c) inductive or resistive
(d) none of the above

Ans: a

8. Iron loss of a transformer can be measured by
(a) low power factor wattmeter
(b) unity power factor wattmeter
(c) frequency meter
(d) any type of wattmeter

Ans: a

9. When secondary of a current transformer is open-circuited its iron core will be
(a) hot because of heavy iron losses taking place in it due to high flux density
(b) hot because primary will carry heavy current
(c) cool as there is no secondary current
(d) none of above will happen

Ans: a

10. The transformer laminations are insulated from each other by
(a) mica strip
(6) thin coat of varnish
(c) paper
(d) any of the above

Ans: b

11. Which type of winding is used in 3phase shell-type transformer ?
(a) Circular type
(b) Sandwich type
(c) Cylindrical type
(d) Rectangular type

Ans: b

12. During open circuit test of a transformer
(a) primary is supplied rated voltage
(b) primary is supplied full-load current
(c) primary is supplied current at reduced voltage
(d) primary is supplied rated kVA

Ans: a

13. Open circuit test on transformers is conducted to determine
(a) hysteresis losses
(b) copper losses
(c) core losses
(d) eddy current losse

Ans: c

14. Short circuit test on transformers is conducted to determine
(a) hysteresis losses
(b) copper losses
(c) core losses
(d) eddy current losse

Ans: b

15. For the parallel operation of single phase transformers it is necessary that they should have
(a) same efficiency
(b) same polarity
(c) same kVA rating
(d) same number of turns on the secondary side.

Ans: b

16. The transformer oil should have volatility and viscosity.
(a) low,low
(b) high,high
(c) low,high
(d) high,low

Ans: a

17. The function of breather in a transformer is
(a) to provide oxygen inside the tank
(b) to cool the coils during reduced load
(c) to cool the transformer oil
(d) to arrest flow of moisture when outside air enters the transformer

Ans: d

18. The secondary winding of which of the following transformers is always kept closed ?
(a) Step-up transformer
(b) Step-down transformer
(c) Potential transformer
(d) Current transformer

Ans: d

19. The size of a transformer core will depend on
(a) frequency
(b) area of the core
(c) flux density of the core material
(d) (a) and (b) both

Ans: d

20. N atural air coo ling is generally restricted for transformers up to
(a) 1.5 MVA
(b) 5 MVA
(c) 15 MVA
(d) 50 MVA

Ans: a

Synchronous motor mcq part -6

1. In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then
(a) the motor is said to be fully loaded
(b) the torque generated is maximum
(c) the excitation is said to be zero per cent
(d) the excitation is said to be hundred per cent

Ans: d

2. In a synchronous motor, the damping winding is generally used to
(a) prevent hunting and provide the starting torque
(b) reduce the eddy currents
(c) provide starting torque only
(d) reduce noise level
(e) none of the above

Ans: a

3. If the field of a synchronous motor is underexcited, the power factor will be
(a) zero
(b) unity
(c) lagging
(d) leading

Ans: c

4. The back e.m.f. in the stator of a synchronous motor depends on
(a) number of poles
(b) flux density
(c) rotor speed
(d) rotor excitation
(e) none of the above

Ans: d

5. The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as
(a) slip torque
(b) pull-out torque
(c) breaking torque
(d) synchronising torque

Ans: d

6. In a synchronous motor, the armature current has large values for
(a) high excitation only
(b) low excitation only
(c) both high and low excitation
(d) none of the above

Ans: c

7. Which of the following losses, in a synchronous motor, does not vary with load?
(a) Windage loss
(b) Copper losses
(c) Any of the above
(d) None of the above

Ans: a

8. The size of a synchronous motor decreases with the increase in
(a) flux density
(b) horse power rating
(c) speed
(d) all of the above

Ans: a

9. Which of the following losses is not dissipated by the stator core surface in a synchronous motor ?
(a) Eddy current losses in the conductors
(b) Iron losses in the stator
(c) Copper losses in the slot portion of the conductors
(d) Windage losses
(e) None of the above

Ans: d

10. The duration of sudden snort-circuit test on a synchronous motor is usually about
(a) one hour
(b) one minute
(c) one second
(d) none of the above

Ans: c

11. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as
(a) pull-up torque
(b) pull-in torque
(c) pull-out torque
(d) none of the above

Ans: b

12. A synchronous machine with low value of short-circuit ratio has
(a) lower stability limit
(6) high stability limit
(c) good speed regulation
(d) good voltage regulation
(e) none of the above

Ans: a

13. The construction of a synchronous motor resembles
(a) a series motor
(b) an induction motor
(c) an alternator
(d) a rotary converter

Ans: c

14. If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will
(a) stop
(b) run as induction motor
(c) function as static condenser
(d) burn with dense smoke

Ans: a

15. For power factor correction, synchronous motors operate at
(a) no-load and greatly over-excited fields
(b) no-load and under-excited fields
(c) normal load with minimum excitation
(d) normal load with zero excitation

Ans: a

16. The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as
(a) locked-rotor torque
(b) synchronous torque
(c) pull up torque
(d) reluctance torque

Ans: a

17. Exciters of synchronous machines are
(a) d.c. shunt machines
(b) d.c. series machines
(c) d.c. compound machines
(d) any of the above

Ans: a

18. The coupling angle or load angle of synchronous motor is defined as the angle between the
(a) rotor and stator teeth
(b) rotor and the stator poles of opposite polarity
(c) rotor and the stator poles of the same polarity
(d) none of the above

Ans: b

19. If the synchronous motor, properly synchronised to the supply is running on no load and is having negligible loss then
(a) the stator current will be zero
(b) the stator current will be very small
(c) the stator current will be very high
(d) the back e.m.f. will be more than the supply voltage
(e) none of the above

Ans: a

20 The armature current of the synchronous motor
(a) has large values for low excitation i niy
(b) has large values for high excitation only
(c) has large values for low and high excitation
(d) any of the above

Ans: c

21. The maximum power developed in a synchronous motor will depend on
(a) the rotor excitation only
(b) the supply voltage only
(c) the rotor excitation and supply volt-age both
(d) the rotor excitation, supply voltage and maximum value of coupling angle (90°)
(e) none of the above

Ans: d

22. A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as
(a) static condenser
(b) condenser
(c) synchronous condenser
(d) none of the above

Ans: c

23. A synchronous motor develops maximum power when load angle is
(a) 45°
(b) 60°
(c) 90°                                                                                                                                   (d) 120°

Ans: c

24. In a synchronous motor, the breakdown torque is
(a) directly proportional to applied voltage
(b) directly proportional to the square of the applied voltage
(c) inversely proportional to applied voltage
(d) none of the above

Ans: a

Synchronous motor mcq part -5

1. The maximum speed variation in a 3-phase synchronous motor is
(a) 10 per cent
(b) 6 per cent
(c) 4 per cent
(d) 2. per cent
(e) zero

Ans: e

2. Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
(a) Phase to phase winding resistance
(b) Stator winding to earthed frame
(c) Rotor winding to earthed shaft
(d) All of the above

Ans: d

3. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?
(a) High field current
(b) Low short circuit ratio
(c) High core losses
(d) Low field current

Ans: d

4. In a synchronous motor, the maximum power developed depends on all of the following except
(a) rotor excitation
(b) maximum value of coupling angle
(c) direction of rotation
(d) supply voltage

Ans: c

5. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
(a) supplied with unbalanced voltage
(b) under-loaded
(c) over-loaded
(d) none of the above

Ans: a

6. In a synchronous motor, damper windings are provided on
(a) stator frame
(b) rotor shaft
(c) pole faces
(d) none of the above

Ans: c

7. The induced e.m.f. in a synchronous motor working on leading power factor will be
(a) more than the supply voltage
(b) less than the supply voltage
(c) equal to the supply voltage

Ans: a

8. The effect of increasing the load on a synchronous motor running with normal excitation is to
(a) decrease both armature current and power factor
(6) decrease armature current but increase power factor
(c) increase armature current but decrease power factor
(d) increase both its armature current and power factor

Ans: c

9. The net armature voltage of a synchronous motor is equal to the
(a) vector sum of Eb and V
(b) arithmetic sum of Eb and V
(c) arithmetic difference of Eb and V
(d) vector difference of Eh and V

Ans: d

10. The ratio of starting torque to running torque in a synchronous motor is
(a) zero
(b) one
(c) two
(d) infinity

Ans: a

11. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(a) load on the motor
(b) d.c. excitation only
(c) both the speed and rotor flux
(d) none of the above

Ans: b

12. A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
(a) the motor will stop
(b) the motor continue to run in the same direction
(c) the winding of the motor will burn
(d) the motor will run in the reverse direction
(e) none of the above

Ans: b

13. The magnitude of field flux in a 3-phase synchronous motor
(a) remains constant at all loads
(b) varies with speed
(c) varies with the load
(d) varies with power factor

Ans: a

14. The torque angle, in a synchronous motor, is the angle between
(a) the supply voltage and the back e.m.f.
(b) magnetising current and back e.m.f.
(c) the rotating stator flux and rotor poles
(d) none of the above

Ans: c

15. Hunting in a synchronous motor cannot be due to
(a) windage friction
(b) variable load
(c) variable frequency
(d) variable supply voltage

Ans: a

16. By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?
(a) By changing the supply frequency
(b) By interchanging any two phases
(c) By changing the applied voltage
(d) By changing the load.

Ans: a

17. In a synchronous motor, V-curves represent relation between
(a) armature current and field current
(b) power factor and speed
(c) field current and speed
(d) field current and power factor

Ans: a

18. In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be
(a) 3000 r.p.m.
(b) 1500 r.p.m.
(c) 750 r.p.m.
(d) none of the above

Ans: b

19. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have
(a) no change of power factor
(b) lagging power factor with over-excitation
(c) leading power factor with under-excitation
(d) leading power factor with over-excitation

Ans: d

20. Which of the following motors is non-self starting ?
(a) D.C. series motor
(b) synchronous motor
(c) Squirrel cage induction motor
(d) Wound round induction motor

Ans: b

Singal phase induction motor mcq part -6

1. After the starting winding of a single phase induction motor is disconnected from supply, it continues to run only on
(a) running winding
(b) rotor winding
(c) field winding
(d) compensating winding

Ans: a

2. Which of the following statements regarding repulsion-start induction motor is incorrect ?
(a) It requires more maintenance of commutator and other mechanical devices
(b) It makes quite a bit of noise on starting
(c) In fractional horse power motors, it has replaced the capacitor motors
(d) It is not easily reversed

Ans: c

3. A.C. series motor as compared to D.C. series motor has
(a) smaller brush width
(b) less number of field turns
(c) more number of armature turns
(d) less air gap
(e) all of the above

Ans: e

4. Locked rotor current of a shaded pole motor is
(a) equal to full load current
(b) less than full load current
(c) slightly more than full load current
(d) several times the full load current

Ans: c

5. Speed control of a universal motor is achieved by
(a) varying field flux with tapped field windings
(b) connecting rheostat in series
(c) applying variable voltage by means of silicon controlled rectifier
(d) applying variable voltage by means of variable auto-transformer
(e) all of the above methods

Ans: e

6. Hysteresis motor is particularly useful for high-quality record players and tape- recorders because
(a) it revolves synchronously
(b) it is not subject to any magnetic or mechanical vibrations
(c) it can be easily manufactured in extremely small sizes of upto 1 W output
(d) it develops hysteresis torque which is extremely steady both in amplitude and phase

Ans: d

7. Which of the following statements regarding hysteresis motor is in incorrect ?
(a) It is extremely sensitive to fluctuations in supply voltage
(b) Its high starting torque is due to its high rotor hysteresis loss
(c) It is extremely quiet in operation
(d) It accelerates from rest to full-speed almost instantaneously

Ans: a

8. Which of the following statements regarding single-phase induction motoris correct
?
(a) It requires only one winding
(b) It can rotate in one direction only
(c) It is self-starting
(d) It is not self-starting

Ans: d

9. The starting winding of a single-phase motor is placed in
(a) armature
(b) field
(c) rotor
(d) stator

Ans: d

10. The speed of a universal motor is usually reduced by using
(a) gearing
(b) belts
(c) brakes
(d) chains

Ans: a