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September 18, 2016

12 Most Common Interview Questions and Answers on Transformer - Part 2

Q. 1: What is induction?

A. 1: The process by which one conductor produces, or induces, a voltage in another conductor, even though there is no mechanical coupling between the two conductors.

Q. 2: What is inductance?

A. 2: The property of a coil in a circuit to oppose any change of existing current flow.

Q. 3: What is self-inductance?

A. 3: The inducing of an emf within the circuit itself, caused by any change of current within that circuit. This induced emf is always in a direction opposite to the applied emf, thus causing opposition to any change in current within the circuit itself.

Q. 4: What is mutual inductance?

A. 4: The linkage of flux between two coils or conductors, caused by the current flowing within one or both of the coils or conductors.

Q. 5: Name several methods by which an emf can be generated.

A. 5: By conductors being cut by-
  • A magnetic field (as in generators) 
  • Chemical reactions (as in batteries)
  • Heat (as in thermocouples)
  • Crystal vibration (as in piezoelectricity) and 
  • Friction (as in static electricity).

Q. 6: What is a transformer?

A. 6: A device that transforms electrical energy from one or more circuits to one or more other circuits at the same frequency but usually at a different voltage and current. It consists of a core of soft-iron laminations surrounded by coils of copper-insulated wire.

Q. 7: Draw a diagram of two coils, such as the coils of a transformer winding, and indicate the self-inductance and the mutual inductance.

A. 7: Self-inductance is produced within the primary coil, and mutual inductance exists between the two transformer coils, as shown bellow this Figure:
Self-inductance and mutual inductance in the coils of a Transformer.
Figure: Self-inductance and mutual inductance in the coils of a Transformer.

Q. 8: What always surrounds a conductor when a current flows through it?

A. 8: A magnetic field.

Q. 9: There are two basic types of transformers. What are they?

A. 9: The isolation type, in which the two windings are physically isolated and electrically insulated from each other, and the autotransformer type, in which there is only one coil with a tap or taps taken off it to secure other voltages (the primary is part of the secondary and the secondary is part of the primary).

Q. 10: What is an oil-immersed transformer?

A. 10: The core and coils are immersed in a high-grade mineral oil, which has high dielectric qualities.

Q. 11: What is Transformer Oil?

A. 11: Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Transformer oil is most often based on mineral oil, but alternative formulations with better engineering and/or environmental properties are growing in popularity.

Q. 12:  Why is oil used in a transformer?

A.12: To increase the dielectric strength of the insulation, to keep down the possibility of arcing between coils, and to dissipate heat to the outer case so that the transformer can carry heavier loads without excessive overheating.

  1. Book of Paul Rosenberg
  2. Wikipedia
  3. Internet.
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 12-most-common-interview-question-and answers on transformer

September 15, 2016

What is optical fiber cable? What are the advantages of optical fiber cable?

Optical fiber cable:

An optical fiber cable is a cable containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed. Different types of cable are used for different applications, for example long distance telecommunication, or providing a high-speed data connection between different parts of a building.

Figure: Optical Fiber Cable.

Advantages of optical fiber cable:

  • High Bandwidth Over Long Distances - Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables. The information carrying capacity increases with frequency. This, however, doesn't mean that optical fiber has infinite bandwidth, but it's certainly greater than coaxial cables. 
  • Less signal degradation - The loss of signal in optical fiber is less than in copper wire. 
  • Data Security - Optical fibers are difficult to tap. As they do not radiate electromagnetic energy, emissions cannot be intercepted. As physically tapping the fiber takes great skill to do undetected, fiber is the most secure medium available for carrying sensitive data.  
  • Safety - Since the fiber is a dielectric, it does not present a spark hazard. 
  • Ease Of Installation - Increasing transmission capacity of wire cables generally makes them thicker and more rigid. Such thick cables can be difficult to install in existing buildings where they must go through walls and cable ducts. Fiber cables are easier to install since they are smaller and more flexible. They can also run along the same routes as electric cables without picking up excessive noise. 
  • Light, flexible and cheap - Optical fiber cable can be used in corrosive environments and is light, flexible and cheap. 
  • Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard.

Read More: What are the various kind of cables used for transmission?

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:

  • Low-tension cables, which can transmit voltage upto 1000 volts.
  • High-tension cables can transmit voltage upto 23000 volts.
  • Super tension cables can transmit voltage 66 kV to 132 kV.

September 14, 2016

How do synchronous motors differ from alternators?

Figure: Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator)
Figure: Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator) 

They may be just like alternators; however, if they are, they won’t be self-starting and will have to be started by some means until they approach synchronous speed, at which time they can be connected to the line and pull into speed. Most synchronous motors have a squirrel cage winding in addition to the dc field. They start as a squirrel cage motor, and when they are about up to the speed of the alternator, the dc field is energized. The poles then lock in position with the revolving field of the armature, and the rotor revolves in synchronization with the supply circuit.

What are the advantages and disadvantages of dc motors compared with ac motors?

Advantages and disadvantages of DC motors compared with AC motors:

Advantages of DC Motor:

  • Speed control over a wide range both above and below the rated speed: Speed control of DC motors is much easier, making them more versatile for use where a wide range of speeds is required. 
  • They may be used for dynamic braking; that is, a motor on an electric train will act as a motor when required, but when going downhill, it can be used as a generator, thereby putting current back into the line. 
  • Free from harmonics, reactive power consumption and many factors which make DC motors more advantageous compared to AC induction motors.
  • In generating, it requires power, so it acts as a brake. 
  • Quick starting, stopping, reversing and acceleration.

Disadvantages of DC Motor:

  • High initial cost.
  • DC motors, however, require more maintenance than most AC motors. 
  • Cannot operate in explosive and hazardous conditions due to sparking occur at brush (risk in commutation failure).
  • In order to use a DC motor, special provisions must be made.

Some of the applications of DC motors are Steel mills, paper mills, cranes and elevators, electric trains and much more etc.

What is the difference between a dc motor and a dc generator?

Fundamentally, there is none. A dc motor will generate electricity if driven by some prime mover, and a dc generator will act as a motor if connected across a dc source.

Read More:  If a dc motor is connected across an ac source, what will be the result? Why?

If a dc motor is connected across an ac source, what will be the result? Why?

Direct-current flow is obstructed only by resistance, whereas alternating current is obstructed by both resistance and inductive reactance. Therefore, when a dc motor is connected across an ac source, the current on ac will be much less than that of dc. The motor would run, however, but it would not carry the same load as it would on dc. There would be more sparking at the brushes. The armature is made up of laminations, but the field is not. The eddy currents in the field would therefore cause the motor to heat up and eventually burn out on ac; this would not happen on dc.

September 13, 2016

What are the conditions for parallel operation of alternators?

Figure: Parallel operation of shunt generators.

To synchronize AC generators, several important factors must be checked:

  • Equal terminal voltages- The terminal voltage of the incoming machine must be approximately equal to bus-bar voltage. This is obtained by adjustment of the incoming generator’s field strength.
  • Equal frequency- The frequency of the incoming machine must be equal to those bus-bars. This is obtained by adjustment of the incoming generator’s prime-mover speed.
  • Phase voltages in proper phase relation- The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e. the phase voltages of the incoming machine and the bus-bar should be in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation (the bus-bars) and the incoming machine.

Note: The above figure shows the shunt generator No.1 is connected to the bus-bars and delivering load. The shunt Generator No.2 is connected in parallel to the Generator No.1. When the load on the generator No.1 increases beyond its rated capacity, immediately the second shunt generator operate and wish the first generator to come across the raised load demand.

13 Most Important Questions and Answers on AC Motor - Part-1

Figure: Asynchronous AC Motor (Induction)

Q1. What are the three basic types of ac motors?

A1. Series, synchronous, induction.

Q2. Series motors are generally used to operate what type of equipment?

A2. To power small appliances.

Q3. Why are series motors sometimes called universal motors?

A3. They operate on either ac or dc.

Q4. What determines the number of field poles required to establish a rotating magnetic field in a multiphase motor stator?

A4. The number of phases in the applied voltage.

Q5. What is the angular displacement between field poles in a two-phase motor stator?

A5. 90º .

Q6. What is the major difference between a two-phase and a three-phase stator?

A6. Number and location of field poles.

Q7. What requirement is the synchronous motor specifically designed to meet?

A7. Constant speed required by some loads.

Q8. Why is the ac induction motor used more often than other types?

A8. They are simple and inexpensive to make.

Q9. The speed of the rotor is always somewhat less than the speed of the rotating field. What is the difference called?

A9. Slip.

Q10. What determines the amount of slip in an induction motor?

A10. Load.

Q11. What type of ac motor is most widely used?

A11. Single-phase induction motor.

Q12. How do split-phase induction motors become self-starting?

A12. By using combinations of inductance and capacitance to apply out-of phase currents in starting windings.

Q13. Why are shaded-pole motors used to drive only very small devices?

A13. They have very weak starting torques.

Download this Article as PDF:
 13 Most Important Question and Answers on AC Motor - Part-1

Related Post: 

  1. 18 Most Important Questions and Answers on DC Motors - Part-1.
  2. 10 Interview Questions and Answers on Motor - Part-2

21 Common Interview Questions & Answers on AC Generators - Part-2

Figure: Three Phase Generator Output

Q1. Magnetic induction occurs when there is relative motion between what two elements?

A1. A conductor and a magnetic field.

Q2. What is the part of an alternator in which the output voltage is generated?

A2. Armature.

Q3. What are the two basic types of alternators?

A3. Rotating armature and rotating field.

Q4. What is the main advantage of the rotating field alternator?

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

Q5. Most large alternators have a small dc generator built into them. What is its purpose?

A5. To provide dc current for the rotating field.

Q6. How are alternators usually rated?

A6. Kilovolt-amperes (volt amperes).

Q7. What type of prime mover requires a specially designed high-speed alternator?

A7. Steam turbine.

Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?

A8. Internal combustion engines, water force and electric motors.

Q9. What does the term single phase indicate?

A9. One voltage (one output).

Q10. In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?

A10. In series.

Q11. What determines the phase relationship between the voltages in a two-phase ac generator?

A11. Placement of armature coils.

Q12. How many voltage outputs are available from a two-phase three-wire alternator?

A12. Three.

Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?

A13. C is 1.414 times greater than A or B.

Q14. In a three-phase alternator, what is the phase relationship between the individual output voltages?

A14. Each phase is displaced 120º from the other two.

Q15. What are the two methods of connecting the outputs from a three-phase alternator to the load?

A15. Wye and Delta.

Q16. Ships’ generators produce 450-volt, three-phase, ac power; however, most equipment uses 117volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?

A16. Three single-phase, delta-delta, step-down transformers.

Q17. What two factors determine the frequency of the output voltage of an alternator?

A17. Speed of rotation and number of poles.

Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

A18. 120 Hz.

Q19. The variation in output voltage as the load changes is referred to as what? How is it expressed?

A19. Voltage regulation. As a percentage.

Q20. How is output voltage controlled in practical alternators?

A20. By varying the voltage applied to the field windings.

Q21. What generator characteristics must be considered when alternators are synchronized for parallel operation?

A21. Output voltage, frequency, and phase relationships.

September 12, 2016

Discuss about different types of Transformers and their Applications.


A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Electromagnetic induction produces an electromotive force across a conductor which is exposed to time varying magnetic fields. Commonly, transformers are used to increase or decrease the voltages of alternating current in electric power applications.

Photo: Dry type Transformer

Types of Transformers:

1.        Power transformers

a)         Laminated core transformer
b)         Toroidal transformer
c)         Autotransformer
d)         Variable autotransformer
e)         Induction regulator transformer
f)          Polyphase transformer
g)         Grounding transformer
h)         Leakage or stray field transformers
i)          Resonant transformer
j)          Constant voltage transformer
k)         Ferrite core transformer
l)          Planar transformer
m)        Oil cooled transformer
n)         Cast resin transformer
o)         Isolating transformer

2.     Instrument transformer

a)         Current transformer
b)         Voltage transformer or potential transformer
c)         Combined instrument transformer

3.      Pulse transformer

4.      RF transformer

a)         Air-core transformer
b)         Ferrite-core transformer
c)         Transmission-line transformer
d)         Balun transformer

5.     Audio transformer

a)         Loudspeaker transformer
b)         Output transformer
c)         Small signal transformer
d)         Interstage and coupling transformers

6.     Other types

a)         Hedgehog
b)         Variometer and variocoupler
c)         Rotary transformer

Overview and Applications of Transformers:

1. Auto-transformer

An auto-transformer has only a single winding with two end terminals, plus a third of an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage taken from one of these and the third terminal. The primary and secondary circuits therefore have a number of windings turns in common. An adjustable auto-transformer is made by the secondary connection through a sliding brush, giving a variable turns ratio.
Figure: Single-phase tapped autotransformer with output voltage range of 40%–115% of input


  • Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks.

2. Polyphase transformers

For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. In this case, the magnetic circuits are connected together, the core, thus containing a three-phase flow of flux. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-∆, ∆-Y, ∆-∆ and Y-Y. If a winding is connected to earth (grounded), the earth connection point is usually at the center point of a Y winding.


  • For higher- power applications, poly-phase transformers are commonly used. 

3. Leakage transformers

A leakage transformer, also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjusted with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditions – even if the secondary is shorted. 
Figure: Leakage Transformer or Stray field Transformer.


  • Leakage transformers are used for arc welding and high voltage discharge lamps. 
  • Other applications are short-circuit-proof extra-low voltage transformers for toys or doorbell installations.

4. Resonant transformers

A resonant transformer is a kind of the leakage transformer. It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits. Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator. 


  • Intermediate frequency (IF) transformer in superheterodyne radio receiver
  • Tank transformers in radio transmitters
  • Tesla coil
  • Oudin coil (or Oudin resonator; named after its inventor Paul Oudin)
  • D'Arsonval apparatus
  • Ignition coil or induction coil used in the ignition system of a petrol engine
  • Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the transformer's secondary is resonated with the cable's capacitance.

5. Instrument transformers

A current transformer is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relaying, where they facilitate the safe measurement of large currents. The current transformer isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. Voltage transformers (VTs)--also referred to as potential transformers (PTs)--are used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential.

Figure: Current transformers used in metering equipment for three-phase 400 ampere electricity supply


  • Used as a portable current measuring instrument.
  • Measurement of high voltages is possible by the potential transformers.

6. Zigzag transformer

A zigzag transformer is a special purpose transformer. It has primary windings but no secondary winding. Its applications are for the creation of a missing neutral connection from an ungrounded 3-phase system to permit the grounding of that neutral to an earth reference point and also harmonic mitigation, as it can suppress triplet (3rd, 9th, 15th, 21st, etc.) harmonic currents, to supply 3-phase power as an autotransformer (serving as the primary and secondary with no isolated circuits)  and to supply non-standard, phase-shifted, 3-phase power.

Figure: Zigzag transformer


  • One application is to derive an earth reference point for an ungrounded electrical system. 
  • Another is to control harmonic currents.

7. Pulse transformers

A pulse transformer is a transformer that is optimized for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and constant amplitude). 

Figure: Bothhand TS6121A pulse transformer


  • Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. 
  • Medium-sized power versions are used in power-control circuits such as camera flash controllers. 
  • Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. 
  • Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.

8. Audio transformer

Audio transformers or Audio Frequency (AF) Transformers are those specifically designed for use in audio circuits to carry audio signal. They can be used to block radio frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide impedance matching between high and low impedance circuits, such as between a high impedance tube (valve) amplifier output and a low impedance loudspeaker, or between a high impedance instrument output and the low impedance input of a mixing console. Audio transformers that operate with loudspeaker voltages and current are larger than those that operate at microphone or line level, which carry much less power.

Figure: Audio Frequency (AF) Transformers


  • Audio transformers are used in car radios and broadcast equipment, and in sound reinforcement applications to steps up the output of the system's amplifier.

9. Isolation transformers 

An isolation transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics. 

Figure: A 230V isolation transformer.


  • It is used as a power supply for medical equipment, when it is necessary to prevent any leakage from the AC power system into devices connected to a patient.
  • Some small transformers are used for isolation in pulse circuits.
  • In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety.
  • Supplying power to ships.

10. Buck boost transformers

A Buck boost transformer is a type of transformer used to make adjustments to the voltage applied to alternating current equipment. Buck boost transformers make small adjustments to the incoming voltage. One major advantages of Buck boost transformers are their low cost, compact size and light weight. 
Figure: Typical multi-tap buck–boost transformer.


  • Buck boost transformers can be used to power low voltage circuits including control, lighting circuits, or applications that require 12, 16, 24, 32 or 48 volts, consistent with the design's secondaries.
  • They are often used to change voltage from 208v to 240v for lighting applications. 

11. Pad mounted transformers 

A padmount or pad-mounted transformer is a ground mounted electric power distribution transformer in a locked steel cabinet mounted on a concrete pad. Since all energized connection points are securely enclosed in a grounded metal housing, a padmount transformer can be installed in places that do not have room for a fenced enclosure. Padmount transformers are used with underground electric power distribution lines at service drops, to step down the primary voltage on the line to the lower secondary voltage supplied to utility customers. A single transformer may serve one large building, or many homes. Pad Mounted Transformers are usually single phase or three phase and is used where safety is a main concern.

Figure: Large pad-mount transformers supplying power to a computer data center. No live wires are exposed.


  • Typical Application is restaurant, commercial building, shopping mall, institutional. 

12. Pole mounted transformers 

Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
Figure: Pole mounted distribution transformer.


  • Pole Mounted Transformers are used for distribution in areas with overhead primary lines. 

13. Oil filled transformers 

Oil-filled transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.


  • Oil filled transformers are used in power distribution or electrical substations.

14. Rotary transformers

A rotary (rotatory) transformer is a specialized transformer used to couple electrical signals between two parts that rotate in relation to each other. They may be either cylindrical or 'pancake' shaped.

Figure: Cross-section diagram of a simple rotary transformer.


  • Rotary transformers are most commonly used in videocassette recorders. 
  • Another use is to transmit the signals from rotary torque sensors installed on electric motors, to allow electronic control of motor speed and torque using feedback.

15. Dry type transformers 

Dry type transformers require minimum maintenance to provide many years of reliable trouble free service. Unlike liquid fill transformers which are cooled with oil or fire resistant liquid dielectric, dry type units utilize only environmentally safe, CSA and UL recognized high temperature insulation systems. Dry type transformers provide a safe and reliable power source which does not require fire proof vaults, catch basins or the venting of toxic gasses. These important safety factors allow the installation of dry type transformers inside buildings close to the load, which improves overall system regulation and reduces costly secondary line losses. 

Dry type transformers are a rather mature product and technology, but of all the components in a power system, a transformer replacement can be a physically challenging event, extended delivery of a replacement or repair unit and expensive transportation costs. These are transformers whose core and coils are not immersed in insulating oil. 

“Dry type” simply means it is cooled by normal air ventilation. The dry type transformer does not require a liquid such as oil or silicone or any other liquid to cool the electrical core and coils


  • Fire-resistant dry type or "cast resin" transformers are well suited for installation in high rise buildingshospitalsunderground tunnelsschoolsteel factories, chemical plants and places where fire safety is a great concern. Hazard free to the environment, dry type transformers have over the years proven to be highly reliable. 

Download this Article as PDF:
 Different Types Of Transformers And Their Applications

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