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September 18, 2016

12 Most Common Interview Question 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.


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.

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