Placement Interview Questions Electrical Engineering - Part 07

Question No. 01: What are the advantages of VSCF wind electrical system?
Answer: Advantages of VSCF wind electrical system are:
  • No complex pitch changing mechanism is needed.
  • Aero turbine always keeps going at maximum efficiency point.
  • Extra energy in the high wind speed region of the speed - duration curve can be extracted
  • Significant reduction in aerodynamic stresses, which are associated with constant - speed operation.

Question No. 02: What are the various kind of cables used for transmission?
Answer: 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.

Question No. 03: Why in a three pin plug the earth pin is thicker and longer than the other pins?
Answer: It depends upon R = rho l/a where area (a) is inversely proportional to resistance (R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Question No. 04: What are the transformer losses?
Answer: TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer

Question No. 05: What is an exciter and how does it work?
Answer: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator. Rotary exciter is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotor poles through slip-ring and brushes( conventional alternator). if it is an ac exciter, output of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.ac exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.

Question No. 06: Why synchronous generators are used for the production of electricity?
Answer: Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence synchronous generators r used for the production of electricity.

Question No. 07: Why, when birds sit on transmission lines or current wires doesn't get shock?
Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock. If birds touch 2 lines than the circuit is closed and they get electrical shock. So if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded, so that means that human who touch the line closes the circuit between phase and neutral.

Question No. 08: Operation carried out in Thermal power station?
Answer: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine; the turbine which is coupled with the generator generates the electricity

Question No. 09: Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?
Answer: 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4 mA is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4 mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument i.e. broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

Question No. 10: What is the full form of KVAR? 
Answer: We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for Kilo Volt Amps with Reactive component.

Question No. 11: What is the difference between delta-delta, delta-star transformer?
Answer:
Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i.e.) generally it is used where the Voltage is high & Current is low. Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena. 

Question No. 12: Explain the application of storage batteries.
Answer: Storage batteries are used for various purposes, some of the applications are mentioned below:
  • For the operation of protective devices and for emergency lighting at generating stations and substations.
  • For starting, ignition and lighting of automobiles, aircrafts etc.
  • For lighting on steam and diesel railways trains.
  • As a supply power source in telephone exchange, laboratories and broad casting stations.
  • For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

Question No. 13: Explain thin film resistors and wire-wound resistors
Answer:
  • Thin film resistors: It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.
  • Wire wound resistors: Length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

Question No. 14: Where should the lighting arrestor be placed in distribution lines?
Answer: Near distribution transformers and outgoing feeders of 11 kV and incoming feeder of 33 kV and near power transformers in sub-stations.

Question No. 15: Why series motor cannot be started on no-load?
Answer: Series motor cannot be started without load because of high starting torque. Series motors are used in Trains, Crane etc.

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