Fresher Electrical Job Interview Questions and Answers - Part 09

Question No. 01: What is the principle of motor?
Answer: Whenever a current carrying conductor is placed in a magnetic field it produce turning or twisting movement is called as torque.

Question No. 02: What is the Polarization index value? Give simple definition of polarization index?
Answer: Its ratio between insulation resistance (IR) i.e. megger value for 10 min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5

Question No. 03: What will happen when power factor is leading in distribution of power?
Answer: If there is high power factor, i.e. if the power factor is close to one:
  1. Losses in form of heat will be reduced,
  2. Cable becomes less bulky and easy to carry, and very cheap to afford, and
  3. It also reduces over heating of transformers.

Question No. 04: What is encoder, how it function?
Answer: An encoder is a device used to change a signal (such as a bit stream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with analog circuitry.

Question No. 05: What is electric traction?
Answer: Traction means using the electric power for traction system i.e. for railways, trams, trolleys etc. electric traction means use of the electricity for all these. Now a days, magnetic traction is also used for bullet trains. Basically dc motors aroused for electric traction systems.

Question No. 06: When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? 
Answer: No, we can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature). So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system, it should have both over voltage & over current relay.

Question No. 07: What will happen if DC supply is given on the primary of a transformer?
Answer: Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So, high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out.

Question No. 08: What are the advantages of freewheeling diode in a Full Wave rectifier?
Answer: It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in an inductive load

Question No. 09: What is the function of interposing current transformer?
Answer: The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.  Interposing current transformer is equipped with a wide range of taps that can be selected by the user to achieve the balance required.

Question No. 10: What is excitation?
Answer: Excitation is applying an external voltage to DC shunt coil in DC motors.

Question No. 11: In three pin plug 6 Amp. 220 V AC rating. Why earth pin diameter is higher than other two pin? What its purpose? 
Answer: Because Current flow in the conductor is inversely proportional to the conductor diameter. So, if any short circuits occur in the system, first high currents bypassed in the Earthling terminal. (R=Pl/a area of the conductor increases resistance value decreases) 

Question No. 12: What is electrical diversity factor in electrical installations? 
Answer: Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

Question No. 13: What is rated speed?
Answer: At the time of motor taking normal current (rated current) the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.

Question No. 14: Which type of motor is used in trains, what is the rating of supply used explain Working principal?
Answer: Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Question No. 15: In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?
Answer: Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Question No. 16: State Maximum power transfer theorem
Answer: The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.

Question No. 17: Define the following terms:-
  • Reliability,
  • Maximum demand,
  • Reserve-generating capacity, 
  • Availability (operational).
Answer:
  • Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.
  • Maximum Demand: It is maximum load demand required in a power station during a given period.
  • Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.
  • Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.

Question No. 18: State the factors, for the choice of electrical system for an aero turbine.
Answer: The choice of electrical system for an aero turbine is guided by three factors:
  • Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
  • Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.
  • Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.

Question No. 19: Define the following: Average demand, Maximum demand, Demand factor, Load factor.
Answer:
  • Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.
  • Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.
  • Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.
  • Load Factor: It is defined as the ratio of the average power to the maximum demand.

Question No. 20: How does Zener phenomenon differ from Avalanche breakdown?
Answer: The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5 V are caused by Zener Effect. Junctions that experience breakdown above 5 V are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

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