Interview Questions and Answers on Electrical Engineering - Part 04

Question No. 01: State Norton’s Theorem
Answer: The Norton’s theorem explains the fact that there are two terminals and they are as follows:
  • One is terminal active network containing voltage sources 
  • Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.
The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.

Question No. 02: State the methods of improving power factor?
Answer: Methods of improving power factor:
  • By connecting static capacitors in parallel with the load operating at lagging power factor. 
  • A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
  • By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.

Question No. 03: What is Difference between Induction Motor & Servo Motor?
Answer:
  1. An induction motor has high inertia and servo motor has a very low inertia. Hence servo motors are used in applications where instant and accurate positioning of load is required.
  2. As in induction motor stator is formed by coils while in servo motor stator is formed with permanent magnet. So in induction motor very high inertia is there while in servo inertia is low. Due to that servo is very easy to stop at instant. So it is used in position control while in  application where at instant position stop is not required induction motor is used
  3. Servo motor is actual normal ac induction motor with added fetcher as
    • Low inertia rotor
    • High torque break and
    • Inbuilt encoder for speed and position feedback. All this components combine work with servo drive.

Question No. 04: What are the advantages of star-delta starter with induction motor?
Answer: The advantages of star-delta starter with induction motor are:
  1. The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting.
  2. Hence the starting current is reduced; the voltage drops during the starting of motor in systems are reduced.

Question No. 05: Why we do 2 types of earthing on transformer, i.e.: body earthing & neutral earthing, what is function. I am going to install a 500 kVA transformer & 380 kVA DG set what should the earthing value?
Answer: The two types of earthing are Familiar as Equipment earthing and System earthing.
In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings.
System Earthing: In this neutral of the supply source (Transformer or Generator) should begrounded. With this, in case of unbalanced loading neutral will not be shifted, so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment (transformer or alternator) and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

Question No. 06: What is SF6 Circuit Breaker?
Answer: SF6 is Sulphur hexafluoride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

Question No. 07: How many types of cooling method of transformers are there?
Answer: Cooling methods of transformers are:
  1. ONAN (oil natural, air natural)
  2. ONAF (oil natural, air forced)
  3. OFAF (oil forced, air forced)
  4. ODWF (oil direct, water forced)
  5. OFAN (oil forced, air forced)

Question No. 08: What is meant by armature reaction?
Answer: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Question No. 09: What is the significance of vector grouping in Power Transformers?
Answer: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. E.g. DYN11 means Delta primary, Wye Secondary and the current is at 11 o’clock referred to the voltage.

Question No. 10: What are the advantages of speed control using thyristor?
Answer: Advantages:
  1. Fast Switching characteristics than Mosfet, BJT, IGBT
  2. Low cost
  3. Higher Accuracy

Question No. 11: How tube light circuit is connected and how it works?
Answer: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Question No. 12: What is reverse power relay?
Answer: Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Question No. 13: What are Motor Generator Sets and explain the different ways the motor generator set can be used? 
Answer: Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator. The motor generator set can be used in the following different ways: 
  1. Alternating current (AC) to direct current (DC)
  2. DC to AC
  3. DC at one voltage to DC at another voltage
  4. AC at one frequency to AC at another harmonically-related frequency

Question No. 14: What are HRC fuses and where it is used?
Answer: HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

Question No. 15: Difference between megger test equipment and contact resistance meter test instruments? 
Answer: Megger test equipment used to measure cable electric resistance, conductor continuity, and phase identification where as contact resistance meter test instruments used to measure low resistance like relays, contactors.


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