Top Important Electrical Interview Questions - Part 06

Question No. 01: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Answer: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.

Question No. 02: Mention the methods for starting an induction motor? 
Answer: The different methods of starting an induction motor
  1. DOL: direct online starter
  2. Star delta starter
  3. Auto transformer starter
  4. Resistance starter
  5. Series reactor starter

Question No. 03: Which type of oil is used as transformer oil?
Answer: Transformer oil, or insulating oil, is usually a highly-refined mineral 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.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

Question No. 04: What is different between resistance grounding system and resistance earthing system?
Answer: Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in electric equipment in order to protect he equipment in occurrence of fault in the system.

Question No. 05: What is inrush current?
Answer:
Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

Question No. 06: In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?
Answer:
In the time of starting a motor the field rheostat is kept in minimum state for the high starting torque.
In the time of starting a motor the armature rheostats is used to control or reduce the high starting current. This high starting current can damage the motors starting systems.

Question No. 07: Comment on the working principle of operation of a single-phase transformer.
Answer: Working principle of operation of a single-phase transformer can be explained as
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.

Question No. 08: Compare JFET’s and MOSFET’s.
Answer: Comparison of JFET’s and MOSFET’s:
  • JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.
  • MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.
  • JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
  • When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.

Question No. 09: Why Delta Star Transformers are used for Lighting Loads?
Answer: For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three-phases. To minimize the current unbalance in the primary, we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Question No. 10: What happens if I connect a capacitor to a generator load?
Answer: Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Question No. 11: What is the one main difference between UPS & inverter?
Answer: Uninterrupted power supply is mainly use for short time means according to UPS VA it gives backup.
UPS is also two types: on line and off-line. Online UPS having high volt and ampere for long time backup with high dc voltage, UPS start with 12 V dc with 7 ampere. But inverter is start with 12 V -24v - 36 V dc and 120-180 ampere battery with long time backup.

Question No. 12: What is the count of hvdc transmission lines in India?
Answer: At present there are three hvdc transmission lines in India 
  1. Chandrapur to Padghe (Mumbai)--(1500 MW at ± 500 kV DC)
  2. Rehand to Delhi (1500 MW at ± 500 kV DC)
  3. Talchal to Kolar (2500 MW) 

Question No. 13: What is use of lockout relay in ht voltage?
Answer: A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Question No. 14: What are the points to be considered for MCB (miniature circuit breaker selection?
Answer: I(L) × 1.25=I(MAX) maximum current. MCB specifications are done on maximum current flow in circuit.

Question No. 15: Battery banks are in connected in series or parallel and why?
Answer: Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex: 24 nos. 2 V, 200 Ah batteries connected in series will give 48 V, 200 Ah output (Ah = Ampere hours)

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