Electrical Engineering Interview Questions

Electrical Engineering Interview Questions

Q: How can you start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

Ans: It's possible by means of Electronic choke, otherwise it's not possible to ionize the particles in tube light, with normal voltage.

Q: What is "p.u" in electrical engineering?

Ans: P.u stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use p.u concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as p.u values. (p.u=actual value/base value).

Q: How the operation carried out in Thermal power station?

Ans: 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

Q: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Ans: Link is provided at a Neutral common point in the circuit from which various connection 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 happen the fuse connected in the particular control circuit alone will blow off.

Q: What is the difference between electronic regulator and ordinary rheostat regulator for fans?

Ans: The difference between the electronic and ordinary regulator is that in electronic reg. power losses are less because as we decrease the speed the electronic reg. give the power needed for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved.In electronic regulator triac is employed for speed control.by varying the firing angle speed is controlled but in rheostatic control resistance is decreased by steps to achieve speed control.

Q: How tube light circuit is connected and how it works?

Ans: 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 1000volts. 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.

Q: What’s MARX CIRCUIT?

Ans: It is used with generators for charging a number of capacitors in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

Q: What is encoder, how it function?

Ans: 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.

Q: What are the advantages of speed control using thyristor?

Ans: Advantages: 1. Fast Switching Characteristics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.

Q: Why Human body feels Electric shock ? and in an Electric train during running, We didn’t feel any Shock. Why?

Ans: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Q: What is the principle of motor?

Ans: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

Q: Why, when birds sit on transmission lines or current wires don't get shock?
Ans: Its true that if birds touch the single / one line (phase or neutral) they don't get electrical shock. If birds touch 2 lines then the circuit will be closed and they get electrical shock. Similarly, if a human touches single / one line(phase) then he doesn't get shock  provided 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 touches the line closes the circuit between phase and neutral.

Q: What is meant by armature reaction?

Ans: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Q: What happen if we give 220 volts dc supply to d bulb r tube light?

Ans: Bulbs [devices] are designed to operate for AC such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

Ans: DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

Q: What is ACSR cable and where we use it?

Ans: ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.
 

Q: What is vacuum circuit breaker; define and where we use it?

Ans: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure the arc formation is interrupted. VCB's can be used up to 11kv.

Q: What will happen when power factor is leading in distribution of power?

Ans: 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, &

3. It also reduces over heating of transformers.

Q: What is 2 phase motor?

Ans: A two phase motor is a motor with the starting winding and the running winding have a phase split. e.g; ac servo motor. Where the auxiliary winding and the control winding have a phase split of 90 degree.

Q: Advantages of vvvf drives over non vvvf drives for EOT cranes?

Ans: 1.smooth start and stop.

2. No jerking of load.

3. Exact positioning

4. Better protection for motor.

5. high/low speed selection.

6. Reliability of break shoe.

7. Programmable break control.

8. Easy circuitry

9. Reduction in controls

10. Increases motor life

Q: Which type of A.C motor is used in the fans (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are used in the houses?

Ans: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Q: Give two basic speed control schemes of DC shunt motor?

Ans:1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled

2. Armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (I_aR_a) can be varied, and since speed is directly proportional to E_b- I_aR_a the speed can be controlled.

Q: What is the principle of (d.c) motor?

Ans: Whenever a current carrying conductor is placed in a magnetic field it produces a turning or twisting movement called torque.

Q: What is meant by armature reaction?

Ans: The effect of armature flu on main flux is called armature reaction. The armature flux may support the main flux or opposes the main flux.

Q: what is the difference between synchronous generator; asynchronous generator?

Ans: In simple, synchronous generator supplies’s both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observe reactive power for magnetizing. These types of generators are used in windmills.

Q: What is the Polarization index value ? (pi value)and simple definition of polarization index ?

Ans: It’s ratio between insulation resistance (IR) (i.e meggar value) for 10min 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.

Q: Why syn. generators r used for the production of electricity?

Ans: Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field and emf. Hence syn. generators are used for the production of electricity.

Q: Enlist types of dc generator?

Ans: D.C.Generators are classified into two types 1) separately excited d.c.generator 2)self excited d.c.generator, which is further classified into;1)series 2)shunt and 3) Compound (which is further classified into cumulative and differential).

Q: What is Automatic Voltage Regulator (AVR)?

Ans: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Q: What is an exciter and how does it work?

Ans: 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 rotary poles through slip ring and brushes (conventional alternator). If it is an ac exciter, out put of ac exciter is rectified by rotating diodes and supply dc to main fixed poles. Ac exciter is the ac generator whose field winding is stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.
 

Q: Difference between a four point starter and three point starter?

Ans: The shunt connection in four point starter is provided separately form the line where as in three point starter it is connected with line which is the drawback in three point starter

Q: Why do we use VCB at High Transmission System ? Why can't use ACB?

Ans: Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the die electric strengths equal to 8 times of air. That y always vacuum used as in HT breaker and air used as in LT.

Q: What is the difference between surge arrestor and Lightning Arrestor?

Ans: LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q: What happens if we connect a capacitor to a generator load?

Ans: Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, other wise 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.

Q: Explain the working principal of the circuit breaker?

Ans: Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

Q: How many types of cooling systems are there for transformers?

Ans: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)

Q: What is stepper motor? What are its uses?

Ans: Stepper motor is the electrical machine which acts upon input pulse applied to it. It is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. so, in automation parts it is used.

Q: Tell me in detail about C.T. and P.T ? (Company:reliance)

Ans: The term C.T means current transformer, and the term P.T means potential transformer. In circuit where measurements of high voltage and high current is involved they are used there. Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device. so, CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed. e.g if C.T is of ratio 5/5000A and it has to measure secondary current of 8000A.then Ans=8000*5\5000=8Aand this result will be given to ammeter .and after measuring 8A we can calculate the primary current. Same is the operation of PT but measuring voltage.

Q: There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?

Ans: The motor has max load current compare to that of transformer because the motor consumes real power. And the transformer is only producing the working flux and its not consuming.. hence the load current in the transformer is because of core loss so it is minimum.

Q: What is the difference between Isolator and Circuit Breaker?

Ans: Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. First the load on it must be made zero and then it can safely operated. its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

Q: What is boucholz relay and the significance of it in to the transformer?

Ans: Boucholz relay is a device which is used for the protection of transformer from its internal faults; it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism.

Q: What is SF6 Circuit Breaker?

Ans: SF6 is Sulpher hexa Flouride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

Q: What is Ferranti effect?

Ans: Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

Q: What is meant by insulation voltage in cables? Explain it?

Ans: It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.