What happens if we give 220 volts dc supply to tube light ?

  Bulbs [devices] for AC are designed to operate 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.

A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses ?

  Single Phase induction motor which mostly squirrels cage rotor and is capacitor start capacitor run.

Advantages of vvvf ( variable voltage variable frequency drive ) drives over non vvvf drives for EOT cranes?

 1. Smooth starts and stops. 

2. No jerking of the load. 

3. Exact positioning 

4. Better protection for the motor. 

5. high/low-speed selection. 

6. Reliability of brake shoe. 

7. Programmable brake control. 

8. Easy circuitry 

9. Reduction in controls 

10. Increases motor life

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

  If there is a high power factor, i.e. if the power factor is close to one: 

1. Losses in the form of heat will be reduced, 

2. Cable becomes less bulky and easy to carry, and very cheap to afford, 

3. It also reduces the overheating of transformers.

What is the Polarization index value? (Pi value) and simple definition of polarization index ?

  Its ratio between insulation resistance (IR) i.e. megger value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for the motor to be in good condition it should be Greater than 2.5. Polarization Index Test (PI) – The PI test measures the ability of the insulation to absorb voltage over a period of time. This gives an indication of the overall insulation quality of the individual pieces of insulation in the transformer. This test is usually performed in conjunction with the Megger Test. It is measured as the ratio of insulation resistance (IR) for 10 minutes to insulation resistance for 1 minute.

DC shunt motor basic speed control scheme ?

 1. By using the 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 (IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.

What is the difference between synchronous generator & asynchronous generator ?

  In simple, synchronous generator supply’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.

D.C. Generators Types ?

 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)

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

  synchronous machines have the capability to work on different power factor (or say different imaginary POW varying the field emf. Hence syn. generators r used for the production of electricity.

What is the difference between surge arrestor and lightning arrestor ?

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

Why use the VCB at High Transmission System? Why can’t use ACB ?

 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.

What is an exciter and how does it work ?

 There are two types of exciters, static exciter, and rotary exciter. The purpose of the exciter is to supply the excitation dc voltage to the fixed poles of the generator. Rotary exciter is an additional small generator mounted on the shaft of the main generator. If it is a dc generator, it will supply dc to the rotary poles through slip ring and brushes (conventional alternator). if it is an ac exciter, the 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.

What is Automatic Voltage regulator (AVR) ?

 AVR is an abbreviation for the Automatic Voltage Regulator. It is an important part of 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.

Why ELCB can't work if N input of ELCB do not connect to ground ?

  ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through a phase and that much current will have to return neutral so the resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safeguard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

Why series motor cannot be started on no-load ?

  Series motor cannot be started without load because of high starting torque. Series motor is used in Trains, Crane, etc.

Why in a three pin plug the earth pin is thicker and longer than the other pins ?

 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.

Why Delta Star Transformers are used for Lighting Loads ?

 For lighting loads, a 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 the delta/star transformer is used for lighting loads.

What are the advantages of star-delta starter with induction motor ?

 (1). The main advantage of using the star delta starter is a 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. 

Why is the starting current high in a DC motor ?

 In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current, Ra = Armature resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

What is meant by regenerative braking ?

 When the supply is cut off for a running motor, it still continues running due to inertia. In order to stop it quickly, we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply. So that back emf voltage is made to apply across the resistor and due to load, the motor stops quickly. This type of breaking is called as "Regenerative Breaking".