Wednesday, February 3, 2010

Electrical Inteview questions Part 12

Query:What is the count of hvdc transmission lines in India?
Resolution: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)

Query:What is meant by regenerative braking?
Resolution:When the supply is cutt off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load(resitor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f 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".

Query:Why is the starting current high in a DC motor?
Resolution:In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia = Armature current,Ra = Aramture 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.

Query:What are the advantages of star-delta starter with induction motor?
Resolution:(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.

Query:Why Delta Star Transformers are used for Lighting Loads?
Resolution: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.

Query:Why in a three pin plug the earth pin is thicker and longer than the other pins?
Resolution: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 The First to make the connection and Last to disconnnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Query:Why series motor cannot be started on no-load?
Resolution:Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.

Query:Why ELCB can't work if N input of ELCB do not connect to ground?
Resolution:ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so 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 wil trip and it will safe guard 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.