121 125

121. What is meant by regenerative braking?

When the supply is cut off for a running motor, it still continue 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".

122. 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.

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

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.

124. Why Delta Star Transformers are used for Lighting Loads?

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.

125. 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.