What is the difference between delta-delta, delta-star transformer?

Ans:
Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

► The three-phase induction motor:

The three-phase induction motor is also called an asynchronous motor and it is the most commonly used type of motor in industrial applications. Specifically, the squirrel cage design is the most extensively used electric motor in industrial applications.

► Advantages:

1. It has simple and rugged construction
2. It is relatively cheap
3. It requires little maintenance
4. It has high efficiency and reasonably good power factor
5. It has self starting torque

How Capacitors Work

► When capacitors are connected in parallel with a battery, electric charge from the negative terminal of the battery travels to the negative terminal of the capacitor where it is accepted. On the other hand, the positive terminal of the capacitor loses electrons which travel to the positive terminal of the battery. Although the electric charge is not stored instantaneously, the process is much faster than the charging process of a battery, or of an accumulator.

The bigger the common surface area between the two metallic plates, the higher the charge that can be stored inside a capacitor, so in the end capacitors may store enormous amounts of electric energy. When the electric charge stored inside a capacitor needs to be used, they are connected in electronic circuits in the same way as the batteries. Again, a big advantage/disadvantage - according to applications - opposed to batteries is that although they can accumulate large amounts of energy in a relatively small amount of time, they release it just as quick.

Why 95% P.C.B.s are of green colour?

ANS:
If the PCB itself is green, that indicates, that its made from glass-epoxy which is naturally green. Also your PCB could be made in every single color that exist. The finishing color is usually a solder mask. This mask is applied for safety reasons, for preventing eventual short circuits and etc. It could be every color, depends on the client's specifications.