Transformers

Question 1

State 3 types of transformers and give an example of their use

Answer 1

• star/star, HV transformer, unbalanced loads
• Delta/delta, LV transformer
• Star/delta, power distribution systems
• Delta/star, Final distribution transformer

Question 2

Explain with the aid of a diagram the operation of an isolation transformer give an example of the use of such a transformer

Answer 2

Diagram will be a very simple transformer with an earth on the secondary.

The primary and secondary are not physically connected, so an earth fault on one side is not seen by the other.

It is used in the isolation of power tools.

Question 3

How does an auto transformer differ in construction from other transformers? Give one advantage and one disadvantage of the auto transformer.

Answer 3

It only has one winding, which is tapped at two points.

Advantage, it is lighter, cheaper and more efficient.

Disadvantage, it cannot provide isolation

Question 4

State the four types of 3 phase transformer

Answer 4

star/star
delta/delta
star/delta
delta/star

Question 5

What type of 3 phase transformer is normally used as a final distribution transformer and why?

Answer 5

Delta/star. This type allows a neutral to be connected to the secondary side

Question 6

Explain why most transformers can provide isolation between their primary and secondary sides. Which type of transformer is unable to provide this type of protection

Answer 6

The primary and secondary windings are not physically connected so current cannot flow directly between them. The auto transformer cannot provide isolation.

Question 7

What danger exists in the use of a current transformer and why?

Answer 7

A high secondary voltage can be produced. This is due to the very high turns ratio of the transformer

Question 8

Describe with the aid of a simple sketch the construction of a 3 phase transformer

Answer 8

A 3 phase transformer comprises 3 single phase transformers wound onto a common magnetic core.

Question 9

When is the connection of a neutral wire in a three phase system desirable? What type of transformer winding can connect to a neutral line and since 2006 what colour is used to identify the neutral?

Answer 9

The neutral is desirable if the load is unbalanced. A star winding can accommodate a neutral connection. The neutral is blue post 2006.