Chapter 20 - Transformers

Question 1

If a Transformer is designed to step-up Voltage, how will Current Change?

Answer 1

Current will decrease

Question 2

Describe 2 types of transformer core?

Answer 2

Both core types are made up of thin sheet, iron or low retentivity steel lamination that are lightly insulated from each other to reduce Eddy Currents. Shell type cores are constructed like a figure 8 on its side and both primary and secondary winding are wound on the middle leg Core type cores is basically a square shape with primary winding wound around one side and secondary winding wound around the opposite side.

Question 3

Describe an application of a single phase transformer with equal numbers of primary and secondary windings?

Answer 3

• This type of transformer is called the insulation transformer
• Used to protect sensitive electronic equipment from external earths.
• The common application for an isolation transformer is on the input connections of an electrical instrument that receives information from an external source.
• The instrument would not be damaged as a result of an earth fault on the input signal line.

Question 4

Explain the advantages & Disadvantages of the Auto-transformer?

Answer 4

Advantages

• Saving in copper coils
• They can be variable Voltage Transformers with movable tapping points on the windings.

Disadvantage

• Auto Transformers povide no electrical isolation between Primary and Secondary windings.

Question 5

State an application for the Auto-transformer?

Answer 5

An Aplication of the Auto Transformer is a reduced Voltage Starter.

Question 6

Explain how the Auto-transformer can be used as a step-up transformer?

Answer 6

The auto transformer could be used as a step up transformer by switching the primary and secondary windings.

Question 7

Explain how a Current Transformer works?

Answer 7

• The core of the Transformer surrounds the conductor (ie bus-bar)
• The magnetic flux created by the bus-bar current flow influences the transformer core in a similar way to a conventional Transformer
• Thus indusing an emf in the secondary winding which is made up of many coils.
• This ensure secondary side current is a small value

Question 8

State 2 Applications for the current Transformer?

Answer 8

1. Clip-on Ammeter.
2. ?

Question 9

Explain the Advantage of using the tap as a reference point for the secondary voltages on the centre-tap Transformer?

Answer 9

Advantages

1.

Question 10

List the uses of 4 different types of three phase transformers?

Answer 10

• Star/Star-Connection: used for relatively small, High Voltage Transformers. Useful also if Neutral connections are needed due to an unbalanced load.
• Delta/Delta-Connection: Used for Large,Low Voltage Transformes. A Disadvantage with this type is the Lack of neutral connections.
• Star/Delta-Conection: Used widely in Distribution Systems. Enployed on ships for 440v to 115v Lighting Transformers. By design of the connection the line voltage is Stepped Down by a factor of 1.73.
• Delta/Star-Connection: Used widely in Power Distribution Systems Somtimes used for stepping Up the voltage.

Question 11

Give a use for the centre tap transformer?

Answer 11

The Centre tap transformer can be used for Full Wave Rectification.