N2 Electrical Trade Theory: Module 8 – Transformers

Q1 – Write down the equation for the turn’s ratio with reference to voltage and current ratios of a transformer.

 Q2 – Name TWO types (shapes) of core that are used in transformers.

Q3 – Name THREE advantages of single-phase transformers.

Q4 – Describe the basic construction of a single-phase transformer.

Q5 – Make a neat, labelled sketch of a single-phase transformer.

Q6 – Briefly state the relationship between the voltage and current in a transformer

Q7 – Why are the cores laminated?

Q8 – Explain why transformers are rated in KVA not KW.

Q9 – An 11-KV /220V single-phase step-down transformer has 3600 primary turns.

Calculate:

a) The transformer ratio

b) The number of secondary turns

c) The primary current if 200A is drawn from the secondary

d) The power output if the secondary power factor is 0.8 when supplying 200A

Q10 – A single-phase transformer has 800turns on the primary winding, which is connected to a 240V AC supply. The voltage and current on the secondary side are 16V and 8A respectively.

Calculate:

a) The number of turns on the secondary side

b) The value of the primary current

c) The turns ratio

d) The voltage per turn

Q11 – Write down the relationship that exits between the line and phase voltage and the line and phase current in a delta connected winding.

Q12 – A three-phase transformer has 420 turns on the primary and 36 turns on the secondary winding. If the supply voltage is 3300V, determine the secondary line voltage when the windings are connected

a) In star /delta

b) In delta / star