Transformer: 2 Marks Important Questions with Answers

CHAPTER 3: TRANSFORMER

2 Marks Important Questions with Answers

1. What is the function of a transformer?

Transformer is a static device that

(a) Transfers electric power from one circuit to another.

(b) It does so without change of frequency.

(c) It accomplishes this by electro‒magnetic induction.

2. State the different types of single phase transformer based on construction.

(i) Core type transformer;

(ii) Shell type transformer

(iii) Berry type transformer.

3. What is an ideal transformer?

An ideal transformer is one that has

(i) no winding resistance.

(ii) no leakage flux.

(iii) no iron losses in the core.

(iv) no I²R Loss (copper loss)

4. What are turns ratio and transformation ratio of transformer?

The transformation ratio is defined as the ratio of the secondary voltage to primary voltage. It is denoted by K,

K = E₂/E₁ = N₂/N₁ = I₁/I₂

Turns ratio

 n or a = V₁/V₂ = E₁/E₂ = N₁/N₂

For step up transformer, K>1 i.e., N₂ > N₁.

5. Write the e.m.f equation of two winding transformer.

E₁= 4.44 f ϕ_m N₁ Volts

E₂ = 4.44 f ϕ_m N₂ Volts

Where

ϕ_m = Maximum flux in the core, in Wb

N₁ = No. of turns in primary winding

N₂ = No. of turns in secondary winding

 f = Supply frequency in Hz

6. What is the purpose of laminating the core in a transformer?

To reduce the eddy current loss, transformer cores are laminated.

7. How does change in frequency affect the operation of a given transformer?

(a) Iron loss increases with decrease in frequency.

(b) Since total loss is greater at lower frequency, the temperature is increased with decrease in frequency.

c) Reactive drop is affected, regulation at low power factors decrease with decrease in frequency.

8. Write the name of material used for transformer core.

Transformer core is made up of high grade of silicon steel.

9. What are the losses in a transformer? How will you minimise them?

There are two losses occur in transformer.

(i) Iron losses or Core losses.

(ii) Copper losses.

Iron losses includes Hysteresis loss and Eddy current loss. Hysteresis loss is reduced by making transformer core high grade of silicon steel. Eddy current loss is minimised by laminating the transformer core.

Copper loss is minimised by reducing the leakage flux which is linked with both primary and secondary windings.

10. What is the condition for maximum efficiency of a transformer?

Copper Loss = Iron Loss is the condition for maximum efficiency of transformer.

11. Mention three phase transformer connections.

(a) Star - Star

(b) Star ‒ Delta

(c) Delta ‒ Delta

(d) Delta - Star

(e) Open ‒ Delta

(f) Scott Connections.

12. Why transformer ratings is expressed in terms of kVA?

Output power of a transformer is depending upon the type of load connected to it (i.e., resistive load, inductive load or capacitive load). The value of output side power factor will vary with the type of loads. Also, the core loss depends on the voltage and copper loss depends on the current. So the transformer cannot be given rating interms of KW (Active power) as other electrical machines given. Hence the transformer is rated in KVA (i.e., apparent power).

13. Give the expressions for the load current when transformer operates its maximum efficiency

Copper loss = Iron loss

is the condition for maximum efficiency of transformer.

The output current corresponding to maximum efficiency is

 I₂ = √(W_i/R₀₂)

14. What are the components of magnetic losses in transformer and on what factors do they depend?

The components of magnetic or iron losses in transformer mention as

(i) Hysteresis loss W_h = μ B_max^1.6 f V  Watts

(ii) Eddy current loss W_e = K_e B_max² f² t² Watts

Where,

Hysteresis loss and Eddy current loss depends on frequency and maximum flux density.

15. Why is the range of efficiency in Transformer higher than those of other electrical machines?

There are 3 types of losses which can be occurred in an electrical machine

• No load loss (or) constant loss.

• Copper loss (or) variable loss (or) I²R loss.

• Mechanical loss.

As the transformer is a static device, there will not be the presence of mechanical losses. Hence, the range of efficiency in Transformer is higher than those of other (rotating) electrical machines.

16. What is a transformer?

It is a static device which will transform electrical energy from one circuit to another circuit, without change in frequency.

It works on mutual - induction principle.

17. Define voltage transformation ratio of transformers. Also write the condition for step‒up transformer.

The transformation ratio is defined as the ratio of the secondary voltage to primary voltage. It is denoted by K.

From the emf equation of a transformer,

 E₂/E₁ = N₂/N₁ = K

• If N₂ > N₁ i.e., K > 1, then the transformer is called as step‒up transformer.

• If N₂ < N₁ i.e., K< 1, then the transformer is called as step‒down transformer.

18. Define efficiency of transformer.

It is defined as the ratio of output power to the input power. It will always represented in percentage.

 % η = [ Output power / Input power ] × 100

% η = [ Output power / (Output power + coreless + copper loss) ] × 100

19. Define voltage regulation.

It is defined as the ratio of change in voltage from no‒load to full load with respect to no‒load voltage.

% Voltage regulation = [ Change in voltage from no load to full load / No load voltage ] × 100

20. What are the constructional difference between core type and shell type transformer?

Core type

i. In this type of transformer, the winding will surround the core.

ii. Here there will be only one closed path for magnetic flux.

iii. There are two limbs in this core type transformer

Shell type

i. In this type of transformer, the core will surround the winding.

ii. Here the will be two closed path for magnetic flux.

iii. There are three limbs in this shell type transformer.

21. A single phase, 2200/250 V, 50 Hz transformer has a net core area of 36 cm² and a maximum flux density of 6 wb/m². Calculate the number of turns of primary and secondary.

Primary Voltage, E₁ = 4.44 f B_m AN₁

N₁ = E₁ / ( 4.44 f B_m A )

= 2200 / (4.44×50×6×36×10^‒4)

N₁ = 458.79 = 459 turns

Secondary voltage, E₂ = 4.44 f B_m AN₂

N₂ = E₂ / ( 4.44 f B_m A )

= 250 / (4.44×50×6×36×10^‒4)

N₂ = 52.14 = 52 turns

22. An ideal 25 kVA transformer has 500 turns on the primary winding and 40 turns on the secondary winding. The primary is connected to 3000 V, 50 Hz supply. Calculate (1) primary and secondary currents on full load. (2) Secondary emf and (3) The maximum core flux.

Solution

(i) Full load primary current, I₁ = Rating / V₁ = (25×10³) / 3000

⇒ I₁ = 18.33 A

By transformation ratio, K = V₂/V₁ = N₂/N₁

 ⇒ V₂ = 3000 × 40/500 = 240 V

(ii) Full load secondary current, I₂ = Rating / V₂ = (25×10³) / 240

 ⇒ I₂ = 104.17 A

(iii) Maximum core flux, ϕ_m

V₁ = 4.44 f ϕ_m N₁

ϕ_m = V₁ / (4.44 f N₁)

 = 3000 /  (4.44×50×500)

⇒ ϕ_m = 0.027 = 27 mwb

23. What are the types of transformers based on construction?

(i) Core type transformer

(ii) Shell type transformer

(iii) Berry type transformer.

24. In a single phase transformer, N_p = 350 turns, Ns=1050 turns, E_p = 400 V. Find Es.

Solution

E_S/E_P = N_S/N_P

E_S = [N_S/N_P] × E_P

= (1050/350) × 400

E_s = 1200 V