# Electrical Machines Questions and Answers – Equivalent Circuit of Transformer

This set of Electrical Machines Assessment Questions and Answers focuses on “Equivalent Circuit of Transformer”.

1. In a transformer if the frequency of the supply is varied then the magnetizing current in the core also varies.
a) True
b) False

Explanation: The magnetizing current does not depend on the frequency of the supply fed to it.

2. CRGO Laminations in a transformer are used to minimize __________
a) eddy current loss
b) hysteresis loss
c) both eddy current and hysteresis loss
d) ohmic loss

Explanation: CRGO steel is rolled grain type domains which not only help for faster reversals but also the reduction of eddy current losses.

3. A single phase transformer has emf per turn having 2310/220 V, 50Hz transformer as 13 V. Then the estimated primary turns will be?
a) 189 turns
b) 179 turns
c) 176 turns
d) 190 turns

Explanation: N2 = Total secondary voltage/Emf per turn
= 220/13 = 16.92 = 17(approx)
For N2 = 17, N1 = 178.5. But it is not an integer, so this approximation is wrong.
If N2 = 18, N1=189 turns.

4. A single phase transformer has emf per turn having 2310/220 V, 50Hz transformer as 13 V. The core area is(in square cm)?
a) 393
b) 277.8
c) 358.92
d) 450.03

Explanation: Emf per turn = 1.44*f*flux density * Area*N2
Area = 220/(18*1.44*50*1.4) = 393 sq.cm.

5. If a transformer is fed from a dc rather than a 1-phase ac supply, then the transformer will __________
a) burn its windings
b) operate normal
c) will not operate
d) will give very small leakage flux

Explanation: Dc is nothing but ac at zero frequency. So the reactance will be zero and the current will be limited solely by the small resistance of the winding which will produce very high amount of current to flow through the windings, so burning them up.

6. For a single phase transformer operating at normal operating conditions has useful flux of 1 Wb. If the machine is loaded at 0.8 p.f., then its mutual flux __________
a) may decrease to 0.98 Wb
b) remains constant
c) may increase 1.02 Wb
d) may decrease to 0.8 Wb

Explanation: The mutual flux will decrease by a very small amount.

7. The flux involved in the emf equation of a transformer has __________
a) rms value
b) average value
c) total value
d) maximum value

Explanation: The flux is always taken at its peak in the practical calculation cases.

8. If the frequency at the primary supply is varied gradually, then the secondary terminal voltage will __________
a) not change
b) vary directly
c) will vary oppositely
d) will vary inverse of frequency

Explanation: The changes in the frequency is not reflected due to the flux density variation correspondingly.

9. There are two identical transformers A and B such that flux density applied to B is doubled. Then magnetizing current of B is?
a) larger than A
b) double of A
c) half of A
d) same as that of A.

Explanation: When we double the flux density, as per the magnetization curve of the core, the magnetizing current is much larger than double of the machine A.

10. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters:

```HV : r = 2.4  ‎Ω, X = 6 ‎Ω
LV: r = 0.03  ‎Ω, X = 0.07 ‎Ω
```

The primary reactance referred to secondary is?
a) 0.024 ‎Ω
b) 0.06 ‎Ω
c) 3 ‎Ω
d) 7 ‎Ω

Explanation: Primary reactance referred to secondary = x1*(N2/N1)^2 = 6*(220/2200)^2
= 0.06 ‎Ω.

11. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters:

```HV : r = 2.4  ‎Ω, X = 6 ‎Ω, Primary
LV: r = 0.03  ‎Ω, X = 0.07 ‎Ω, Secondary
```

The secondary reactance referred to primary is?
a) 0.024 ‎Ω
b) 0.06 ‎Ω
c) 3 ‎Ω
d) 7 ‎Ω

Explanation: Secondary reactance referred to primary = x2*(N1/N2)^2 = 0.07*(2200/220)^2 = 7.0 Ω

12. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters:

```HV : r = 2.4  ‎Ω, X = 6 ‎Ω
LV: r = 0.03  ‎Ω, X = 0.07 ‎Ω
```

The primary resistance referred to secondary is?
a) 0.024 ‎Ω
b) 0.06 ‎Ω
c) 3 ‎Ω
d) 7 ‎Ω

Explanation: Primary resistance referred to secondary = r1*(N2/N1)^2 = 2.4*(220/2200)^2
= 0.024 ‎Ω.

13. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters:

```HV : r = 2.4  ‎Ω, X = 6 ‎Ω
LV: r = 0.03  ‎Ω, X = 0.07 ‎Ω
```

The secondary resistance referred to primary is?
a) 0.024 ‎Ω
b) 0.06 ‎Ω
c) 3 ‎Ω
d) 7 ‎Ω

Explanation: Secondary resistance referred to primary = r2*(N1/N2)^2 = 0.03*(2200/220)^2 = 3.0 Ω.

14. In a transformer ______ decreases with increase of the leakage flux.
a) secondary terminal voltage
b) secondary induced voltage
c) primary induced voltage
d) all of the mentioned

Explanation: The leakage is modelled as the reactance which incurrs as a loss to the terminal voltage not the induced voltage.

15. The components needed to draw the phasor diagram of the transformer is?

```A. load current
B. Equivalent circuit parameters

a) A, B, C
b) B
c) B, C
d) A, B

Explanation: All the mentioned quantities are must to draw the phasor diagram of a transformer.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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