# Electrical Machines Questions and Answers – Transformer as a Magnetically Coupled Circuit

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Transformer as a Magnetically Coupled Circuit”.

1. In power transformers, it is found that the iron or core losses practically varies very less, this is because ____________
a) constant core flux
b) constant leakage flux
c) constant leakage and core flux

Explanation: Due to the constant core flux, the iron losses also remain constant.

2. It is advised to coat the laminations of the core with some enamel, to ensure __________
a) insulation
c) reduction in humming sound
d) all of the mentioned

Explanation: It is done to provide insulation.

3. Primary and the secondary winding of the transformer are interlaced mainly for __________
a) reducing leakage flux
b) reducing cost
c) reducing heating
d) saving the copper of the winding

Explanation: Interlacing is done to provide less air gaps and improved efficiency by reduction of leakage fluxes around the core.
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4. Transients currents are maximum in the transformer when it is switched on with secondary ______ and when input voltage wave is passing through ________
a) open, zero
b) open, peak
c) closed, zero
d) closed, peak

Explanation: At t=0, the current flowing will be very high as emf is very high due to t = 0.

5. Identify the matching which is not matched correctly?

```I. Silicon steel - Power transformer
II. Ferrite  - High frequency transformers
III.  Alnico - permanent magnet
```

a) I, II, III
b) I, III
c) II, I
d) None of the mentioned

Explanation: All are matched correct. High frequency transformer uses ferrites as the magnetic domains reverse very fastly.

6. Identify the matching which is not matched correctly?

```I. Silicon steel - Power transformer
II. Ferrite  - High frequency transformers
III. Alnico - permanent magnet
```

a) I, II, III
b) I, III
c) II, I
d) I

Explanation: All are matched correct. High frequency transformer uses ferrites as the magnetic domains reverse very fastly.

7. Choose the matching which is not matched correctly?

```I. Silicon steel - High frequency transformers
II. Ferrite  - Power transformer
III.  Alnico - permanent magnet
```

a) I, II, III
b) I, III
c) II, I
d) III

Explanation: High frequency transformer uses ferrites as the magnetic domains reverse very fastly. And power transformer uses CRGO steel.

8. Which of the following matches are matched correctly?

```I. OC test - Copper and iron losses
II. SC test - Copper losses
III. Load Test  - Total losses
IV. Sumpner's Test - Iron losses
```

a) II, III
b) II, III, IV
c) I, IV
d) II, IV

Explanation: OC test gives iron losses. Sumpner’s test gives iron and core losses.

9. In a transformer,eddy-current loss is 1000 watts which is half of the total core loss. If thickness of lamination and frequency are increased by 10%, What is the new core-losses?
a) 3200W
b) 3300W
c) 2200W
d) 3100W

Explanation: Pe = 1000W, Pi = 2000W
Here, V/f = constant
Pe will be same 1000W, Pi = 200+2000 = 2200W
Total core losses = 1000+2200 = 3200W.

10. The voltage conversion ratio of the transformer is inverse of the current transformation.
a) True
b) False

Explanation: It is true because, V1/V2 = N2/N1; I2/I1=1/N.

11. The voltage regulation of a transformer at full-load 0.8 p.f lagging is 2%. Its voltage regulation at full load 0.8 p.f leading _____________
a) will be positive
b) will be negative
c) may be positive
d) may be negative

Explanation: The leading p.f. has negative v.r.

12. The Transformer equivalent circuit referred to primary with converting two winding transformer in impedance forms is?
a)
b)
c)
d)

Explanation: When referred to primary, secondary inductance winding will be turns ratio squared.

13. Consider a 10 KVA, 2000/200 V, single phase transformer having below following parameters are r1=10Ω, r2=0.1Ω, l1 = 40mH, l2=0.4 mH, M = 10H. Then the expected primary self inductance will be?
a) 100.04H
b) 99.06H
c) 1.0004H
d) 10.04H

Explanation: Primary self inductance = L1 = Lm1 + l1
= (N1/N2)*M + l1
= 2000/200*10 + 0.04
=100.04 H.

14. There is a 10 KVA, 2000/200 V, single phase transformer having below following parameters are r1=10Ω, r2=0.1Ω, l1 = 40mH, l2=0.4 mH, M = 10H. Then the expected secondary self inductance will be?
a) 100.04H
b) 99.06H
c) 1.0004H
d) 10.04H

Explanation: Secondary self inductance = L2 = Lm2 + l2
= (N2/N1)*M + l2
= 200/2000*10 + 0.0004H
= 0.0004 H.

15. There is a 10 KVA, 2000/200 V, single phase transformer operating at 50 Hz. Maximum flux linkages in secondary will be?
a) 0.9 AT
b) 9 AT
c) 1 AT
d) 0.09 AT

Explanation: Flux linkages on secondary = N*φ = E/(4.44*f)
= 200/(4.44*50) = 0.9 AT

16. A 10 KVA, 2000/200 V, single phase transformer operating at 50 Hz with a current of 50 A flowing through the l.v. winding. Then the Self inductance of l.v. winding is?
a) 12.7mH
b) 1.27 mH
c) 12.7H
d) 0.127 H

Explanation: Flux linkages on l.v. = N*φ = E/(4.44*f)
= 200/(4.44*50) = 0.9 AT
Self inductance = mutual flux linkage/peak current
= 0.9/(1.414*50)
= 0.0127 H.

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