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

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.

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
```
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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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