# Transformers Questions and Answers – Ideal Transformer

This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Ideal Transformer”.

1. A transformer cannot work on the DC supply because __________________
a) There is no need to change the DC voltage
b) A DC circuit has more losses
c) Faraday’s laws of electromagnetic induction are not valid since the rate of change of flux is zero
d) Cannot be determined

Explanation: For DC supply the direction and the magnitude of the supply remains constant, produced flux will be constant. Thus, rate of change of flux through the windings will be equal to zero. As a result, voltage at secondary will always be equal to 0.

2. An ideal transformer has infinite primary and secondary inductances.
a) True
b) False

Explanation: The primary and secondary windings have zero resistance. It means that there is no ohmic power loss and no resistive voltage drop in the ideal transformer. An actual transformer has finite but small winding resistances.

3. In a transformer the resistance between its primary and secondary is ______________
a) Zero
b) Very small
c) Cannot be predicted
d) Infinite

Explanation: Since the primary and secondary windings are not connected to each other, one can say there exists the resistance of infinite ohms. These windings are connected to each other magnetically not electrically.

4. Identify the correct statement relating to the ideal transformer.
a) no losses and magnetic leakage
b) interleaved primary and secondary windings
c) a common core for its primary and secondary windings
d) core of stainless steel and winding of pure copper metal

Explanation: There is no leakage flux so that all the flux is confined to the core and links both the windings. An actual transformer does have a small amount of leakage flux which can be accounted in detailed analysis by appropriate circuit modelling.

5. An ideal transformer will have maximum efficiency at a load such that _____________
a) copper loss = iron loss
b) copper loss < iron loss
c) copper loss > iron loss
d) cannot be determined

Explanation: Maximum efficiency of a transformer is defined at the that values when, copper losses become completely equal to the iron losses. In all other cases the efficiency will be lower than the maximum value.
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6. Which of the following statement regarding an ideal single-phase transformer is incorrect? Transformer is having a turn ratio of 1: 2 and drawing a current of 10 A from 200 V AC supply is incorrect?
a) It’s a step-up transformer
b) Its secondary voltage is 400 V
c) Its rating is 2 kVA
d) Its secondary current is 20 A

Explanation: Since turns ratio is equal to 1:2 the transformer will give higher voltage at secondary with respect to the primary voltage, and current in secondary thus will be halved. In last option current is doubled which is opposite to the ratings given.

7. Ideal transformer core has permeability equal to _____
a) Zero
b) Non-zero finite
c) Negative
d) Infinite

Explanation: The core has infinite permeability so that zero magnetizing current is needed to establish the requisite amount of flux in the core. The core-loss (hysteresis as well as eddy-current loss) is considered zero.

8.Turns ratio of the transformer is directly proportional to ____________
a) Resistance ratio
b) Currents ratio
c) Voltage ratio
d) Not proportional to any terms

Explanation: According to the voltage expression, emf induced in the primary is directly proportional to the change in the flux with respect to the time and number of turns of the primary winding. Similarly, for secondary winding.

9. Which of the following statement is correct regarding turns ratio?
a) Current ratio and turns ratio are inverse of each other
b) Current ratio is exactly same to the voltage ratio
c) Currents ratio is exactly same to the turns ratio
d) Voltage ratio and turns ratio are inverse of each other

Explanation: Voltage ratio of transformer winding is exactly similar to the turns ratio of transformer, while voltage ratio and turns ratio is exactly inverse of the currents ratio. Hence, by knowing any of these quantities on can identify the type of transformer.

10. Which of the following is the expression for emf induced in primary with voltage applied to primary of an ideal transformer?
a) e=V
b) V= √2*e*cos ωt
c) e= √2*V*cos ωt
d) Cannot say

Explanation: For an ideal transformer having a primary of N1 turns and a secondary of N2 turns on a common magnetic core. The voltage of the source to which the primary is connected is v = √2 V cos wt. while the secondary is initially assumed to be an open circuited. As a consequence, flux f is established in the core such that e = v = N1 dφ/dt.

11. Which of the following is the wrong expression?
a) i1N1=i2N2
b) i1v1=i2v2
c) i1N2=i2N1
d) v2N1=v1N2

Explanation: According to the transformation ratio, current flowing through the transformer is inversely proportional to the number turns of winding and voltage applied across it. While, voltage applied is directly proportional to the number of turns.

12. For transformer given, turns ratio is equal to a, what will be the impedance of primary with respect to secondary?
a) a2 times the secondary impedance
b) a times secondary impedance
c) secondary impedance/a
d) secondary impedance/a2

Explanation: The ratio of impedances on primary to the secondary is directly proportional to the inverse of square of turns ratio of transformer. Hence primary impedance to the secondary impedance ratio will be 1/ a2.

13. Power transformed in the ideal transformer with turns ratio a is _______
a) a2 times primary
b) a times primary
c) primary power/ a2
d) primary power

Explanation: In an ideal transformer, voltages are transformed in the direct ratio of turns, currents in the inverse ratio and impedances in the direct ratio squared; while power and VA remain unaltered. Thus, primary power= secondary power.

14. For a transformer with primary turns 100, secondary turns 400, if 200 V is applied at primary we will get ___________
a) 80 V at secondary
b) 800 V at secondary
c) 1600 V at secondary
d) 3200 V at secondary

Explanation: Voltage in the primary of the transformer will get modified in the transformer secondary, according to the number of turns. Thus, turns are modified with 4 times the primary, we’ll get 4 times higher voltage at secondary.

15. For a transformer with primary turns 400, secondary turns 100, if 20A current is flowing through primary, we will get ___________
a) 80A at secondary
b) 5A at secondary
c) 800A at secondary
d) 40A at secondary

Explanation: Current in the primary of the transformer will get modified in the transformer secondary, according to the number of turns, in inverse proportion. Thus, turns are modified with 1/4 times the primary, we’ll get 4 times higher current at secondary.

Sanfoundry Global Education & Learning Series – Transformers.

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