TRANSFORMERS MCQs SET 07

TRANSFORMERS MCQs SET 07

1. Third harmonic current in transformer at no-load is ______________
a) 3% of exciting current
b) 10% of exciting current
c) 25% of exciting current
d) 35% of exciting current
 

Answer: d 

Explanation: The effect of saturation nonlinearity is to create a family of odd-harmonic components in the exciting current, the predominant being the third harmonic; this may constitute as large as 35–40% of the exciting current.
 

2. Ii in no-load test is responsible for ______________
a) Production of flux
b) Reactive power drawn from the supply
c) Active power drawn from the supply
d) No significance
 

Answer: c 

Explanation: It will be assumed here that the current Io and its magnetizing component Im and its core-loss component Ii are sinusoidal on equivalent rms basis. In other words, Im is the magnetizing current and is responsible for the production of flux, while Ii is the core-loss current responsible for the active power being drawn from the source to provide the hysteresis and eddy-current loss.
 

3. The parallel circuit model is drawn because _________________
a) Conductance Gi accounts for core-loss current
b) Inductive susceptance Bm accounts for magnetizing current
c) Gi for core – loss current and Bm for magnetizing current
d) Cannot say
 

Answer: c 

Explanation: The parallel circuit model of exciting current can be easily imagined wherein conductance Gi accounts for core-loss current Ii and inductive susceptance Bm for magnetizing current Im. Both these currents are drawn at induced emf E1 = V1 for resistance-less, no-leakage primary coil; even otherwise E1 =V1.
 

4. 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
 

Answer: c 

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.
 

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

Answer: b 

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.
 

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

Answer: d 

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.
 

7. 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
 

Answer: a 

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.
 

8. 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
 

Answer: a 

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.
 

9. 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
 

Answer: d 

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.
 

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

Answer: d 

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.