# Transformers Questions and Answers – Autotransformer

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

1. Which of the following is the main advantage of an auto-transformer over a two-winding transformer?
a) Hysteresis losses are reduced
b) Saving in winding material
c) Copper losses are negligible
d) Eddy losses are totally eliminated

Explanation: Auto transformer is a special type of transformer which has primary and secondary winding both located on same winding. Thus, winding material required for a transformer is very less in the case of autotransformer.

2. Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is
a) Approximately equal to one
b) Less than one
c) Great than one
d) Cannot be found

Explanation: Copper In auto transformer /copper in two-winding transformer = 1- T2/T1. This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns. Hence, if the transformation ratio is approximately equal to one, then the copper saving is good and the copper loss is less.

3. Total windings present in a autotransformer are __________
a) 1
b) 2
c) 3
d) 4

Explanation: Autotransformer is the special transformer for which the single winding acts as a primary and secondary both. Thus, by taking the appropriate winding into consideration a variable secondary voltage is obtained.

4. Autotransformers are particularly economical when _________
a) Voltage ratio is less than 2
b) Voltage ratio is very high
c) Voltage ratio is higher than 2 in smaller range
d) Can be used anywhere

Explanation: Autotransformer is economical where the voltage ratio is less than 2 in which case electrical isolation of the two windings is not essential. The major applications are induction motor starters, interconnection of HV systems at voltage levels with ratio less than 2, and in obtaining variable voltage power supplies (low voltage and current levels).

5. Which of the following is not true regarding the autotransformer compare to two-winding transformer?
a) Lower reactance
b) Lower losses
c) Higher exciting current
d) Better voltage regulation

Explanation: Autotransformer is the advance version of normal transformer. It is having better voltage regulation, higher efficiency due to lower losses, lower reactance and thus it also requires very small exciting current.

6. Two-winding transformer of a given VA rating if connected as an autotransformer can handle ___________
a) Higher VA
b) Lower VA
c) Same VA
d) Cannot be found

Explanation: A two-winding transformer of a given VA rating when connected as an autotransformer can handle higher VA. This is because in the autotransformer connection part of the VA is transferred conductively.

7. When auto-transformation ratio becomes equal to 1, which of the following statement is true?
a) VA rating of the autotransformer becomes far greater than VA rating of two winding transformer
b) VA rating of the autotransformer becomes far lower than VA rating of two winding transformer
c) VA rating of the autotransformer becomes equal to VA rating of two winding transformer
d) Can’t comment

Explanation: VA rating of autotransformer is = [1/1-a] * VA of two-winding transformer, thus, when a i.e. transformation ratio of autotransformer becomes closer to 1 one gets very high value of VA rating of an autotransformer.

8. An autotransformer compared to its two-winding counterpart has a higher operating efficiency.
a) True
b) False

Explanation: The losses are less in autotransformer compare to two-winding transformer. Thus, for the given same input to autotransformer as that of two-winding transformer more output will be available to secondary side.

9. Ratio of winding material needed for autotransformer to thr two winding transformer is ______
a) 1- V2/V1
b) 1- N2/N1
c) 1- V2/V1 and 1- N2/N1
d) 1- V1/V2

Explanation: Gauto/GTW= 1- V2/V1, as voltage is directly proportional to the number of turns we can say the value is also equal to the 1- N2/N1. Thus, one can write GTW-Gauto = 1/a’ * GTW = saving of the conductor material using autotransformer.

10. For an auto-transformation ratio tending to the unity value, saving of the conductor material will be ___________
a) Tend towards 90% or more
b) Tend towards 0%
c) Can’t say
d) Wil remain fix

Explanation: GTW-Gauto = 1/a’ * GTW = saving of the conductor material using autotransformer. So, if a’=10, saving is only 10% but for a’=1.1, saving is as high as 90%. Hence it is more economical when the turn-ratio is closer to unity.

11. What are the modes in which power can be transferred in an autotransformer?
a) Conduction
b) Induction
c) Conduction and Induction
d) Cannot be said

Explanation: In two winding transformer there is no electrical connection between primary and secondary. So, the power is transferred through induction. But in auto-transformer there is a common electrical path between primary and secondary. So, power is transferred through both conduction and induction processes.

12. A 100/10, 50 VA double winding transformer is converted to 100/110 V auto transformer. tWhat will be the rating of auto transformer?
a) 500 VA
b) 550 VA
c) 100 VA
d) 110 VA

Explanation: Secondary current of the two-winding transformer at rated voltage supply = 50/10= 5 A
Thus, autotransformer will also carry the same rated current in secondary giving the power output as 5*110= 550 VA.

13. For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred inductively?
a) 8kVA
b) 12kVA
c) 10kVA
d) 50kVA

Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred inductively is given by (1-k)*total power= 0.6*20= 12kVA.

14. For a 20kVA transformer with turn ratio 0.4 what amount of total power is transferred conductively?
a) 8kVA
b) 12kVA
c) 10kVA
d) 50kVA

Explanation: For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred conductively is given by (k)*total power= 0.4*20= 8kVA.

Sanfoundry Global Education & Learning Series – Transformers.

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