Question

ons Nse-12) Nc(-120) Q1: A 100 VA 120/12-V transformer is to be connected so as to form a step-up autotransformer (see Figure). A primary voltage of 120 v is applied to the transformer. a) What is the secondary voltage of the transformer? Q1: A 100 VA 120/12-V transformer is to be connected so as to form a step-up autotransformer (see Figure). A primary voltage of 120 v is applied to the transformer. a) What is the secondary voltage of the transformer ? b) What is its maximum voltampre rating in this mode of operation? c) Calculte the rating advantage of this autotransformer connection over the transformer's rating in conventional 120/12-V operation? 02: What are the advantages and disadvantages of autotransformers?

Answer 1

1)

Hope you understood part a and b with clarity !!

lets move to other REMAINING part C and Question 2 given below !!

C)

you can see from the discussion and calculations given above that

VA ( 2 Winding ) = 100 VA----> Only the power transfered by inductive effect or mutual induction

VA ( Auto T/F ) = 1100 VA----> Power 100 VA is trasfered by mutual induction like the 2 winding Transformer But ADDITIONALLY WE HAVE 1000 VA POWER TRANSFERED TO THE SECONDRY SIDE , WHICH IS DUE TO CONDUCTIVE POWER TRANSFER OR POWER TRANSFER DUE TO SHARING OF THE SAME CORE LEADING POWER TO DIRECTLY CONDUCT FROM PRIMARY TO SECONDRY SIDE .

THIS ADDITIONAL 1000 VA IS THE RATING ADVANTAGE OF AUTOTRANSFORMER CAUSING IT TO HAVE 1100 VA RATING POWER VALUE FROM THE OUTPUT WHICH IS 11 TIMES THE GIVEN RATING OF 100 VA OF THE CONVENTIONAL 2 WINDING TRANSFORMER  POWER OUTPUT.

2)

ADVANTAGES OF AUTO TRANSFORMER ( Relative to two winding tansformer )

a) Less amount of copper is used as windings to build the autotransformer

b) Since the amount of copper has been reduced , so do the losses in autotransformer ( i meant I² R losses ) and due to this the autotransformer have higher efficiency .

c) Less losses means better voltage regulation at the output side .

DISADVANTAGES OF AUTO TRANSFORMER

a) Isolation Loss as the single core hosts both primary and secondry windings ( can be fatal in case of faults / overvoltage cases)

b) Large short circuit current can flow in the windings as the reactance is less in case of autotransformers causing more severe damage to the setup of autotransformer .