Auto transformer is a transformer in which a part of
the winding is common to both primary and secondary circuits. The total power
transfer consists of inductive transfer and conductive transfer.
In a two winding transformer, there is only inductive
power transfer. In auto-transformer, there is additional power transfer on
account of physical connection between source and the load through a series
winding. This conductive power transfer is the main factor responsible for massive
saving and increased kva output in an auto-transformer as compared to a two
winding transformer of same capacity and voltage rating.
Consider the auto transformer shown below.
Let AC winding has NH and BC (common
winding) has NL turns respectively. Then, AB (series winding) has (NH-NL)
turns.
Copper Saving :-
Since a part of the winding is common to both source and load, therefore,
copper is saved. Lets drive an expression for the amount of copper (Cu) saved.
Cu weight = Cu volume ×
Cu density
α Cu volume
α cross-section ×
length
Since conductor cross-section depends on the current I carried
by it and length depends on the number of turns N in the winding, therefore
Cu weight α NI
Components
of power transfer :-
SLV = VLIL
= VL (IL-IH)
+ VLIH
VL (IL-IH) = power transfer by
induction
VLIH = power transfer by conduction
Advantages
of auto-transformer :-
(1) An auto-transformer uses less winding and core material,
hence, it is economical to a two-winding transformer.
(2) Besides, an auto-transformer has higher efficiency, lower
voltage regulation and lower per unit impedance as compared to a two-winding
transformer.
(3) It has smaller no-load current and hence no-load (or light
load) power factor of auto transformer is higher than 2-winding transformer.
Disadvantages
of auto-transformer :-
(1) Lower per unit impedance results into high short circuit
current.
(2) It does not provide electrical isolation between load and
source. This can be dangerous to the instruments acting as load and the
personnel operating.
(3) If the auto-transformer is operated in step-down mode and
suddenly the common winding develops open circuit, thenthe load is subjected to
almost full high voltage.
Auto-transformer Solved Example
Q. Convert 20kva,
2300/230V two winding transformer into an auto-transformer with constant source
voltage of 2300V and load voltage of 2530V. Calculate
(a) kva rating of auto transformer and ita comparison with the original 2-winding transformer
(b) kva transformed
(c) kva conducted
Sol :- Let the two winding transformer is shown below.
AB winding current rating = (20000/2300) = 8.69 A
CD winding current rating = (20000/230) = 86.9 A
Now connect A and D to get 2530V output terminals BC.
2300V side is source therefore current Is must leave
+ve terminal.
2530V is load side therefore current IL must enter
the +ve terminal.
CD winding rating = 86.9 A . Therefore, IL=86.9 A
CD and AB mmf must balance each other, therefore, since IL
leaves the dot, 8.69A (current rating of AB) must enter the dot of AB.
Applying KCL at point A or D we get ,
Is = (8.69+86.9) A
= 95.59 A
Related concepts :-
