Lecture 6 Auto-Transformers

Lecture 6
Auto-Transformers
• Another type of transformer is known as the autotransformer, having a part of its
windings common to the primary and secondary. It consists of a single tapped primary
where the center tap is common to both primary and secondary (not isolated).
• Some of the turns on the coil are used for the primary and some are used for the
secondary.
• An example of a variable autotransformer is known as a variac. A variac is a single
coil with a sweeping arm for the center common, this allows the ratio of primary turns:
secondary turns to be altered easily.
• When a load is connected across b and c, then a current I 2 will flow through the load.
The current I 2 will produce an m.m.f. in the core which will be balanced by a current I 1
flowing in the complete winding.
Features of Auto-transformers
• Same operation as two windings transformer
• Physical connection from primary to secondary
• Sliding connection allows for variable voltage
• Higher kVA delivery than two windings connection
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The voltages and currents are related by the same turns-ratio as in a two-winding
transformer:
V1 N1 I1 N2
1
a; ; I N I N
V N I N a
2 2 2 1
Example:
1 1 2 2
A single-phase, 100 kVA, 2000/200V, two winding transformer is connected as an
autotransformer to supply a load at 2200V from a 2000V supply. Calculate the
following:
a) kVA rating as an autotransformer
b) Apparent power transferred by electromagnetic induction
c) Apparent power transferred by conduction
d) If the total losses of the two-winding transformer at full-load is 1450 W, find
the efficiency of the auto-transformer operating at full-load and 0.8 pf lagging.
Solution:
The single-phase, two-winding transformer is reconnected as an autotransformer as
shown in Fig below. The current ratings of the windings are given by
The current ratings of the windings
are:
I
I
ab
bc
100,000
500A
200
100,000
50A
2000
Therefore, for full-load operation of the autotransformer, the terminal currents are:
I H = 500 A ;
I L = (500+50) = 550 A
Now, V L = 2000 V, V H = (2000 + 200) = 2200 V
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Therefore, the kVA rating of the autotransformer is:
kVA
kVA
L
H
2000550 1100 kVA
1000
2200500 1100 kVA
1000
(b) The apparent power transferred by induction: S indction = 100 kVA
(c) The apparent power transferred by conduction: S condction = 1100 – 100 = 1000 kVA
(d) The output of the auto-transformer:
( )
Considering transformer losses remains same, the efficiency is given by:
Pout
Pout
880,000
100% 100% 100% 99.83%
P P P
880,000 1450
in out loss
Advantages of Auto-transformer
• It effects a saving in winding material (copper or aluminum), since the secondary
winding is part of the primary current.
• Lower copper loss, therefore efficiency is higher than in the two winding
transformer.
• Lower leakage reactances, lower exciting current.
• Variable output voltage can be obtained.
Disadvantages of Auto-Transformer
• There is a direct connection between the primary and secondary sides.
• Should an open-circuit develop between points b and c, the full mains voltage would
be applied to the secondary.
• The short-circuit current is much larger than for normal two-winding transformer
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