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This presentation explains in brief about the auto-transformers, their concept, math and applications.
2. INDEX
ABOUT AUTO TRANSFORMER
A BRIEF DIFFERENCE BETWEEN AUTO TRANSFORMER AND THE
CONVENTIONAL ONE
EQUIVALENT CIRCUIT DIAGRAM
PHASOR DIAGRAMS
PROS AND CONS
APPLICATIONS
3. AUTO
TRANSFORMER
• Auto transformer is kind
of electrical
primary and secondary
same common single
So basically it’s a one
transformer.
• This single winding is tapped
various points along its
to provide a percentage of
primary voltage supply
its secondary load.
4. • Autotransformers are often
used to step up or step down
voltages in the 110-115-120 V
range and voltages in the 220-
230-240 V range - for example,
providing 110 V or 120 V (with
taps) from 230 V input, allowing
equipment designed for 100 or
120 V to be used with a 230 V
supply.
5. AUTO TRANSFORMER
Only one winding
Self induction
Power transfer, partly by
process of transformation, and
partly by the direct current
Better voltage regulation
Non-electrically insulated
windings
Higher efficiency
Less leakage flux, resistance,
impedance, losses and cost
Variable output voltage
CONVENTIONAL
TRANSFORMER
Two windings
Mutual induction
Power transfer through
electrical transformation, due
to which power losses occur
Lower than auto transformer
Electrically insulated windings
Lower when compared to
auto transformer
More when compared to auto
transformer
Fixed output voltage
DIFFERENCE BETWEEN AUTO
TRANSFORMER AND A TRANSFORMER
8. PROs
For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two
winding transformer. For transformation ratio say 20 however the size would be 95 %. The saving in cost of the material is of course
not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2. Thus auto
transformer is smaller in size and cheaper.
An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to
reduction of transformer material.
Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.
9. CONS:
Because of electrical conductivity of the primary and secondary windings the lower voltage circuit is liable to be impressed
upon by higher voltage. To avoid breakdown in the lower voltage circuit, it becomes necessary to design the low voltage
circuit to withstand higher voltage.
The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results into
severer short circuit currents under fault conditions.
The connections on primary and secondary sides have necessarily needs to be same, except when using interconnected
starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the
case of delta/delta connection.
Because of common neutral in a star/star connected auto transformer it is not possible to earth neutral of one side only. Both
their sides should have their neutrality either earth or isolated.
It is more difficult to maintain the electromagnetic balance of the winding when voltage adjustment tappings are provided. It
should be known that the provision of tapping on an auto transformer increases considerably the frame size of
the transformer. If the range of tapping is very large, the advantages gained in initial cost is lost to a great event.
10. APPLICATIONS:
Compensating voltage drops by boosting supply voltage in distribution systems.
Auto transformers with a number of tapping are used for starting induction and synchronous motors.
Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.
required.
