Auto Transformer
Principle Of Operation
Transformer works on the
principle of mutual
induction of two coils.
When current in the primary
coil is changed the flux
linked to the secondary coil
also changes. Consequently
an EMF is induced in the
secondary coil.
AUTO-TRANSFORMERS
An autotransformer
(sometimes called auto step
down transformer)is an
electrical transformer with
only one winding. The
"auto" (Greek for "self")
prefix refers to the single
coil acting on itself and not
to any kind of automatic
mechanism.
In autotransformer winding
is common for primary and
secondary
AUTO-TRANSFORMERS as step down transformer
Two fixed terminals A and
B connected to the single-
phase AC supply V1
AB winding is primary
winding
CB terminal acts as the
secondary winding across
the load
Operating principle is
same as the normal
transformer
AUTO-TRANSFORMERS as step down transformer
N1=primary turn(AB)
N2=secondary turn(CB)
I1=primary current
I2=secondary current
V1=primary voltage
V2=secondary votage
Load voltage
AUTO-TRANSFORMERS as step down transformer
If losses, the magnetizing current and the leakage reactance are neglected
the transformation ratio is defined as
AUTO-TRANSFORMERS as step up transformer
Two fixed terminals C and
B connected to the single-
phase AC supply V1
AB winding is primary
winding
AB terminal acts as the
secondary winding across
the load
Operating principle is
same as the normal
transformer
AUTO-TRANSFORMERS as step up transformer
If losses, the magnetizing current and the
leakage reactance are neglected the
transformation ratio is defined as
A
Copper saving in Auto transformer
The same output and voltage transformation ratio
an autotransformer requires less copper than the 2-
winding transformer
Types of AUTOTransformer
Step UP Transformer :
A transformer in which
Ns>Np is called a step up
transformer. A step up
transformer is a
transformer which
converts low alternatic
voltage to high alternatic
voltage.
Step DOWNTransformer :
A transformer in which Np>Ns is
called a step down transformer. A
step down transformer is a
transformer which converts high
alternating voltage to low
alternating voltage.
Conversion of 2-winding transformer
into
autotrans former
ADDITIVE POLARITY (SEP-UP)
SUBSTRACTIVE POLARITY (STEP DOWN)
ADDITIVE POLARITY
In this case common current flow towards the common
terminal
SUBSTRACTIVE POLARITY
In this case common current flow away from common
terminal
Advantages
An autotransformer requires less Cu than a two-winding
transformer of similar rating.
The size and hence the cost is reduced as compared to the
conventional transformer
An autotransformer operates at a higher efficiency than a
two-winding transformer of similar rating.
An autotransformer has better voltage regulation than a
two-windingtransformer of the same rating.
An autotransformer has smaller size than a two-winding
transformer of the same rating.
An autotransformer requires smaller exciting current than
a two-windingtransformer of the same rating.
Disadvantages
There is no electrical isolation between the primary
and secondary windings. So it is dangerous for high
voltage applications
If the common part of the winding breaks(O/C) then
the transformer action is lost and full primary voltage
appears across the secondary.
It possed a low impedance, hence if the secondary
circuit is S/C, then a large current will flow on the
secondary side.
Application of Auto-transformer
Autotransformers are used to transmit power transmission
and distribution
Autotransformers are used in audio systems and for
impedence matching
Autotransformers are used in railways
CompareAuto-transformer with two-winding transformer
Conventional
Basis For Differences Autortransformer
transformer
Definition A transformer, having It is a static machine
only one winding a part which transfers
of which acts as a electrical energy from
primary and the other one end to another
as a secondary. without changing
frequency.
Number of Windings Auto-transformer has It has two separate
only one winding wound winding, i.e., primary
on a laminated core and secondary winding.
Insulation The primary and The primary and
secondary winding are secondary winding are
not electrically electrically insulated
insulated. from each other.
Induction Self Induction Mutual Induction
CompareAuto-transformer with two-winding transformer
Conventional
Basis For Differences Autortransformer
transformer
Size Small Large
Power Transfer Partly by transformation Through transformation
and partly by direct
electrical connection.
Voltage Regulation Better Good
Winding Material Less requires More requires
Circuit The primary and The primary and
secondary winding secondary winding
circuits are connected circuits are connected
magnetically. both electrically and
magnetically.
Connection Depends upon the Connect directly to the
tapping load.
Starting current Decreases Decreases by 1/3 times.
CompareAuto-transformer with two-winding transformer
Conventional
Basis For Differences Autortransformer
transformer
Excitation current Small Large
Economical More Less
Cost Less costly More costly
Efficient More Less
Leakage flux and Low High
resistance
Impedance Less High
Cost Cheap Very costly
Losses Low High
Output voltage Variable Constant.
symbol
LIMITATION
Because it requires both fewer windings and a smaller core, an
autotransformer for power applications is typically lighter and
less costly than a two-winding transformer, up to a voltage
ratio of about 3:1; beyond that range, a two-winding
transformer is usually more economical.
Like multiple-winding transformers, autotransformers operate
on time-varying magnetic fields and so will not function with
DC.
A failure of the insulation of the windings of an
autotransformer can result in full input voltage applied to the
output. Also, a break in the part of the winding that is used as
both primary and secondary will result in the transformer
acting as an inductor in series with the load .
