Auto Transformer
An autotransformer is a one winding transformer in which a part of the winding
is common to both primary and secondary windings (Autotransformer is a
special type of transformer).
An autotransformer has a single continuous winding with a tap point between the
primary and secondary windings. The tap point can be adjusted to obtain the
desired output voltage, hence this is an obvious advantage of the autotransformer.
The main disadvantage of an autotransformer is that the secondary winding is not
electrically isolated from the primary.
Obviously the two windings are electrically connected and it works on the
principle of conduction as well as induction. The power transfer in 2 winding
transformer is fully inductively while in autotransformer the power is transferred
from primary to secondary by both inductively as well as conductively.
In autotransformer the output voltage can be stepped down (see Fig.b below) or
stepped up (see Fig. c below)
In step down autotransformer, the entire winding acts as a primary while
the part of the winding is used common to both primary and secondary.
Thus AB forms the primary having N1 turns while BC forms the secondary
with N2 turns. As N2 < N1, the output voltage V2 < V1 and it acts as a step
down autotransformer.
In step up autotransformer, the entire winding acts as secondary while the
part of the winding is used common to both primary and secondary. Thus
AB forms the secondary having N2 turns while BC forms the primary with
N1 turns. As N2 > N1, the output voltage V2 > V1 and it acts as a step up
autotransformer.
Autotransformer on Load
When the load is connected to the secondary of an autotransformer, then it is said
to be loaded. The figure shows the circuit diagram of a loaded autotransformer
(step-down and step-up).
In which the current I1 is the primary input current and the current I2 is the
secondary output current or load current. The current in common portion of the
primary and secondary windings is the difference of the currents I1 and I2,
regardless of whether the autotransformer is step down or step-up.
For the step-down autotransformer the current I2 > I1, thus the current flowing in
the common portion is
Current in common portion=I2−I1
For the step-up autotransformer the current I2 < I1, thus the current in the common
portion is
Current in common portion=I1−I2
The transformation ratio of autotransformer can be obtained as:
NOTE: K is greater than unity for step up autotransformer while K is less
than unity for step down autotransformer.
Using the figure below:
Input power = V1 I1
Output power = V2 I2
where:
I1 =the current drawn from the supply,
V1 = the input voltage.
I2 =the load current,
V2= the load voltage.
Now BC portion has N2 turns and acts as secondary. The current induced in this
secondary due to transformer action is I2- I1, while secondary induced voltage is
V2.
Pt = Power transformed inductively i.e. transformer action
Advantages of Autotransformer
The advantages of an autotransformer are given as follows −
1. An autotransformer has smaller core and copper losses and hence higher
efficiency as compared to an ordinary 2-winding transformer.
2. Autotransformer requires less conductor material as compared to the 2-
winding transformer.
3. It has better voltage regulation due to reduced voltage drops in resistance
and leakage reactance.
4. It has smaller size and cheaper than a 2-winding transformer.
5. It requires smaller excitation current.
6. An autotransformer can produce variable output voltage.
Disadvantages of Autotransformer
The disadvantages of an autotransformer are given as follows −
1. There is a direct connection between the primary and secondary sides. In
case of an open circuit in the common portion bc of the windings, the full
primary voltage would be applied to the load on the secondary side. This
may damage the equipment connected to the secondary side.
2. An autotransformer has reduced internal impedance as compared to a 2-
winding transformer which results in a larger short circuit current.
3. The autotransformer is not electrically isolated, thus cannot be used to
isolate two circuits electrically.
Applications of Autotransformer
The various applications of an autotransformer are,
1. For safely starting the machines like induction motors, synchronous
motors i.e. as a starter.
2. To give a small boost to a distribution cable to compensate for a voltage
drop i.e. as a booster.
3. As a furnace transformer to supply power to the furnaces at the required
supply voltage.
4. For interconnecting the systems which are operating roughly at same
voltage level.
5. It can be used to vary the voltage to the load, smoothly from zero to the
rated voltage. Such a device giving smooth and continuous supply using
an autotransformer is called variac (see Figure below).
Solved Problems
