Auto Transformer

1. Autotransformer
By
A.Nandhakumar

2. Autotransformer Construction

3. Construction:
• The core is constructed by taking a
rectangular long strip of magnetic material
(say CRGO) and rolled to give the radial
thickness.
• Over the core, a continuous single coil is
wound the free terminals of which are
marked as C and A.
• A carbon brush attached to a manually
rotating handle makes contact with different
number of turns and brought out as a
terminal, marked E.
• The number of turns between E & C,
denoted by NEC can be varied from zero to a
maximum of total number of turns between
A & C i.e, NAC.
• The output voltage can be varied smoothly
from zero to the value of the input voltage
simply by rotating the handle in the
clockwise direction.
• This type of autotransformers are
commercially known as varic or dimmerstat
and is an important piece of equipment in
any laboratory.

4. Autotransformer (Simple Circuit Representation)
• An auto-transformer is a
transformer with only one winding
(HV) wound on a laminated core.
• Part of the HV acts as LV due to
the presence of tappings.
• A part of winding is common to
the both primary and secondary
circuits.
• Transformation ratio of auto
transformer is
K = LV/HV (always <1)
= % of tappings
• If secondary is tapimg at 50% of
primary winding , then K = 0.5
• If it is 75%, then K = 0.75
N1- N2 V1-V2
Secondary Tapping Point
Single Core Series Winding
Common
Winding

5. Working:
At no-load
• When we apply a AC voltage to the primary side of a auto-
transformer, the exciting current flows from A to C and it
establishes a working m.m.f. directed vertically downward in the
core
At load
• When switch S is called, the current in winding BC must flow
from C to B, in order to create an m.m.f. opposing the exciting or
working m.m.f. , according to Lenz’s Law.
• Since the working m.m.f. in a transformer remains substantially
constant at its no-load value, the primary must draw additional
current I1, from the source, in order to neutralize the current of I2
– I1.
• In winding AC, I1 flows from A to C, whereas in winding BC I2 –
I1 flows from C to B.

7. Savings of Copper
• Volume and weight of Cu, is proportional to
the length and area of the cross-section of the
conductors.
• Now, length of conductors is proportional to
the number of turns and cross-section depends
on current.
• Hence, weight is proportional to the product of
the current and number of turns.

8. • Wt. of Cu in section AC is ∝ (N1 – N2)I1
• Wt. of Cu in section BC is ∝ N2 (I2 –I1)
• Therefore total Wt. Cu in auto transformer
∝ (N1 – N2)I1 + N2 (I2 –I1)
• If a two-winding transformer were to perform the
same duty, then
• Wt. of Cu on its primary ∝ N1 I1
• Wt. of Cu on secondary ∝ N2 I2
• Total Wt. of Cu ∝ N1 I1 + N2 I2

9. • Wt. of Cu in auto-transformer (Wa) = (1 - K) × (Wt. of Cu
in ordinary transformer W0)
∴ Saving = W0 - Wa = W0 - (1 - K) W0 = KW0
∴ Saving = K × (Wt. of Cu in ordinary transformer)
• Hence, saving will increase as K approaches unity.
• It can be proved that power transformed inductively is
input (1 - K).
• The rest of the power = (K × input) is conducted directly
from the source to the load i.e., it is transferred
conductively to the load.

10. Advantages of Autotransformer when compared to 2 winding
transformer of same rating
• Auto transformer requires less volumes of core material.
• No insulation is required between windings, because part of
primary winding is secondary winding.
• Autotransformer requires less amount of insulating material when
compared to 2 – winding transformer.
• Auto transformer requires less amount of copper for windings.
• Due to above reasons size, weight and cost of transformer are less.
• Auto transformer requires less volume of core and Cu materials ,
so that iron and copper losses are less and its efficiency is more
than 2 winding transformer.
• Auto transformer has less leakage flux and less leakage reactance.
Therefore its voltage regulation is less tan 2 winding transformer.

11. • The feasibility of auto transformer depends on the
tapping point.
• Advantages increases if K approaches to 100% (K =
1)
• Advantages decreases if K approaches to 0% (K = 0)
• Eample:
1. N1: N2 = 3:1(breaking Point), K = 33%
2. N1: N2 > 3:1 , K < 33.3% , low percentage of
tapping and advantages are very less
3. N1: N2 < 3:1, K > 33.3%, feasible autotransformer

12. Applications:
• 33 kV/ 11kV – Autotransformer can be used.
• 11 kv/6.6kV – Autotransformer can be used.
• 132 kV/33 kV - Autotransformer cannot be
used.
• In auto transformer power is not only
transferred by induction process but also by
conduction process.

13. Realization of auto transformer from
two winding transformer
• By using two winding transformer we can realize two
different auto transformers one with series additive
polarity and the other with series subtractive polarity.
+ +
- -

14. (a) Series Additive Polarity
?

15. (a) Series Additive Polarity

18. (b) Series Subtractive Polarity

19. (b) Series Subtractive Polarity

21. Drawbacks of Autotransformer
1. Auto transformers are not feasible if N1 :N2 > 3: 1
or K < 33.3%
2. There is no electrical isolation between the
primary and secondary if any fault occurs on load
side it will de directly reflected to source side.
3. As low leakage reactance and low percentage
impedance, then fault current level is more in
Autotransformer.
4. Whenever accidental open circuit occurred in
common winding of auto transformer it will cause
appearance of high voltage across LV terminals
which may damage the load.

22. Applications of Autotransformer
1. Used in both synchronous motors and
induction motors at starting.
2. Used in electrical apparatus testing labs since
the voltage can be smoothly and continuously
varied.
3. They find application as boosters in as feeders
to increase the voltage levels.