Auto Transformer Advantages Disadv Application Limitations & Connection

Auto Transformer Applications Advantages Disadvantages Limitations and Connection. Today we will discuss hot topic of Electrical Transformer topic i.e. Auto Transformer. At least one questions always comes in any competitive exam. One numerical problem always asked in GATE exam. So GATE 2018 point of view its important topic. Many time its application or advantages asked as a Objective type Questions. So dear reader read full article.


1.Used in both Synchronous motors and induction

2.Used in electrical apparatus testing labs since the
voltage can be smoothly and continuously varied.
3.They find application as boosters in AC feeders to
increase the voltage levels.
4.Used in HV Substation due to following reasons.
●If we use normal transformer the size of
transformer will be very high which leads to heavy
weight, more copper and high cost.
●The tertiary winding used in Auto transformer
balances single phase unbalanced loads connected to secondary and it does not pass on these unbalanced currents to Primary side. Hence
Harmonics and voltage unbalance are eliminated.
Tertiary winding in the Auto Transformer balances
amp turns so that Auto transformer achieves
magnetic separation like two winding transformers.
●Autotransformers are also used on
radial distribution feeder circuits as voltage

Auto Transformer Connection detail - 

1.As we know Ordinary Transformer consists of two windings its
called primary winding and secondary winding. They coupled magnetically and
Isolated electrically .
2.Defination- the transformer in which a
part of windings is common to both primary and secondary is called Auto Transformer.
3.In Auto Transformer two windings are magnetically coupled as well as electrically coupled.
4.The input to the transformer is constant in  Power or voltage but the
output voltage can be varied by changing the tapings.

The connection is shown in Figure

5.Due to a common mutual flux the primary and secondary windings of  transformer induced emf  and are in phase.
6.The currents run by these two windings are out of phase by 180◦.
7.The common section need to have a cross sectional area of the conductor to carry (I2−I1) ampere.
8.Total number of turns between A and C are T1.  At point B a connection is taken. Section AB has T2 turns. As the volts per turn, which is proportional to the flux in the machine, is the same for the whole
winding, V1 : V2 = T1 : T2
When the secondary winding delivers a load current I2 Ampere the demagnetizing ampere turns is I2T2. This will be countered by a current I1 flowing
from the source through the T1 turns such that,
I1T1 = I2T2
9.A current of I1 ampere flows through the winding between B and C. So the current in the winding between A and B is (I2 − I1) ampere. The cross section of the wire to be selected for AB is proportional to this current assuming a constant current density for the whole winding.
10.Thus some amount of material saving can be achieved compared to a two winding transformer.
11.This means that an auto transformer requires the use of lesser copper given by the ratio
of turns. This ratio therefore the savings in copper.
12.As the space for the second winding need not be there, the window space can be less for an auto transformer, giving some saving in the lamination weight also. The larger the ratio of the voltages, smaller is the savings. As T2 approaches T1 the savings become significant. Thus auto transformers become ideal choice for close ratio transformations.

13.The auto transformer shown in Figure is connected as a boosting auto transformer because the series winding boosts the output voltage.
14.In two-winding transformers, the primary voltage is
associated with the primary winding, the secondary
voltage is associated with the secondary winding,
and the primary voltage is normally considered to
be greater than the secondary voltage. In the case
of a boosting autotransformer, however, the
primary (or high) voltage is associated with the
series winding, and the secondary (or low) voltage
is associated with the common winding; but the
voltage across the common winding is higher than
across the series winding.

Limitation of the autotransformer

1.Not all types of three-phase
connections are possible. For example, the ∆-Y and Y- ∆ connections are not possible using the autotransformer. The Y-Y connection must share a
common neutral between the high-voltage and low-
voltage windings, so the neutrals of the circuits
connected to these windings cannot be isolated.

2.A ∆- ∆ autotransformer connection is theoretically possible; however, this will create a peculiar phase
shift. The phase shift is a function of the ratio of the primary to secondary voltages and it can be calculated from the vector diagram. This phase shift cannot be changed or eliminated and for this
reason, autotransformers are very seldom connected as ∆ – ∆ transformers.

Advantages of the autotransformer

1.There are copper savings in size and weight.
2.Decrease in losses for a given KVA
3.Lower series impedances and better regulation.
4.Its efficiency is more when
compared with the conventional transformer.
5.Its size is relatively smaller.
6.Much better voltage regulator.
7.Lower cost due to less use of copper.
8.Lower excitation current.
9.In conventional transformer the step up or step down voltage is fixed while in autotransformer, we can vary the output voltage i.e. increase or decrease as per our requirements.

Disadvantages of the autotransformer:

1.Not available with three-phase connections.
2.Higher short-circuit currents can result from a lower series impedance.
3.Short circuits can impress voltages higher than operating voltages across the windings of an autotransformer.
4.For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a two-winding
5.Autotransformer consists of a single winding around an iron core, which creates a change in voltage from one end to the other. In other words, the self-inductance of the winding around the core
changes the voltage potential, but there is no isolation of the high and low voltage ends of the winding. So any noise or other voltage anomaly coming in on one side is passed through to the
other. For that reason, Autotransformers are typically only used where there is already some sort of filtering or conditioning ahead of it, as in
electronic applications, or the downstream device is
unaffected by those anomalies, such as an AC motor during starting

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