Electrical Engineering Portal.com Autotransformer Connection Explained

7/27/2019 Electrical Engineering Portal.com Autotransformer Connection Explained

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Autotransformer Connection Explained

Autotransformer Connection

An Ordinary Transformerconsist s o f two windings called primary winding and secondary winding. These two

windings are magnetically coupled and electrically iso lated. But the trans f ormer in which a part o f windings is

common to both primary and secondary is calledAutotransformer.

In Auto transf ormer two windings are not only magnetically coupled but also electrically coupled. The input to

the transf ormer is constant but the output can be varied by varying the tapings.

The auto transf ormer is both the most simple and the most f ascinating of the connections involving twowindings. It is used quite extensively in bulk power transmission s ystems because of its ability to multiply the

ef f ective KVA capacity of a transf ormer. Autot ransf ormers are also used on radial distribution f eeder circuits

as voltage regulators.

The connection is shown in Figure 1 below.
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Figure1 - Boo sting autotransforme r co nnection

The primary and secondary windings of a two winding transf ormer

have induced emfin them due to a common mutual f lux and hence

are in phase. The currents drawn by these two windings are out

of phase by 180. This prompted the use o f a part o f the primary

as secondary. This is equivalent t o common the secondary turns

into primary turns.

The common section need to have a cross sectional area of the

conductor to carry (I2I1) ampere. Total number of turns between

A and Care T1. At point B a connection is taken. Section AB has

T2 turns. As the volts per turn, which is proportional to the f lux in

the machine, is the same f or the whole winding, V1 : V2 = T1 : T2

When the secondary winding delivers a load current o f I2Ampere

the demagnetizing ampere turns is I2T2. This will be countered by

a current I1 f lowing f rom the source through the T1 turns such that,

I1T1 = I2T2

A current of I1 ampere f lows through the winding between B and C. The current in the winding betweenA and

is ( I2 I1 ) ampere. The cross section of the wire to be selected fo rAB is proportional to this current

assuming a constant current density f or t he whole winding. Thus so me amount of material saving can be

achieved compared to a two winding transf ormer. The magnetic circuit is assumed to be identical and hence

there is no saving in the same.

To quantif y the saving the total quantity of copper used in an autot ransformer is expressed as a f raction o f

that used in a two winding transf ormer as:

Copper in autotransf ormer / copper in two winding transf ormer

= ( ( T1 T2 ) I1 + T2 ( I2 I1 ) ) / T1I1 + T2I2

Copper in autotransf ormer / copper in two winding transf ormer

= 1 ( 2T2I1 / ( T1I1 + T2I2 ) )

But T1I1 = T2I2so,

The Ratio = 1 ( 2T2I1 / 2T1I1 ) = 1 ( T2/T1 )

This means that an autot ransformer requires the use of lesser quantity of copper given by the ratio o

turns. This ratio theref ore the savings in copper.

As the space f or the second winding need not be there, the window space can be less f or an autotransf orme

giving some saving in the lamination weight also. The larger the rat io o f the voltages, smaller is the savings. A

T2approaches T1 the savings become signif icant. Thus auto transf ormers become ideal choice f or close ratiotransformations.


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Figure 2 - Close ratio transformations

The autotransf ormer shown in Figure 2above is

connected as a boosting auto transf ormer because the

series winding boosts the output voltage. Care must be

exercised when discussing primary and secondary

voltages in relationship to windings in an auto transf ormer.

In two-winding transf ormers, the primary voltage is

associated with the primary winding, the secondary voltage

is associated with t he secondary winding, and the primary

voltage is normally considered to be greater than the

secondary voltage. In the case of a boosting

auto transf ormer, 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

One of the limitations of the autot ransformer connection is that not all types o f three-phase connections are

possible. For example, the -Y and Y- connections are not possible using the auto transf ormer.

The Y-Y connection must share a common neut ral between the high-voltage and low-voltage windings, so th

neutrals of the circuits connected to these windings cannot be isolated.

A autotransformer connection is theoret ically possible; however, this will create a peculiar phase shif t.

The phase shift is a f unction o f the ratio o f the primary to secondary voltages and it can be calculated f rom

the vecto r 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 considerable savings in size and weight.

2. There are decreased losses f or a given KVA capacity.

3. Using an autot ransf ormer connection provides an opportunity f or achieving lower series impedances

and better regulation. Its ef f iciency is more when compared with the conventional one.

4. Its size is relatively very smaller.

5. Voltage regulation of autot ransf ormer is much better.

6. Lower cost

7. Low requirements o f excitation current.

8. Less copper is used in its design and const ruction.

9. In conventional transf ormer the voltage step up or st ep down value is f ixed while in autotransf ormer, we

can vary the output voltage as per out requirements and can smoo thly increase or decrease its value as

per our requirement.
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Disadvantages of the autot ransformer

1. The auto transf ormer connection is not available with certain three-phase connections.

2. Higher (and poss ibly more damaging) short- circuit currents can result f rom a lower series impedance.

3. Short circuits can impress voltages signif icantly higher than operating voltages across the windings o f

an autot ransf ormer.

4. For t he same voltage surge at the line terminals, the impressed and induced voltages are greater f or an

autot ransf ormer than f or a two-winding transformer.

5. Autot ransf ormer cons ists of a single winding around an iron core, which creates a change in voltage

f rom 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 iso lation 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,

Auto transf ormers are t ypically only used where there is already so me sort of f iltering or condit ioning

ahead of it, as in electronic applications, or the downst ream device is unaf f ected by those anomalies,

such as an AC mot or during starting.

Application

Used in both Synchronous motors and induction motors.

Used in electrical apparatus tes ting labs since the voltage can be smoothly and continuously varied.

They f ind application as boos ters in AC feeders to increase the voltage levels.

Used in HV Substat ion due to following reasons:

1. If we use normal transf ormer the size of transf ormer will be very high which leads to heavy weight, more

copperand high cost .

2. The t ertiary winding used in Autot ransf ormer balances single phase unbalanced loads connected to

secondary and it does no t pass on these unbalanced currents to Primary side. Hence Harmonics and

voltage unbalance are eliminated.

3. Tertiary winding in the Auto transf ormer balances amp turns so t hat Auto transf ormer achieves magnetic

separation like two winding transf ormers.
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Electrical Engineering Portal.com Autotransformer Connection Explained