Autotransformer|Application Of Autotransformer

AUTOTRANSFORMER : An autotransformer is a special type of single-phase transformer, consisting of a single winding wound on a laminated core.

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What is an Autotransformer?

An autotransformer is a special type of single-phase transformer, consisting of a single winding wound on a laminated core.

Here, some part of this single winding acts as the primary winding and some part acts as
the secondary winding.

The number of turns of the primary and secondary windings can be varied using the switch contact.

What is a Variac?

Since the output voltage of the auto-transformer can be varied by changing the number
of turns of the windings, it is also known as VARIAC (VARIable AC) or voltage regulator.

difference between the single-phase transformer and the auto-transformer

The principal difference between the single-phase transformer and the auto-transformer is based on the way the primary and secondary windings are interrelated.

In a single-phase transformer, both primary and secondary windings are magnetically connected but electrically insulated, whereas, in an auto-transformer these windings
are connected both magnetically and electrically.

The schematic diagram of an auto-transformer is shown in Figure given below.

Schematic diagram of an Autotransformer

In an auto-transformer, a continuous copper wire wound on a laminated silicon steel core is used for the windings.

A single or multiple tappings or a smooth sliding brush over the winding helps in varying
the turns ratio, which determines the secondary voltage of the auto-transformer.

The schematic diagram of an auto-transformer with a sliding brush and with tappings are shown in Figures given above.

Advantages, Disadvantages and Applications of Autotransformer is given below.

Advantages of autotransformer

Since the short circuit impedance of the auto-transformer is very low, it has higher efficiency and better regulation.
Since only one major winding is used, it is relatively smaller in size.
As the requirement of copper material is less, it is very cheap and more economical.
Requires low level of current for excitation.
Output voltage of the auto-transformer can be increased or decreased smoothly as per requirement.

Disadvantages of autotransformer

Since the winding is not insulated electrically, it cannot be used as an interconnection between high and low-voltage systems.
As it is electrically connected, the harmonics in the load get transferred to the supply.
The short-circuit current of the auto-transformer is high.
Highly sensitive to over-voltages.

Application of autotransformer

Used as a starter in both synchronous and induction motors.
Since smooth variation of output voltage exists, it is used in electrical apparatus-testing labs.
Used as a booster to increase voltage levels in AC feeders.
Used as a voltage regulator.
Used in a power system (i.e., in transmission and distribution), audio systems and railways.

Comparison between Autotransformer and Conventional Transformer

Table given below lists the comparison between an auto-transformer and a conventional transformer.

Auto-transformer vs. Conventional Transformer

Autotransformer	Conventional transformer
It has only one winding, wound on a laminated core.	It has two separate windings, i.e., primary and secondary windings.
The windings are not electrically insulated.	The windings are electrically insulated.
Size of this transformer is small.	Size of this transformer is large.
The primary and secondary winding circuits are connected magnetically and electrically.	The primary and secondary winding circuits are connected magnetically.
It is more efficient and has better voltage regulation.	Efficiency and voltage regulation are less when compared to an auto-transformer
Has low leakage flux and resistance.	Has high leakage flux and resistance
Since the copper material required is less, it is very cheap and more economical.	Since the copper material required is high, it is very costly and less economical.
Loss is low.	Loss is high.
Generates variable output voltage.	Generates constant voltage.
It is mostly used as a starter in an induction motor.	It is mostly used to step-up or step-down the voltage in power system.
Also called as VARIAC	Also called as two-winding transformer