Construction and Working of AutoTransformer

In a conventional transformer, the primary and the secondary windings are electrically insulated from one another but connected magnetically as shown in the figure below. In an auto Transformer, the primary and secondary windings are linked both magnetically and electrically.

Auto transformers can be categorized into two types based on their construction. In the first type, a continuous winding is employed with taps strategically positioned to achieve the desired secondary voltage. Conversely, the second type features at least two distinct coils that are electrically interconnected to create a seamless winding. The schematic representation of the auto transformer construction is depicted in the figure below.

The primary winding AB from which a tapping at C is taken, that CB goes about as an optional winding. The supply voltage is applied across the AB, and the load is connected across the CB. The tapping could be constant or fluctuating. When AC voltage V1 is applied across AB, an alternating flux is set up in the core, thus, an emf E1 is induced in the winding AB. A part of this induced emf is taken in the secondary side circuit.

Let,

V1 is Primary applied voltage

V2 is Secondary voltage across the load

I1 is Primary current

I2 is load current

N1 is number of turns among A and B

N2 is number of turns among C and B

Neglecting no-load current, leakage reactance and losses,

V₁ = E₁ and V₂ = E₂

Therefore, the transformation ratio:

K = V₂ / V₁ = N₂ / N₁ = I₁ / I₂

As the secondary ampere-turns are opposite to primary ampere-turns, so the current I2 is in phase opposition to I1. The secondary voltage is less than the primary. Accordingly current I2 is more than the current I1. As a result, the current that flows through section BC is (I2–I1).

The ampere-turns because of section BC = current x turns

Ampere-turns because of section BC

= ( I₁ – I₂) N₂

= ( I₁ /K – I₁) x N₁ K

= I₁ N₁ (1 – K) – – – ( Equation 1)

Ampere-goes because of section AC

=I₁ (N₁ – N₂ )

= I₁N₁ ( 1 – N₂/N₁)

= I₁N₁(1 – K) – – – (Equation 2)

Auto-transformer-Equation (1) and (2) shows that the ampere-turns because of section BC and AC balance each other which is characteristics for the transformer action.

An Auto Transformer refers to a transformer that features a single winding wound around a laminated core. An autotransformer is like a two-winding transformer however contrast in the manner the primary winding and secondary winding are interrelated. A piece of the winding is common to both the primary side and secondary sides.

In this article, we will be going through Auto Transformer, First we will start our Article with What is an AutoTransformer, next, we will go through the construction and Working of an Autotransformer, Then we will go through the Autotransformer Theory and Copper Savings in an Auto Transformer, At last, we will conclude our Article with Advantages, Disadvantages, Applications and Some FAQs.