Auto Transformer: Construction and it’s Working

As we know that a transformer includes two windings and the main function of these windings is to change the voltage level to the desired level. The two winding transformer includes two separately coupled magnetic coils without electrical connection.

Auto Transformer: Construction and it’s Working

What is an Auto Transformer?

Definition: A transformer that has a single winding is called an Auto Transformer. The working principle of the autotransformer is similar to a 2-winding transformer but the only difference is, the portions of the single winding in this transformer will work at both sides of the windings like primary & secondary. In a normal transformer, it includes two separate windings that are not allied with each other. 

Auto Transformer Construction

We know that the transformer includes two windings namely primary and secondary which are connected magnetically but insulated electrically. There are two types of autotransformer based on construction. In one type of transformer, there is continuous winding with the taps brought out at convenient points. However, in another type of autotransformer, there are two or more distinct coils that are electrically connected to form a continuous winding.

The primary winding AB from which tapping is taken, CB acts as a secondary winding.  Here, the tapping may be fixed or variable. When an AC voltage V1 is applied across AB, an alternating flux is set up in the core, as a result, an emf E1 is induced in the winding AB. A part of this induced emf is taken in the secondary circuit.

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If the voltage ‘V1’ is applied across the winding , then the voltage for each turn will be V1/N1.

Therefore, the voltage across the section will be (V1/N1)*N2

V2/V1 = N2/N1 = K

When the other section in the winding can be considered secondary. So ‘K’ is the constant value, it is nothing but the ratio of voltage or turns in the transformer.

Whenever the load is connected in between the terminals, then the load current like ‘I2’ will starts flowing. The flow of current within the secondary winding will be the main difference of currents ‘I1&I2’.

Auto Transformer: Construction and it’s Working

Copper Savings

In the above winding, the weight of copper mainly depends on its length as well as the cross-sectional area.

Again conductor’s length within the winding can be proportional to the no. of turns as well as cross-sectional area changes with the rated current. So copper weight within the winding can be directly proportional to the product of no. of turns & rated current of the winding.

Thus, the weight of copper within the AC section is proportional to I1 (N1-N2). Similarly, the copper weight within the BC section is proportional to N2 (I2-I1).

Therefore, the whole copper weight of this transformer is proportional to,

= I1 (N1-N2) + N2 (I2-I1)

= I1N1-I1N2 + I2N2-N2I1

= I1N1+I2N2-2I1N2

We know that N1I1=N2I2

= I1N1+I1N1-2I1N2

= 2I1N1-2I1N2 = 2(I1N1-I1N2)

In this way, it is proved, then the copper weight within two winding transformers can be proportional to N1I1-N2I2

Since in a transformer, N1I1=N2I2

2N1I1 (Since in a transformer N1I1 = N2I2)

In autotransformer, let’s assume the weights of copper like Wa & Wtw as well as two windings respectively,


Thus, Wa/Wtw = 2 (N1I1-N2I1)/2N1I1

= N1I1-N2I1/2N1I1 = 1-N2I1/N1I1

= 1-N2/N1 = 1-K

Therefore, Wa = Wtw (1-K) = Wtw- k Wtw

So, saving of copper within the transformer when we evaluated with two winding transformers is

Wtw- Wa = k Wtw

This transformer uses simply winding for each phase as against two particularly separate windings.

Auto Transformer: Construction and it’s Working

Advantages of Auto Transformer

The advantages are:

It uses single winding, so these are smaller & cost-effective.

These transformers are more efficient

It needs lesser excitation currents to compare with the conventional type transformers.

The voltage can be changed easily and smoothly

Enhanced regulation

Fewer losses

It needs less copper

Efficiency is high due to low losses in ohmic and core.

Disadvantages of Auto Transformer

The disadvantages are:

The secondary winding cannot be insulated from the primary.

It is applicable in restricted areas where a small difference in the o/p voltage from i/p voltage is necessary.

The leakage flux is small among the two windings so the impedance will below.

It can be dangerous to the load while we are utilizing an autotransformer like a step-down transformer. This transformer is used only to make small changes.

Applications of Auto Transformer

The applications are:

It increases the voltage drop for the distribution cable

It is used as a voltage regulator

It is used in audio, distribution, power transmission and railways

 It is used in laboratories to obtain a varying voltage continuously.

It is used like regulating transformers in voltage stabilizers.

It increases the voltage in AC feeders

It is applicable in electronics testing centers wherever frequently changing voltages are required.

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It is used where high voltages are necessary like boosters or amplifiers

It is used in audio devices like speakers as well as to adjust the device for nonstop voltage supply.

It is used in power stations where the voltage needs to step down and step up to equal the voltage at the receiving end which is necessary for the device.

Three-phase autotransformers

Three-phase reversible autotransformers, dry type, intended for use as a voltage adapter when an economical solution is required in applications where the galvanic isolation or attenuation of disturbances is not required. Connection with terminal blocks or with screws for flat terminals. Manufactured with steel with low losses and copper windings.