Difference between Two-Winding Transformer and Autotransformer

Differences Autotransformer Electronics & Electrical

A transformer is a static electric machine which changes the level of voltage and current in the circuit without changing the power and supply frequency. The electrical transformer works on the principle of mutual inductance.

Based upon the number of windings used in the transformer, they can be grouped into two different categories −

Two-winding transformer (or typical transformer or conventional transformer)
Autotransformer

Read through this article is to find out the major differences between two-winding transformer and autotransformer.

What is a Two-Winding Transformer?

A two-winding transformer or conventional transformer is one which consists of two magnetically coupled windings (primary and secondary) which are electrically insulated and transfers electrical energy from one circuit to another at the same frequency but different voltage and current. The circuit symbol of a two winding transformer is shown in Figure-1.

In case of two winding transformer, the primary winding is the one which takes the input from the supply. While the secondary winding is connected to a load for supplying energy to the load.

What is an Autotransformer?

A type of electrical transformer which has only one winding, a part of which acts as a primary winding and the other part as the secondary is known as autotransformer. Thus, in an autotransformer only a single winding acts as both primary and secondary. Consequently, the windings of the autotransformer are connected magnetically as well as electrically. The circuit symbol of an autotransformer is shown in Figure-2.

In case of an autotransformer, when the primary winding voltage is more than the secondary winding voltage, then it is known as step-down autotransformer. When the primary winding voltage is less than secondary voltage, it is known as step-up autotransformer.

Difference between Two-Winding Transformer and Autotransformer

The following table shows the key differences between two winding transformer and autotransformer.

Point of Comparison	Two-Winding Transformer	Autotransformer
Definition	A static electric machine which consists of two magnetically coupled windings, which are electrically insulated from one another for the transfer of electrical energy from one circuit to another at the same frequency but at different voltage is known as two winding transformer.	A type of electrical transformer which consists of only one winding, a part of which acts as primary and the other part as the secondary is called the autotransformer.
Number of windings	Two-winding transformer consists two electrically insulated windings viz. primary winding and secondary winding.	Autotransformer consists of only one winding which acts as both primary and secondary.
Insulation between	In a two winding transformer, primary and secondary	In autotransformer, primary and secondary
primary and secondary	windings are electrically insulated from each other.	windings are not electrically insulated.
Working Principle	Two-winding transformer works on the principle of mutual inductance.	Autotransformer works on the principle of self-inductance.
Mode of power transfer	In a two-winding transformer, the electrical power transfers from one circuit to another circuit magnetically.	In an autotransformer, power transfers from input to output partly magnetically and partly electrically.
Physical size	Two-winding transformers are larger in size.	Autotransformers are relatively smaller in size.
Conductor material	Two-winding transformer requires more winding conductor material.	For the same rating, an autotransformer requires less conductor material.
Losses	Two-winding transformer has more losses.	The losses in an autotransformer are low as compared to two winding transformer.
Efficiency	Efficiency of a two winding transformer is less due to more copper loss in the two windings.	An autotransformer has more efficiency.
Leakage flux	Two-winding transformer has high leakage flux.	The leakage flux is low in an autotransformer.
Excitation current	Two-winding transformer requires large excitation current.	The excitation current of an autotransformer is relatively smaller.
Internal impedance	The internal impedance of a two winding transformer is high.	Autotransformer has less internal impedance.
Secondary voltage	Two-winding transformer generally provides constant output voltage.	For an autotransformer, the output voltage variable.
Voltage regulation	Two-winding transformer has good voltage regulation.	Autotransformer has better voltage regulation.
Economical	Two-winding transformers are less economically due to larger size and high maintenance cost.	Autotransformers are more economical than two winding transformers.
Cost	For the same VA rating, the cost of two winding transformer is more.	Autotransformer is less costly than two winding transformer.
Applications	Two-winding transformer are employed for step-up and step-down the voltage for transmission and distribution purposes.	Autotransformer is used as laboratory transformer, starter for induction motor, voltage regulator, traction transformer, etc.

Conclusion

From the above discussion, it can be concluded that there are various differences between two winding transformer and autotransformer. Both of these transformers are extensively used in different electrical applications such as power transmission and distribution, induction motor starting, laboratory, etc. for stepping up and down the voltage level. Thus, the knowledge of these differences can help an individual to select either of them according to the requirements.