You are currently viewing AUTO TRANSFORMERS | What is an Auto Transformer?

AUTO TRANSFORMERS | What is an Auto Transformer?


An autotransformer is a transformer that has part of its winding common to the primary and secondary circuits. Fig. 1(a) shows the circuit for a double-wound transformer and Fig. 1(b) that for an autotransformer. The latter shows that the secondary is actually part of the primary, the current in the secondary being (I2-I1). Since the current is less in this section, the cross-sectional area of the winding can be reduced, which reduces the amount of material necessary.

Figure 1: (a) double-wound transformer (b) autotransformer


The advantages of autotransformers over double-wound transformers include:

1) a saving in cost since less copper is needed

2) less volume, hence less weight

3) a higher efficiency, resulting from lower I2R losses

4) a continuously variable output voltage is achievable if a sliding contact is used

5) a smaller percentage of voltage regulation.


The primary and secondary windings are not electrically separate, hence if an open circuit occurs in the secondary winding the full primary voltage appears across the secondary.


Autotransformers are used for reducing the voltage when starting induction motors and for interconnecting systems that are operating at approximately the same voltage.

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Aanchal Gupta

Welcome to my website! I'm Aanchal Gupta, an expert in Electrical Technology, and I'm excited to share my knowledge and insights with you. With a strong educational background and practical experience, I aim to provide valuable information and solutions related to the field of electrical engineering. I hold a Bachelor of Engineering (BE) degree in Electrical Engineering, which has equipped me with a solid foundation in the principles and applications of electrical technology. Throughout my academic journey, I focused on developing a deep understanding of various electrical systems, circuits, and power distribution networks.

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