What is the difference between autotransformer and ordinary transformer?

Transformers are of many types and widely used. Today, Daelim Transformer will introduce the difference between autotransformers and ordinary transformers in power transformers.

Definition of Autotransformer

An autotransformer has primary and secondary windings on the same winding, with the primary and secondary windings directly connected in series, resulting in self-coupling. Based on its structure, an autotransformer can be adjustable voltage type or fixed type. The coupling in an autotransformer is electromagnetic, whereas in ordinary transformers, energy is transferred solely through the electromagnetic coupling of primary and secondary coils without direct electrical connection between them. In autotransformers, the primary and secondary sides have direct electrical connections, and the low-voltage coil is part of the high-voltage coil. Autotransformers, along with other protective equipment, are used in the protection equipment of communication lines.

Definition of Ordinary Transformer

Ordinary transformers are used to change voltage (current, impedance) and transmit electrical energy. They consist of a primary coil, one or more secondary coils, and an iron core (hollow coils are often used at high frequencies). Differential transformers, used mainly as displacement sensors, typically consist of a primary coil, two secondary coils, and an iron core. The two secondary coils of this sensor produce increased and decreased induced potentials, respectively. Connected in series in reverse, this configuration is called a differential voltage device. The displacement of the middle iron core causes a change in the output voltage of the secondary coil, which is amplified to indicate the displacement size.

Differences Between Autotransformers and Ordinary Transformers

• Connection: Autotransformers have both magnetic and electrical connections between the primary and secondary sides, while ordinary transformers only have magnetic connections.
• Power Capacity: The capacity of the power supply through an autotransformer comprises two parts: the electromagnetic induction power between the primary winding and the common winding, and the conduction power directly conducted by the primary winding.
• Winding and Reactance: The autotransformer winding, composed of the primary winding and the common winding, has fewer turns than the primary winding of an ordinary transformer. This results in reduced height, current of the common winding, and leakage reactance. The short-circuit reactance XselfX_{self}Xself​ of the autotransformer is (1−1/k)(1-1/k)(1−1/k) times that of an ordinary transformer, where kkk is the transformer ratio.
• Third Winding: If an autotransformer is equipped with a third winding, it will occupy the capacity of the common winding, affecting the operation mode and exchange capacity of the autotransformer.
• Relay Protection: The neutral point of an autotransformer must be grounded, complicating the setting and configuration of relay protection.
• Size and Cost: Autotransformers are smaller, lighter, easier to transport, and lower in cost compared to ordinary transformers.

These distinctions highlight the specific applications and operational considerations for both autotransformers and ordinary transformers in power transmission and distribution.