Auto-Transformers Explained: How They Work, Benefits and Applications

 


Auto-transformers are affordable and energy-efficient solutions to regulate voltage levels in modern power systems.

The primary function of transformers is to adjust the voltage levels between two or more alternating-current (AC) circuits. They are made of two main parts: a core (made of laminated iron sheets) and a wire coil (usually two). An increase or decrease in voltage levels is achieved by altering the ratio of coils.

In this guide, we explore the basics of auto-transformers. 

What is an Auto-Transformer?

Unlike the conventional transformer, which has two sets of wire coils, the auto transformer has a core with a single coil acting as both primary and secondary windings. Therefore, it is also known as a single winding transformer.

Using electromagnetic induction, the single winding transformer adjusts the voltage levels (steps up or steps down) using a portion of the winding as the primary and another portion of the same winding as the secondary in electrical applications.

Construction & Operation

  • The single winding has two end terminals with one or more intermediate tap points wrapped around a ferromagnetic iron core.
  • When an AC voltage is applied to the input terminals, it creates a fluctuating magnetic field in the core. This magnetic field produces an electromotive force (voltage) along the entire length of the winding. 
  • The voltage is regulated by tapping the winding at different points. Tapping closer to the input produces a lower voltage while tapping farther away produces a higher voltage.
  • Thanks to the single winding, auto-transformers have minimal energy loss.
  • Advantages and Disadvantages of Single Winding Transformers

The following are the advantages and disadvantages of auto-transformers:

Advantages:

  • It is relatively less expensive due to the single winding design and copper savings.
  • It comes with a reduced size and weight for the same power rating.
  • It provides more efficiency as losses are minimized.
  • It can withstand short-circuit currents better.
  • It requires less maintenance and has improved capacity utilization for high-power applications.

Disadvantages:

  • It requires more insulation since it is both electrically and magnetically coupled.
  • With no electrical isolation between the primary and secondary windings, auto-transformers pose significant safety risks as a fault in either of the winding portions can directly affect the other. This might cause equipment or personnel damage.
  • It can experience higher short circuit currents than conventional transformers, leading to dangerous conditions during faults.

Applications of Auto-Transformers


Single winding transformers are commonly used in low-voltage ranges. Some of their applications include:


  • They have applications in power transmission and distribution to increase or decrease voltage levels between different power systems. This compensates for drops along long transmission lines and makes sure that consumers receive stable voltage.
  • They are most commonly used in voltage stabilizers for residential and industrial sectors as they help maintain a consistent voltage output against the fluctuating input supply.
  • They also find use in large welding installations to control the voltage. Laboratories use them to obtain a varying voltage continuously. They are used in special applications like X-ray machines and impedance matching.
  • Transformers with several tappings can be used to start the induction motors. They gradually increase the voltage which prevents damage during startup. Additionally, they are also used to boost the voltage in AC feeders.
  • Electronics testing centers utilize them to test devices post-repair, preventing component damage. In audio devices such as speakers, they are used to balance the impedance and also to equip the device for continuous voltage supply.
  • They control the temperature of hot wires by adjusting the voltage levels in manufacturing processes like cutting styrofoam.

Are Power & Generator Transformers the same as Auto-Transformers?


No. There is a difference between auto, generator, and power transformers:

  • Generator Transformer: There is a generator transformer for each electricity generating unit according to the unit’s size, connecting the generator output to a power grid. It increases the voltage produced by a power plant and is commonly used in thermal and hydropower plants.

  • Power Transformer: Power transformers help adjust voltage levels for power transmission. They transfer electrical energy between circuits in power generation and distribution networks without changing the frequency.



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