Isolation Transformers and Auto Transformers for Electrical Professionals

Isolation Transformers

A transformer is a device that transfers electric power from one circuit to another circuit without changing frequency. It contains primary and secondary winding wherein the primary winding is connected to the main circuit and the secondary winding to a required load circuit. An Isolation transformer is defined as the primary and secondary windings of the transformer separated from each other.

Isolation Transformers
Isolation Transformers
Winding of isolation transformer
Winding of isolation transformer

The input and output powers in a transformer are magnetically coupled since the transformer design is made by using a dielectric insulation barrier. An isolation transformer isolates load in an electrical system in order to prevent the equipment from getting spikes and harmonics from the mains as shown in the figure. Such a transformer is also known as insulating transformer.

An Isolation transformer with an electro static shield is used for sensitive equipments such as computers and laboratory instruments. The turn ratio determines whether the transformer is used: to step-up or step-down or for unchanged voltages. This transformer can be used in different applications like portable electric tools and electric traction, and so on.

Isolation transformers’ classification depends on the winding arrangement, construction and the alternating current type involved.

Classification Based on Winding Arrangement

  • Some transformers capable of producing an output voltage that is identical to their input are known as 1:1 isolation transformers.
  • A Step-up transformer produces an output voltage greater than its input voltage.
  • A Step-down transformer produces smaller output relative to its input.
Step-up transformer
Step-up transformer

Step-Up Transformer: This type of transformer has more number of turns in the secondary winding and less in the primary such that the voltage is more in the secondary when compared to the primary as shown in the figure. The number of turns in both the windings is decided by the application rating requirement. Step-up transformers are used as boosters power transmission lines.

Step-down transformer
Step-down transformer

Step-Down Transformer: This type of transformer reduces the mains supply voltage to low values depending on the load requirement. In a step-down transformer, the primary winding consists of more number of turns compared with the secondary winding.

The Relationship between currents, voltages and turns are in transformation ratio equations that are given below.

Voltage transformation ratio = Secondary Turns/ Primary Turns
Current transformation ratio = Primary Turns / Secondary Turns

Classification Based on the Nature of Power Supply

An isolation transformer can be manufactured to operate on a single and three-phase AC supplies.

Single Phase Transformer: This is manufactured to operate on a single-phase AC supply and is mostly used for low-power applications like residential lighting, air conditioning and heating, etc. Single phase transformer can be reconnected in series or parallel based on the requirement of the load.

Single phase transformer
Single phase transformer

A Single phase transformer consists of two windings on a common iron core. If one of the winding is connected to an AC voltage, an alternative magnetic field is set on iron core. This field coupled with the secondary winding produces an EMF in it. As a result, this EMF drives the current to pass to the load circuit.

Three phase transformer: This transformer is designed and constructed for specific voltages particularly for higher voltages. Three-phase transformer has three types of windings as both primary and secondary windings are included as three phases.

Three phase transformer
Three phase transformer

These winidngs can be connected in wye (Star) or delta form. The combination of primary and secondary windings can be delta- delta, wye- delta, wye-wye and delta-wye. This type of configuration depends on the application – like, in distribution side, delta to star connections transforners are used.

Auto Transformers

An auto transformer consists of only one winding, and part of it acts as a secondary winding. It is smaller, lighter and cheaper than the dual winding transformer and also possesses lower leakage reactance, higher efficiency, good power quality and less copper requirements.

Though it is advantageous over conventional one, yet it does not provide any electrical isolation to load from the mains, and is more prone to faults. This type of transformer can be used to step up or step down the voltage by connecting the windings in various configurations.

Auto Transformers
Auto Transformers

Auto-transformers are used in power transmission, distribution, railways and audio applications. The turns ratio of a step- down transformer is less than ‘1’, and the turn ratio of a step-up transformer is always greater than ‘1’.

Step-up auto transformer: This type of auto transformer in which the source voltage is connected to the main winding and the load is connected across the part of a main winding is called as a step-up auto transformer.

Step-down auto transformer: This type of auto transformer in which the source voltage is applied to the part of a main winding, and the load is connected to the whole main winding, as shown in the figure.

Variable Auto Transformer

Variable Auto Transformer
Variable Auto Transformer

A variable auto transformer is also known as variac in which the secondary connection through a sliding brush allows the voltage to get vary over a given range. This type of transformer is an AC voltage control that provides a variable AC voltage to the various circuits. Variac transformers can boost the output voltage, which is in excess and twice than that of the input voltage.

This autotransformer is equipped with many taps and automatic switch gears that allow it to act as automatic voltage regulators. The top features of the variable auto transformer are high efficiency, low temperature rise and short time overload capacity.

After going through the above information, one can easily compare both these transformers. The following are some of the differences that emerge after comparing them.

Isolation tranformer Vs Auto Transformer
Isolation tranformer Vs Auto Transformer

1. In an isolation transformer, the input is isolated from the output, whereas in an auto transformer, there is no electrical isolation between the input and output.

2. An isolation transformer consists of both primary and secondary windings that are wounded on an iron core, whereas an auto transformer consists of one coil that acts both as primary and secondary windings.

3. Due to more windings, isolation transformers require more copper, so the weight is significantly high, whereas auto transformers require fewer windings and small core so these are lighter in weight and less costly for the same rating of isolation transformers.

4. If any surge takes place in the primary winding of the isolation transformer, it sustains to load but the auto transformer maintains the outputs to a specified level, irrespective of the input fluctuations.

5. Low voltage regulation occurs in an insulation transformers due to large voltage swings, whereas high voltage regulation takes place in an auto transformer due to smaller voltage swings.

This is all about the transformers. We hope that you might have gained some valuable insights and concepts out of this article, after thoroughly reading it. Furthermore, we encourage you to share your knowledge on this particular topic or electrical and electronic projects as that would become a valuable proposition for us. However, for further details, suggestions, and comments, you can comment in the comment section below.

Photo Credits

4 Comments

  1. nice article!! i read all these thing about the isolation transformer.
    i heard servomax is manufacturing company gives one step solution about the isolation transformer.

  2. jeewan garg says:

    Nice post full of information. You can also get some more information about Isolation Transformer here.

Add Comment