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.

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 to prevent the equipment from getting spikes and harmonics from the mains as shown in the figure. Such a transformer is also known as an insulating transformer.

An Isolation transformer with an electrostatic shield is used for sensitive equipment 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 a smaller output relative to its input.

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: This type of transformer reduces the mains supply voltage too 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 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.

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 an 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. The three-phase transformer has three types of windings as both primary and secondary windings are included as three phases.

These windings 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 the distribution side, delta to star connections transformers are used.

Auto Transformers

An autotransformer 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 fewer copper requirements.

Though it is advantageous over the 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 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 autotransformer: This type of autotransformer in which the source voltage is connected to the main winding and the load is connected across the part of the main winding is called a step-up autotransformer.

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

Variable Auto Transformer

A variable autotransformer 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 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 autotransformer 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.

1. In an isolation transformer, the input is isolated from the output, whereas in an autotransformer, 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 autotransformer 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 autotransformers 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 autotransformer maintains the outputs to a specified level, irrespective of the input fluctuations.

5. Low voltage regulation occurs in insulation transformers due to large voltage swings, whereas high voltage regulation takes place in an autotransformer 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 value proposition for us. However, for further details, suggestions, and comments, you can comment in the comment section below.

Photo Credits

• Isolation Transformers by imimg
• Winding of isolation transformer by bp.blogspot
• Step-up transformer by electricityforum
• Step-down transformer by electricityforum
• Single phase transformer by wikimedia
• Three phase transformer by mathworks
• Auto Transformers by itacanet
• Variable Auto Transformer by ete
• Isolation tranformer Vs Auto Transformer by acmefaq