Toggle Nav

Sign in to see your recent orders, quotes and account activity.

Sign in

Transformers - Explaining The Basics


A transformer is an electrical device which, by the principles of electromagnetic induction, transfers electrical energy from one electric circuit to another, without changing the frequency. The energy transfer usually takes place with a change of voltage and current. Transformers either increases or decreases AC voltage.

Transformers are used to meet a wide variety of needs. Some transformers can be several stories high, like the type found at a generating station or small enough to hold in your hand, which might be used with the charging cradle for a video camera. No matter what the shape or size, a transformers purpose remains the same: transforming electrical power from one type to another.

There are many different types of transformers in use today. This resource will take a closer look at Power Transformers, Auto Transformers, Distribution Transformers, Instrument Transformers, Isolation Transformers, Potential Transformers and Current Transformers.

Constant Voltage Transformers Overview
How Transformers Work

It is important to remember that transformers do not generate electrical power; they transfer electrical power from one AC circuit to another using magnetic coupling. The core of the transformer is used to provide a controlled path for the magnetic flux generated in the transformer by the current flowing through the windings, which are also known as coils. There are four primary parts to the basic transformer. The parts include the Input Connection, the Output Connection, the Windings or Coils and the Core.

  • Input Connections - The input side of a transformer is called the primary side because the main electrical power to be changed is connected at this point.
  • Output Connections - The output side or secondary side of the transformer is where the electrical power is sent to the load. Depending on the requirement of the load, the incoming electric power is either increased or decreased.
  • Winding - Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load. Usually, these two coils are subdivided into several coils in order to reduce the creation of flux.
  • Core - The transformer core is used to provide a controlled path for the magnetic flux generated in the transformer. The core is generally not a solid bar of steel, but rather a construction of many thin laminated steel sheets or layers. This construction is used to help eliminate and reduce heating.

    Transformers generally have one of two types of cores: Core Type and Shell Type. These two types are distinguished from each other by the manner in which the primary and secondary coils are placed around the steel core.

    • Core type - With this type, the windings surround the laminated core.
    • Shell type - With this type, the windings are surrounded by the laminated core.

When an input voltage is applied to the primary winding, alternating current starts to flow in the primary winding. As the current flows, a changing magnetic field is set up in the transformer core. As this magnetic field cuts across the secondary winding, alternating voltage is produced in the secondary winding.

Basic Transformer Design

The ratio between the number of actual turns of wire in each coil is the key in determining the type of transformer and what the output voltage will be. The ratio between output voltage and input voltage is the same as the ratio of the number of turns between the two windings.

A transformers output voltage is greater than the input voltage if the secondary winding has more turns of wire than the primary winding. The output voltage is stepped up, and considered to be a "step-up transformer". If the secondary winding has fewer turns than the primary winding, the output voltage is lower. This is a "step-down transformer".

Diagram of Step-down Transformer Diagram of Step-up Transformer
Transformer Configurations

There are different configurations for both single-phase and three-phase systems.

  • Single-phase Power - Single-phase transformers are often used to supply power for residential lighting, receptacle, air-conditioning, and heating needs. Single phase transformers can be made even more versatile by having both the primary winding and secondary winding made in two equal parts. The two parts of either winding can then be reconnected in series or parallel configurations.
  • Three-phase Power - Power may be supplied through a three-phase circuit containing transformers in which a set of three single-phase transformers is used, or on three-phase transformer is used. When a considerable amount of power is involved in the transformation of three-phase power, it is more economical to use a three-phase transformer. The unique arrangement of the windings and core saves a lot of iron.


  • Delta and Wye Defined - There are two connection configurations for three-phase power: Delta and Wye. Delta and Wye are Greek letters that represent the way the conductors on the transformers are configured. In a delta connection, the three conductors are connected end to end in a triangle or delta shape. For a wye, all the conductors radiate from the center, meaning they are connected at one common point.
  • Three-phase Transformers - Three-phase transformers have six windings; three primary and three secondary. The six windings are connected by the manufacturer as either delta or wye. As previously stated, the primary windings and secondary windings may each be connected in a delta or wye configuration. They do not have to be connected in the same configuration in the same transformer. The actual connection configurations used depend upon the application.
Types of Transformers
Power Transformer

A power transformer is used primarily to couple electrical energy from a power supply line to a circuit system, or to one or more components of the system. A power transformer used with solid state circuits is called a rectifier transformer. A power transformer's rating is given in terms of the secondary's maximum voltage and current-delivering capacity.

Isolation Transformer

An isolation transformer is a very unique transformer. It has a 1:1 turn's ratio. Therefore, it does not step voltage up or down. Instead, it serves as a safety device. It is used to isolate the grounded conductor of a power line from the chassis or any portion of a circuit load. Using an isolation transformer does not reduce the danger or shock if contact is made across the transformer's secondary winding.

Technically, any true transformer, whether used to transfer signals or power, is isolating, as the primary and secondary are not connected by conductors but only by induction. However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer function of voltage conversion), are routinely described as isolation transformers.

Distribution Transformer

A pole-type distribution transformer is used to supply relatively small amounts of power to residences. It is used at the end of the electrical utility's delivery system.


The autotransformer is a special type of power transformer. It consists of a single, continuous winding that is tapped on one side to provide either a step-up or a step-down function. This is different from a conventional two-winding transformer, which has the primary and secondary completely isolated from each other, but magnetically linked by a common core. The autotransformer's windings are both electrically and magnetically interconnected.

An autotransformer is initially cheaper than a similarly-rated two-winding transformer. It also has better regulation (smaller voltage drops), and greater efficiency. Furthermore, it can be used to obtain the neutral wire of a three-wire 240/120-volt service, just like the secondary of a two-winding transformer. Autotransformers are considered unsafe for use on ordinary distribution circuits. This is because the high-voltage primary circuits are connected directly to the low-voltage secondary circuit.

Instrument Transformer

For measuring high values of current or voltage, it is desirable to use standard low-range measuring instruments together with specially-constructed instrument transformers, also called accurate ratio transformers. An accurate ratio transformer does just as the name suggests. It transforms at an accurate ratio to allow an attached instrument to gauge the current or voltage without actually running full power through the instrument. It is required to transform relatively small amounts of power because it's only load, called a burden, is the delicate moving elements of an ammeter, voltmeter or wattmeter.

There are two types of instrument transformers:

  1. Current - Used with an ammeter to measure current in AC voltages
  2. Potential - Used with a voltmeter to measure voltage (potential difference) in AC.
Current Transformer

A current transformer has a primary coil of one or more turns of heavy wire. It is always connected in series in the circuit in which current is to be measured. The secondary coil is made up of many turns of fine wire, which must always be connected across the ammeter terminals. The secondary of a current transformer must never be open-circuited. This is because the primary is not connected to a constant source. There is a wide range of possible primary voltages, because the device can be connected to many types of conductors. The secondary must always be available (closed-circuited) to react with the primary, to prevent the core from becoming completely magnetized. If this happens, the instruments will no longer read accurately.

A clamp-on ammeter works in a similar way. By opening the clamp and placing it around a current carrying conductor, the conductor itself acts as a single turn primary. The secondary and the ammeter are conveniently mounted in the handle of the device. The dial allows a number of current ranges to be gauged accurately.

Potential Transformer

A potential transformer is a carefully designed, extremely accurate step-down transformer. It is normally used with a standard 120-volt voltmeter. By multiplying the reading on the voltmeter (called the deflections) by the ratio of transformation, the user can determine the voltage on the high side. Common transformation ratios are 10:1, 20:1, 40:1, 80:1, 100:1, 120:1, and even higher.

In general, a potential transformer is very similar to a standard two-winding transformer, except that it handles a very small amount of power. Transformers for this service are always the shell type, because this construction has been proven to provide better accuracy.

Potential Transformers are designed for monitoring single-phase and three-phase power line voltages in power metering applications.

© Copyright 2023 Galco Industrial Electronics, All Rights Reserved