Engineering Tutorials

Welcome to Module 4, which is about the Transformer.

In this module you will begin to appreciate how the topics discussed in previous modules form the critical foundation for new topics. Topics from Modules 2 and 3-such as electromagnetism, power, utility systems, and electrical distribution-all contribute to a more solid understanding of transformers.

Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully then answer the questions without referring back to what you've just read. You are the best judge of how well you grasp the material. Review material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

A Note on Font Styles
Key points are in bold.
Glossary items are italicized and underlined the first time they appear.

Introduction
Transformers are all around you. They come in a wide range of shapes, sizes and application purposes. To get a general idea of where transformers are used, let's look at a simple electrical utility system.


Once electricity is generated, the voltage is increased by transformers. It is then transported to substations, where transformers decrease the voltage to usable levels for industrial plants, shopping centers and homes. These large amounts of electricity are moved at high voltages for a number of reasons, such as lower losses of power and overall Efficiency. The bottom line is: it costs less.

In order to understand the advantage and use of a transformer, let's first look at the basic transformer. There are four basic parts:

• Input connections
• Output connections
• Windings or coils
• Core



Input Connections
The input side is called the Primary Side of the transformer because this is where the main electrical power to be changed is connected.

Output Connections
The output side is called the Secondary Side of the transformer. This is where the electrical power is sent to the load. Depending upon the requirement of the load, the incoming electric power is either increased or decreased.

Windings
The transformer has two windings, called the Primary Winding and the Secondary Winding, wound around an iron core. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at a transformed or changed voltage to the load.

The primary and secondary windings of practically all transformers are subdivided into several coils. This is to reduce the creation of flux that does not link both primary and secondary. The transforming action can only exist when flux (mutual flux) couples both the primary and the secondary. Flux that does not do so is, in effect, leakage flux.
The windings are also subdivided to reduce the voltage per coil. This is important in high voltage transformers, in which insulation thicknesses make up a considerable part of the construction. In practice it is customary to subdivide a winding so that the voltage across each coil does not exceed about 5,000 volts.

Core
The transformer core is used to provide a controlled path for the magnetic flux generated in the transformer. The core is not a solid bar of steel, but is constructed of many layers (laminations) of thin sheet steel. It is laminated to help reduce heating, which creates power losses. Because the two circuits are not electrically connected, the core serves the very important part of transferring electrical power into the secondary winding through magnetic induction. The core usually takes the shape of a square or a ring.

There are two general types of cores: Core Type, and Shell Type. They are distinguished from each other by the manner in which the primary and secondary coils are placed around the laminated steel core.

Core type: In this type, the windings surround the laminated iron core. Figure 4 is a picture of an assembled core type transformer.

The primary and secondary coils are wrapped around the core side, or leg, with the low-voltage coil leads at the top and the high-voltage leads at the bottom. In practice, the primary winding is divided into an even number of separate coils, with half of them placed around one leg and the other half around the other leg. The same arrangement is used for the secondary winding. In some constructions the primary and secondary coils for each leg are assembled together to form a single unit, after which the assembly is dipped in insulating varnish and baked. This type is typically used with distribution transformers.
 


Shell type:
In this type, the core surrounds the windings. Figure 5 is a picture of a partially assembled shell type transformer.

All primary and secondary coils are assembled insulated from each other after which the entire coil assembly is dipped in an insulating varnish and baked.
This type is typically used in very large transformers with high voltages. If a core transformer was used at very high voltages, a relatively large amount of the flux produced by the primary windings would fail to link the secondary windings, and a large leakage flux would result.