Electrical Fundamentals – Overview of Transformers

A. Bhatia, B.E.

Course Outline

A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to "step up" or "step down" voltages and works on the magnetic induction principle. A "transformer" only changes one voltage to another; it doesn't change power levels. If you put 100 watts into a transformer, 100 watts come out the other end. [Actually, there are minor losses in the transformer because nothing in the real world is 100% perfect. But transformers come pretty darn close; perhaps 95% efficient].

This course provides you with insight to the fundamentals and applications of transformers.

This 3-hr course material is based entirely on Naval Education and Training Materials (NAVEDTRA 14174), Electricity and Electronic Training Series; Module-2, Chapter 5, titled “Transformers”.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.

Learning Objective

At the conclusion of this course, the student will be able to:

• State the meaning of transformer action;
• State the physical characteristics of transformers, including the basic parts, common core materials and the main core types;
• State the meaning of “no-load condition” and “exciting current” relative to transformer;
• State what causes voltage to be developed across the secondary of a transformer;
• State the meaning of leakage flux and its effect on the coefficient of coupling;
• Identify a transformer as step-up or step-down when given the turns ratio;
• State the three power loss in a transformer and describe how to calculate the efficiency of transformer;
• List five different type of transformers according to their applications; and
• State the general safety precautions one should observe when working on the transformers.

Intended Audience

This course is aimed at students, professional engineers, service technicians, energy auditors, operational & maintenance personnel, facility engineers and general audience.

Course Introduction

A transformer is an electrical device that allows an AC input signal to produce a related AC output signal without the input and output being physically connected together. This is accomplished by having two (or more) coils of insulated wire wound around a magnetic metal core. These wire coils are called windings.

Power is fed into one winding (the "primary"), which creates a magnetic field. The magnetic field causes current to flow in the other winding (the "secondary") via a phenomenon called inductive coupling. By changing the number of wire turns in each winding, transformers can be manufactured to have various impedance ratios. The ratio between the input and output impedances provides a gain or loss of signal level as the signal passes through the transformer. Transformers are bidirectional so that an input winding can become the output winding and an output can become an input. Because of a transformer's bidirectional nature, it can provide a gain in signal level when used in one direction or a loss when used in reverse.

Course Content

In this course, you are required to study Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-2, Chapter 5, titled “Transformers”:

Transformers (Chapter 5, NAVEDTRA 14173)

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Course Summary

A transformer converts alternating current from one voltage to another. Transformer, in its simplest form consists of two or more coils of insulated wire wound on a laminated steel core. When an alternating voltage is introduced to one coil, called the primary, it creates a fluctuating magnetic field in the iron core. This fluctuating field then induces an alternating voltage in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils. The number of times the wires are wrapped around the core ("turns") is very important and determines how the transformer changes the voltage.

• If the primary has fewer turns than the secondary, you have a step-up transformer that increases the voltage.
• If the primary has more turns than the secondary, you have a step-down transformer that reduces the voltage.
• If the primary has the same number of turns as the secondary, the outgoing voltage will be the same as what comes in. This is the case for an isolation transformer.
• In certain exceptional cases, one large coil of wire can serve as both primary and secondary. This is the case with variable auto-transformers.

Modern electric power systems use transformers to convert electricity into different voltages. Transformers step up voltage levels at the generation point because higher voltages can be transmitted more efficiently over long distances. At the substation the voltage may be transformed down to lower levels for further transfer on the distribution system. Another set of transformers step down voltage near the point of use.

To size transformers, you must calculate voltage, current, and power. To do that, you need the transformer's "turns ratio," primary voltage, and total load. The secondary voltage is the primary voltage divided by the turns ratio. Secondary current is the primary current times the turns ratio. A transformer has no moving parts and is a completely static solid state device, which ensures under normal conditions, a long and trouble-free life. Note that a transformer doesn't work for direct current (DC): the incoming voltage needs to change over time - alternating current (AC) or pulsed DC.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.