Transformers

A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling, where relative motion between the parts is not required to transfer energy between the circuits. more...

It is often used to convert between high and low voltages and for impedance transformation.

Introduction

The transformer is one of the simplest of electrical devices. Its design and principles have changed little over the last one hundred years, yet the transformer still plays a substantial role in the continuing development of high-voltage power transmission. It permits the economical transmission of power over large distances, allowing generating stations to be located physically further from sites of electricity demand and nearer to their sources of fuel. The simplicity, reliability, and economy of conversion of voltages by transformers was the principal factor in the selection of alternating current power transmission in the "War of Currents".

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to gigawatt units used to interconnect large portions of national power grids, all operating with the same basic principles and with many similarities in their parts.

Basic principles

An analogy

The transformer may be considered as a simple two wheel 'gearbox' for electrical voltage and current. The primary winding is analogous the the input shaft and the secondary winding to the output shaft. In this comparison, voltage is equivalent to shaft speed, current to shaft torque. In a gearbox, mechanical power (speed multiplied by torque) is constant (neglecting losses) and is equivalent to electrical power (voltage multiplied by current) which is also constant. The gear ratio is equivalent to the transformer step up or down ratio.

Flux coupling

A simple single phase transformer consists of two electrical conductors called the primary winding and the secondary winding. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field around the conductor and in the magnetic core (shaded grey). The secondary, which is placed in this varying magnetic flux, develops an electromotive force or EMF. If the ends of the secondary are connected together to form an electric circuit, this EMF will cause a current to flow in the secondary. In this way, the electrical energy fed into the primary winding is delivered to the secondary winding. In most practical transformers, the primary and secondary conductors are coils of conducting wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor.

Electrical laws

Transformers alone cannot do the following:

• Convert DC to AC or vice versa
• Change the voltage or current of DC
• Change the AC supply frequency.

However, transformers are components of the systems that perform all these functions.

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