Capacitors, Magnetic Circuits, and Transformers is a free introductory textbook on the physics of capacitors, coils, and transformers. See the editorial for more information....

The Transformer

The transformer is largely responsible for the prevalence of the a-c power system. In the earliest systems electric power was generated, transmitted, distributed, and utilized in the form of direct current. The limitations of the d-c generator restricted operation to low values of generating voltages as well as transmission voltages, which not only severely restricted the amounts of power that could be transmitted efficiently to small values by present-day standards but also limited the transmission of even these small amounts of power to relatively short distances. Because of its high efficiency, exceeding 99 percent in ratings of 10000 kva and above, and also because of its almost unlimited ratio of voltage transformation, the transformer makes it possible to generate power at voltages consistent with the most economical design of generators, 22000 v being a common generating voltage in the larger systems. The transformer also makes it possible to step these voltages up to values approaching a million v for efficient transmission over distances as great as several hundred miles, and eventually to step these voltages down to values at which the power can be utilized most effectively. Most household appliances operate at about 120 v.

The transformer is also a vital component in many low-voltage, low-power applications - as in electronic circuits, where it may be used to provide the optimum ratio of load impedance to source impedance so that signals may be amplified with small distortion, or in some cases to effect maximum power transfer from a source to a load. It is not unusual to have a source of several thousand ohms impedance supplying a load that has an impedance as low as several ohms and still get maximum power transfer by means of an impedance-matching transformer.

The transformer may be defined as a device in which two or more stationary circuits are coupled magnetically. Some of the principles that underlie its operation can be extended to the analysis of more complex arrangements in which one or more circuits may be in motion relative to other circuits, as in the case of motors and generators.

Last Update: 2011-02-16