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 efficiency of a device is defined as the ratio of the energy output to the energy input. It is also defined as the ratio of power output to power input.
In these definitions the term "output" means useful or desired output. In that sense of the term, the output of a generator or transformer is the energy or the power transmitted to the connected load. The generator or transformer, as the case may be, in addition to supplying useful energy also gives up energy in the form of heat, vibration, and noise to surrounding space. According to the Law of Conservation of Energy there can be no loss of energy because energy can neither be created nor destroyed. However, in the case of the generator or transformer the energy given up in the form of heat, vibration, and sound is not put to use. Such energy, as far as utilization is concerned, is wasted and is therefore considered lost. Hence the terms, copper losses, core losses, friction and windage losses. The relationship between efficiency, input, and losses is expressed by


The efficiencies of large power transformers are frequently greater than 99 percent at rated output. On the other hand the overall efficiency of commercial heat engines is below 40 percent. In many applications, such as in communication and control systems, efficiency is of minor importance in relation to quality and precision of performance. As an example, the efficiency of a cone loud-speaker is generally less than 5 percent.

Last Update: 2011-02-16