Power dissipation

When power loss is defined as the difference between the recorded power ( power consumption ), and in the form desired actual output power (power output) of a device or process. Power loss is mainly released as heat flow .

The power loss is particularly important in energy transmission and energy conversion , such as in gears (mechanical energy ), transformers (electrical energy), lamps (conversion of electrical energy into light energy), motors (conversion of chemical or electrical energy into mechanical energy); it should be kept as small as possible. Part of the power loss of a motor is the drag power . The dissipation of the resulting heat loss takes place directly, through radiation or heat transfer ( heat conduction and convection ), partly with the help of a cooler .

Electrical engineering

Declaration of loss on lines

In electrical engineering, that part of the active power is referred to as power loss that is undesirably converted into heat flow in a device or component. ${\ displaystyle P _ {\ text {loss}}}$

{\ displaystyle P _ {\ text {effective}} = P _ {\ text {loss}} + P _ {\ text {useful}} \,}

.

Capacitors and coils available from AC voltage in addition reactive power , which however is returned again to the generator. It can be reduced by means of reactive power compensation . Ideally, no power is lost in the form of a heat flow at such reactances , but their transmission creates losses in the power grid .

Since electrical components such as cables or even microelectronic circuits may only be operated up to a maximum permitted working temperature (otherwise there is often a risk of destruction of the component), the maximum power loss depends on the cooling conditions, i.e. H. the heat dissipation, depending. This is usually specified by the manufacturer. The power loss therefore plays an important role in the dimensioning of semiconductors, since large amounts of energy are often converted into heat loss in the relatively small components. To discharge them into the air, u. a. Heat sink inserted. The larger the surface, the lower the temperature at which the thermal energy is released. Since the output of power is usually not one of the tasks of integrated circuits and they therefore have no desired active power, the power loss in this case corresponds to the total power consumed. ${\ displaystyle P _ {\ text {dead}}}$

The line loss during the transmission of electrical energy depends directly on the line resistance , thus on the line thickness and the material used, as well as on the electrical current flowing . It can be calculated with or , where denotes the voltage drop across the line resistance . ${\ displaystyle R}$ ${\ displaystyle I}$ ${\ displaystyle P = R \ cdot I ^ {2}}$ ${\ displaystyle \ P = U ^ {2} / R}$ ${\ displaystyle U}$ ${\ displaystyle R}$

Power loss in electronic switches

→ Main article: Switching losses

In switched-mode power supplies , electronic switches ( bipolar transistors or MOSFETs ) are used to switch current with a frequency in the kilohertz range. It is important to avoid intermediate states in which both the voltage at the transistor (with bipolar transistors between collector and emitter, with MOSFETs between drain and source) and the flowing current are high at the same time , because then the product is very large, so that the junction in the transistor is overheated and destroyed faster than the heat can be dissipated. ${\ displaystyle P = U \ cdot I}$

When the transistor blocks, the current and thus also the power are approximately 0, even if the voltage across the transistor is 300 V.

When the transistor turns on fully, decreases with bipolar transistors on a saturation voltage of about 0.5 V, at MOSFETs sinks deeper. With a collector current of 30 A, the power loss at the transistor is only 15 W, although a load of 9000 W is switched. ${\ displaystyle U _ {\ text {CE}}}$ ${\ displaystyle U _ {\ text {DS}}}$

During the switchover, the values do not change suddenly, and it can happen that the current has already risen to 10 A, for example, and the voltage at the transistor has only dropped to 40 V. The power loss increases briefly to 400 W in the smallest of spaces. The average switching losses are proportional to the switching frequency.

literature

Wolfgang Nerreter: Fundamentals of electrical engineering . Hanser, Munich 2011, ISBN 3-446-40414-7 .

Web links

www.weltderphysik.de: Why does current transfer take place at high voltages?