No-load loss (electrical engineering)

No-load losses occur in electrical engineering in machines when electrical power is supplied without the machine producing any useful output.

The efficiency is zero percent when idling , the power consumed is mainly given off as waste heat to the environment. For economic reasons, no-load losses should be kept as small as possible and ideally avoided entirely by disconnecting the device from the power supply. In addition to rotating machines such as electric motors , electrical machines also include transformers and, in the broadest sense, also various forms of power supply units that can be operated without a connected load.

A distinction must be made between the no-load losses and the load losses that occur when a service is withdrawn and are caused by this withdrawal. The no-load losses together with the load losses result in the total losses. Since the efficiency usually depends on the power drawn, the efficiency for electrical machines is usually related to the operating range at nominal load .

Types of no-load losses

No-load losses occur in various electrical engineering devices. Some of the causes and magnitudes of the no-load losses are shown below.

transformer

The no-load losses of a transformer are determined at nominal voltage and without load. In the case of larger transformers, especially power transformers, these no-load losses arise primarily from the iron losses in the magnetic core . The core is periodically magnetized by the alternating current, which leads to losses through hysteresis and eddy currents . With smaller transformers, in the range below 20 VA , the copper losses due to the ohmic resistance of the windings also play a greater role.

To minimize iron losses, the transformer sheet is built up in layers in the magnetic core. The smaller the layer thickness, the lower the eddy current losses. Layer thicknesses less than 0.5 mm are common. Other improvements concern the choice of material for the magnetic core, the geometry of the core and the absence of air gaps.

Specifically, the no-load losses in a power transformer with 1 MVA, as used as a machine transformer and with a year of construction from the 1990s, are around 6.5 kW. In the case of a local network transformer with a nominal output of 250 kVA to supply a low-voltage network , such as that used in smaller transformer stations or as a mast transformer , the no-load losses amount to a few 100 W. In the case of small-power transformers below 20 VA and operation at mains frequency , such as those used in the household sector in simple plug-in power supplies , the no-load losses are around 2 W. The absolute power loss hardly decreases with these small powers at mains frequency downwards. At powers below 4 VA, the power consumption is already in the same order of magnitude as the nominal power consumption, according to idling. To avoid no-load losses, switched-mode power supplies are preferably used in these power ranges, which because of the higher operating frequency generally have lower no-load losses. As an alternative to this, toroidal transformers are also used, which consume around 100 times less no-load current than square transformers.

Electric motor

In the case of electric motors , the no-load losses are made up of two main components: the iron losses in the magnetic core, which have the same causes as in transformers, and the losses due to the friction in the bearings , which occur when the electric motor rotates even without external load. In order to record the frictional losses in the machine elements when idling, it is necessary that the motor rotates at the rated speed , and for electric motors with magnetic excitation the excitation current , which is decisive for the iron losses in the magnetic core in all excitation machines , must correspond to the nominal value .

Some electrical machines, e.g. B. series motors , can "run away" when idling, ie without braking payload. They can reach such high speeds that they are mechanically destroyed.

The no-load losses of electric motors play an important role in materials handling technology , where there is a cyclical change between different operating areas such as full load, part load and idling. As a result of improvements in conveyor system technology, the electric motors are increasingly no longer mechanically decoupled from the conveyor system, but rather operated more efficiently by electronic converters even in partial load operation and switched off completely when idling. In this way, the no-load losses of the motors can be avoided, only the significantly lower losses due to the standby mode of the converter occur.

literature

Rolf Fischer: Electrical machines . 16th edition. Carl Hanser Verlag, 2013, ISBN 978-3-446-43813-2 .

Individual evidence

↑ ^a ^b Reduction of losses from network transformers. Federal Office for Energy, 1997, accessed on March 27, 2014 .
↑ Hans-Ulrich Giersch, Hans Harthus, Norbert Vogelsang: Electrical machines: testing, standardization, power electronics . 5th edition. Vieweg + Teubner, 2003, ISBN 978-3-519-46821-9 , pp. 158 .

[ener1-1] Reduction of losses from network transformers. Federal Office for Energy, 1997, accessed on March 27, 2014 .

[elek1-2] Hans-Ulrich Giersch, Hans Harthus, Norbert Vogelsang: Electrical machines: testing, standardization, power electronics . 5th edition. Vieweg + Teubner, 2003, ISBN 978-3-519-46821-9 , pp. 158 .