Brush fire

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Brush fire of a defective motor

Brush fire of a simple electric motor

Brush fire describes the creation of sparks on the commutator of electric motors or on the slip rings of synchronous generators with static excitation .

In electric motors with commutators, such as the DC machines and some AC motors such as the universal motor , carbon brushes are used. These brushes make contact with the rotating part of the machine, the rotor . Two effects cause electrical flashovers to occur on these brushes, which become visible as sparks:

On the one hand, the neutral zone rotates under load from the center of the pole gap, which means that the induction voltage is not equal to zero at the time of the current reversal. At exactly this moment, however, the brushes are galvanically separated from the commutator , so that tear-off sparks arise, which are called brush fires.

There are also motors in which the brushes touch two lamellas of the commutator for a short time and thus cause a short-term short circuit , which also causes a brush fire.

In three-phase synchronous generators with static excitation, brush fires and high brush wear can also occur if the current distribution of the brushes connected in parallel is disturbed. This can be caused by both the wrong number of brushes and climatic conditions.

Brush fire has several disadvantageous consequences: First, it causes soiling of the lamellae and leads to increased wear on the carbon brushes.

In addition, it represents a broadband source of interference . The current interruption has a large d i / d t , that is, a large change in current in a short time and thus a high self-induction voltage

${\displaystyle u_{L}=L\cdot {\frac {{\text{d}}i}{{\text{d}}t}}}$

result. This tension can under unfavorable circumstances such. B. if there is no filtering, feed an interference voltage into neighboring circuits. ${\displaystyle u_{L}}$

See also

• Brush fire meter

literature

• Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3 .

Individual evidence

1. ↑ Christof Lechner, Jörg Seume (ed.): Stationary gas turbines, VDI . 1st edition. Springer, 2006, ISBN 978-3-540-42831-2 .
2. ↑ Archive link ( Memento of the original from December 24, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.