Brushless excitation device

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The picture shows a three-phase synchronous generator (SG), the excitation current (I E ), the rotating diode rectifier (V1), the excitation machine (G1) and the auxiliary excitation machine (G2) with the thyristor actuator (V2).
The picture shows a three-phase synchronous generator (SG), the excitation current (I E ), the rotating diode rectifier (V1), the excitation machine (G) and the excitation transformer (ET) with the thyristor actuator (V2).

The brushless excitation device (also excitation device without slip rings ) is an excitation system for synchronous machines . When large generators are excited , several k A flow , which in excitation devices with slip rings requires very wide slip rings and many brushes connected in parallel and entails corresponding losses. In addition, brush fires can occur when using brushes . These problems can be avoided with the principle of brushless excitation with a rotating diode rectifier .

construction

An external- pole three-phase generator (exciter) and a (mostly) permanently excited three-phase synchronous generator (auxiliary exciter) are coupled directly to the synchronous machine (main machine) . The auxiliary exciter feeds the external pole winding of the exciter via a thyristor actuator with auxiliary exciter power. This is regulated depending on the stator voltage of the main machine. It is also possible to obtain the auxiliary excitation power from the internal power supply or the stator circuit of the main machine. A transformer , which in this case is referred to as an excitation transformer, is often used to adjust the voltage . The excitation winding of the main machine is fed with excitation power from the rotor winding of the excitation machine directly via the rotating diode rectifier. Since the excitation winding of the main machine, the diode rectifier and the rotor winding of the excitation machine are located on rotating parts and the excitation machine receives the auxiliary excitation power via its outer poles, no slip rings and brushes are required.

In addition to its use in large generators, brushless excitation is mainly used in synchronous motors. Since the excitation device is located on the shaft and together in a housing with the synchronous machine, a compact design is possible.

Advantages and disadvantages

advantages

  • No brushes and slip rings and the associated losses and wear
  • No brush fire
  • Use in Ex-protected areas possible

disadvantage

  • Maintenance of the diode rectifier is only possible when it is not running
  • The excitation current of the main machine is influenced indirectly via the auxiliary excitation current of the excitation machine. The achievable changes over time dI E / dt are reduced by the time constant of the exciter. The settling times in the event of faults are therefore longer than with static excitation systems.
  • No rapid de- energization possible
  • The size of the excitation device increases in proportion to the speed of the main engine → unattractive for low speeds.
  • A frequency start-up, as is required for gas turbine sets, cannot be performed with RG excitations (rotating rectifier).

See also

literature

  • Rolf Fischer: Electrical machines. 14th, updated and expanded edition. Carl Hanser Verlag, Munich 2007, ISBN 978-3-446-41754-0 .
  • Germar Müller, Bernd Ponick: Fundamentals of electrical machines. 9th, completely revised edition. Wiley-VCH, Berlin 2006, ISBN 3-527-40524-0 .