Brushless excitation, generators

Another concept is brushless excitation, in which an ac generator (exciter) is direc tfy coupled to or mounted on the motor shaft. The ac exciter has a stator field and an ac rotor armature which is directly connected to a static controllable rectifier on the motor rotor (or a shaft-mounted drum). Static control elements (to sense synchronizing speed, phase angle, etc.) are also rotor-mounted, as is the field discharge resistor. Changing the exciter field adjusts the motor field current without the necessity of brushes or slip rings. Brushless excitation is suitable for use in hazardous atmospheres, where conventional brush-type motors must have protective brush and slip-ring enclosures. 

Brushless excitation Eliminates need for slip rings in the generator cavity. Slip rings can cause reliability problems. 

Steam turbine generation of electricity for example, via condensate-type steam turbine with simple automatic extraction and two-pole synchronous generators with a cylindrical rotor and brushless exciter system. 

FIGURE 6.43 AC generator with rotating rectifier (brushless) excitation system. 

Rotary brushless excitation system. The winding of the generator rotor is supplied via a rectifler mounted on the shaft, from a directly connected 3-phase AC exciter. The system includes a pilot AC exciter. 

The excitation system provides the magnetizing current necessary for the generator to operate at the desired voltage and, when in parallel with other generators, supplies the required amount of reactive current. In modern practice the excitation system can be either brushless or static. 

In a brushless system an a.c. exciter with a rotating armature and stationary field system is provided. The voltage applied to the stationary field system is varied, thus changing the output of the rotating armature. This output is rectified via shaft-mounted diodes to produce a direct current (D.C.) supply that is connected to the main generator field. 

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