Excitation brushless

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. 

Modern technology has reduced the size of motors, increased their expected life and improved their resistance to dirt and corrosion. Other important developments of the last 30 years are brushless excitation for synchronous motors and new two-speed, single-winding, induction motors. 

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. 

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. 

When comparing excitation systems, each has its advantages. While the brushless system eliminates collector rings, a failure of the rotating rectifier can cause a shutdown. In contrast, the static excitation system normally provides parallel sets of stationary rectifiers, so a full load can be carried with one bank out of service. However, this system requires periodic brush maintenance, which can be done while operating. 

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

The modular design of the ttubogenerator permits the selection of a standard version with either an open or closed cooling S3 stem and static or brushless excitation systems. 

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. 

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