Transformer, induction motor and synchronous motor
Since it is necessary to supply a magnetizing current to an inductive load such as a transformer and an induction motor, the synchronous motor in an overexcited state has the ability to provide a hysteresis current, which is a great advantage of the synchronous motor and has good economic significance. In effect, the overexcited state synchronous motor acts as a generator that produces hysteretic reactive power and relieves the power required to provide this reactive component. Therefore, they can perform the same function as the compensation capacitor device. Sometimes a synchronous motor running at no load is connected to the power system, only to adjust the power factor or control the reactive power. Such synchronous motors are often referred to as synchronous compensators and are more economical than stationary capacitors in larger sizes.
Turbine generator excitation system
As turbo generators can continue to increase in capacity, it is increasingly difficult to provide DC excitation currents (up to 1000A or higher in larger units). A commonly used excitation power source is a DC generator that is driven coaxially with the generator, the output of which is excited by the brush and the slip ring to the field winding of the alternator. Alternatively, a conventional coaxial drive alternator can be used as the main exciter to provide excitation to the main generator. The exciter has a stationary armature and a rotating field winding, the frequency of which may be 180 Hz or 240 Hz, the output of which is sent to a stationary solid state rectifier, and the output of the rectifier provides excitation to the turbine generator through a (brush and) slip ring .
Rotary rectifier and brushless excitation
Slip rings, commutators, and brushes inevitably involve cooling and maintenance issues. Many modern excitation systems avoid such problems by using as few sliding contacts as possible and brushes. For example, some excitation systems use coaxially driven alternators, but the field windings of the exciter are stationary and their AC armature windings rotate with the shaft. With a rotary rectifier, DC excitation can be applied directly to the field winding of the main generator without slip rings.
