How a AC Generator Works
Electromagnetic Induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field. It is this same principle that your generator uses to produce electricity.
The alternator part of your generator consists of two basic parts. The Stator and the Rotor. The Stator is the part you draw your power from and the Rotor is the electromagnet that rotates its magnetic field around the stator windings. The output from the Stator depends on the strength of the rotors electromagnet. That strength is controlled by increasing or decreasing the DC (Direct Current) applied to it.
This rotating magnetic field requires direct current to energize it. The DC field current is obtained from a separate source called an exciter. Either rotating or static-type exciters are used for AC power generation systems. There are two types of rotating exciters: brush and brushless. The primary difference between brush and brushless exciters is the method used to transfer the DC exciting current to the generator fields.
Static excitation for the generator fields is provided in several forms including field-flash voltage from storage batteries and voltage from a system of solid-state components. DC generators are either separately excited or self-excited.
Excitation systems in current use include direct-connected or gear-connected shaft-driven DC generators, belt-driven or separate prime mover or motor-driven DC generators, and DC supplied through static rectifiers.
The brush-type exciter can be mounted on the same shaft as the AC generator armature or can be housed separately from, but adjacent to, the generator. When it is housed separately, the exciter is rotated by the AC generator through a drive belt.
The distinguishing feature of the brush-type generator is that stationary brushes are used to transfer the DC exciting current to the rotating generator field. Current transfer is made via rotating slip rings (collector rings) that are in contact with the brushes.
Each collector ring is a hardened-steel forging that is mounted on the exciter shaft. Two collector rings are used on each exciter, each ring is fully insulated from the shaft and each other. The inner ring is usually wired for negative polarity, the outer ring for positive polarity.
A rotating-rectifier exciter is one example of brushless field excitation. In rotating-rectifier exciters, the brushes and slip rings are replaced by a rotating, solid-state rectifier assembly. The exciter armature, generator rotating assembly, and rectifier assembly are mounted on a common shaft. The rectifier assembly rotates with, but is insulated from, the generator shaft as well as from each winding.
Static exciters contain no moving parts. A portion of the AC from each phase of generator output is fed back to the field windings, as DC excitations, through a system of transformers, rectifiers, and reactors. An external source of DC is necessary for initial excitation of the field windings. On engine driven generators, the initial excitation may be obtained from the storage batteries used to start the engine or from control voltage at the switchgear.
What is Better Brush or Brushless?
The biggest difference is how D/C “gets” to the rotor to produce a magnetic field.
Brush type alternators make contact with the rotor using brushes but because of the contact with the moving parts there is friction over time they ware and need to be replaced. It is easy to do this on a open frame generator where access is easy. The enclosed or silent type generators it requires more work to move things like the muffler out of the way. The advantage with Brushes is it is easier to control this DC current used to control the magnetic field using electronic circuits called AVRs or Automatic Voltage Regulators.
Brushless is a lot more complicated to explain but a far more simpler design. There is really nothing to worry about other then it is a bit slower to respond to changes in voltage. A brushless alternator is composed of two alternators built end-to-end on one shaft. The purpose is to eliminate moving parts and brushes to make physical contact with the rotor to apply voltage to it and in turn create a electromagnetic field. Now that the rotor has its own source of power induced into it it can be rectified, converted to DC voltage and used to energize the main rotor windings necessary for the larger magnetic field that is used to produce the main output.
So, we recommend on a Silent Diesel Generator you pick brushless and on a open frame diesel generator you pick one with brushes and AVR.
