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BRUSHLESS ALTERNATOR SUBMITTED BY –

RAJESH MALIK 2110678-EEE

Several different technologies are used in the generator portion of wind turbines (wind generators). One of the older, more reliable technologies is the brushless DC alternator. The operation of them is often poorly understood by the owners and in some cases, poorly described by the turbine manufacturers. I was prompted to write this after reading one particularly bad circuit description on the Internet. (Name withheld to protect the guilty.) I’m not an expert on brushless alternators, but when I acquired a damaged Dunlite wind turbine, I became seriously interested in how they work

Introduction

A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary

BRUSHLESS ALTERNATOR

field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on a plate attached to the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).

Construction of synchronous Alternator

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In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then turned by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.

•Field windings are the windings producing the main magnetic field (rotor windings

Type of Brushless Alternator:-

•armature windings are the windings where the main voltage is induced (stator windings)

Construction of Synchronous Rotor:-

Non-salient-pole rotor: usually two- and four-pole rotors. Salient-pole rotor: four and more poles.

The rotor of a synchronous machine is a large electromagnet. The magnetic poles can be either salient (sticking out of rotor surface) or non-salient construction.

Rotors are made laminated to reduce eddy current losses.

BRUSHLESS EXCITER:-

Synchronous Alternator1.Supply the DC power from an external DC source to the rotor by means of slip rings and brushes;

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2.Supply the DC power from a special DC power source mounted directly on the shaft of the machine. Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. Graphite-like carbon brushes connected to DC terminals ride on each slip ring supplying DC voltage to field windings.

Construction of synchronous machines

•On large generators and motors, brushless exciters are used. •A brushless exciter is a small AC generator whose field circuits are mounted on the stator and armature circuits are mounted on the rotor shaft. •The exciter generator’s 3-phase output is rectified to DC by a 3-phase rectifier (mounted on the shaft) and fed into the main DC field circuit. •It is possible to adjust the field current on the main machine by controlling the small DC field current of the exciter generator (located on the stator).

Brushless excitation synchronous alternator:-

A brushless exciter: a low 3-phase current is rectified and used to supply the field circuit of the exciter (located on the stator). The output of the exciter’s armature circuit (on the rotor) is rectified and used as the field current of the main machine.

Construction of synchronous Alternator

A rotor of large synchronous machine with a brushless exciter mounted on the same shaft. Construction of synchronous machines

1)BrushlessDesign 2) No brushes or slip rings to fail. 3)No rotating windings 4) Non-Electrical unitized rotor shaft. 5)Large Diameter stator wire 6) Lower I²R winding loss. 7)Cooler stator operation 8) High amperage output. 9)High mechanical to electrical efficiency, saves fuel. 10)Replaceable diodes 11)Positive, Negative and Isolated ground options 12)Integral alternator and bearing housing casting providing permanent bearing alignment & oversize grease reservoir.

Advantages of Brushless Alternator are:

Difference between Brushless and Brushed AlternatorBrushed alternator: In brushed alternator DC voltage for field excitation (located on rotar) is provided from DC source through contacts of slip ring and brushes. Brushless alternators: In brushless alternator DC voltage for field excitation (located on rotar) is supply through rotating bridge rectifire (located on rotar) and high also located on rotar. To provide excitation supply for main alternator field winding we are not require to use brush hence it is called brushless. Permanent magnet alternator: This type of alternators are usually small capacity alternators, for this permanent magnet is place on rotar hence we are not require to provide any field magnetisation supply.

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