Brushless Alternators© 2000 Graig Pearen

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IntroductionSeveral different technologies are used in the generatorportion of wind turbines (wind generators). One of theolder, more reliable technologies is the brushless DCalternator. The operation of them is often poorlyunderstood by the owners and in some cases, poorlydescribed by the turbine manufacturers. I was promptedto write this after reading one particularly bad circuitdescription on the Internet. (Name withheld to protect the

guilty.) I’m not an expert on brushless alternators, butwhen I acquired a damaged Dunlite wind turbine, Ibecame seriously interested in how they work.

Basic TheoryWhen an electric current is passed through a coil of wire,a magnetic field is produced (an electromagnet).Conversely, when a magnetic field is moved through acoil of wire, a voltage is induced in the wire. Theinduced voltage becomes a current when the electronshave some place to go such as into a battery or otherload. [See Word~Power in issue ___ ] Both of theseactions take place in alternators, motors and generatorsor dynamos.

Voltage is generated when a coil of wire is movedthrough a magnetic field. It doesn’t matter whether thecoil is moving or the magnetic field is moving. Eitherconfiguration works equally well and both are usedseparately or in combination depending on mechanical,electrical and other objectives. The old DC generators(dynamos) used a stationary field and rotating armature.Automotive alternators use the opposite configurationwith a rotating field and stationary armature. In abrushless alternator, both configurations are used in onemachine.

TerminologyThe stationary part of a motor or alternator is called thestator and the rotating part is called the rotor. The coilsof wire that are used to produce a magnetic field arecalled the field and the coils that produce the power arecalled the armature.

This can be confusing because most of us equate thearmature with the rotor. Traditionally, the armature wason the rotor but this is not necessarily the case. The twoterms are not synonymous. For example, in the commonautomotive alternator, the field is on the rotor and thearmature is on the stator. The rotor and stator are themechanical configuration. The field and the armature are

the electrical components. Either electrical componentcan be located on either of the two mechanical parts.

The coils of wire that are used to create the field and thearmature are sometimes referred to as the “windings”.

ConstructionA brushless alternator is composed of two alternatorsbuilt end-to-end on one shaft. Smaller brushlessalternators may look like one unit but the two parts arereadily identifiable on the large versions. The larger ofthe two sections is the main alternator and the smallerone is the exciter. The exciter has stationary field coilsand a rotating armature (power coils). The mainalternator uses the opposite configuration with a rotatingfield and stationary armature.

ExciterThe exciter field coils are on the stator and its armature ison the rotor. The AC output from the exciter armature isfed through a set of diodes that are also mounted on therotor to produce a DC voltage. This is fed directly to thefield coils of the main alternator, which are also locatedon the rotor. With this arrangement, brushes and sliprings are not required to feed current to the rotating fieldcoils. This can be contrasted with a simple automotivealternator where brushes and slip rings are used to supplycurrent to the rotating field.

Main AlternatorThe main alternator has a rotating field as describedabove and a stationary armature (power generationwindings). This is the part that can be confusing so takenote that in this case, the armature is the stator, not therotor.

With the armature in the stationary portion of thealternator, the high current output does not have to gothrough brushes and slip rings. Although the electricaldesign is more complex, it results in a very reliablealternator because the only parts subject to wear are thebearings.

EXCITERARMATURE

EXCITER FIELD

EXCITER FIELD

MAIN ARMATURE

MAIN ARMATURE

MAIN FIELD

DIO

DE

MO

UN

TIN

G P

LAT

E

Control SystemVarying the amount of current through the stationaryexciter field coils controls the strength of the magneticfield in the exciter. This in turn controls the output fromthe exciter. The exciter output is fed into the rotatingfield of the main alternator to supply the magnetic fieldfor it. The strength of the magnetic field in the mainalternator then controls its output. The result of all this isthat a small current, in the field of the exciter indirectlycontrols the output of the main alternator and none of ithas to go through brushes and slip-rings.

AVRIn many diagrams and explanations, you will encounterthe term “AVR”, with no explanation of what it is. AVRis an abbreviation for Automatic Voltage Regulator. AnAVR serves the same function as the “voltage regulator”in an automobile or the “regulator” or “controller” in ahome power system.

VariationsAs with any other electronic or electrical device, therecan be numerous variations in the design. The Dunlite2kw wind machines that I have, use an auxiliary windingon the main stator to generate the voltage for the exciterfield. These machines rely on residual magnetism toexcite the auxiliary winding while some machines usepermanent magnets for this purpose. On the Dunlitemachines, both the exciter and main field coils use 8poles. (3-phases multiplied by 8 poles is 24 coils ofwire.) The exciter armature is wound in a 3-phase, 4-wirewye (star) configuration and the main armature is a 3-phase, 3-wire design.

Three-Phase BasicsA three phase alternator has a minimum of 3 sets ofwindings spaced 120° apart around the stationary

armature (stator). As a result, there are 3 outputs fromthe alternator and they are electrically spaced 120° out ofphase with each other. A multi-pole design will havemultiple sets of 3 windings. These sets of windings(poles) are spaced evenly around the circumference ofthe machine. The more poles there are, the slower thealternator turns for a given voltage and frequency. Morepoles increase the complexity of the alternator and that inpart accounts for the higher price of slow speed versions.

Other than in single-phase power plants, mostalternators, including the automotive type, generate 3-phase power. A three-phase AC alternator will not haveany diodes in it. If the output is DC, it will probably have6 diodes to convert the output from the main alternator toDC. This is the configuration used in automotivealternators. A 3-phase brushless alternator may have 4 or6 diodes on the rotor for the exciter output in addition tothe diodes that may be on the stator.

There are two ways that 3-phase machines can be wired.One is the delta (triangle) configuration with one wirecoming off each “point of the triangle”. The other is thewye (Y) or star configuration. They have one wire fromeach branch of the “Y” and in some cases a 4th commonwire is added from the center/centre point of the “Y” (thecommon connection point between the windings).

Multiple voltage machines will have additional wires toallow them to be configured for the desired systemvoltage.

If you ever have to service your wind turbine, this briefdescription may help you to understand how it works orat least let you talk the same language as the repair shopstaff.

Armature

(AVR)

Voltage Regulator

Sliprings

Brushes

Automotive Alternator

Field

Automatic Voltage Regulator

AVR

Exciter Field

Exciter Armature

Main Field

Main Armature

Stator

Basic 3-phase Brushless Alternator

Main Field

Main Armature

Exciter Armature

G+

G-

White

White

White

Brown

Bla

ck

F

Exciter Field Exciter Field Supply

Stator

Red

Blue

Bu

Y

Yellow

White

Slate

Yellow

Yellow

Red

Red

120v= S2BN3032v= S3AN40

32v= S3AR40120v= S2BR30

Motorola MR328Siemens E1140

ITT EM402