Alternators

68 home power 97 / october & november 2003

Several different technologies are used inthe generator portion of wind turbines(wind generators). One of the older, morereliable technologies is the brushless DC alternator.Its operation is often poorly understood by theowners, and in some cases, poorly described by themanufacturers. I’m not an expert on brushlessalternators, but when I acquired some damagedDunlite wind turbines, I became seriously inter-ested in how they work.

Graig Pearen©2003 Graig Pearen

AlternatorsBru

shless

The author prepares to repair a Dunlite BL, 2 KW wind generatorthat uses a brushless, 3-phase alternator.

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alternator technologyBasic Theory

When 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 a coil of wire, anelectric current is produced in the wire. Both of these actionstake place in alternators, motors, and generators ordynamos.

The stationary part of a motor or alternator is called thestator and the rotating part is called the rotor. The coils ofwire that are used to produce a magnetic field are called thefield, and the coils that produce electricity are called thearmature. The coils of wire are sometimes referred to as the“windings.”

Electrical energy is generated when a coil of wire ismoved through a magnetic field. It doesn’t matter whetherthe coil 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 configuration—arotating field and stationary armature. In a brushlessalternator, both configurations are used in one machine.

ConstructionA brushless alternator is composed of two alternators

built end-to-end on one shaft. Smaller brushless alternatorsmay look like one unit, but the two parts are readilyidentifiable on the large versions. The larger of the twosections is the main alternator and the smaller one is theexciter.

The exciter has stationary field coils and a rotatingarmature (power coils). The main alternator uses theopposite configuration with a rotating field and stationaryarmature. This configuration is required to implementbrushless technology without using permanent magnets.Eliminating the brushes eliminates brush and slip ring wearand maintenance. The only moving parts subject to wear inthe basic alternator are the bearings.

Although rugged and reliable, brushless alternators areexpensive to manufacture and therefore were only used inpremium quality wind turbines. The high cost of productionputs them solidly in the industrial grade category. With theadvent of powerful, cheap permanent magnets, theresidential-sized wind industry has switched to the cheaperpermanent magnet alternator (PMA) technology. Brushlessalternators are also used extensively in large engine-drivenpower plants and in utility-sized power generation.

ExciterThe exciter field coils are on the stator, and its armature

is on 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 the fieldcoils of the main alternator, which are also located on therotor. With this arrangement, brushes and slip rings are notrequired to feed a current to the rotating field coils. This canbe contrasted with a simple automotive alternator, wherebrushes and slip rings are used to supply a current to therotating field.

Main FieldExciter Armature

Exciter Field

Main Armature

Main Armature

Exciter Field

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Mo

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late

ShaftRotor Stator

Brushless AlternatorConfiguration

AlternatorTerminology

The parts of an alternator or related equipmentcan be expressed in either mechanical terms orelectrical terms. These two sets of terminology arefrequently used interchangeably or in combi-nations that include one mechanical term and oneelectrical term. This causes great confusion whenworking with compound machines such as abrushless alternator, or when conversing withpeople who are used to working on a machine thatis configured differently than the machines thatthe speaker is used to.

MechanicalRotor:The rotating part of an alternator, generator,dynamo, or motor.

Stator: The stationary part of an alternator,generator, dynamo, or motor.

ElectricalArmature: The power-producing component of analternator, generator, dynamo, or motor. Thearmature can be on either the rotor or the stator.

Field: The magnetic field component of analternator, generator, dynamo, or motor. The fieldcan be on either the rotor or the stator, and can beeither an electromagnet or a permanent magnet.

70 home power 97 / october & november 2003

alternator technology

Main AlternatorThe main alternator has a rotating field as described

above and a stationary armature (power generationwindings). With the armature in the stationary portion ofthe alternator, 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.

Control SystemVarying the current through the stationary exciter field

coils controls the strength of the magnetic field in the exciter.This in turn controls the output from the exciter. The exciteroutput is fed into the rotating field of the main alternator tosupply the magnetic field for it. The strength of the magneticfield in the main alternator then controls its output.

The result of all this is that a small current in the field ofthe exciter indirectly controls the output of the mainalternator, and none of it has to go through brushes and sliprings.

In a wind generator, this means that the loading can bematched to the speed, unlike in a permanent magnetalternator. Although some of the wind generator’s output isused to generate the magnetic field, there is no coggingeffect, so low wind start-up characteristics are improved.

Automatic Voltage RegulatorAn automatic voltage regulator (AVR) serves the same

function as the voltage regulator in an automobile or thecharge controller in a solar-electric system. It measures thebattery voltage and uses that information to adjust the levelof current fed into the field coil. As the battery voltage goesdown, the AVR increases the current to the field coils toproduce a stronger magnetic field.

The stronger magnetic field causes a higher output fromthe alternator to bring the battery voltage back up to the setpoint. Conversely, when the voltage rises too high, the fieldcurrent is decreased, lowering the output from thealternator.

VariationsAs with any other electronic or electrical device, there

can be numerous variations in the design. The Dunlite 2 KWwind machines use an auxiliary winding on the main statorto generate the voltage for the exciter field. These machinesrely on residual magnetism to excite the auxiliary winding,while some machines use permanent magnets for thispurpose.

On the Dunlite machines, both the exciter and main fieldcoils use eight poles. (Three phases, multiplied by eightpoles, is twenty-four coils of wire.) The exciter armature iswound in a three-phase, four-wire wye (star) configuration,and the main armature is a three-phase, three-wire deltadesign.

Three-Phase BasicsA three-phase alternator has a minimum of three sets of

windings, spaced 120 degrees apart around the stationaryarmature (stator). As a result, there are three outputs from

Stator:Armature

Rotor:Field

Slip Rings& Brushes

VoltageRegulator:Automatic

BridgeRectifier:

Six Diodes

Positive

Negative

The rotor of a brushless alternator: The smaller section at left is the exciter armature; the larger section at right is the main alternatorfield.The diodes are mounted on the aluminum disk at the left end.

Automotive Alternator

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alternator technology

the alternator, and they are electrically spaced 120 degreesout of phase with each other. A multi-pole design will havemultiple sets of three windings. The more poles there are,the slower the alternator turns for a given voltage andfrequency.

More poles increase the complexity of the alternator, andthat in part accounts for the higher price of slow speedversions. Other than in single-phase power plants, mostalternators, including the automotive type, generate three-phase power. A three-phase AC alternator will not have anydiodes in it.

If the output is DC, it will probably have six diodes toconvert the output from the main alternator to DC. This isthe configuration used in automotive alternators. A three-phase brushless alternator may have four or six diodes onthe rotor for the exciter output in addition to the diodes thatmay be on the stator.

There are two ways that three-phase machines can bewired. One is the delta (triangle) configuration, with one

wire coming off each “point of the triangle.” The other is thewye (Y) or star configuration. They have one wire from eachbranch of the Y and a fourth common wire is added from thecenter point of the Y (the common connection point betweenthe windings). Multiple voltage machines will haveadditional wires to allow them to be configured for thedesired system voltage.

If you ever have to service an old wind turbine with abrushless alternator, this brief description may help you tounderstand how it works, or at least let you talk the samelanguage as the repair shop.

AccessGraig Pearen • gpearen@telus.net •www3.telus.net/pearen

See “Yer Basic Alternator,” Bob-O Schultze, HP20 , available in print or on the Solar2 CD.

Rotor:Exciter Armature

Stator:Exciter Field

VoltageRegulator:Automatic

BridgeRectifier:

Six Diodes

Stator:Main Armature

BridgeRectifier:

Six Diodes

Positive

Negative

Rotor:Main Field

Brushless, 3-Phase Alternator