Brushless Alternators

Brushless Alternators


Introduction

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.

Basic 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, a voltage is induced in the wire. The

induced voltage becomes a current when the electrons

have some place to go such as into a battery or other

load. [See Word~Power in issue ___ ] Both of these

actions take place in alternators, motors and generators

or dynamos.

Voltage is generated when a coil of wire is moved

through a magnetic field. It doesn’t matter whether the

coil is moving or the magnetic field is moving. Either

configuration works equally well and both are used

separately 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

with a rotating field and stationary armature. In a

brushless alternator, both configurations are used in one

machine.

Terminology

The stationary part of a motor or alternator is called the

stator and the rotating part is called the rotor. The coils

of wire that are used to produce a magnetic field are

called the field and the coils that produce the power are

called the armature.

This can be confusing because most of us equate the

armature with the rotor. Traditionally, the armature was

on the rotor but this is not necessarily the case. The two

terms are not synonymous. For example, in the common

automotive alternator, the field is on the rotor and the

armature is on the stator. The rotor and stator are the

mechanical configuration. The field and the armature are

the electrical components. Either electrical component

can be located on either of the two mechanical parts.

The coils of wire that are used to create the field and the

armature are sometimes referred to as the “windings”.

Construction

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 field coils

and a rotating armature (power coils). The main

alternator uses the opposite configuration with a rotating

field and stationary armature.

Exciter

The exciter field coils are on the stator and its armature is

on the rotor. The AC output from the exciter armature is

fed through a set of diodes that are also mounted on the

rotor to produce a DC voltage. This is fed directly to the

field coils of the main alternator, which are also located

on the rotor. With this arrangement, brushes and slip

rings are not required to feed current to the rotating field

coils. This can be contrasted with a simple automotive

alternator where brushes and slip rings are used to supply

current to the rotating field.

Main Alternator

The main alternator has a rotating field as described

above and a stationary armature (power generation

windings). This is the part that can be confusing so take

note that in this case, the armature is the stator, not the

rotor.

With the armature in the stationary portion of the

alternator, the high current output does not have to go

through brushes and slip rings. Although the electrical

design is more complex, it results in a very reliable

alternator because the only parts subject to wear are the

bearings.

Control System

Varying the amount of 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 exciter output is fed into the rotating

field of the main alternator to supply the magnetic field

for it. The strength of the magnetic field in the main

alternator then controls its output. The result of all this is

that a small current, in the field of the exciter indirectly

controls the output of the main alternator and none of it

has to go through brushes and slip-rings.

AVR

In many diagrams and explanations, you will encounter

the term “AVR”, with no explanation of what it is. AVR

is an abbreviation for Automatic Voltage Regulator. An

AVR serves the same function as the “voltage regulator”

in an automobile or the “regulator” or “controller” in a

home power system.

Variations

As with any other electronic or electrical device, there

can be numerous variations in the design. The Dunlite

2kw wind machines that I have, use an auxiliary winding

on the main stator to generate the voltage for the exciter

field. These machines rely on residual magnetism to

excite the auxiliary winding while some machines use

permanent magnets for this purpose. On the Dunlite

machines, both the exciter and main field coils use 8

poles. (3-phases multiplied by 8 poles is 24 coils of

wire.) The exciter armature is wound in a 3-phase, 4-wire

wye (star) configuration and the main armature is a 3-

phase, 3-wire design.

Three-Phase Basics

A three phase alternator has a minimum of 3 sets of

windings spaced 120° apart around the stationary

armature (stator). As a result, there are 3 outputs from

the alternator and they are electrically spaced 120° out of

phase with each other. A multi-pole design will have

multiple sets of 3 windings. These sets of windings

(poles) are spaced evenly around the circumference of

the machine. The more poles there are, the slower the

alternator turns for a given voltage and frequency. More

poles increase the complexity of the alternator and that in

part accounts for the higher price of slow speed versions.

Other than in single-phase power plants, most

alternators, including the automotive type, generate 3-

phase power. A three-phase AC alternator will not have

any diodes in it. If the output is DC, it will probably have

6 diodes to convert the output from the main alternator to

DC. This is the configuration used in automotive

alternators. A 3-phase brushless alternator may have 4 or

6 diodes on the rotor for the exciter output in addition to

the 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 wire

coming off each “point of the triangle”. The other is the

wye (Y) or star configuration. They have one wire from

each branch of the “Y” and in some cases a 4th common

wire is added from the center/centre point of the “Y” (the

common connection point between the windings).

Multiple voltage machines will have additional wires to

allow them to be configured for the desired system

voltage.

If you ever have to service your wind turbine, this brief

description may help you to understand how it works or

at least let you talk the same language as the repair shop

staff.