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Many folks discourage using alternators for wind power due to
the requirement of the high RPM (common automotive alternator) and
loss of power through field excitation (most alternators don't use magnets to
generate electricity) and loss of power through belt-drive pulleys required
to get that high RPM.
You can either accept the performance characteristics of a common alternator which when considered against their cheap availability, can still
work out quite well, or modify a common alternator (our classes and many others out there
show you how,) or pay a premium price for a PM alternator or even more for a brushless
design. These expensive alternator solutions are just about as good as any PM DC motor, but usually more expensive for the
cost per amp or kwh and many are 'garage built'; some are tougher than factory made and some are not.
For the record, NASA Jerry hates alternators as a solution, has tested them in a variety
of conditions and never liked the power losses. Hugh Piggott isn't too fond of these solutions
either. Most wind gen pros will tell you that automotive alternators are not suitable for wind power.
A Kiwi Sailor we know, a chemist by trade, loves his cheap belt-drive car alternator wind gen.
You decide.
We think with the right mount running several cheap scrap yard alternators, you can build a very powerful rig for a fraction of
the price of other systems, as long as you can accept belt and field current losses.
ADVANTAGES:
- A cheap, common automotive alternator has no magenets to come unglued during high speed operation. Designed for 8000-12000 RPM, they can take abuse that can destroy a cheap PM DC motor such as an ametek.
- #1 Advantage: PRICE PER AMP! Common alternators are inexpensive ($5.00 at a scrap yard for 60 amps) - easy to find and replace when worn out. When done this way, a common automotive alternator (or series of them on one blade) is the cheapest way to make power from the wind, hands down. You pay $5.00-$10.00 (scrap yards) for each HP of power you want to bring online.
At $10.00 each (scrap) it works out to a cost of about $12-$20/kilowatt plus tower (scrap) and batteries (scrap). Use our
Battery class to save $1000's on your battery bank.
A mega-watt tower, with all that 'big wind' efficiency costs $1000.00 or more per kilowatt hour.
Most home-sized wind gen systems cost $1500-$3000 per kilowatt hour.
Buying your blades from us and putting the rest of your rig together
with scrap pipe, scrap batteries, scrap welding cable and a nice 220VAC (or 110VAC)
3000-5000 watt inverter (less than $1000.00) means your cost per
kilowatt hour is less than $500.00 and can be less than $50.00 if you make your own props, use scrap parts for
everything else.
That's what
we teach here at WindGenZen
Common car alternators with a 3 to 1 or 4 or 5 to 1 pulley/fanbelt drive, are hands down, the cheapest way to make a lot of power, despite all the talk about their inefficient designs and they have some advantages over PM DC motors (below).
- Nonlinear rapid rise in output (powerband) which helps limit blade rpm in gusty and stormy conditions.
- Most are designed for 12 volt charging, making a battery based system a no-brainer.
- Many are self-regulating so when batteries are charged, they stop producing power. A problem with that is that your blade has no load and will run-away at high speed. It would be wise to incorporate a physical brake (driven by that excess power) to slow the gen down when batteries are charged.
- Case has holes for bolts - fabricating a bracket is straightforward.
- At about 2000-2500 RPM, alternators are putting out about 60-70% of their capacity. With a 4 to 1 pulley ratio (below) this means the blade is only spinning 500-600 RPM - a good speed for wind gen blades in the 4 to 10 foot range.
- Alternators have a wide RPM range and can take very high RPMs well. The number one reasons PM DC motors fail is because they were too small for a big wind blade and get hot, magnets come unglued and are destroyed inside a whirling case, or the varnish on the windings cooks after 4-12 hours of storm conditions. This 'heat' problem can happen to either alternators or PM DC motors, but fortunately common car alternators have no magnets to come unglued. Capt. Steve always takes his ameteks (and other PM DC motors) apart and rebeds the magnets with high-temp epoxy to insure they stay put.
- Furling, RPM protection circuits or manual shutoff/brake are required no matter what type of generating system you use to be safe.
- Ten 60-80 amp automotive alternators, running off a single large prop, making 40-50 amps each or 400-500 amps at 12 volts is 5kw-6kw of power and the cost for these 10 alternators could be as low as $100.00 total from a junk yard. Not a bad price per kwh ratio, in fact it's unbeatable compared to nearly any other method.
Disadvantages:
- To stop/brake PM DC motors, simply lift the positive gen wire off the battery and clamp it to ground - a simple switch does the job. Alternators require a physical brake assembly or controlled furling (rotation) to brake/stop.
- While the graph to the right shows a higher output at the low end of operation, when you deduct the field current and pulley inefficiencies, the PM DC motors actually make more NET power at the low end than common alternators, due to power for the field and pulley/belt losses. If your goal is ultimate efficiency in light winds, a PM DC motor is a good choice. If your goal is tough, cheap and plenty of power once winds pick up, common alternators and larger ones may be a good choice.
The graph shows that a larger gen is also a key ingredident. While the cheap 60 amp alternator levels off early, the 150 amp PM DC gen can keep going up. The way to get around that problem is to put mulitple alternators on a single mount and perhaps 'engage' them one by one as wind speeds and power output rise.
The obvious choice is to get a 150-200 amp alternator.
We have them for sale, but they aren't cheap.
Barry Breeden, Vice Chairman of the American Welding Society and a heck of a welder, can weld you up a single or multiple alternator mount for any mast/tower you may have for $300.00-$500.00. Barry only does stainless (sometimes aluminum, but he prefers the strength of steel) and his work is some of the best on Earth. He wrote most of the technical books and standards for the welding industry and knows his stuff.
- Alternators have diodes and circuits inside that can be fragile (fry easily) and consume some power (less than 5%) under load. PM DC motors are more efficient - what you see is what you get.
- Most alternators have a coil that needs to be excited to produce the magnetism for charging. This requires some current - an amp or more - and that means in light winds, low end performance suffers. If you have light winds, you'll need a PM alternator (below) or a PM DC motor (much more efficient).
- Most alternators require 1000 RPM before they put out even 1 amp. Blades usually kick-in/start-up at 200-300 RPM and only in 40-50 knots will they get to 1000 RPM.
- This means you will need a pulley and fanbelt or gearbox or friction drive (rubber rollers) to gain a 3 to 1 or 4 or 5 to 1 ratio to use a common alternator. The power loss is 5-15% and you have another component that wears out - a belt or friction drive wheel. A Kiwi (New Zealander) on a large sailboat said both the belt loss and added maintenance were acceptable when weighed against the ease of finding a replacement and the power he gets from a 60 amp (1 HP) alternator and his 5 foot prop.
- Permanent Magnet Brushless Alternators (no coil to excite and so they are more efficient) are much more expensive (cost per amp) compared to a PM DC motor/generator. Getting 200 amps from a PM DC motor for less than $500.00 is easy. Getting that much power from a Permanent Magnet brushless Alternator is a pricey equation.
- Most alternators have brushes that wear out just like a PM DC motor and need to be changed. Changing them in a PM DC motor is a bit easier to do.
- Like PM DC motors, alternators aren't designed to take the thrust of the wind pushing back on a blade. A special face plate and thrust bearing is advised or you'll shave the life of your gear by 20-50%. A large PM DC gen can take this thrust a bit better. Most alternators don't and the pulley arrangment above solves this problem. A shaft and bearing set leading to a pulley that drives one or more alternators is the common way of doing this and it works.
- Many 'high power' alternators are producing wild-cat AC that varies with RPM. To insure an even charge, many wind generators using these 'cut out' the charge when the RPM isn't just right and you lose power that way-it spins, but no charge is in the line. Others have to either rectify it for 12 volt charging (power lost) or use transformers to boost it up or down for AC useage (power lost). For a battery based charging system, alternators are not as efficient as PM DC motors if you are trying to squeeze every amp from your rig.
The big advantage to alternators, besides availabilty, is the nonlinear rapid rise in output (powerband) which helps limit blade rpm in gusty and stormy conditions and quickly provides
high output and increased load on the shaft (slowing down a blade) with just a slight rise in blade RPM (gusts). This helps dampen/even out performance in windy conditions. However, the disadvantages of power losses
through exciting the field (common alternators), the high prices of PM alternators and the pulleys and belts make some folks shy away from this solution. At WindGenZen, we suggest you try different solutions and see what works for you. If a Kiwi sailor can rig up a 4 to 1 pulley and be happy with the output you might consider a similar arrangement for a large 7 to 12 foot blade and
in fact, mega-watt wind gens are usually alternator based with gear boxes. Note: those 'transmissions' are usually the weak points and often fail first.
ALTERNATOR WIRING - THE BASIC WARNING LIGHT
"What does that little red light that says ALT mean when it comes
on?" Very basically, it means that either the alternator output voltage is
lower than the battery voltage, or the battery voltage is lower than the
alternator output voltage. If the light gets dimmer as you rev up the engine,
then you most likely have a problem with the alternator. If it gets brighter,
then the battery is most likely bad.
That's all well and good, but just exactly what does all that mean? To get a
good idea, it is first necessary to understand how an alternator works. You
don't need an engineering degree, just a basic understanding of the general
principles. Figure 1, below, is a block diagram, or a "functional"
diagram, of an alternator, and its connections to the remainder of the
automobile electrical system. Following the figure is a description of the
various components that make up an alternator, and a description of how each
operates to keep the battery charged in your car.
ALTERNATOR ROTOR
We'll start our tour of the alternator where it all starts in the alternator
itself - at the alternator rotor. The rotor consists of a coil of wire wrapped
around an iron core. Current through the wire coil - called "field"
current - produces a magnetic field around the core. The strength of the field
current determines the strength of the magnetic field. The field current is D/C,
or direct current. In other words, the current flows in one direction only, and
is supplied to the wire coil by a set of brushes and slip rings. The magnetic
field produced has, as any magnet, a north and a south pole. The rotor is driven
by the alternator pulley, rotating as the engine runs, hence the name
"rotor."
STATOR
Surrounding the rotor is another set of coils, three in number, called the
stator. The stator is fixed to the shell of the alternator, and does not turn.
As the rotor turns within the stator windings, the magnetic field of the rotor
sweeps through the stator windings, producing an electrical current in the
windings. Because of the rotation of the rotor, an alternating current is
produced. As, for example, the north pole of the magnetic field approaches one
of the stator windings, there is little coupling taking place, and a weak
current is produced, As the rotation continues, the magnetic field moves to the
center of the winding, where maximum coupling takes place, and the induced
current is at its peak. As the rotation continues to the point that the magnetic
field is leaving the stator winding, the induced current is small. By this time,
the south pole is approaching the winding, producing a weak current in the
opposite direction. As this continues, the current produced in each winding
plotted against the angle of rotation of the rotor has the form shown in figure
2. The three stator windings are spaced inside the alternator 120 degrees apart,
producing three separate sets, or "phases," of output voltages, spaced
120 degrees apart, as shown in figure 3.
OUTPUT DIODES
A/C voltage is of little use in a D/C system, such as used in an automobile,
so it has to be converted to D/C before it can be used. This conversion to D/C
takes place in the "output diodes" and in the "diode trio."
Diodes have the property of allowing current to flow in only one direction,
while blocking current flow in the other direction. The output diodes consist of
six diodes, one pair for each winding. One of the pair is for the negative half
cycle, and the other for the positive half cycle. As a result of this diode
rectification, the output of the alternator looks as shown in figure 4.
Surprisingly enough, the output of the alternator is not a pure D/C as one
might expect, but a pulsating D/C. Because there are three windings, each with a
positive and a negative half, by the time the voltage is passed through the
diodes, there are six pulsations for each rotation of the rotor. This is close
enough to D/C for most automotive components. Critical components, such as
radios, have their own internal filtering circuits to further smooth out the
waveform to a purer D/C.
DIODE TRIO
The diode trio consists, as the name suggests, of three diodes, one per
phase, which provides field current to the alternator regulator. This output
will be discussed in more detail later in the "field current supply"
section.
REGULATOR
The regulator has two inputs and one output. The inputs are the field current
supply and the control voltage input, and the output is the field current to the
rotor. The regulator uses the control voltage input to control the amount of
field current input that is allow to pass through to the rotor winding. If the
battery voltage drops, the regulator senses this, by means of the connection to
the battery, and allows more of the field current input to reach the rotor,
which increases the magnetic field strength, which ultimately increases the
voltage output of the alternator. Conversely, if the battery voltage goes up,
less field current goes through the rotor windings, and the output voltage is
reduced.
FIELD CURRENT SUPPLY
Field current supply is provided from two different sources - from the
alternator itself, via the diode trio, and from the battery, via the alternator
warning lamp. When you first get in the car and turn the key on, the engine is
not running and the alternator is not spinning. At this time, the
voltage/current source for the field current is from the battery, through the
ignition switch, and through the warning lamp. After the engine is started, and
the alternator is up to speed, the output of the diode trio is fed back to the
regulator, and serves as a source of current for the field current. At this
time, the alternator is self sustaining, and the battery is no longer needed to
power the automobiles electrical system WARNING!!! This is theoretical only - in
actual practice, the voltage surges resulting from disconnecting the battery can
seriously damage the regulator circuitry. All alternator manufacturers strongly
advise NOT doing this! This test will not prove the functionality of the
alternator anyway, as the engine may still run with a weak alternator output.
WARNING LAMP
This brings us back full circle to the starting point - the alternator
warning lamp. As can be seen from figure 5, a schematic for an actual
alternator, there is a path to ground from the field current supply input [1] to
the regulator. As a result, when the key is turned on, current flows through the
warning lamp, through the resisters, transistors, and field coil, and then to
ground, causing the lamp to illuminate. Once the alternator is at full output,
voltage from the diode trio, also applied to [1], equals the battery voltage. At
this time, with 12 volts on both sides, the lamp is out.
If the alternator should fail, voltage from the diode trio would drop, and
once again the lamp would light from the battery voltage. If the alternator
output is only a little low, the lamp will be dimly lit. If the alternator fails
completely, and the output voltage goes to zero, the lamp will be lit at full
brilliance. Conversely, if the battery should fail, and the battery voltage
drops, with the output voltage of the alternator on one side and the low battery
voltage on the other, the lamp will also light.
As stated earlier, if the light grows dimmer as the engine is revved up, it
is because the alternator voltage is rising with the RPM, producing more voltage
on the alternator side of the lamp. The closer the output voltage gets to the
battery voltage, the dimmer the bulb becomes. By the same way, if the light gets
brighter with increasing RPM, it is because as the alternator voltage increases,
it is getting higher than the battery voltage. The higher the voltage with
respect to the battery voltage, the greater the voltage difference across the
lamp, and the brighter it gets.
SUMMATION
In summary, then, we can say that field current through the rotor coils
produces a magnetic field, which is coupled over to the stator coils, producing
an AC voltage. This AC voltage is converted by the output diodes into pulsating
DC voltage, which charges the battery.
The field current is supplied from either the battery, via the warning lamp,
or from the diode trio. The amount of field current allowed to pass through the
regulator to the rotor, or field coil, is controlled by the voltage feedback
from the battery.
And there you have it - the complete operation of an alternator in a
nutshell. The next time you see the little red light, you will know exactly what
it is trying to tell you.
The above article has been provided courtesy of
Dan Masters
danmas@aol.com
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