|>> Aren't alternators AC and Generators DC? Wouldn't it take more circuitry
|>> use an alternator as opposed to a generator to charge batteries?
|>> I have 2 12volt DC generators I bought from Thermax over 20 years ago
|>> how time flies) and I've never used them.
|>> Do people use alternaters because they are easier and cheaper to find, or
|>> because they are more efficient?
|> Another advantage to using an alternator is that you can use smaller,
|> wires to run "wild" AC to your "power station" and rectify to DC just
|> it gets to your batteries. Trying to run DC any distance would require
|> fat, expensive cables and you would probably still have some voltage drop.
| Might want to rethink this paragraph. Wire size would be specified by
| current, not
| whether it ac or dc. Voltage drop would be simular whether ac or dc.
AC allows the use of a smaller wire size ... but requires a transformer to
step the voltage way up as part of doing that, and possibly back down again
at the destination.
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| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |
I think you lost me. If you have regulated DC coming out of an alternator
wouldn't that require larger cables than if you ran wire the same distance
with unregulated AC from the same source, at the same RPM? The AC voltage
would be higher because it is unregulated. That is what I meant by "wild."
Sure, you would still have some voltage drop with the AC but it would be
less significant percentagewise because the voltage would likely be much
higher if everything else is the same. The apparent current would also be
lower while still in the AC form so smaller wires could be used.
from firstname.lastname@example.org contains these words:
The benefit of this arrangement ("wild" AC feed from the generator) is
that it gives a stabilised voltage at the point of consumption. For a
wind turbine source, this is usually at the lead acid _accumulator_
terminals where an accurately set and maintained voltage is desirable
for maximising the service life of the battery.
Any variations in volt drop in the supply cable due to variations in
demand current are automatically accounted for (within reason) by
regulating the voltage seen at the battery terminals rather than the
output voltage seen at the generator terminals.
This same technique is used in vehicles where the alternator output
wire used to connect to the battery is considered too excessive for
satisfactory voltage regulation. In this case, an alternator with a
remote sense input to the VR circuit is fitted and a light gauge sense
wire is run back from the battery terminal. This compensates for,
typically, a one (maybe two) volt drop in the generator feed line to the
However, if the line running from the generator to the point of
consumption is accounting for a volt drop of more than 20% of the
designed delivery voltage, it's time to consider either a heavier gauge
feed line or the use of a pair of transformers (or sets of, if using 3
phase transmission) in order to transmit the power at higher voltage and
If the generator is located a hundred yards or more from the house,
stepping the generator voltage up to a maximum of 2 or 3 hundred volts
would probably be the optimum for generator ouputs in the 0.1KW to 3 KW
range. At this sort of voltage, you could use standard 20 amp mains
voltage cable for the feed.
For very long runs and even higher powers you might consider an open
wire feeder mounted on pylons with ceramic insulator supports and a 500
to 1KV maximum transmission line voltage. The step down transformer need
not necessarily drop the voltage back down to generator voltage if you
want to be able to trickle charge the battery in light winds.
In fact you could use the transformers as a means of raising the
voltage from a PM generator that would otherwise need to be rewound with
more turns of lighter gauge wire to get the desired minimum speed
Insufficient voltage from a wind turbine mounted PM generator is a
common problem met by many DIYers. If transmission losses justify the
use of transformers, they can also, conveniently, provide the means to
overcome any such generator voltage mismatches.
When selecting transformers for wind driven PM alternators, you need to
know the frequency and no-load voltage range. Since open circuit output
very closely approximates directly to the speed (and hence the
frequency), you only need to get a single frequency and corresponding
voltage figure for a given wind speed. Once you have these three figures
(wind speed, frequency and voltage) you can extrapolate the maximum and
minimum values and also the voltage corresponding to 50 or 60Hz.
Once you know the 50 or 60Hz output no-load voltage, you can choose a
suitable transformer. If the 50 or 60 Hz corresponds to half the maximum
speed and the generator voltage is 30 volts, you could use a 240 volt
line transformer with a 60 volt secondary. The generator would be
connected to the 60 volt "secondary" and the "primary" would be
connected to your 300v max transmission line.
If the insulation on a 120 volt mains transformer could be trusted to
withstand 300 volts, you could use a suitably VA rated one with a 30v
"secondary" to connect to the PM alternator output. As long as you pick
a secondary voltage equal to or greater than the open circuit output
voltage of the generator at the transformer's design frequency, you will
automatically avoid the risk of excessive transformer magnetising
 I don't have any actual figures but I'm guessing a frequency range
for useful power output of a simple wind turbine driven PM alternator to
be from 20 to 100Hz (give or take).
 The VA rating of a transformer for a given amount of iron increases
in proportion to the frequency provided it is either rewound with less
turns of thicker wire for a given fixed voltage or else the voltage
increases in direct proportion to the supply frequency (which is true in
this case). The losses reduce with increased VA ratings so, apart from
financial considerations, it won't do any harm to use a larger than
minimum VA rating. In fact, even if the PM alternator output is less
than 100 watts at the 60 Hz speed, I'd choose a 500VA transformer simply
to keep the losses down to the 2% or less mark (per transformer).
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