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[#3103 - March 2010]
I have been experimenting with a
10,000 watt AC alternator that outputs
110/220 VAC for use as a backup power
I am using the variable speed from
the gas engine that turns the alternator to
create a variable AC voltage output. An
alternator of this type has no permanent
magnet so it needs to get up to some
RPM before it generates ANY voltage at
all. On mine, this kicks in somewhere
between 35 and 50 VAC.
How does this type of alternator
actually work? Is it possible to excite the
fields externally so the output voltage
kicks in consistently at low RPM?
The alternating current synchronous generator is a fairly simple machine;
as you know, some laws apply. The theory and math are readily found online.
My assumption is that your generator is
a two pole revolving-field direct drive
unit turning at 3,600 RPM originally. The
current for the field may be supplied
through slip-rings and brushes, or the
generator may be brushless with the field
current supplied by secondary windings
or other means. This is typical. The term
synchronous describes the fact that
3,600 RPM at the input (gas engine,
turbine, etc.) of your generator will give
the rated output voltage/current at 60
Hz. This again is determined by the
number of poles on the field and on the
stator. The mechanism that makes the 60
Hz stay at 60 Hz is the governor on your
engine; it makes sure the engine runs at
a fixed 3,600 RPM regardless of load.
The voltage regulator on the generator is
designed to keep the output voltage
stable at this rated input RPM, regardless
of changes in load current.
Now to get it all started, how is this
done with no external source of current
or permanent magnets? Some residual
magnetism is in the iron and steel that
makes up the core of the field and the
lamination where the stator is wound.
This is where the exciter does its job. As
the generator starts to reach its rated
speed, some current will be produced in
either the field or seperate exciter
windings. This current will build up and
be fed back to the field winding either by
rectification or an autotransformer.
Magnetism in the field will rapidly build
up — as will the generator output voltage
— until the voltage regulator limits the
field current to keep the output at the
rated voltage. All is accomplished without the need for an external field current
supply, but when you slow the generator
below the designed speed the
abovementioned system will reach a
point where it can no longer self-sustain
field current and drop out.
[#3107 - March 2010]
I'm looking for an anemometer
circuit using the cooling effect on transistors. A circuit with an analog output I can
interface to a microcontroller would be
great as I plan to add other sensors.
Mechanical anemometers need to be
calibrated which I'd like to avoid (and I’d
rather build a circuit anyway).
It should measure "normal" wind
speeds (up to maybe 50 mph).
There is a circuit in the Linear
Technology Corp LT1013/LT1014
op-amp datasheet for a hot wire
anemometer. It uses a small incandescent lamp with the glass envelope
removed as the hot wire. The link
10134fc.pdf goes to the LT1013/LT1014
datasheet. The anemometer circuit is on
page 11 under "Typical Applications."
Searching for or going to the
datasheet for "LT1013" produces a
different datasheet for LT1013 alone and
it does not have the anemometer
circuit on it. A search for "hot wire
anemometers" also yields a lot of
information and some circuits.
[#4101 - April 2010]
Using PIC Processor to
I am looking for info to use PICs to
generate DTMF. I am using a PIC
18F2431. Any assembly code would be
of great help.
#1 The Scenix SX28 (now from Ubicom)
is quite similar to many of the PIC family
processors. There are some excellent
ap-notes for a DTMF tone generator —
including theory of operation and fully
commented source code — on their site.
This requires minimal hardware and
you should be able to adapt the source
code. Here are some current links:
perf.htm = Virtualperipherals
DTMF_gen.PDF = Theory of Operation
gen_1_3_4.SRC = Source code
If your PIC doesn't support a high
clock rate, you should still be able to use
this by adjusting the loop counts to scale
to your clock rate.
August 2010 79