|> | Note that a typical light dimmer will use triacs which are essentially
|> | to back SCR's. You control the turn on time but the turn off time is at
|> | current zero. These are so cheap that there would appear to be little
|> | incentive to go to pulse width modulation.
|> Based on some waveforms I've seen, some turn on shortly after the zero
|> and turn off somewhere mid way through the half cycle. The zero cross
|> to be uninvolved. Not all are like that, but many are.
| If that is the case- I don't know what you are looking at. Which zero
| crossing - voltage or current ?
| SCR's or Triacs that I have known don't do that. Are you sure something is
| not reversed or out of whack in the scope?\
For resistive lights, the zero-crossing is pretty much the same. But as
I am referring to waveforms that rise up from the zero-crossing and are
then just suddenly turned off, there is no ambiguity unless this involves
radically low power factors.
These are waveforms I have seen published. I don't know what is being
tested. But the point is that _something_
shut off at
least a few amps of current. Whether it is a Triac, SCR, Thyristor, or
what, I either didn't see, didn't care, or didn't remember at the time.
I was only concerned with the harmonics of the waveform, not in how to
achieve the waveform, back then.
My concern today is finding devices to turn voltage and current on and
off at points specifically controlled. I'm not dealing with the power
levels of transmission grid AC/DC or 50/60 converstion. What I do want
to look at is DC regulators and DC to AC inverters in power levels up
to 10kw or 20kw. But that's NOT per individual component. A design
could parallel many components if need be. Ideally, such a device would
have fast switching that would allow using pulse density modulation and
be easier to filter. But pulse width modulation is certainly usable.
|> And optical fiber to control?
| Yes that is used.
I don't know if I will need an optical control for turn on/off operations.
The DC voltage levels could be from 48 to 600 volts open-circuit.
|> I'm interested in the 600 volt and below range. Can something switch a
|> amps on and off at times the current (and voltage) is not at zero
|> The idea is to build an inverter that uses high frequency PWM or PDM to
|> chop up the voltage from DC into an LC circuit to filter the high
|> out and produce AC at 50 or 60 Hz (or whatever frequency in that range is
|> programmed in the pulse pattern synthesizer). I also want to do similar
|> as a DC to DC regulator for controlling charge from solar arrays and AC to
|> DC to control and diverter power from wind generators.
| OK- this is low power/voltage stuff and is doable. It appears from the
| reference that I gave that there have been advances in such switching. It
| very much depends on the circuit inductance. Chopping involves high Ldi/dt
| voltages which can cause problems. Obviously inverters do exist at this
| level and are available on the market.
I don't know what is obvious. A number of inverters do exist on the
market but so far not a single one of them reveals what technological
method they use to convert DC to AC. Given the efficiency figures of
of them, I'll confidently speculate they are not using class A
analog amplifiers. For a few, I could not rule out a class B or maybe
even a class C. I don't know if they are doing digital or analog.
One idea I want to explore in an inverter design based on pulse density
modulation is to use pulse scattering among the individual components
doing the pulse gating. Instead of having every component all gate on
or off at exactly the same time, it would be rotated. So if the density
level is at say 12.5% and there are 8 components in parallel, each one
could be on for 12.5% of the time and all at different times. When the
density is at 75% (voltage peaks under load), there would be overlap of
on time, but the off times would now be rotated with some overlap.
12.5% on 25% pdm 25% pwm 50% pdm 50% pwm 50% hybrid
gate0 10000000 10001000 11000000 10101010 11110000 11001100
gate1 01000000 01000100 01100000 01010101 01111000 01100110
gate2 00100000 00100010 00110000 10101010 00111100 00110011
gate3 00010000 00010001 00011000 01010101 00011110 10011001
gate4 00001000 10001000 00001100 10101010 00001111 11001100
gate5 00000100 01000100 00000110 01010101 10000111 01100110
gate6 00000010 00100010 00000011 10101010 11000011 00110011
gate7 00000001 00010001 10000001 01010101 11100001 10011001
This is just multiple gates for an individual phase. Multiple phases
would each be under a different density percentage according to that
phase's current point in the cycle, and any load imbalance.
Most other percentages would be more complicated. There would be more
pulse intervals per cycle, variable cycle times, and a lot more gates.
Control circuitry would be designed to ensure a balance of on-times among
all the gates, possibly influenced by sensed temperature. The input LC
circuit would smooth out the incoming DC current, and the output LC
circuit would be there to smooth the pulses to a relatively clean sine
wave and supply at least some reactive current. The control circuit
would adjust the desnity/width of pulses to maintain the appropriate
voltage in the output tank based on the power demand. It might well be
measuring both voltage and current and making adjustments accordingly.
Other projects that would use some of this would be charging regulators
for charging batteries from photovoltaic sources, and diverters for wind
generators to manage the alternator loading under varying usage loading
and wind speeds. It might also manage the exciter power for alternators
not using permanent magnets.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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