6 ms·

DC is synonymous with 0 frequency. Your metaphor for rectified circuits is not crude, a half-wave rectifier will clip the negative portion of the signal to 0,

by zild3d 10y ago

DC is synonymous with 0 frequency. Your metaphor for rectified circuits is not crude, a half-wave rectifier will clip the negative portion of the signal to 0, a full-wave rectifier will actually invert the negative portion. I wouldn't say you push it through sufficient capacitance or impedance, but you do want to smooth out this bumpy (now positive) wavy signal, so you low pass filter it, a simple resistor-capacitor circuit. what we're describing is an AC/DC convertor

dredmorbius 10y ago
Thanks for your patience. There's a distinction between a current which reverses flow, and one which varies, perhaps regularly, but without reversing flow. The reason coordination of AC generation is crucial is that if you pump opposing cycles at each other, they simply cancel out. If you pump out-of-phase varying DC currents at each other, they amplify one another -- the sign is always positive, but the magnitude varies. I suspect that might also cause problems, but not in the same way as a going-nowhere-fast out-of-phase AC circuit would. The phase-damping (and I very vaguely recall damped driven oscillators from physics: friends don't let friends drive damped ...), wouldn't be an AC/DC converter, but a form of power smoothing / power conditioning, I believe.
- euyyn 10y ago
  > There's a distinction between a current which reverses flow, and one which varies, perhaps regularly, but without reversing flow. [5, 3, 5, 3, ...] = [4, 4, 4, 4, ...] + [1, -1, 1, -1, ...] Every current that varies with a regular frequency is just the sum of a DC current and an AC current that reverses flow. I guess in most applications one can decompose them and analyze the two components separately.
  - dredmorbius 10y ago
    Sure, you can decompose them. But if you're outputting DC, again, you never have to worry about subtracting from an existing DC line voltage. If you're outputting AC, you have to worry about wave interference.
    - euyyn 10y ago
      I was just talking about your distinction between a current that reverses flow, and one that varies without reversing flow.
Animats 10y ago
We're kind of off topic here, but ... Ideal DC has zero ripple, and looks like a straight line on a scope. Ideal AC is a sinusoid. AC-DC conversion tends to produce some ripple. At low power levels, a capacitor across the line and/or an inductor in series will level out the ripple. At high power and voltage, such filters become huge, and there are lots of tricks used to reduce the need for filtering. Converting 3 phase power to DC works better than single phase, since at least two phases are always delivering power. There's no zero-power point, as there is with single phase, and the ripple is at 6x the AC frequency. (The higher the frequency, the less filtering required.) There's also a trick with transformers for converting 3-phase power to 6-phase power. This gets the ripple frequency up to 720Hz. This is an old trick; the NYC subway system has used it for many decades. Converting DC to clean AC used to be tough. The home solar power industry took a long time to get inverters with good output waveforms. Early inverters produced square waves, which overheat motors and transformers. There's been a lot of progress, and power semiconductors have improved enormously in recent years. Now everything from electric cars to Diesel-electric locomotives to electric model airplanes have 3-phase inverters. Anything with a "brushless DC" (really synchronous AC) motor has an inverter driving it. The output waveform is usually a stairstep approximation to a sine wave. This can be filtered into a sine wave if necessary. Power MOSFETs make this all possible. It's incredible how good they are as electrical switches. Turn-on resistances are in the milli-ohm range. That's as good as a mechanical switch, and better than a carbon-brush commutator.
- dredmorbius 10y ago
  I think this is one of those allowed off-topic digressions :) Thanks for the info.

by zild3d 10y ago

dredmorbius 10y ago

Thanks for your patience. There's a distinction between a current which reverses flow, and one which varies, perhaps regularly, but without reversing flow. The reason coordination of AC generation is crucial is that if you pump opposing cycles at each other, they simply cancel out. If you pump out-of-phase varying DC currents at each other, they amplify one another -- the sign is always positive, but the magnitude varies. I suspect that might also cause problems, but not in the same way as a going-nowhere-fast out-of-phase AC circuit would. The phase-damping (and I very vaguely recall damped driven oscillators from physics: friends don't let friends drive damped ...), wouldn't be an AC/DC converter, but a form of power smoothing / power conditioning, I believe.

euyyn 10y ago
> There's a distinction between a current which reverses flow, and one which varies, perhaps regularly, but without reversing flow. [5, 3, 5, 3, ...] = [4, 4, 4, 4, ...] + [1, -1, 1, -1, ...] Every current that varies with a regular frequency is just the sum of a DC current and an AC current that reverses flow. I guess in most applications one can decompose them and analyze the two components separately.
- dredmorbius 10y ago
  Sure, you can decompose them. But if you're outputting DC, again, you never have to worry about subtracting from an existing DC line voltage. If you're outputting AC, you have to worry about wave interference.
  - euyyn 10y ago
    I was just talking about your distinction between a current that reverses flow, and one that varies without reversing flow.

Animats 10y ago

We're kind of off topic here, but ... Ideal DC has zero ripple, and looks like a straight line on a scope. Ideal AC is a sinusoid. AC-DC conversion tends to produce some ripple. At low power levels, a capacitor across the line and/or an inductor in series will level out the ripple. At high power and voltage, such filters become huge, and there are lots of tricks used to reduce the need for filtering. Converting 3 phase power to DC works better than single phase, since at least two phases are always delivering power. There's no zero-power point, as there is with single phase, and the ripple is at 6x the AC frequency. (The higher the frequency, the less filtering required.) There's also a trick with transformers for converting 3-phase power to 6-phase power. This gets the ripple frequency up to 720Hz. This is an old trick; the NYC subway system has used it for many decades. Converting DC to clean AC used to be tough. The home solar power industry took a long time to get inverters with good output waveforms. Early inverters produced square waves, which overheat motors and transformers. There's been a lot of progress, and power semiconductors have improved enormously in recent years. Now everything from electric cars to Diesel-electric locomotives to electric model airplanes have 3-phase inverters. Anything with a "brushless DC" (really synchronous AC) motor has an inverter driving it. The output waveform is usually a stairstep approximation to a sine wave. This can be filtered into a sine wave if necessary. Power MOSFETs make this all possible. It's incredible how good they are as electrical switches. Turn-on resistances are in the milli-ohm range. That's as good as a mechanical switch, and better than a carbon-brush commutator.

dredmorbius 10y ago
I think this is one of those allowed off-topic digressions :) Thanks for the info.