3 ms·

What? WHAT? DC does not have a "frequency".

by fnj 10y ago

dredmorbius 10y ago
Damn, I guess it's been too long since I've done (or thought about) circuit theory. But yeah, the general idea of HVDC is a frequency-free current: http://www.cleanlineenergy.com/technology/hvdc/how http://www.cleanlineenergy.com/technology/hvdc/how And now I'm trying to think my way through it: There is definitely DC with no frequency component, e.g., battery circuits. You've got a cathode, an anode, and a flow between them. I'm trying to sort out if DC can have a frequency component. I seem to recall that rectified circuits can operate by basically (pardon crude metaphors) chopping off the negative side of the phase. You'll have a wave pattern, but it's all going in one direction. Pushed through sufficient capacitance or impedance and you'll likely smooth that out as well. Which is getting me to wonder just how a DC generator works.
- nicky0 10y ago
  There is no DC generator as such, just a converter from AC to DC.
  - dredmorbius 10y ago
    Um, isn't a dynamo a DC generator? https://en.wikipedia.org/wiki/Dynamo https://en.wikipedia.org/wiki/Dynamo It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.
    - dmoy 10y ago
      iirc (been awhile since EE so could be wrong): Dynamo is basically an AC gen with some crap in front to make output look DC. "Pulsing DC current" is actually ac with >50% of the duty cycle zeroed out. Again, could be very wrong here my memory is fuzzy as hell.
      - y4mi 10y ago
        wikipedia seems to agree with you, if i read this right. > A dynamo is an electrical generator that produces direct current with the use of a commutator. >> A commutator is a moving part of a rotary electrical switch [...] that periodically reverses the current direction between the rotor and the external circuit
      - dredmorbius 10y ago
        If I'm remembering and understanding correctly: the commutator swaps the + and - ends of the energised coil. So you're not actually zeroing out the inverted end of the wave, you're swapping its connections to the circuit. Where an AC generator coil applies a positive and negative voltage to a single lead, the DC generator swaps leads as the current reverses. I'm glad I'm not the only fuzzy one here....
  - planteen 10y ago
    Take a DC motor, attach a load resistor, and spin it at constant speed. Look at the output voltage on a voltmeter. You will get DC output.
- 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.

What? WHAT? DC does not have a "frequency".

by fnj 10y ago

What? WHAT? DC does not have a "frequency".

Damn, I guess it's been too long since I've done (or thought about) circuit theory. But yeah, the general idea of HVDC is a frequency-free current: http://www.cleanlineenergy.com/technology/hvdc/how http://www.cleanlineenergy.com/technology/hvdc/how And now I'm trying to think my way through it: There is definitely DC with no frequency component, e.g., battery circuits. You've got a cathode, an anode, and a flow between them. I'm trying to sort out if DC can have a frequency component. I seem to recall that rectified circuits can operate by basically (pardon crude metaphors) chopping off the negative side of the phase. You'll have a wave pattern, but it's all going in one direction. Pushed through sufficient capacitance or impedance and you'll likely smooth that out as well. Which is getting me to wonder just how a DC generator works.

nicky0 10y ago
There is no DC generator as such, just a converter from AC to DC.
- dredmorbius 10y ago
  Um, isn't a dynamo a DC generator? https://en.wikipedia.org/wiki/Dynamo https://en.wikipedia.org/wiki/Dynamo It's described (and matches my memory) of producing a "pulsing direct current". The commutator, a physical switch, maintains the polarity of current by breaking and switching between generator coils.
  - dmoy 10y ago
    iirc (been awhile since EE so could be wrong): Dynamo is basically an AC gen with some crap in front to make output look DC. "Pulsing DC current" is actually ac with >50% of the duty cycle zeroed out. Again, could be very wrong here my memory is fuzzy as hell.
    - y4mi 10y ago
      wikipedia seems to agree with you, if i read this right. > A dynamo is an electrical generator that produces direct current with the use of a commutator. >> A commutator is a moving part of a rotary electrical switch [...] that periodically reverses the current direction between the rotor and the external circuit
    - dredmorbius 10y ago
      If I'm remembering and understanding correctly: the commutator swaps the + and - ends of the energised coil. So you're not actually zeroing out the inverted end of the wave, you're swapping its connections to the circuit. Where an AC generator coil applies a positive and negative voltage to a single lead, the DC generator swaps leads as the current reverses. I'm glad I'm not the only fuzzy one here....
- planteen 10y ago
  Take a DC motor, attach a load resistor, and spin it at constant speed. Look at the output voltage on a voltmeter. You will get DC output.
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.