Is DC Current Truly Direct or Does it Have Alternating Components?

[Femme_physics]

Or does it have a hint of alternation in it as well?

I know that if AC current is "direct enough" it's considered DC current. I was wondering if DC current can also be "alternating enough", and therefor might be even considered AC current.

 

[Evil Bunny]

I'm not seeing how there could be any gray area here. "direct enough" isn't something I've heard of when discussing AC or DC.

The charge is either moving in one direction... Or it's periodically changing directions...

It's one or the other. Don't see how it can be both.

 

[I_am_learning]

There can be these kind of signals
1. Pure DC.
2. DC with some ripples.
3. AC with DC offset
4. Pure AC

If you can distinguish these, then probably you will have little more understanding.

 

[Femme_physics]

I think I get it now. Thanks :)

 

[Dmytry]

dc with some ripples is same as ac with dc offset, only a matter of relative amplitude. You can do the Fourier transform, then the frequency 0's amplitude is dc, all other are ac with specific frequencies.

 

[Evil Bunny]

A key point here is the changing of direction... ripple DC changes magnitude but never changes direction. This is a very important distinction that actually defines the terms in question .

 

[Dmytry]

A key point here is the changing of direction... ripple DC changes magnitude but never changes direction. This is a very important distinction that actually defines the terms in question .

well, I've seen 'ripple' used to describe situation when you have some real big but brief ripples in the current, with short duration transient reversals. Say, you have a ripple DC source, you connect it to resistor with inductor in series, you can have voltage reversals across the inductor.

Even with something as simple as powering brushed DC motor with a battery (especially the textbook simple DC motor that has just 2 rotor poles), you could get ripple with current reversal when motor is not loaded (is free-spinning), but nobody is going to call it "AC with DC offset". The difference is really quantitative and quite informal.

 

[Averagesupernova]

While it is likely that DC with ripple won't be called AC with an offset it is technically true. It is just a much larger offset than we are accustomed to working with if we had for instance 5 volts peak to peak with a 20 volt offset.
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Suppose we have a sine wave that is for instance 20 volts peak to peak that still goes above and below zero but is slightly offset by 5 volts DC. THAT situation would be more correctly called AC with an offset. Well experienced and trained individuals will understand it referred to in either manner although they may have to read it twice before it sinks in.

 

[Dmytry]

While it is likely that DC with ripple won't be called AC with an offset it is technically true. It is just a much larger offset than we are accustomed to working with if we had for instance 5 volts peak to peak with a 20 volt offset.
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Suppose we have a sine wave that is for instance 20 volts peak to peak that still goes above and below zero but is slightly offset by 5 volts DC. THAT situation would be more correctly called AC with an offset. Well experienced and trained individuals will understand it referred to in either manner although they may have to read it twice before it sinks in.

There's also some very curious cases... suppose you connect a battery to an free spinning brushed DC motor, the kind that's depicted here:

assuming zero winding resistance (but nonzero inductance) and zero friction. (It won't speed up to infinite speed, by the way). Think, what the actual current between battery and motor be? You actually get alternating current here (part of the time current flowing from battery into motor, part of the time from motor into battery). Motor ripple can be used to sense motor speed, this has practical application.

 

[Jiggy-Ninja]

There's also some very curious cases... suppose you connect a battery to an free spinning brushed DC motor, the kind that's depicted here:

assuming zero winding resistance (but nonzero inductance) and zero friction. (It won't speed up to infinite speed, by the way). Think, what the actual current between battery and motor be? You actually get alternating current here (part of the time current flowing from battery into motor, part of the time from motor into battery). Motor ripple can be used to sense motor speed, this has practical application.

The current doesn't alternate in that example. It goes the same direction each rotation.

The whole point of the brushes is to make the current through the windings switch, and it does that by switching the direction of voltage in the coil very half-rotation. The current rom the batteyr is always the same direction though.

 

[phinds]

I'm not seeing how there could be any gray area here. "direct enough" isn't something I've heard of when discussing AC or DC.

Theoretically, you can have an AC current that is driven by an alternating voltage that has such a low period that it acts a lot like a DC current. I'm not aware of this happening in any real-world situation, but mathematically, it it perfectly sensical to talk about a sine wave voltage with a period of, say, 1 cycle per year. At any point in time, it is going to act more like a DC system than a AC system. Again, I emphasize that I don't think this is relevant to real world situations, but it certainly fits the term "direct enough".

 

[Averagesupernova]

Theoretically, you can have an AC current that is driven by an alternating voltage that has such a low period that it acts a lot like a DC current. I'm not aware of this happening in any real-world situation, but mathematically, it it perfectly sensical to talk about a sine wave voltage with a period of, say, 1 cycle per year. At any point in time, it is going to act more like a DC system than a AC system. Again, I emphasize that I don't think this is relevant to real world situations, but it certainly fits the term "direct enough".

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Interesting that you bring that up. When I first saw the title of this thread I thought about posting: Do you define direct current as having been a constant current since the beginning of time? I'm sure you see where I am going with this which is similar to what you pointed out.

 

[Dmytry]

The current doesn't alternate in that example. It goes the same direction each rotation.

The whole point of the brushes is to make the current through the windings switch, and it does that by switching the direction of voltage in the coil very half-rotation. The current rom the batteyr is always the same direction though.

It actually does when the motor spins up to max speed, that's the amazing thing.
If you disconnect the battery from the DC motor that has spun up to it maximal speed, the DC motor will work as the generator with voltage varying as abs(sin(t*w)) . Like on bottom of this image: http://en.wikipedia.org/wiki/File:Rectification.svg
and the peaks will exceed battery voltage. Ideally, with no friction, and zero winding resistance (nonzero inductance, nonzero inertia), the average current flowing from the battery is zero when motor has reached it's maximum speed. On the early and late part of half-rotation, the current is flowing one way, speeding the motor up (motor works as motor), and on the middle, it is flowing other way, slowing the motor down (motor works as generator) because the rate of flux change through the windings is larger in the middle of half rotation and at that time the electromotive force of the coil exceeds that of the battery. That condition actually defines the maximum speed of the free-wheeling motor at given voltage in absence of friction. When you use higher flux stator magnets in DC motor, the motor's top speed gets slower and vice versa.

I just thought it's relevant because it is a very practical example of ripple current that can go all way from small comparing to direct current (for a loaded motor) to the current reversal (for un-loaded motor).