The historical war of currents in Mains Power Distribution: AC vs DC

In summary, the pulsating DC is unsuitable for practical transformers because the steady component saturates the iron core.
  • #1
FranzDiCoccio
342
41
TL;DR Summary
I think I understand the basics of why AC prevailed over DC, but I'm not sure how exactly the "basic" argument applies to the current generated by Edison's dynamos
So, if I get it right, the basic argument goes like this: AC was preferred to DC because its voltage can be stepped up by a transformer. This limits losses while the current is transported from the production plant to the final user. The voltage is subsequently stepped down when delivered to the user.

Of course, this cannot work with a (trivially uniform) direct current.

But Edison used dynamos, so I expect that his current was direct in the sense that it always flowed in the same direction, but pulsed, i.e. time dependent. This would not prevent the use of transformers to step up the voltage, transport the current and step it down again, would it?

So what was the problem there? Did Edison just refuse the whole high voltage idea, although possible?

Or am I wrong on the current generated by Edison's dynamos? I'm thinking something working along the lines of this nice "vintage" animation by W. Fendt (case "with commutator").
Maybe this is too naive, and Edison did actually generate a constant, or almost constant current by using a more complex apparatus?

I tried to imagine a dynamo generating a constant direct current. I guess this would be in principle possible with a conducting rod rotating about the axis of an axysimmetric magnetic field.

Thanks a lot for any insight
Francesco
 
Physics news on Phys.org
  • #2
The pulsating DC is unsuitable for practical transformers because the steady component saturates the iron core.
 
  • Like
Likes FranzDiCoccio, berkeman, cnh1995 and 2 others
  • #3
Oh, I think I sort of see what you mean. I did not think of the role of the iron core.

I'm not exactly sure I know what you mean by "steady component", though. My knowledge of this field is extremely limited.

Can I understand it like this?

Since the current changes in time but flows always in the same direction, it produces a magnetic field which never changes its direction. This ends up magnetizing the iron core in that direction.
This in turn means that the flux throught the secondary coil is kind of dominated by the magnetic field of the core, and is changed only a little by the field of the primary coil. This sorts of defeats the purpose of the iron core. Instead of magnifying the change in the flux produced by the primary coil, it dampens it.
The magnetic field through the core sorts of gets stuck.

This of course makes the transformer very ineffective.

On the other hand, with AC, the field in the core reverses completely its direction, although along a hysteresis cycle. This variation makes the transformer worth using.

I guess that if no iron core was present there would not be much difference between AC and pulsating current, but probably the result would be equally bad.
 
  • #4
FranzDiCoccio said:
But Edison used dynamos, so I expect that his current was direct in the sense that it always flowed in the same direction, but pulsed, i.e. time dependent.

Is this true? I don't know much about generators/dynamos but I have always thought of Edison's DC as just that, DC like you get from a battery.

Or am I wrong on the current generated by Edison's dynamos? I'm thinking something working along the lines of this nice "vintage" animation by W. Fendt (case "with commutator").
Maybe this is too naive, and Edison did actually generate a constant, or almost constant current by using a more complex apparatus?

Can anyone answer this? maybe @anorlunda ?
 
  • #5
I found this old book; it shows the output as more like (how I imagine) DC, with a "ripple" on top. I guess this is what you mean by "pulsating DC?"

Dynamo-electric Machinery: A Manual for Students of Electrotechnics
Silvanus Phillips Thompson

https://play.google.com/books/reader?id=0Uk5AQAAMAAJ&pg=GBS.PA38&hl=en

dynamo.jpg
 

Attachments

  • dynamo.jpg
    dynamo.jpg
    80.5 KB · Views: 65
  • Like
Likes FranzDiCoccio
  • #6
FranzDiCoccio said:
Can I understand it like this?

Since the current changes in time but flows always in the same direction, it produces a magnetic field which never changes its direction. This ends up magnetizing the iron core in that direction.
This in turn means that the flux throught the secondary coil is kind of dominated by the magnetic field of the core, and is changed only a little by the field of the primary coil. This sorts of defeats the purpose of the iron core. Instead of magnifying the change in the flux produced by the primary coil, it dampens it.
The magnetic field through the core sorts of gets stuck.
Umm... No. Just no. I'm not sure where to go with this to fix it.

If you imagine a graph of the voltage applied to the primary of a transformer (or inductor) vs. time, then the area under that graph (in volt⋅seconds) determines the flux increase (or decrease for the other polarity). A magnetic material can only tolerate a fixed amount of this applied volt⋅second excitation, or flux, before it saturates (i.e. fills up) after which it stops working like a magnetic core and bad things transpire. So that waveform must have an area below the zero volt line to reduce the accumulation of the sum (or integral) area accumulated over time. A small amount of DC offset for a long time will saturate the core material and make the transformer stop working. Sort of like water flowing into or out of a bucket, it has to balance over time to keep the bucket from overflowing.

Honestly, it's a bit too complicated to explain well if you haven't studied physics or electronics much.
 
  • #7
Hi, i think I've got my answer, which is along the lines of what gmax137 suggested.
A friend suggested that W. Fendt's is actually a very simplified version of a real dynamo, and that one could use more than one armature. I made some diagrams.

This is what happens with W. Fendt "single armature" dynamo
oneArmature.png

(the dashed line is what would happen with the "two ring" configuration of the alternator in the same animation).
If two armatures are used, whose arc leads span 90° instead of 180°, one gets two graphs similar to the previous one.
twoArmatures.png

with three armatures whose arc leads span 60° the results is similar, with three graphs instead of two:
threeArmatures.png

The emf is pulsed but, as the number of armature increases, the pulses become very small bumps over a constant background .

With a sufficient number of armatures the resulting current is basically continuous, and Faraday's law becomes pretty much irrelevant, whether there's an iron core or not.
 
  • Like
Likes KurtLudwig
  • #8
DaveE said:
If you imagine a graph of the voltage applied [...]

Honestly, it's a bit too complicated to explain well if you haven't studied physics or electronics much.

Thank you for your answer. I have a really little knowledge of magnetism in ferromagnetic materials, but your explanation is more or less what I figured from tech99 reply.
I tried to put my understanding into words, but couldn't explain myself well.

gmax137 said:
I don't know much about generators/dynamos but I have always thought of Edison's DC as just that, DC like you get from a battery.

I thought that as well, but then I read that Edison used dynamos.
Thanks for the excerpt from the book! I am happy I could figure that out on my own (although a chat with a friend helped too, and I cannot exclude I studied something similar many years ago in a textbook).
 

Similar threads

  • Electrical Engineering
Replies
15
Views
832
Replies
13
Views
3K
Replies
30
Views
2K
  • Electrical Engineering
Replies
10
Views
2K
  • Electrical Engineering
Replies
8
Views
1K
Replies
2
Views
786
Replies
7
Views
1K
  • Electrical Engineering
Replies
32
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
1K
Back
Top