Electrics, somewhat explained

"Cé hé sin" (michael-m-mouse) 12/18/2018 at 18:25 • Filed to: Electricity

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I sympathise with James May. He says he doesn’t believe in electricity.

He has a point. External combustion, I can understand it. You light a fire, it heats water, that turns to steam which expands with sufficient force to make an up and down thing go up and down which in turn makes a turning around thing turn around and off you go. Internal combustion too. You have a suck-squeeze-bang-blow process (if you have four strokes, otherwise it’s simultaneous suck-and-squeeze followed by bang merging into blow) which also makes an up and down thing go up and down and so on. Electricity though involves a mysterious invisible something travelling through solid metal and into a thing which it makes turn around, light up, heat up or do complicated sums.

Anyway, if we just suspend disbelief for a while we can consider that electricity can come in two forms. It can just go from one place to another in a steady stream like water or it can go in one direction and then almost immediately turn back on itself and go the other way. That would be alternating current or AC and it’s what you get in your domestic supply.

AC doesn’t just suddenly change direction though, it does so somewhat gradually and it can be shown as a graph like this, complete with complicated abbreviations which needn’t concern us ( they certainly don’t concern me as I haven’t an idea what they mean) :

So your electricity is going from maximum voltage in one direction through zero to maximum in the other direction. Believers in electricity call this single phase and it’ s good for most of us. Not so good for more demanding users though because larger motors don’t like the fluctuations.

Enter three phase. Now you get three supplies of electrons, each of which follows a curve just like the one above. Single phase is (where I am) 230v so three phase should obviously be 230*3 or 690v. Except that it’s not. It’s 400v.

There’s a reason for that. Each of the three phases follows the same kind of curve, but they don’t overlap. Each is deliberately arranged to be one third of a cycle or 120deg apart, so we get this:

At any point on the horizontal axis (time) the total voltage is the total of the three phases. This, though, is never three or even two maxima. It’s always less than that. If you apply really complicated sums involving difficult things like square roots and the calculus which I was never able to understand you discover that (assuming that each phase is equal) the total voltage is the individual voltage, 230 in my case, multiplied by the square root of three which gives 400. So there we have it.

The real point though is that three phase has far less voltage variation than si ngle phase - it never reaches zero - and large motors run much better on it.

If you find yourself of an American persuasion you get your dom estic supply as two phases of about 120v each. Combine them and you actually do get 240v. Why? Because the two phases are 180deg apart and peaks coincide ( albeit in opposite directions) so the maximum is indeed twice the individual voltage.

lone_liberal > Cé hé sin 12/18/2018 at 18:35

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RMS explained . Good luck everybody!

Cé hé sin > lone_liberal 12/18/2018 at 18:42

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farscythe - makin da cawfee! > Cé hé sin 12/18/2018 at 18:48

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i dont know what any of this means and my job involves hitting an industrial powered water cooled robot with a hammer and screwdriver most days

its cool... i have rubber soles

(at this point im not sure if my mad einstein hair is natural or a side effect of the job tho)

Phyrxes once again has a wagon! > Cé hé sin 12/18/2018 at 19:09

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And this is why my physics students call this "black gypsy magic" every year when I cover basic charges, fields, and DC circuits.

jimz > Cé hé sin 12/18/2018 at 19:20

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the reason induction motors work best on 3-phase (also called polyphase) is that if you wire the stator coils such that there are an even number of poles per phase, that 120 degrees between each phase creates a natural “rotating” magnetic field around the motor’s stator, “dragging” the armature (rotor) along with it. so a 2-pole (per phase) motor has a field rotating around the stator at 3,600 rpm (60 Hz regions) or 3,000 rpm (50 Hz regions) and the rotor will attempt to follow it minus a bit of slip. typical full load rpm for a 60 Hz polyphase motor is ~3,450 rpm. And because the three phases inherently create a rotating field, no special tricks are needed to start a polyphase motor.

whereas an induction motor for single phase needs silly stuff like start capacitors, special startup poles, etc. to simulate a rotating field.  otherwise if you tried to start them on single phase the rotor would just sit there oscillating at the mains frequency and something gonna burn slap up. 

You can tell a Finn but you can't tell him much > Cé hé sin 12/18/2018 at 19:55

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If electricity was real why don’t electrons pile up underneath outlets when you unplug things?

MrDakka > Cé hé sin 12/18/2018 at 20:27

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Electricity is made up of angry little pixies.

Amperage is how many of them you can squeeze throu gh a tube. Voltage is the difference in how many are upstream vs downstream. Resistance is how hard you’re making the pixies move through said tube.

Multiphase AC is simply how many times you shove those pixies back and forth in said tube.

Urambo Tauro > Cé hé sin 12/18/2018 at 20:40

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jimz > Cé hé sin 12/18/2018 at 21:15

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I posted a comment to add to this, and the shit pile that is Kinja updates ate it.  so fuck it.

Cé hé sin > You can tell a Finn but you can't tell him much 12/19/2018 at 08:25

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Maybe they do? They're invisible after all!

Cé hé sin > jimz 12/19/2018 at 09:41

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I’ll just have to take your word for all that!

Akio Ohtori - RIP Oppo > Cé hé sin 12/19/2018 at 10:28

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Reminds me of this gem...

But they start at the basics...