Diesel Car Ban

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Mandrake
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Re: Diesel Car Ban

Post by Mandrake »

EDC5 wrote:
25 Aug 2017, 23:04
Ah I see, so because of the 120 degree shift of the phases they can all share the same neutral as they're not using it at the same time? is that right?

Wikipedia covers it pretty well:

https://en.wikipedia.org/wiki/Three-pha ... tric_power

But basically, yes because of the 120 degree phase shift in the case of equal current on each phase (and unity power factor loads, like a resistive heater) all the current actually flows between the phases and none of it flows through the neutral wire.

Whenever there is an imbalance either in amplitude or phase (such as a reactive load) the difference or imbalance current flows through the neutral. But this difference current can never be greater than the current of a single phase - hence the neutral wire doesn't need to be any bigger than the three individual phase wires, and in practice it can often be smaller (or even non-existent) depending on how well balanced the load is.

If the load is something like a 3 phase motor (which is inherently balanced) no neutral wire is needed at all.

In fact the motors in all modern EV's are 3 phase AC motors, either permanent magnet synchronous or induction, and they only use 3 phase wires and no neutral wire at all.

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EDC5
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Re: Diesel Car Ban

Post by EDC5 »

Mandrake wrote:
25 Aug 2017, 23:08
EDC5 wrote:
25 Aug 2017, 23:04
Ah I see, so because of the 120 degree shift of the phases they can all share the same neutral as they're not using it at the same time? is that right?

Wikipedia covers it pretty well:

https://en.wikipedia.org/wiki/Three-pha ... tric_power

But basically, yes because of the 120 degree phase shift in the case of equal current on each phase (and unity power factor loads, like a resistive heater) all the current actually flows between the phases and none of it flows through the neutral wire.

Whenever there is an imbalance either in amplitude or phase (such as a reactive load) the difference current flows through the neutral. But this difference current can never be greater than the current of a single phase - hence the neutral wire doesn't need to be any bigger than the three individual phase wires, and in practice it can often be smaller (or even non-existent) depending on how well balanced the load is.

If the load is something like a 3 phase motor (which is inherently balanced) no neutral wire is needed at all.


Yeah I gather that's how it works in a three phase system; the remainder due to imbalance goes down the neutral. But for 3 separate single phase loads connected to a 3P+N supply can they all run at the maximum current and share the same neutral?

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Re: Diesel Car Ban

Post by elma »

Beaten to it by Simon again, heres my version.
Simon are you an Engineer? You definitely have a good knowledge of this stuff.

Yes you've got it but the words you are using aren't quite right.
They do use it at the same time but are out of time with each other so when the currents are added together they add up to approximately zero.
On an unbalanced load they add up to approximately the imbalance which will always be smaller than the individual loads.
Heres 2 diagrams that make it far easier to understand..

As you can see in the first diagram unequal 3 phase loads as you described use a shared neutral and barely put any current through it compared to the phases
If they were all maximum, say 100amps, there would be no current in the neutral in theory but as it's 1ph loads there would be a little as they don't balance perfectly.
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Mandrake
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Re: Diesel Car Ban

Post by Mandrake »

EDC5 wrote:
25 Aug 2017, 23:12

Yeah I gather that's how it works in a three phase system; the remainder due to imbalance goes down the neutral. But for 3 separate single phase loads connected to a 3P+N supply can they all run at the maximum current and share the same neutral?

Yes.

Refer to the Y configuration diagram in the Wikipedia article.

This could equally represent 3 Windings at 120 degrees in a single 3 phase motor, or three separate but equal single phase loads connected between their own phase's and a common neutral. If there is no imbalance between them nothing will flow on the common neutral but in practice while an actual three phase load can be almost perfectly balanced multiple single phase loads are unlikely to be well balanced so there will be some neutral current, but typically less current than a single phase.

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Mandrake
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Re: Diesel Car Ban

Post by Mandrake »

elma wrote:
25 Aug 2017, 23:18
Beaten to it by Simon again, heres my version.
Simon are you an Engineer? You definitely have a good knowledge of this stuff.
Sort of, but not practising in my day job, which unfortunately went in the IT direction... :twisted: I did a Diploma in Electronics and Electrical Engineering in NZ. Roughly equivalent to HND level over here. I sometimes wish I'd pursued it instead of falling accidentally into the IT career path. Too many interests and too little time to follow up on them all! :(

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Re: Diesel Car Ban

Post by elma »

It shows, eyes usually roll when I talk about this stuff.
I'm enjoying having someone else thats on my wavelength.

I'm troubled this week deciding weather to put my efforts into growing mushrooms or go back to college and assume the number 2 position in the companies electrical division. The age difference between me and number 1 would mean I'd become number one at the same age he did in about 15 years.
It's a case of big gamble that may pay off big or fail vs good steady job on decent wage with security.
Greedily I want both but thats not realistic, maybe I pursue the Mushrooms as a hobby and give up on them as a business idea.
Thats a hard call though as I have the space and the funding but not the time at the moment.

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EDC5
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Re: Diesel Car Ban

Post by EDC5 »

Mandrake wrote:
25 Aug 2017, 23:29
EDC5 wrote:
25 Aug 2017, 23:12

Yeah I gather that's how it works in a three phase system; the remainder due to imbalance goes down the neutral. But for 3 separate single phase loads connected to a 3P+N supply can they all run at the maximum current and share the same neutral?

Yes.

Refer to the Y configuration diagram in the Wikipedia article.

This could equally represent 3 Windings at 120 degrees in a single 3 phase motor, or three separate but equal single phase loads connected between their own phase's and a common neutral. If there is no imbalance between them nothing will flow on the common neutral but in practice while an actual three phase load can be almost perfectly balanced multiple single phase loads are unlikely to be well balanced so there will be some neutral current, but typically less current than a single phase.

elma wrote:
25 Aug 2017, 23:18
Beaten to it by Simon again, heres my version.
Simon are you an Engineer? You definitely have a good knowledge of this stuff.

Yes you've got it but the words you are using aren't quite right.
They do use it at the same time but are out of time with each other so when the currents are added together they add up to approximately zero.
On an unbalanced load they add up to approximately the imbalance which will always be smaller than the individual loads.
Heres 2 diagrams that make it far easier to understand..

As you can see in the first diagram unequal 3 phase loads as you described use a shared neutral and barely put any current through it compared to the phases
If they were all maximum, say 100amps, there would be no current in the neutral in theory but as it's 1ph loads there would be a little as they don't balance perfectly.
popup_2.jpg
3PQJx.gif
Thanks for that, I understand the principle now. I'm pretty interested in electronics and micro controllers but 3 phase power has always interested me.

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Re: Diesel Car Ban

Post by elma »

Maybe you can give me some lessons on the electronics at some point, I have an interest but a complete lack of knowledge once it gets past individual components and simple circuits.

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CitroJim
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Re: Diesel Car Ban

Post by CitroJim »

So, when 3-phase supplies are installed into a domestic property what precautions are made to ensure hapless souls do not get themselves stuck between two phases and thus receive a 415V belt?

I assume the regs have plenty of rules about phase separation in domestic situations...

Years back it was almost impossible to get 3-phase supplies into domestic properties; my old uncle who owned a lovely big lathe with a 3-phase motor tried and failed...

He looked into a 3-phase inverter to generate 3-phase from a single-phase supply but it was no good. In the end he replaced the motor with a giant and somewhat unsatisfactory single-phase motor...

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white exec
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Re: Diesel Car Ban

Post by white exec »

When we moved into our (2001-built) house here, we found a 3ph installation.
Allocation of phases within the property was completely haphazard, with everything spur wired (no ring mains here).
Worst case, Jim, was the main bathroom: double unswitched socket next to the wash basin . . . LH socket on one phase, RH socket on another, light above the basin mirror on the third!
Everything now rationalised - underfloor heating on ph1, cooking on ph2, power/lighting on ph3. Not best balanced, but safe.

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CitroJim
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Re: Diesel Car Ban

Post by CitroJim »

white exec wrote:
26 Aug 2017, 07:47
When we moved into our (2001-built) house here, we found a 3ph installation.
Allocation of phases within the property was completely haphazard, with everything spur wired (no ring mains here).
Worst case, Jim, was the main bathroom: double unswitched socket next to the wash basin . . . LH socket on one phase, RH socket on another, light above the basin mirror on the third!


Gosh :shock: That was an accident waiting to happen!!! That lot would never have passed the 16th regs current in the UK at the time :evil: Pleased you've rationalised things...

I'm shocked and amazed one EU country can be so lax abut electrical installations when the UK has perhaps the toughest regs in the world... So much for EU harmonisation :roll:

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Re: Diesel Car Ban

Post by elma »

Thats a good question Jim and the answer is basically separation.
You shouldn't have anywhere you can actually reach the separate phases.
I'll look over the regs later and see what I've forgotten as there is more on it.

Where I've seen it installed domestically the whole property was on a single phase as normal but the workshop in the garden had a 3ph supply brought over by a single armoured cable.
The person who's house that is in probably installed it themselves as they are quite handy, think the rules are bull and would never ask for permission to do something to their own house.

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bobins
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Re: Diesel Car Ban

Post by bobins »

In Germany it's the norm to have sockets in the bathrooms. They'll happily install them right next to the basins - they'll even hard wire hairdriers and put them next to the basins :shock: Never worked out if we've got it wrong or if it's them ! :)

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Re: Diesel Car Ban

Post by elma »

Over here nothing electrical can go within arm's reach of the basin. Doesn't stop people doing it anyway though, I often see bathrooms that fall way short of the regs. Mines a good example, someone's put a double socket almost touching the sink. I really must get rid of that actually, it gets splashed.
My picture, someone else's dodgy work.
My picture, someone else's dodgy work.

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demag
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Re: Diesel Car Ban

Post by demag »

Is that a trailing twin and earth beneath it? :-)
I seem to remember that Continental (Europe) wiring is all radial and no rings. Better for fault finding IMHO.
My sons first house was a strange split level affair with separate breakers for up and down lighting. He wanted a new lamp in the bathroom so we killed the upstairs breaker and double checked with multi meter. Yep still live and this was a brand new house! How did that get through inspection?