Understanding three phase.

[Rampage]

I've heard many a time on here that people don't understand three phase enough to work on it.
Im just wondering what exactly people don't understand?

 

[Silly Sausage]

Not being one for maths, would the phase - phase voltage of a 12 phase system be:
SQUARE ROOT 12 x Uo ?
So if the Uo is 230v, the phase - phase voltage would be 796v ?

Eeeeee....possibly, I'd have to check out my old course notes on that.
I remember that the more phases there are increases transmission efficiency, but 3 is the optimum system, cost etc....

 

[johnboy6083]

just about your earlier comment about level 3 2330 as far as im aware there are no entry requirements im sure if you knew your stuff and explained it to the college they would put you onto a course after all your paying.

i myself dont get to do alot of 3 phase work normally do domestic, however i was on holiday last week and my grandfathers old house in france. and found out the immersion had broken and needed a like for like swap, took out the old immersion to find 3 phases and earth for my new 230 immersion, i was under the impression that you could connect between 2 phases and get 230v. not having my tester on me at the time i decided to get a light and connect it between the 2 phases to see if it would blow, and it did then i added a 2nd light in series and they both lit up brightly.

ive actually used this theory to fit 110V underwater lights before, i fitted 2 in series at every point. For some reason 110v ones were the only ones available and my bop
ss told me to go with it. It works fine though, but obviously they blow as pairs, but we tended to change the lamps as whole lots anyway.

 

[yellowvanman]

the 3[SUP]rd[/SUP] harmonic isn’t a problem (it’s the natural harmonic of 3 phase), 5[SUP]th[/SUP], 7[SUP]th[/SUP] and so on are the problem.

Not wanting to correct a distinguished poster! but!

The 3rd harmonic is a problem and any multiple of 3rd harmonic (eg. 9th, 12th etc). See example in Appendix 11.

With 3rd harmonics the frequency is 150Hz, line currents meet at the neutral point but don't cancel each other out (as opposed to 50Hz) because they are all in phase with each other, so the neutral carrys the sum of 3rd harmonic currents in the 3 phases.

So if the 3rd harmonic is 40% of the line current (50Hz) the neutral 3rd harmonic current will be 3 x 40% = 120%

 

[Knobhead]

12 phase and up to 48 phases were used for Mercury Arc Rectifiers, very rare beasts. The multiple phases being derived from 3 phase by using Zig-Zag and inter phase transformers.

As for the phase to phase voltages, you can see for yourself. You have to remember these transformers were used phase to neutral for rectifiers, the star point being the negative of to DC system. They were not used phase to phase.

Vector diagram.
View attachment 14347

Conections for a 2 phase MAR
View attachment 14346

A six phase MAR
View attachment 14345

 

[Rampage]

Are 3rd harmonics caused in inductive circuits because the current is lagging the voltage? Or is that totally wrong?

 

[topquark]

No, you wouldn't get 796V from a nominal 230V supply if it was 12 phase (assuming every phase was a nominal 230V). The most you could ever get would be 460V (between two phases 180 degrees apart).

 

[Rampage]

Ah yes, two coils are opposite so they make the maximum of 460v, that makes sense now thanks.

 

[oldtimer]

3 phase is the most efficiant way to transmit electricity ie you dont see one big transmission cable on a pylon nope you 2 sets of 3 phases that are around us more than some realize ie domestic sparks deal with and understand a single supply coming into a property but it is a single phase supply coming from a 3 phase supply in the street hence if a phase goes down in a area it is every third house that has no electricity.

Also in commercial or industrial buildings 3 phase is the norm for AHU,s,Chiller,pumps plus a lot of sparks for some reason when they add a circuit to a 3 phase DB they connect it to the red or as they call it no L1 phase.

I got called out to a job where the 3 phase breaker for one section of a building was tripping because the red phase was 150-180 amp out of balance so I asked whats changed then and the reply was nothing really but we got new communial photocopiers fitted last week so when I checked them out yep 30 amp single phase supply and every on wired to the red or L1 phase and by pure luck every one was wired to the bottom of each DB happy days and after a couple of hours the were moved around and the phase imbalance was gone.

PS and that where our trade is falling down guys training in domestics then doing work in a 3 phase environment

 

[topquark]

Ah yes, two coils are opposite so they make the maximum of 460v, that makes sense now thanks.

You're welcome.

So if I had four 230V nominal voltage phases (all 90 degrees apart from one another) could I derive 460V between phases 1 and 3 and 460V between phases 2 and 4 .... then could I derive a PD of 920V between those two? Explain your reasoning.

 

[Rampage]

You're welcome.

So if I had four 230V nominal voltage phases (all 90 degrees apart from one another) could I derive 460V between phases 1 and 3 and 460V between phases 2 and 4 .... then could I derive a PD of 920V between those two? Explain your reasoning.

Phases 1 & 3 and 2 & 4 would produce 460v between each other as they are 90 degrees apart and thus 'fully immersed' (can't think of a better word!).
I don't think connecting them together wouldn't do anything unless you are using two phases, connecting the other two phases would effectively double the frequency.
You can't get 920v as only two phases are 'fully immersed' at once.

 

[Knobhead]

Phase to phase voltages for a 12 phase transformer based on phase to neutral value of 10

30° 5.2
60° 10
90° 14
120° 17
150° 19
180° 20

View attachment 14348

 

[ian.settle1]

reminds me of a memorable mistake i made in my youth was removing a drill bit from behind a 3 phase air handling unit motor and had to take off the perspex plate to get my hand down, i turned the breaker off and then went on a coffee break, while i was away the machine next to it tripped and a member of the team came in and isolated the unit that had tripped and put my unit back intthe breaker back onto my unit, i returned and poked my head into the board and then shoved my hand down to get the drill bit, i understand 3 phase

Thats not three phase thats pure stupidity for not isolating it correctly in the first place.:banghead::censored:

 

[topquark]

Phases 1 & 3 and 2 & 4 would produce 460v between each other as they are 90 degrees apart and thus 'fully immersed' (can't think of a better word!).
I don't think connecting them together wouldn't do anything unless you are using two phases, connecting the other two phases would effectively double the frequency.
You can't get 920v as only two phases are 'fully immersed' at once.

Phases 1 & 3 (or 2 & 4) are 90 degrees apart giving you 460V?

 

[Knobhead]

Phases 1 & 3 and 2 & 4 would produce 460v between each other as they are 90 degrees apart and thus 'fully immersed' (can't think of a better word!).
I don't think connecting them together wouldn't do anything unless you are using two phases, connecting the other two phases would effectively double the frequency.
You can't get 920v as only two phases are 'fully immersed' at once.

Why are we talking about 6 and 12 phase, you’ll never come across it.

You definitely don’t understand the maths or physics of them.

 

[plugsandsparks]

Are 3rd harmonics caused in inductive circuits because the current is lagging the voltage? Or is that totally wrong?

Harmonics are caused by non-linear loads,

To understand non-linearities in electrical engineering and their effects on power distribution and on other system components is a whole new box to open.
Certain electrical components are non-linear in their load demand. E.g. a diode has dead-space non linearity, because you get not much current until you pass the forward threshold voltage about 0.7V.
Switching type power supplies and phase cutting regulators create impulse type loads which will generate the widest range of harmonics. discharge lighting
generates its largest harmonic in the 3rd , 150Hz
In todays world, one really does have to be aware of these effects. As always manufacturers data is the best source.


I know of a recent lighting panel that was built with equal TP+N cores, the neutral burnt out completely. My advice to the company was to double the size of SWA cables they were using for a given load, or half the load for a given SWA cable size for that type of application. e.g. Use 16mm up to 60amps etc.


It does make that much difference