Its been over 30 years since I studied 3 phase at Norwich City College, and as I do mainly domestic I don't come into contact with it a great deal, but I just have a passion to learn about it now, particularly as I have a small 3 phase commercial cold store to wire up in the near future. I have been reading 'Electrical craft Principle' by John Whitfield, who was my head course tutor back in the late 70's, and although an excellent book, some of the basics are just not explained. Anyway enough of my history!

To the question, when cables, MCB's, Isolators etc are used on a 400V 3 phase supply, my assumption is that the current rating refers to the sum of the L1, L2 & L3, and not individual phases?
I have a lot of questions in my head, but this will do to kick off with

Julian

The current rating will apply to individual phases - eg a 32A 3-phase supply will have three 32A protective devices, one per phase. The power (kW) is additive.


Regards,

Alan.

On 3 phase loads the current is normally stated per phase, the power being total.
So a 3 phase oven say rated at 3kw would be 1kw per phase so around 4A per phase.

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Empty barrels make the most noise.

Thanks Guy's, so likewise when 3 core & N swa cable is for arguments sake rated at 30A, thats 30A per conductor?

Correct.


Regards,

Alan.

If you look at the correct table then a cable rated at a particular current for 3 phase will be the current carrying capacity per core (ie per phase)

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Empty barrels make the most noise.

I'm ashamed to ask such simple questions, but like most things, when you don't use it you lose it. I was just googling and found this on another forum, is the following explanation correct?..



Three phase power is the most efficient way to generate and distribute electricity. The basics are easy, but it gets more complicated when you start taking about 3ph phasor diagrams, phase balancing and looking at how three phase motors work.

Three phase motors are perfectly blanced loads, in that they draw the same current from each phase, so they don't need a neutural. Each live takes it in turns to act as the neutural return path for the other lives. This happens 50 times a second or 50Hz.

If you are drawing different currents from each live then you need a neutural to return the difference in current between all the phases.

Yes that is basically correct.
The efficiency thing is due to the fact that you are spreading the load over 3 cables so less current per cable so less I²R losses.

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Empty barrels make the most noise.

Ok.
I know I might be accused of splitting hairs but it is 100 times a second as a 50Hz sine wave has two current peaks.

Hmmm. But isn't a 'complete' wave one 'up' and one 'down'? (thus equalling 'one' hertz?)

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I am prone to talking complete bol***ks at times, please accept my apologies in advance.

Don't be.
Like you, I haven't done a single bit of three phase work since leaving college, and have forgotton just about anything to do with it.
(I've probably laid 50,000 + bricks since tho' )

If I had a three phase fridge/container to wire up I'd sub it out to an electrical firm (I figure the time taken to learn how to do it competently would be better used actually earning coins of the realm.

Yet I'm registered with ElecSa as an Approved Contractor

Surely the purpose of most forums is so that answers are readily available... quite where the outrage comes from at having to 'spoon feed' other members with the audacity to ask questions is beyond me.

Wipe the foam from your mouth bod, you look daft

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I am prone to talking complete bol***ks at times, please accept my apologies in advance.

This could be a very immotive statement to those who believe that DC, albeit losing out to AC in times gone by, consider that the world would have been better off if this alternative had been taken up

The advantage with AC is that it can be easily transformed to different voltages. This enables transmission over long distances as losses are lower with a low current and high voltage compared to a high current and low voltage.

Also, anyone who's into motors will have a comment to make about the relative reliability of AC and DC motors - brushes and commutators have a lot to answer for.

I'm not sure that three phase is the most "efficient" though. I was always taught it provides the optimum balance between efficiency and cost. The extra efficiency you get from more than three phases is outweighed by the increased cost. So perhaps most economical is the best term, reflecting the fact that money is a key input into engineering design decisions.

How true is that - engineers are expected to do for a shilling what any idiot can do for a pound

Regards

OMS

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Failure is always an option

Although I'm sure you can understand a certain reservation at the concept of not bothering to think, not bothering to learn, not bothering to look and not bothering to invest in your own skills and knowledge being slightly irritating to those who have

Regards

OMS

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Failure is always an option

This applies to their pay as well as the cost of the hardware!

LoL - yes, well - another thread perhaps

Regards

OMS

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Failure is always an option

I agree. Three-phase is also the lowest number of phases that you need to define a direction of rotation. We have a couple of historic electric railways here, which use a 600v dc traction supply. In some of the original substations, the dc is provided using a mercury arc rectifier. In order to reduce the amount of filtering required on the dc, the substations use a transformer to change the incoming three-phase into a six-phase supply and it is this that is rectified.


Regards,

Alan.

Ghostly green-blue glow from them when in operation!

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Ross Currie TMIET

I used to look after a rubber mixing machine where it's incoming supply was via a phase shifting transformer giving six phases. The motor was controlled by two thyristor drives giving a total 'twelve pulse DC' output.

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Empty barrels make the most noise.