3 phase

[geezer]

i was told in a 3 phase system if you cross any of the live phases you get a bang how come? i thought live was live and whe you cross live and neutral you get a bang

also why do they say 415volts or 230/415v is each phase coming in at 415 volts also would that mean that each circuit would be having current at 415v?

 

[Steve]

each phase is 230 volts with respect to neutral.

each phase is 415 volts or thereabouts to any other phase. hence the big bang. phase to phase faults are worse than phase to neutral.

search wikipedia for 3-phase, and read about it.

 

[simon_d]

addin to above, each phase is 120 degrees apart. which is why you dont go from 230 to 460. phase-phase gives 400V.

now, if there was no difference between each of the 3 phases, it would be easier just to have phase and neutral...


in a balanced 3 phase circuit, there is nothin travelling back along neutral. 3 phase is used because you get more power outta it. look at the diagrams of the waves and youll see what i mean about it (basically, is has less down time)

 

[JohnD]

I don't know if this will help, but when I found it difficult to grasp (thinking of a 6v battery plus a 6v battery making 12v, so how did 240v plus 240v make 415v?) an aid was looking at a picture.

Think of a circle of radius 240mm. The difference from the centre to any point on the circumference of the circle is alway 240mm.

But if you take three equally spaced points around the circle, the distance from each to the other in a straight line (as a chord) is not 480mm but (I think) 415mm. And the voltages on the generator work like that.

Somone may know if I've got it right. It's not the sort of thing I usually do.

 

[breezer]

almost.

imagine a do-nut (no, not the ones with jam in them)
now wind a coil of wire round round it, that will only give you single phase as there is only the one coil

now try 3 coils, all sitting side by side, but covering the whole of the do-nut.

now stick a bar magnet in the middle on a shaft now spin the shaft hey presto, 3 phase.

that is very basic but it is how its done.

the speed of the magnet determines the frequency

in the uk its 50hz (50 times / second)

The size of the wire used determines how much curent you can get out*

the voltage is determined by the number of turns (i am not 100% on that one)

in reality they dont make 415v its too low, they make a couple of thousand as its cheaper / better.

now note this.

The higher the voltage the less current is required to operate a device, BUT if the voltage is too igh it will fry the device. That is a fact.

so when they generate electricity they need to send it all over the place, since you cant buy it in packets, and it runs down cables they have a grid to send it all over the country.

lets play with some numbers too ilustrate this.

town A need 100Amps @ 500v

now town A is 10 miles away

to get 100 A to go 10 miles would need a cable that is 10 inches in diameter (remeber i am playing with numbers)

now as a 10 inch cable (3 required 1 for each phase) is expensive what they do is increase the voltage.

so instead of 500v they increas it to 5000 volts.

as the voltage is now 10 times as big, they can use a cable that is 10 times as small

so you now only need a 1 inch cable / phase (much cheaper)

when it arrives at town A 5000 v is way too much

they install a transformer

this transformer transformes the voltage back to the 500v that is required at 100 amps

The transformer is a star delta transformer

there is a triangle each side has one phase

in the middle is another set of 3 coils in a "star shape" Y (almost)

The centre of the Y is earthed

AND that is also where the neutral comes from

there is no electrical connection between the star and delta coils, but as its moving at 50 hz it generates a magnetic filed in the delta coils

as the star coils are sitting so close (but not connected) to the dellt coils magnetism induces a current / voltage in the star coils

which is why transformers do not work on dc but only on ac , because ac moves and dc doesnt


simple but clever isnt it.

Reality check

I invented the figures to make it easier to follow.

the laws of physics say you will get some voltage loss

but the idea is how its done.

I trust this helps / easy to understand

 

[Big_Spark]

Just to add to the above post by Breezer.

When electricity is distributed locally, stepdown Tx's are used to reduce the supply voltage from 11kV 3 phase to 415V 3 phase. Now the peak power of any electrical system is the root mean square of the peak of the sinewave, which is 0.707 of the peak. This means the actual peak voltages are HIGHER than you might think.

If you want to know the voltage on each phase of the 3 phase system, then you need to divide the voltage across the phases by the square root of 3 (which is 1.73)

Now if you take 415/1.73 = 239.8.

Now the peaks of the signwave are, respectively, the rms value multiplied by 1.41 (1/0.707 = 1.41), so the peaks are actually 415 x 1.41 = 585.15 and 239.8 x 1.41 = 338.18

 

[ricicle]

John D the circle theory is correct as I have used it to evaluate line voltages from a phase shifting Tx.Following on from Breezers explanation,the voltage being high aids in the reduction of volts drop but also as the current is lower the power ( I2R) losses in the cable are lower making the system more efficient for a given power demand

 

[Big_Spark]

Ricicle, if you need to do the I²R in future and want it to look as I have posted it, all you need to do to get the superscript 2 is hold down the ALT key on your keyboard whilst typing in 0178, then you get ², if you want the cube then it is ALT 0179 for ³

Hope this helps.

 

[securespark]

Breezer, that must be one big f'in doughnut at the generating station...

Yum, yum, I'm getting hungry. Anyone know where the nearest GS is to SK7?? My satnav is waiting...

 

[Stoday]

to get 100 A to go 10 miles would need a cable that is 10 inches in diameter (remeber i am playing with numbers)

now as a 10 inch cable (3 required 1 for each phase) is expensive what they do is increase the voltage.

so instead of 500v they increas it to 5000 volts.

as the voltage is now 10 times as big, they can use a cable that is 10 times as small

so you now only need a 1 inch cable / phase (much cheaper)

I hate to be pedantic, but a 10" dia cable is 100 times bigger than a 1" dia. It'll weigh 100 times as much too.

But the losses (in the example) are the same in both cases, which is the point.

 

[breezer]

yes that is true, but i did say i was playing with the figures just to ilustrate the point.

and if you dont believe stoday ty this










The top item is one "unit"

The middle item although it is 2 units it is only 50% bigger (surface area) than the 1st item

The bottom item is 100% bigger (surface area) since which ever way you look at it there are two "units"

BUT the total number of units is actualy 4 in the third item