3 PHASE EXPLANATION
- Thread starter TOPSURV
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Domestic circuits are “single phase” AC (alternating current) the neutral is grounded and the voltage on the phase is in the form of a sine wave rising and falling in comparison to the neutral.
However, the “three-phase” system uses three wires instead of two. This is more efficient in that it requires only one-and-a-half times the number of wires to transmit three times as much power. The three phases carry voltages and currents which are 120 degrees out of phase with each other.
The voltage between any two of the three phases is 1.73 (the square root of three) times larger than the voltage between any one phase and earth.
It's a better way of distributing power to heavy users.
However, the “three-phase” system uses three wires instead of two. This is more efficient in that it requires only one-and-a-half times the number of wires to transmit three times as much power. The three phases carry voltages and currents which are 120 degrees out of phase with each other.
The voltage between any two of the three phases is 1.73 (the square root of three) times larger than the voltage between any one phase and earth.
It's a better way of distributing power to heavy users.
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http://en.wikipedia.org/wiki/Three-phase
The graph there may help you to understand, but the equations won't unless you're an engineer
The graph there may help you to understand, but the equations won't unless you're an engineer
It's a simply as I can explain something that is complicated. You should see the unabridged version....ah I see jwilliams has posted the unabridged version aboveTOPSURV said:
stem said:
This is all true, however, in this day and age we have become have used to three phase. It is no longer used as a benefit because it can transmit more power over less lines........The cost is actually offset inside an install.
For example.
A dairy wants a pump for the mil tank. They could have either a 6kw single phase, or a 6kw three phase. Most people would go for a three phase, as three phase motors are less likely to fail (usually).
BUT.
A three phase motor will be more expensive to purchase, will require more conductors to be run (yes smaller), and will require a TP MCB rather than a SP MCB - a TP one being 3 times the cost (or more) of a SP one. You also require TP Isolators etc.
Working to a larger scale, you would then also require larger TP boards to accomodate all three phase circuits as apposed to SP ones - You may infact find you need more than one board due to capacity issues.
DNO's will always prefer to give an install three phase rather than single phase if the demand is above 15kw. They don't care if you use three phase kit. I have worked on a couple of 200amp SP supplies (actually, also a 300amp) in rural areas for hotels etc, but this is very rare.
Doing a large job at the moment, and three phase machinary verses single phase is costing a substatial amount more due to machine costs, cabling costs, DB requirements/sizes etc.
Is Three phase really there to save money?!?
hi
if you draw one sine wave you will note that it only spends a short time at or around its maximum(peak bit). when it is at or near the "zero line" there is little or no voltage so therefore little or no power. you should see that it is quite inefficient for the job. now if you draw 3 sinewaves spread evenly (120 degrees apart)you will see the power is never zero. in other words as one phase is at zero the other two are producing power. it is a far more efficient way of doing things but as posters have stated there are drawbacks of "more of everything" especially the risk of a 400V shock as opposed to a 230V one
hope this helps
if you draw one sine wave you will note that it only spends a short time at or around its maximum(peak bit). when it is at or near the "zero line" there is little or no voltage so therefore little or no power. you should see that it is quite inefficient for the job. now if you draw 3 sinewaves spread evenly (120 degrees apart)you will see the power is never zero. in other words as one phase is at zero the other two are producing power. it is a far more efficient way of doing things but as posters have stated there are drawbacks of "more of everything" especially the risk of a 400V shock as opposed to a 230V one
hope this helps
Crafty said:
Not if the protective device is correctly selected
Partly though it won't for all loads depending on run lengths, size of the load and other installation conditions.Lectrician said:
For smaller loads as you say the extra costs of more cores and more switchgear may well outweigh the advantages.
The BIG advantages of three phase power over single phase (other polyphase systems have also been tried but mostly abandoned in favour of 3 phase though some are derived from grid power using transformer networks to power REALLY large rectifiers) are constant power transfer and the ability to easilly create rotating magnetic fields.
The former is of vital importance for larger equipment. Without it large motors and generators would simply shake themselves to peices. Even for smaller motors the single phase motor is likely to have less good power/weight because it has to absorb the energy in pulses.
The latter is fairly important because it makes it easier to build motors that actually start. There are cheats that allow single phase motors to start but they come at a price in either equipment cost or efficiancy.
What many DIYers won't realise is that their 240v power tools actually have DC motors (look for the commutator/brushes) running on AC because of the difficulties of making single phase AC motors work.
I would guess that washing machines have DC motors as well.
Three phase makes life much easier.
I would guess that washing machines have DC motors as well.
Three phase makes life much easier.
IJWS15 said:
That's not the reason. Single phase induction motors work fine once they've run up to speed.
The real reasons for the use of the universal motor (not dc) is the power for its frame size: it's smaller for a given power output. Secondly it's easier to control the speed. Lastly, it's cheaper. If you buy power tools such as pillar drills, table saws, planers etc, the cheapo B&Q versions use a universal motor but the pro versions an induction motor (single or three phase).
As for three phase with smaller motors (say less than 5 to 10 HP), the advantage used to be the smaller starting transient. The single phase direct on line starting transient is three times that of the same size three phase motor. If you have air conditioning, that used to cause voltage dips on a small supply such as a house. Three pahse was the old solution.
Now that single phase electronic soft start starters are available, that consideration no longer applies.
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