Why 3-Phase?

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Jraef said:
. . . but later decided that 60Hz offered the advantage of being easily divisible into 360 degrees of rotation, which made rotor design a bit easier . . .
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In fact, 360 is used to divide a circle because it's evenly divisible by every number from 1 through 12 (except for 7 and 11).

I also think that the use of fluorescent lighting contributed to the move from 50Hz to 60 Hz.
 
LarryFine said:
Inversely: higher capacity for a given mass. Translation: more efficient.
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Or better cooling.
We make high speed motors, typically in the 10,000 - 20,000 rpm range and up to about 150 kW.
The high speed makes them very much smaller and lighter than standard motors of the same power rating. Efficiency is a little higher which reduces the losses some so we still need similar cooling but within the smaller envelope.
 
Besoeker said:
Then let's ban HVDC transmission........
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HVDC isn't DC.

By this, what I mean is that all of the transformation stages used in HVDC transmission use alternating current of one form or another. I don't know what the frequency is; a quick look suggests that most systems use AC at the 'local' mains frequency, normal size AC transformers, and high voltage switching/rectifying elements.

Small DC-DC converters operate at 50kHz up into the low MHz range, with very small magnetics made using various magnetic composite materials. I don't know if this sort of high frequency switching has found its way into HVDC transmission.

For actually going down the wires, DC is more efficient than AC. AC is more efficient over-all for most applications because it is more efficient and robust at the generation and transformation stage.

-Jon
 
winnie said:
HVDC isn't DC.

By this, what I mean is that all of the transformation stages used in HVDC transmission use alternating current of one form or another. I don't know what the frequency is; a quick look suggests that most systems use AC at the 'local' mains frequency, normal size AC transformers, and high voltage switching/rectifying elements.

Small DC-DC converters operate at 50kHz up into the low MHz range, with very small magnetics made using various magnetic composite materials. I don't know if this sort of high frequency switching has found its way into HVDC transmission.

For actually going down the wires, DC is more efficient than AC. AC is more efficient over-all for most applications because it is more efficient and robust at the generation and transformation stage.

-Jon
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By transmission I meant the stuff that is transmitted along the transmission lines.
That is DC. And high voltage.
The other parts of the system at each end of the line are conversion and inversion rather than transmission. The conversion and inversion stages are, as you say, at local power frequencies as a rule.
 
philly said:
I got to thiniking over the weekend why 3-phase electricity was adopted for use as the standard for multiphase electricity? Why was three phase adopted as opposed to 4-phase with phases being seperated by 90deg or some other multiple like say 5 or 6 phases? Is the 120deg offset the only phase difference that will work for balanced systems?

I understand most of the math behind three phase systems but wasn't sure if there was some underlying issue why 3-phase was an advantange of invening or adopting some other multi-phase system as mentioned.

Just something I was pondering and would be curious to hear if there were any explanations or references out there.
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Three phases are the lowest number of multiples that can produce rotational force with the simplest equipment. Eg. the most economical choice.
 
winnie said:
This subject is near and dear to my wallet (I get paid to research 'high phase order' electric motors).

You can build a balanced polyphase system with any number of phases.

The initial 'polyphase' systems were _two_ phase; however they were either balanced (requiring 4 wires) or unblanced (requiring 3). It was quickly realized that with the same 3 wires you could supply balanced 3 phase power and deliver more power over the same wires.

Going to more than three phases adds to the cost of your transformers, switchgear, etc, and is of benefit only in specific circumstances.

EPRI tested out some high phase order power transmission systems. The benefit was that the phase-phase voltage was significantly reduced, so you could space the transmission wires more closely, thus with the same phase current and system voltage you could pack more power into the same right of way. This required transformer arrays to go between three phase and high phase; the thought was that this would be cheaper than the land for a wider transmission right of way.

In the motor realm, the high phase counts are produced by a specialized VSD, so you only have high phase counts between the VSD and the motor; it is still 3 phase power going into the system. The benefits there have to do with motor overload capability and inverter limitations.

-Jon
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Aren't even (ex. 4) phase motors have two opposing magnetic force thus producing a more difficult mechanical element to balance?
 
h
weressl said:
Aren't even (ex. 4) phase motors have two opposing magnetic force thus producing a more difficult mechanical element to balance?
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Not necessarily, because a single phase will generally show up at different locations on the stator, and can easily be connected to provide 'inverse' slot current flow. Thus with any number of phases you can connect the machine for balanced magnetic forces.

Consider a conventional three phase 2 pole machine. You have a total of _6_ phase bands in such a machine: A (0?) C' (60?) B (120?) A' (180?) C (240?) B' (300?) and back to A. The primed phase bands are simply the normal electrical phases, but the direction of the wire in the slots is in the opposite direction, so the effective slot current produced by current flow in these wires is 180? out of phase with the corresponding non-prime phase band.

It is this symmetry that ends up being a problem for certain systems, eg. 4 phases. Because your A phase also provides your A' phase simply be reversing the direction of the wires, it is a waste to externally supply phases that are 180? apart. So a 4 phase motor will be supplied with phases that are some multiple of 45? apart. However there is no arrangement of four 45? multiples that will evenly divide 360 degrees, so you cannot use the 4 phase analog to a wye connected motor; either you are forced to provide a neutral wire or you are forced to use 8 wires. If you provided the neutral, then you could drive such a motor, and have balanced magnetic pulls...but geting a neutral out of a VFD is a pain because you either need to synthesize it or you need to generate your DC with a neutral.

If you want to build a 6 phase machine, you could supply 0, 60, 120, 180, 240, and 300 degree phases, but you have the same issue with inversion; such a '6 phase' machine is simply a different way of powering a conventional 3 phase machine. Another approach is to build a machine supplied with 0, 30, 120, 150, 240, 270 degree phases; essentially you have two balanced 3 phase sets, offset by 30 degees from each other.

-Jon
 
philly said:
I got to thiniking over the weekend why 3-phase electricity was adopted for use as the standard for multiphase electricity?
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Okay, guys, here's the definitive reason: Just like a 3-legged ladder; it's inherently stable. :cool:
 
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