three phase power

[Omegachi]

what is the advantage of using three phase power and how does it work?

 

[FrankSchwab]

Man, Google must be on the fritz in your area.

Try Wikipedia

 

[bobsmith1492]

Basically there are three separate lines with AC current running 120 degrees out of phase with each other. Why?

Well, at work all the motors are three phase because that way, instead of only having one sine wave that bottoms out to zero half the time, there are always one or two coils pushing the thing allowing for more power (and more consistant, I believe).

 

[PowerEngineer]

Originally posted by: bobsmith1492
Basically there are three separate lines with AC current running 120 degrees out of phase with each other. Why?

Well, at work all the motors are three phase because that way, instead of only having one sine wave that bottoms out to zero half the time, there are always one or two coils pushing the thing allowing for more power (and more consistant, I believe).

Yes, that is generally correct.

mechanical torque is created when two out-of-phase magnetic fluxes cross each other, and therefore creating fluxes in three windings arranged in a circle (120 degrees apart) with three voltages also 120 degrees apart produces a steady torque on the motor's rotor.

Now there are clearly ways to make motors work on a single "phase", but these are less efficient. Bigger motors tend to be three phase.

 

[imported_Soldier]

Omegachi you didnt mention motors specificaly so I will go in the other direction and say three phase power is used for it's superior "smoothing" ability in electronics devices. Basicaly if you think of power as water then you can think that 3 hoses pointing at a water wheel would be more efficent and more powerful than one water hose. As power applies its force while performing work it is much more efficent to have 3 phases in electronics (and motors) just like it is for the water wheel. Your power is more consistant, more reliable, and can even be delivered with cheaper components because the need to have very rugged and heat resistant components is minimized. Electronic stability is maximized by reducing the "peaks and valleys" in the AC voltage swing, so its easy to see that 3 phases would aid voltage stability when compared to single phase power. Many DC circuits use 3 phase power designs also for the same reasons.

Here's a link to some 3 phase info on the motor side also ...

http://www.answers.com/main/ntquery;jse...b=2222_1&hl=single&hl=phase&sbid=lc03b

 

[pnb263]

Being an EE graduate, I think Ill chime in here...

The biggest advantage to using three phase power is the ability to do more work than one, or two phase power. It also does more work than DC power...

Soldier and PowerEngineer are both correct... when using machines (motors), the three phases allow the motor to have a smoother, more constant torque, and to maintain the motion with smaller ammounts of power input. while the three phases are 120 degrees out of phase with each other, each has an identical sin wave function. because of the high-low nature of a sin wave, being 120 degrees out of phase with each other creates a contant power output in the positive range (think of a series of small hills)...

You are probably thinking that they should just create 30 phase power for an even bigger stability, and you are right, it would create a much smoother output... but the logistics of utilizing anything more than 3 phases make the cost/benifit ratio go down the crapper faster than a liquid turd.

As you probably know, AC power is much easier to transmit over longer distances than DC power. but what you probably dont know is that anything that does not involve a moving (rotating) motor uses DC as the real power source (computers, monitors, anything with batteries, clocks, A/V systems, etc). in order to get better price/transmission for the power, it is transfered using three phases of AC power. The AC power, since it is easier to transmit with fewer losses, is the ideal choice. Once it has reached the specific piece of equipment that will use it, it is either converted to DC or kept as AC (for rotating motors). The real benifit comes in that it is far easier to convert three phases of AC to DC than it is to convert 1 phase of AC to DC. The fact that 3 phases of AC makes for a smoother, more constant power output gives the converter a head start in the actual power conversion. If you were to use 1 phase AC to convert to DC, then the converter would be far less efficient, and therefore would waste most of what is being supplied to it.

AC to DC converters rely on 3 phases of AC to convert with less wasted power (put off in the form of heat). the more phases you have, the less work the AC/DC converter needs to do in order to achieve the linear power line that makes up "DC" power. just keep in mind that DC power has limits (high and low) that it MUST stay within in order to constitute "DC". If the high or low points of the line exceed that tollerance, then it no longer is DC, and is just garbage.

so, in short...

AC power is used for transmission, and DC power is used for electrically powered systems (most of them).
AC is used for transmission to the home due to its superior ability to be transfered with smaller losses over longer distances
DC is used for its "safer" factor in most electronics that are used in the home today

Three phases of AC is used to aid in the conversion to DC.

<did i miss anything all you EE/CompE people out there?>

 

[pnb263]

in adition to my post from above...

also keep in mind that DC power (used in electronics in the home) is actually AC power behaving as DC power. if you were to look VERY closely at a DC power line on an ocilliscope, you would see that it has a small, but detectable 'ripple'. this 'ripple' is the AC power passing itself off as DC power after the 'conversion'

the only true "DC" power source is a battery

 

[BitByBit]

So how does phasing in this case affect frequency, if at all?

 

[AdamNuhfer]

Omegachi: You need to be more specific. Do you want three phase power, or are you just asking in general terms?

pnb263: "the AC power passing itself off as DC power after the 'conversion' "

Beg to differ, It's DC. The ripple effect is from the voltage drop off of the sine wave in
the AC source voltage. After the AC goes through the rectifiers, it's DC. Three phase AC will give you less ripple than single phase AC due to the voltage overlap by the other two phases.
Capacitors help smooth out the ripple. A good quality Power supply will have a better DC sine wave than a poor quality Power supply, in general. The proper voltage and voltage regulation is what we as computers users need.

In the USA- Home users get single phase-60 HZ AC-240/120 volts
Very few residential customers have three phase in their home. A few do however.
For three phase in your home, this is what most will offer you, if they will at all.

120/208 or 480/277 three phase
240/120 three phase - not true three phase as is derived from two primary phases, but your three phase motors wil run just fine, plus you get regular 240/120 single phase out of it .

Unless you have a lot of large electric motors or heating elements, their is no real reason to have three phase power in a home. A kilo watt of power used is still a Kilo watt used regardless of voltage or number of phases. Now, if you are talking about the differnce in size and cost in reards to a 7.5 hp 240/120 single phase electric motor vs. a 7.5 hp 480/240 three phase motor, that's a different matter all togeather.

 

[pnb263]

Originally posted by: AdamNuhfer
Omegachi: You need to be more specific. What do you want three phase for, or are you just asking in general terms?

pnb263: "the AC power passing itself off as DC power after the 'conversion' "

Beg to differ, It's DC. The ripple effect is from the voltage drop off of the sine wave in
the AC source voltage. After the AC goes through the rectifiers, it's DC. Three phase AC will give you less ripple than single phase AC due to the voltage overlap by the other two phases.
Capacitors help smooth out the ripple. A good quality Power supply will have a better DC sine wave than a poor quality Power supply, in general. The proper voltage and voltage regulation is what we as computers users need. I need to understand Omegachi's want of three phase power.

I am trying to explain what the DC wave function will look like if you look at it close enough. The ripple of the DC wave will look like the very peaks of a whole lot of sin waves (which is what it is in essence). The more peaks you can sqeeze into a given space, the 'cleaner' the DC power is (less ripple) because the more you can sqeeze in, the less the actual ripple is (straighter) [IE, as you approach infinite sine waves of the DC function, the actual voltage level approaches the very peak of the sine wave, thus making a perfectly straight function (or line) of rippleless power]

 

[pnb263]

Originally posted by: BitByBit
So how does phasing in this case affect frequency, if at all?

Phasing doesnt affect frequency at all, frequency is set at 60 Hz in the US and 50 Hz in Europe. the three phases simply are 120 degrees out of phase with each other (on a circle if you will visualize it). if you want to get more down to it, since the three phase power is operating at 60 cycles per second, and there are three phases, in one second you will still see 60 cycles (total) of all three phases. The 60 can be thought of as 60 "pulses" or "peaks" per second for AC power. <crap> now i cant remember if that is 60 per phase or all together... its been three years since Ive taken this stuff, and I dont work with it on a daily basis ... someone chime in here

 

[AdamNuhfer]

pnb263: Ahhh, I'm not trying to give you a hard time, so please don't take it as such. I'm trying to figure out this guys angle on his question in regards to three phase power.

If I was trying to give you a hard time, I'd follow up on this:

"but what you probably dont know is that anything that does not involve a moving (rotating) motor uses DC as the real power source (computers, monitors, anything with batteries, clocks, A/V systems, etc)."

I do work with it every day. Thanks

 

[BitByBit]

Ah.
I imagined three lots of peaks and troughs which I thought would in effect triple the frequency.

 

[pnb263]

I didnt take it as so... but its good to get some feedback from someone who does work with it daily. It took me 6 years to graduate with a BS, and I went into Law Enforcement <hmpf>... I like these questions though.. lets me use the old noodle again like I used to..

<oooo... popcorn is done.. yum>

 

[AdamNuhfer]

Bit: Nope, just good old 60 HZ, wheter it be in single phase or three phase. We at the power company aren't trick like Intel, just old school supply the power to you.

pnb263: You took up EE and got into law enforcement. Now's there has got to be a good story behind that one. Thanks

 

[pnb263]

Adam... Mainly has to do with post 9/11 stuff. I graduated a year after that happened,and the job market was, well, in the pooper... I figured it would rebound quickly and took the Law Enforcement job as a stop-gap measure until then. Well, that was a few years ago, and I have yet to make the switch back to EE. I dont know if I will considering how far I have come, and the people I work with (I like my current job a LOT)... sure the money isnt there like an EE job, but there is also far less pressure, and I dont HAVE to remember all those little things about EE

Anyhow... thats the short n sweet of it (just in case you were curious)

 

[AdamNuhfer]

pnb263: Thanks for the reply. If you enjoy what your doing now, stick with it. A man has got to be happy in his work. Me, I've been in the Electrical field most of my life, I'm 49. The computer stuff is a hobby I enjoy when I have the time. Thanks

 

[Painkiller]

Originally posted by: pnb263
Being an EE graduate, I think Ill chime in here...

The biggest advantage to using three phase power is the ability to do more work than one, or two phase power. It also does more work than DC power...

Soldier and PowerEngineer are both correct... when using machines (motors), the three phases allow the motor to have a smoother, more constant torque, and to maintain the motion with smaller ammounts of power input. while the three phases are 120 degrees out of phase with each other, each has an identical sin wave function. because of the high-low nature of a sin wave, being 120 degrees out of phase with each other creates a contant power output in the positive range (think of a series of small hills)...

You are probably thinking that they should just create 30 phase power for an even bigger stability, and you are right, it would create a much smoother output... but the logistics of utilizing anything more than 3 phases make the cost/benifit ratio go down the crapper faster than a liquid turd.

As you probably know, AC power is much easier to transmit over longer distances than DC power. but what you probably dont know is that anything that does not involve a moving (rotating) motor uses DC as the real power source (computers, monitors, anything with batteries, clocks, A/V systems, etc). in order to get better price/transmission for the power, it is transfered using three phases of AC power. The AC power, since it is easier to transmit with fewer losses, is the ideal choice. Once it has reached the specific piece of equipment that will use it, it is either converted to DC or kept as AC (for rotating motors). The real benifit comes in that it is far easier to convert three phases of AC to DC than it is to convert 1 phase of AC to DC. The fact that 3 phases of AC makes for a smoother, more constant power output gives the converter a head start in the actual power conversion. If you were to use 1 phase AC to convert to DC, then the converter would be far less efficient, and therefore would waste most of what is being supplied to it.

AC to DC converters rely on 3 phases of AC to convert with less wasted power (put off in the form of heat). the more phases you have, the less work the AC/DC converter needs to do in order to achieve the linear power line that makes up "DC" power. just keep in mind that DC power has limits (high and low) that it MUST stay within in order to constitute "DC". If the high or low points of the line exceed that tollerance, then it no longer is DC, and is just garbage.

so, in short...

AC power is used for transmission, and DC power is used for electrically powered systems (most of them).
AC is used for transmission to the home due to its superior ability to be transfered with smaller losses over longer distances
DC is used for its "safer" factor in most electronics that are used in the home today

Three phases of AC is used to aid in the conversion to DC.

<did i miss anything all you EE/CompE people out there?>

AC transmission has more losses than DC. This is because impedance is a combination of resistance and reactance. Since DC has is zero frequency we can scratch out the reactance (i.e. inductive reactance = 2*pi*f*L) leaving only resistance. AC is subjected to both resistive and reactive losses. AC is used primarily because it is easy to step up and down. Also DC and RMS AC are equal in power.

 

[PowerEngineer]

Hmmmm... Let me add a few clarifications:

pnb263: I think it's fair to say that you can deliver more power to a three-phase motor of the same physical size as a single-phase motor (and single-phase motors actually have to split off that single phase into a slightly lagging second phase to get those two out-of-phase magnetic fluxes needed to get it rolling from a dead stop, and this is less efficient).

It's not really any easier or more efficient to convert three-phase AC power into DC power, however it is true that the 'ripple' will be higher in frequency and smaller in magnitude (all other things being equal) which means better power quality.

AC power is used for transmission because its voltage can be easily stepped up and down using transformers, and higher voltages require less current (and therefore smaller conductors and less losses) than lower voltages. Putting that advantage aside, DC transmission is actually more efficient that AC transmission for the same voltage level. That's why you'll see several really long-distance transmission circuits that are DC; the savings in conductor costs and line losses pay for the expensive converter stations. This (and very large motor control set-ups) are the only places you'll see three-phse AC conversion to DC. The vast majority of homes have just single-phase service (if you have a well, you may get a second phase for the pump).

The larger loads in most homes use AC directly. These include all the motor loads you referred to (furnace blowers, refigerator motors, wash machines, dryers) but also the incadensent lighting and the heating loads (ovens, stove tops, toasters, hair dryers, water heaters, and electric furnaces).

BitByBit: The phase shift between the three phases is to make it easy to produce that intersection of out-of-phase fluxes needed to produce torque on the motor's rotor. If you want the smoothest possible result for a motor, you want that phase shift to be 360 divided by the number of phases (and a winding for each phase spaced that same number of degrees apart around the rotor). So, for three phases that's 120 degrees. If we'd picked 6 phases as our standard, then we'd want a phase shift of 60 degrees between them.

Painkiller: As I said above, AC transmission does have higher losses than DC but it is not a direct result of reactance. Current passing passing through reactive impedance does not generate real power losses; "scratching out" the reactance does not by itself reduce real power losses. Instead, it's the phase angle between AC voltage and AC current (and there almost always is one) that lowers the amount of power delivered at tthat amount of current. The higher the phase angle, the higher the current you need to deliver the power, and the more losses generated through the resistance in the line.

RMS stands for "root mean square" which means that you take your sinusiodal shape, square the values through the complete cycle, that the mean (average) of its value over the complete cycle, and then take the square root of that. The RMS value for a sinusoidal wave is its peak value divided by the sqaure root of two. RMS is handy because the RMS values of voltage and current can be mutilplied together (with due regard for phase angle) to give you the average power delivered over the complete cycle -- just like DC.

:beer:

 

[AdamNuhfer]

PowerEngineer: I think your going to start and confuse folks here. Your starting off in the direction in dealing with the slight shift in the current phase in realtion to the voltage phase due to inductance. {current lags voltage}. Hence Capacitor packs in Sub stations and out on the High voltage lines to use the reactance of the capacitors to offset the effects of inductance. I think this in depth sort of talk will do more to confuse than to assist in the three phase vs single phase. Thanks

 

[Delbert]

Glad I stumbled across this thread. I teach aircraft systems to aircraft techs and the aircraft I teach all use 3 phase systems. Looks like most of what I understood was correct so thanks for the confirmation.
Here?s a question for the experts however. Aircraft AC generators are typically 3 phase, 115 volts @ 400 Hz. In the aircraft galleys they have a single leg (phase) connected to a standard three prong outlet. My question is what would happen if you connected a regular 60 Hz item to this outlet. Say a power drill perhaps? I suspect it would run really really fast for a very short time, but I?m not sure. What about just a wall wart transformer for say charging a cell phone?

 

[Mark R]

My question is what would happen if you connected a regular 60 Hz item to this outlet. Say a power drill perhaps?

It depends on what type of appliance it is.

A hand-held power drill will almost always use a universal motor, which works equally well on DC or AC. (Interestingly a real power drill will generally work much better on DC, because there is no hysteresis loss in the iron). Conversely, at higher frequencies, the hysteresis loss increases - so at 400 Hz I would expect it to work, but be noticably weaker and run hotter. If it has an electronic speed contoller, all bets are off, as the controller will likely be tuned for 60 Hz power.

Induction motors (e.g. a bench grinder) will try to run really fast and probably destroy themselves - either through overspeed, or through overheating because they can't get up to their new operating speed.

Power adaptors - it depends on how they are designed. An electronic voltage converter like on modern phones would almost certainly work fine. A transformer based design will probably also work but will overheat because of increased hysteresis loss in the iron core.

 

[Painkiller]

Originally posted by: PowerEngineer
Hmmmm... Let me add a few clarifications:

Painkiller: As I said above, AC transmission does have higher losses than DC but it is not a direct result of reactance. Current passing passing through reactive impedance does not generate real power losses; "scratching out" the reactance does not by itself reduce real power losses. Instead, it's the phase angle between AC voltage and AC current (and there almost always is one) that lowers the amount of power delivered at tthat amount of current. The higher the phase angle, the higher the current you need to deliver the power, and the more losses generated through the resistance in the line.

RMS stands for "root mean square" which means that you take your sinusiodal shape, square the values through the complete cycle, that the mean (average) of its value over the complete cycle, and then take the square root of that. The RMS value for a sinusoidal wave is its peak value divided by the sqaure root of two. RMS is handy because the RMS values of voltage and current can be mutilplied together (with due regard for phase angle) to give you the average power delivered over the complete cycle -- just like DC.

:beer:

:thumbsup: Agreed, maybe it was best to describe it in terms of real power efficiency. More kVA to deliver the same amount of kW on AC than DC.

 

[imported_Soldier]

The drill motor would burn out rather quickly...to put it into computer terms, you'd be overclocking to high.

The charger would most likely fail also because you would be exceeding the input by a large margin.

 

[imported_Soldier]

Originally posted by: Painkiller

AC transmission has more losses than DC. This is because impedance is a combination of resistance and reactance. Since DC has is zero frequency we can scratch out the reactance (i.e. inductive reactance = 2*pi*f*L) leaving only resistance. AC is subjected to both resistive and reactive losses. AC is used primarily because it is easy to step up and down. Also DC and RMS AC are equal in power.

Painkiller, you make it sound like AC is inferior to DC due to reactance. This is not true at all, AC power is the only reason the world is the way it is today. DC could never replace AC in daily use. The resistive losses you mention for DC limit the practical uses for DC power to very short range, in most cases it is economicaly unfeasible to supply DC power more than half a mile or so. This is due to the fact that as load increases so does heat, the power lines would have to be massive to carry the load of a DC powered world. DC power is also much much more dangerous than AC power, by its very nature DC requires much higher current to perform work where AC can be delivered at higher voltages and then easily stepped up or down at need....as you mentioned.