Why have three phase power?

Factual Questions
1

My incomplete understanding of AC current is that it flickers because the back and forth reversal of current has a moment of “no-current” as it changes from one direction to the other. So adding more phases that are offset means that while one phase is at that “no current” moment, other phases are still pushing electrons. Right? so this keeps the flicker from being too bad.

But at the same time I remember that the rate of flicker is kept somewhere like 60 cycles per second which is so fast that a person can’t detect it.

So why have phases if the rate of flickering is not noticable? I ask this because after reading a book about modern infrastructure, I saw that our power lines downtown come in threes. This means that the current is being split so that each wire carries one of the phases and those wires can go individually to buildings. This works so long as they all come together again before heading back to the power station. But it makes wonder even more why we have the phases if a single phase line can do the job for a location.

2

Three phases are really only used in commercial and industrial applications. Big motors and resistance heaters need the extra power that 1 phase just can’t provide. It doesn’t have anything to do with you not not noticing the lights flickering.

3

Huh, I never thought about it.
Good topic.

4

You have it partly right but partly wrong.

The use of three phases does not reduce flicker. Any 110V circuit in your house uses only one of the three phases*, and would flicker at 60Hz if the light intensity varies fast enough. (I not only see 60 Hz flicker, I find it very distracting/annoying, so I always have to set monitor rates higher.)

Also, all three phases are delivered to every building; they don’t distribute the phases differently and hope it sorts out later.

  • I believe this is an oversimplification.

I’ll let someone with more knowledge provide a better answer to the whole “why three phases” issue. Meanwhile, you should read Wiki to better understand the answers.

5

Electricity flows in a circuit. You have a wire going out from the generator to the load and another wire coming back from the load to the generator. If you have three circuits, you have six wires, three out and three back.

In 3 phase systems, each phase is 120 degrees off from the other. Three sine waves 120 degrees apart add up to zero. If you took your three circuits and tied all of the return wires together, and the loads were perfectly balanced, there would be zero current running through the return wire. If there’s no current, then you really don’t need the wire and you can get rid of it completely.

Of course, in reality you’ll never have the currents perfectly balanced. So what you end up with is 3 big wires and one little return wire, which is still a pretty big savings over 6 big wires.

3 phase also has an advantage for large motors. In a single phase motor, the torque “pulses” along with the AC current. When the sine wave is zero, at that instant there is zero torque coming from the motor. In a 3 phase motor though, when one phase is at the zero point in the sine wave, the other 2 phases aren’t, so there’s always torque on the motor. This makes for a smoother running motor (less vibration), which gets important when you are talking about very big motors.

If you need more power on a single phase motor, you need to increase the size of the two wires and the insulation to handle the greater voltage and current. For the same size wire, a 3 phase motor will give you significantly more power at a cost of only one extra wire. It’s basically 3 times the power with 1.5 times as much wire.

There are two types of residential electrical service in the U.S. By far the most common is that once the three phase reaches the distribution level, it is split out into three separate single phases. Transformers on the distribution line then take this single phase down to the power that you get into your house, so called “split phase” because they split the transformer with a center tap. The center tap becomes your neutral, and the two lines from either end of the transformer winding become your two “hot” wires. Line to line voltage is 240 volts, and line to neutral (from either “hot” to the neutral center tap) is 120 volts.

Much less common, but still in use in some areas, you get two phases out of the three. This still gives you 120 volts between either “hot” wire and neutral, but only gives you 208 volts from line to line. Since 240/208 is typically only used by things like electric ovens and electric clothes dryers, the only real side effect of the lower 208 voltage is that it takes longer for your oven to heat up and longer for your clothes to dry.

6

The part about electric motors is very interesting. Mostly because it’s the part I understand, especially after seeing the GIF on the wiki link. This makes me ask: is this why we have 120-volt AC current in the US? Because it derives from 120 degree offset of a three phase current?

ETA, it’s interesting to yet another place where the concept of a 360 degree circle has influenced terminology.

7

What you are missing is the requirements of induction motors, as well as the best way to generate a.c.

Each phase rises and falls, but not together, they are separated in time. If you plot this on a rotating graph, you will see there is a difference of 120degrees between the timing of each phase.

Next, if you feed each phase separately into its own coil, and arrange those coil as if they were on a clock face, one coil would be at 12, the next at 4, the third at 8. What will happen is that they will combine their individual magnetic fields into a single field, which would tend to point in one direction or another, but as time passes, this would rotate.

Its this rotating magnetic field that is essential for an induction motor to operate. You can operate electric motors on single phase, but only up to a certain power output, after which it becomes difficult and uneconomic.

Generation is simply the reverse of driving a motor, you rotate coil of wire within a magnetic field and the coil produce current.
Instead of having just one coil on one rotating generator, its generally more efficient to have 3 sets of coils, that way you can have 3 lots of current coming out of one machine.

8

The 120 volt standard has nothing to do with the 120 degree separation of phases.

Exactly how we ended up at 120 volts and 60 Hz is a bit of a mystery. Early electrical systems were all over the place, with all different voltages and frequencies. There was even Edison’s famous battle with Westinghouse over AC vs. DC. Eventually, systems in the 100 to 250 volt range proved to be the best tradeoff between lower wire cost and difficulty of insulation, and AC won out over DC. Since the most cost effective and reliable early systems were 120 (or 110) volts and 60 Hz, once those systems became established everything that came after them followed the same standard.

Exactly why those first AC systems ran 120 volts and 60 Hz isn’t exactly clear though. One story I heard from a very old power engineer back in the early days of my career was that one of the early demonstration systems was designed for the nice even numbers of 100 volts and 50 Hz. But then it wasn’t quite producing enough power, so they cranked up the generator a bit and ended up with 120 volts and 60 Hz. That satisfied the test, and the system was delivered using the higher voltage and current, and everything followed using the same thing after that.

I’ve never found a good cite to back this story up though.

ETA: I don’t know if it’s still in operation, but I know that at least as recently as a few years ago there was still a 25 Hz system running at Niagra Falls that had its origins in the old days before the 60 Hz standard.

9

So with 3-phase power, one is tripling the amount of current generated per turn of the rotor?

ETA: thanks E_C_G!

10

So we can lord it over those puny single-phase countries. USA! USA!

11

Yep. A 3 phase generator has 3 sets of coils in it (again, spaced 120 degrees apart) so it generates 3 times the power. Of course it also takes 3 times as much mechanical power to make it spin.

12

Not Niagara, but there is an extensive 25Hz, 138kV, single-phase system built by the Penn Central for railroad electrification on the Northeast Corridor (now inherited by Amtrak), and a similar 25Hz system used by the New York & New Haven Railroad (now inherited by Metro-North as the New Haven line.)

The power is stepped down to around 11 or 12kV for the train caternary wires.

13

I worked as the maintenance electrician at an industrial plant for a while. What Joey P said.

14

Well, the OP was more correct than I thought:

This doesn’t apply to household lighting, but would apply to commercial/industrial settings.

15

Cool! Then, if I balance my loads properly, I don’t even need a generator!

(What ECG really means here is that you need only a tiny amount of wire, vanishingly small. But you do still need some wire, unless your power distribution system uses the psychic network.)

Thanks! Ignorance fought. I remember now that I once knew this.

16

In fact the field vector in a three-phase synchronous motor is (theoretically) of constant strength and rotates perfectly smoothly.

ECG has covered the why of 120 volts and 60 Hz. I’ll expand on that by noting that the alternator at the power plant does not produce 120 volts; its output is much higher, in the thousands of volts. Note also that some countries have electrical systems operating at 50 Hz. Conversely, commercial aircraft have systems that operate at 400 Hz, enabling compact, high-efficiency (light-weight) alternators.

We have AC mains electricity instead of DC because it’s more versatile and efficient (although this wasn’t necessarily true in the early days). An alternator can generate AC power without any sliding contacts that cause electrical resistance or mechanical wear; a direct DC electrical generator incorporates a commutator that introduces electrical resistance and mechanical wear that necessitates regular maintenance (note that it’s also possible to generate AC powe with an alternator and then just run it through a rectifier to create DC power; this is what happens under the hood of your car). At the power plant, you can easily run AC power through a transformer to convert it to high voltage/low current for efficient cross-country transmission, and at the consumer end, you can run it through a couple more transformers to step the voltage down to a usable level. If you’re in a house, you have access to 120/240 volts; in an industrial setting, you may also have access to 480 volts or more, depending on what sort of equipment you’re running.

Just as AC power can be produced by an alternator with no sliding electrical contacts, an AC motor can operate with no sliding contacts. A DC motor requires a commutator that decreases efficiency, increases maintenance requirements, and (at very high currents) can produce plasma and ozone that cause localized damage.

AC power has some interesting inefficiencies associated with cross-country transmission, owing in part to capacitive coupling with the earth itself. Modern technology is beginning to make it cost effective to convert AC power at the plant to high-voltage DC for cross-country transmission, and then convert back to AC for consumption.

17

Which would these single-phase countries be?

18

Sylvania?

19

Whichever ones aren’t on this list (I’m guessing that’s zero).

20

One other advantage of three-phase power is in those rectifiers. The simplest sort of rectifier is a single diode, which sets the waveform to zero anywhere it would be negative, but that throws away half of your power. Better but still relatively simple is a four-diode rectifier, which effectively takes the absolute value of your waveform. That’s always positive and never throws anything away, but it’s still very bumpy. But if you take three different rectified waveforms at different phases and add them together, you get something that’s significantly less bumpy.