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  #1  
Old 02-11-2014, 11:56 AM
Hypno-Toad Hypno-Toad is offline
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Why have three phase power?

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.

Last edited by Hypno-Toad; 02-11-2014 at 11:58 AM..
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  #2  
Old 02-11-2014, 11:59 AM
Joey P Joey P is offline
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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.
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Old 02-11-2014, 12:01 PM
zoid zoid is offline
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Huh, I never thought about it.
Good topic.
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  #4  
Old 02-11-2014, 12:05 PM
Learjeff Learjeff is offline
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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.

Last edited by Learjeff; 02-11-2014 at 12:07 PM..
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  #5  
Old 02-11-2014, 12:20 PM
engineer_comp_geek engineer_comp_geek is offline
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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.
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Old 02-11-2014, 12:28 PM
Hypno-Toad Hypno-Toad is offline
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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.
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  #7  
Old 02-11-2014, 12:29 PM
casdave casdave is offline
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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.
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Old 02-11-2014, 12:39 PM
engineer_comp_geek engineer_comp_geek is offline
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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.

Last edited by engineer_comp_geek; 02-11-2014 at 12:40 PM..
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  #9  
Old 02-11-2014, 12:40 PM
Hypno-Toad Hypno-Toad is offline
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So with 3-phase power, one is tripling the amount of current generated per turn of the rotor?

ETA: thanks E_C_G!
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  #10  
Old 02-11-2014, 12:41 PM
Chefguy Chefguy is offline
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So we can lord it over those puny single-phase countries. USA! USA!
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  #11  
Old 02-11-2014, 12:49 PM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by Hypno-Toad View Post
So with 3-phase power, one is tripling the amount of current generated per turn of the rotor?
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.
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Old 02-11-2014, 12:58 PM
friedo friedo is offline
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Quote:
Originally Posted by engineer_comp_geek View Post
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.
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.
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  #13  
Old 02-11-2014, 01:08 PM
Ranger Jeff Ranger Jeff is offline
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I worked as the maintenance electrician at an industrial plant for a while. What Joey P said.
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  #14  
Old 02-11-2014, 01:10 PM
Learjeff Learjeff is offline
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Well, the OP was more correct than I thought:

Quote:
Originally Posted by wiki
Line frequency flicker in light can be reduced by evenly spreading three phases across line frequency operated light sources so that illuminated area is provided light from all three phases.
This doesn't apply to household lighting, but would apply to commercial/industrial settings.
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  #15  
Old 02-11-2014, 01:23 PM
Learjeff Learjeff is offline
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Originally Posted by engineer_comp_geek View Post
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.
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.)

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

Last edited by Learjeff; 02-11-2014 at 01:24 PM..
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Old 02-11-2014, 01:59 PM
Machine Elf Machine Elf is offline
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Originally Posted by engineer_comp_geek View Post
This makes for a smoother running motor (less vibration), which gets important when you are talking about very big motors.
In fact the field vector in a three-phase synchronous motor is (theoretically) of constant strength and rotates perfectly smoothly.

Quote:
Originally Posted by hypno-toad
is this why we have 120-volt AC current in the US? Because it derives from 120 degree offset of a three phase current?
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.
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Old 02-11-2014, 02:31 PM
naita naita is online now
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So we can lord it over those puny single-phase countries. USA! USA!
Which would these single-phase countries be?
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  #18  
Old 02-11-2014, 02:45 PM
Hypno-Toad Hypno-Toad is offline
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Sylvania?
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  #19  
Old 02-11-2014, 02:48 PM
Machine Elf Machine Elf is offline
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Which would these single-phase countries be?
Whichever ones aren't on this list (I'm guessing that's zero).
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  #20  
Old 02-11-2014, 04:45 PM
Chronos Chronos is offline
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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.
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Old 02-11-2014, 04:51 PM
Napier Napier is offline
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casdave and MachineElf kind of got to this, a bit, but I think we're all missing the most important thing.

Three phase power ROTATES! The power itself is pumping in a cyclic manner on those three wires. To run an induction motor, you have to have a magnetic field that rotates. If you have three phase power, you can get that field with three electromagnets at 120 degree angles to one another (or six at 60 degree angles, et cetera). Three phase motors have coils in them, and nothing else.

With single phase power, you still need the field to rotate, so there are various cheesy ways of making it. You can run one of two sets of coils through a capacitor. You can do something funny with the windings to make part of the magnetic field lag behind the rest. But these tricks require centrifugal switches or other troublesome parts, and they're not that efficient.

Notice that the way we run power, we use two hots and a neutral. We are running three wires anyway. Seems kind of a shame we didn't spend those three on three phases. I think three phase power is cool, and wish I had it in my house. Of course, I think a lot of off-kilter stuff....
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Old 02-11-2014, 05:07 PM
Chronos Chronos is offline
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You can also make a simple, no-switches AC motor with a single phase of power, but it'll rotate backwards as readily as it does forwards. This is acceptable in a few applications (such as the turntable in a microwave), but not in most.
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Old 02-11-2014, 05:17 PM
Chefguy Chefguy is offline
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Which would these single-phase countries be?
It's a joke, you see, crafted to give you a smile.
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  #24  
Old 02-11-2014, 06:31 PM
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We get 3 phase to the end of the street, where there is a small transformer. It used to be quite big, but is now a small box in a fenced off space with "DANGER OF DEATH" SIGNS ON IT.

From that point, the phases are split and the cable under the street carries 240 volts, single phase to all the houses.

Way back in the 60s I used to deliver catering machinery. In Sheffield, a large industrial town, there were still shops that ran on DC. This was because it came from the nearby steel works which was yet to modernise. We exported to countries all over the world with all kinds of voltages. In Australia we had motors specially wound for strange voltages like 160 volts or 339/3 phase, because the place was so far away from the generator that the voltage dropped in the line.

Last edited by bob++; 02-11-2014 at 06:34 PM..
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Old 02-11-2014, 06:42 PM
HardlySanguine HardlySanguine is offline
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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.
Meh. That's what giant caps are for.
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Old 02-11-2014, 06:53 PM
appleciders appleciders is offline
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Originally Posted by engineer_comp_geek View Post
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.
Lots of places in the world still use 50 Hz power. I work in entertainment lighting and traveling shows have to be aware of that because some systems use the 60 Hz frequency to establish time. If you don't convert over, you might find your moving lights moving at 5/6 speed (or 6/5, I suppose). Japan is half-and-half; one side of the country uses 50 Hz, one side 60 Hz.
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  #27  
Old 02-11-2014, 07:05 PM
Crafter_Man Crafter_Man is online now
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Here's the deal:

If power is delivered using a single sine wave at 60 Hz – like the 120 VAC at your wall receptacle – then the voltage is at exactly 0 V every 8.3 milliseconds. That's a bummer, because 0 V = 0 power. So every 8.3 milliseconds, the power delivered by the receptacle is exactly zero. And this obviously means the power is very very low when the voltage is near zero.

This sucks all around, because you expect anything you plug into the outlet to deliver constant, uninterrupted power. So how is an appliance supposed to do this if the damn power delivered by the outlet is at or close to zero much of the time?

The most common solution is to incorporate devices into the appliance that store energy. These devices will deliver energy when the power delivered by the receptacle is at zero or close to zero. The most common devices are capacitors and inductors, and they "take up the slack" so to speak. Other appliances use thermal heat capacity (e.g. electric heater) or momentum (e.g. motor) as energy storage devices.

For some appliances, the situation sucks supremely when you initially try to start the appliance. Such as a motor. It's really difficult to start a big honkin motor when the power delivered by the receptacle is at zero or close to zero much of the time. So motor designers are forced to employ clever tricks in order to get the motor spinning from a standstill.

The solution to this problem, of course, is to use a voltage at the receptacle that has lots of power available 100% of the time.

The most obvious solution is to use DC. But DC has a lot of drawbacks. Large DC motors really suck, since they must employ a commutator. And it's difficult to bump up/down DC, which leads to problems when you're trying to deliver power over long distances.

So that's where three phase power comes in. With three phase power, there will always be a healthy voltage (in one of the three wires) that can deliver lots of power, right now. And when the power on that wire starts to decrease, you can switch over to another wire where the power is increasing. And so on and so on. In other words, when an appliance runs on three phase power, there will always be lots of good power available to the appliance, 100% of the time, at every nanosecond. This is especially advantageous with motors, since the designer doesn't have to pull any tricks to get it started. Since power is always available, it's a piece of cake to start.

Last edited by Crafter_Man; 02-11-2014 at 07:08 PM..
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Old 02-11-2014, 08:23 PM
Melbourne Melbourne is online now
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Originally Posted by engineer_comp_geek View Post

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.
With you till you got to that: are you saying that in America dryers and ovens are installed with the same heating elements in 208V installations and 240V installations?

Heating elements are normally replaceable parts, and in AUS we get appliances from all over the world: you just specify the heating element and the mains plug to make them work at local voltages.

Same question for 240V dryer motors used in 208V dryers: I would have expected the same model to be available in 208/240V versions, with different motor windings?
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  #29  
Old 02-11-2014, 08:43 PM
JWT Kottekoe JWT Kottekoe is offline
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Nikola Tesla's biggest contributions to electrical engineering were his brilliant independent invention of the rotating magnetic field, his use of that concept to invent efficient polyphase motors and generators, and his work on transformers and AC power distribution systems. He got rich off these inventions, but went bankrupt in later years pursuing the ideas that have made him so popular recently with conspiracy theorists and pseudo scientists.
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Old 02-11-2014, 11:22 PM
Mdcastle Mdcastle is offline
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You can order commercial equipment with elements for either 208 or 240, but outside of apartment buildings 208 is uncommon for residential use so electric ranges and clothes dryers and the like are all rated for 240 only. It will take longer to heat up, but usually you have a thermostat turning it on and off in a duty cycle, so a 240 unit run at 208 will just have the heating element on longer.
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Old 02-11-2014, 11:57 PM
Magiver Magiver is offline
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Originally Posted by engineer_comp_geek View Post

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.
how is the 208 created if not from a split in the transformer?
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Old 02-12-2014, 12:15 AM
realmarine realmarine is offline
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Engr. comp GK , Craftsrman & many others...

You've all done amazing explanations of a complex que.You guys are all brlilliant electricians[ Though, much more than that title explains].You are setting yourselves up for ques. like why nuetral can be shared on two differant circuits & thousands of other questions. But, a great and learned thread ! Thanks , realmarine
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Old 02-12-2014, 01:33 AM
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how is the 208 created if not from a split in the transformer?
208 is created by vector-addition of two out of the three 120-volt signals which are 120-degrees out of phase. Here's a picture. On the right-hand side of the diagram, the red, black, and blue dots are 120-degrees apart. Imagine each of them is the output of an independent step-down transformer which converts a high-voltage signal from the generator to a 120v signal. Remember, the generator outputs three signals that are 120-degrees apart, so these three transformers will give you three 120v signals 120-degrees apart.

Measure from any phase to the center and you get a distance of 120. Apply a little Pythagoras and you'll find the distance between any two adjacent hot wires is 208.

The moral of the story is that complex numbers are just a way of screwing around with right triangles. Hooray!
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Old 02-12-2014, 03:51 AM
engineer_comp_geek engineer_comp_geek is offline
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With you till you got to that: are you saying that in America dryers and ovens are installed with the same heating elements in 208V installations and 240V installations?
208V in the U.S. is pretty rare, at least for residential use. Parts of New York City still have it and there are a few other areas using it scattered across the U.S., and larger apartment buildings are usually fed by 3 phase power, but most homes have split phase 120/240V. You can usually buy 208V heating elements from the manufacturer but an off the shelf oven or dryer is going to be designed for 240V.

Commercial ovens (like for restaurants) on the other hand are often specified for 208V or 240V.

Since 208 is also common in office buildings, large office photocopiers and printers are ordered with the voltage specified as well. With some of them you just connect the electrical cord to different terminals inside the unit. Others actually have a different power transformer installed.
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Old 02-12-2014, 04:06 AM
engineer_comp_geek engineer_comp_geek is offline
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Originally Posted by Magiver View Post
how is the 208 created if not from a split in the transformer?
There are two types of 3 phase transformers, Y and Delta. In a Y transformer the 3 coils are arranged in the shape of the letter Y, and the center of the Y becomes your "neutral" connection. In a delta transformer, the coils are arranged in a triangle and there is no center neutral connection. The primary side of the transformer and the secondary side don't have the be connected the same way, so you can have Y-Y, Y-Delta, Delta-Y and Delta Delta transformers. The important thing out of this is that if you want a neutral, you need a Y transformer.

This picture should help if you are having trouble picturing what a Y and a Delta transformer looks like:

http://www.itacanet.org/wp-content/u...ar-600x207.jpg

In a 3 phase Y transformer, the voltage from any line to neutral is 120 volts, and the voltage from any line to any other line is 208 volts.

Y transformers are also called wye transformers or star transformers. Googling these three terms will give you lots of reading material.
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Old 02-12-2014, 05:15 AM
t-bonham@scc.net t-bonham@scc.net is offline
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Originally Posted by Mdcastle View Post
You can order commercial equipment with elements for either 208 or 240, but outside of apartment buildings 208 is uncommon for residential use so electric ranges and clothes dryers and the like are all rated for 240 only. It will take longer to heat up, but usually you have a thermostat turning it on and off in a duty cycle, so a 240 unit run at 208 will just have the heating element on longer.
That's why some apartment laundry room dryers are slow to dry. S cheapskate landlord got a deal on 240V dryers, and installed them rather than getting 208V ones at a higher price.

+
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Old 02-12-2014, 07:00 AM
Mdcastle Mdcastle is offline
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Originally Posted by Machine Elf View Post
Whichever ones aren't on this list (I'm guessing that's zero).
Liberia probably would have been for a while. The entire electrical infrastructure got looted and destroyed so the only electricity was from people with generators. They're now rebuilding it to European specs.
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  #38  
Old 02-12-2014, 09:25 AM
Cheshire Human Cheshire Human is offline
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Originally Posted by friedo View Post
The moral of the story is that complex numbers are just a way of screwing around with right triangles. Hooray!
HOLY SHIT! You're right! I never thought of that! Mind blown!

:head asplode smilie:

Last edited by Cheshire Human; 02-12-2014 at 09:25 AM..
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  #39  
Old 02-12-2014, 06:52 PM
Mr. Goob Mr. Goob is offline
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I run a machine shop and took it as a matter of faith that most of my lathes, mills and punches have 3 phase because it delivers more power for the bigger motors. I'm a machinist and only think about electricity when the magic smoke is leaking out. I've sort of wondered how it worked but never took the time to find out. Now I understand and thank you all.

So real world application, Two of the lathes are a similar size, one is 3 phase, the other is 2 phase. Now I understand why the 2 phase one takes longer to get up to speed from a dead stop.

As an aside question into electric motors, I've had to buy replacements over the years. For my big 8 foot engine lathe ( not relevant but a cool fact that it runs great and was made in 1906 ) I needed a reversable motor so it can run it in forward and reverse. Why did that motor cost so much more than an otherwise 3hp motor that only goes one way?
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Old 02-12-2014, 09:06 PM
Napier Napier is offline
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Originally Posted by Mr. Goob View Post
As an aside question into electric motors, I've had to buy replacements over the years. For my big 8 foot engine lathe ( not relevant but a cool fact that it runs great and was made in 1906 ) I needed a reversable motor so it can run it in forward and reverse. Why did that motor cost so much more than an otherwise 3hp motor that only goes one way?
Was this a three phase motor? Because, as far as I know, a three phase induction motor is ALWAYS reversible. Swap any two of the three leads, and it runs the other way.

If it's a single phase motor, in that horsepower range, it would have a starting capacitor and possibly a run capacitor, and a centrifugal switch to control the starting capacitor. You'd have to have a means to switch which of the windings the capacitors worked with. Also, single phase induction motors would have to be bigger to have the same starting torque, if starting torque was the thing necessarily constrained to be some given value.
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Old 02-12-2014, 09:36 PM
CurtC CurtC is offline
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Originally Posted by Learjeff View Post
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.)
An incandescent light will flicker at 120 Hz, because it gets bright when the current is pushing in one direction, then again when the current pushes the other direction, so twice for every cycle of the power line. That's why you don't see light bulbs flicker, even though you can see it with a 60 Hz monitor (I can too, especially in my peripheral vision).
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Old 02-12-2014, 09:46 PM
Chronos Chronos is offline
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An incandescent light won't actually flicker all that much, though, at any frequency, because it takes more than 1/120 of a second to cool down.
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Old 02-13-2014, 06:44 PM
Mr. Goob Mr. Goob is offline
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Napier, yes it's a 3 phase, but if I read you right you said switching the leads. This is the only machine in my shop that has a switch where I can flip back and forth forward or reverse. Every other machine is on/off and it spins the way it was set up.

Is it all in the switch or is the motor different somehow? Practically speaking is the motor different and did the electrical shop take me for a noob and charge me more?
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  #44  
Old 02-13-2014, 06:47 PM
friedo friedo is offline
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I'm sure the motor is the same as any other three-phase motor and the switch just swaps two of the three power leads. You should be able to accomplish the same thing by swapping two (any two) of the leads on your other machines.
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Old 02-13-2014, 09:08 PM
Snnipe 70E Snnipe 70E is offline
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Originally Posted by Mr. Goob View Post
Napier, yes it's a 3 phase, but if I read you right you said switching the leads. This is the only machine in my shop that has a switch where I can flip back and forth forward or reverse. Every other machine is on/off and it spins the way it was set up.

Is it all in the switch or is the motor different somehow? Practically speaking is the motor different and did the electrical shop take me for a noob and charge me more?
The switch probably just switching two of the legs.
But the motor may be different. If the motor has thrust bearings being reversible will require double acting thrust bearings. The motor may be more heavy duty incase the operator switches direction to the other without stopping the motor first.
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Old 02-14-2014, 12:08 AM
48Willys 48Willys is offline
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Originally Posted by Snnipe 70E View Post
The switch probably just switching two of the legs.
But the motor may be different. If the motor has thrust bearings being reversible will require double acting thrust bearings. The motor may be more heavy duty incase the operator switches direction to the other without stopping the motor first.
Both good points.

The cooling fan is often directional. So a reversible motor would have to contend with that issue also. Instead of curved or angled vanes for the cooling fan it needs straight ones. Straight vane cooling fans do not move as much air as curved or angled ones do. That is why they are curved or angled. Thus, the motor has to be able to dissipate more heat then a non reversible motor.

Any three phase motor can be a reversing motor. Unless they are designed for reversing, their longevity will be compromised. In its "forward" direction, it will cool just fine. In its "reverse" direction the cooling air flows slower and backwards over the motor. this will cause the motor to get very hot fairly fast. If you are using the motor intermittently, you MAY get by with just any three phase motor. In a machine shop, buy the correct motor. Do not trip over a dollar to save a dime.

IHTH, 48.
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  #47  
Old 02-14-2014, 02:13 PM
Napier Napier is offline
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Originally Posted by 48Willys View Post
Both good points.

The cooling fan is often directional. So a reversible motor would have to contend with that issue also. Instead of curved or angled vanes for the cooling fan it needs straight ones. Straight vane cooling fans do not move as much air as curved or angled ones do. That is why they are curved or angled. Thus, the motor has to be able to dissipate more heat then a non reversible motor.

Any three phase motor can be a reversing motor. Unless they are designed for reversing, their longevity will be compromised. In its "forward" direction, it will cool just fine. In its "reverse" direction the cooling air flows slower and backwards over the motor. this will cause the motor to get very hot fairly fast. If you are using the motor intermittently, you MAY get by with just any three phase motor. In a machine shop, buy the correct motor. Do not trip over a dollar to save a dime.

IHTH, 48.
OK, I've never heard of a directional fan built into a motor. They'd have to sell twice as many motor models if they did this, because whether you consider the motor application one that requires reversing per se, there are plenty of motor applications that require CW rotation and plenty that require CCW.

Blowers in general, yes, unless they're fairly high pressure ones, have some kind of handedness to the impeller blades. The whole job of the blower is moving air. But every motor with cooling I've seen has a radial blade impeller on the exterior mounted to the shaft (if it's a TEFC or Totally Enclosed Fan Cooled) or it has radial fins molded into the ends of the rotor or a radial blade impeller pressed onto the shaft inside (if it has some kind of open frame). The impeller is a kind of an afterthought not designed for high efficiency -- and I bet it consumes much less than 1% of the motor power.

Where could I see an example of a motor whose integral cooling impeller is not symmetric with respect to direction of rotation?
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  #48  
Old 02-14-2014, 07:44 PM
Mr. Goob Mr. Goob is offline
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I looked inside the motor in question and it does indeed have straight fins. So I got curious and looked in 6 other motors I can easily get to and they all have straight fins.
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  #49  
Old 05-14-2014, 12:36 AM
AaronX AaronX is offline
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I thought of something regarding 3-phase power. For normal 1-phase you can calculate the power used by multiplying voltage by current (ignoring power factor and stuff). For 3-phase, do you have to multiply the voltage by current by 3?
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  #50  
Old 05-14-2014, 01:41 AM
Isilder Isilder is offline
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Originally Posted by AaronX View Post
I thought of something regarding 3-phase power. For normal 1-phase you can calculate the power used by multiplying voltage by current (ignoring power factor and stuff). For 3-phase, do you have to multiply the voltage by current by 3?

Well the design aim would be to have the same voltage and current on the three legs, but the end result may be that it varies a little.
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