# Need help understanding single and three phase power.



## Rbeckett (Feb 5, 2012)

Could one of the smarter gents on this subforum please write a down to earth readable definition of the difference between single and three phase power.  A discussion of the voltages, leg arraingements and plusses would be great.  I know there is a difference and that it must be wired by a pro, but I would love to understand exactly what it is and how it is made.  As always electrical wiring should be left to a pro unless you are well versed in the systems you intend to work on.  Thanks in advance for any thoughts and your inputs


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## British Steel (Feb 5, 2012)

Hi RB,

I'll give it a go - I guess I'm qualified, as I do a lot of work with 3-phase machines...
 I'll try to start at the basics, and use simple analogies, apologies if I condescend!

OK, first of all single-phase:

The single-phase voltage swings about the Neutral potential (usually tied to earth at the electricity co's transformer), describing a "Sine wave". Why a Sine wave, not a square, a triangle? Imagine if you will the generator - a coil carrying a steady "exciter" current (the generator armature) spins inside another winding causing a voltage to be induced in the stationary winding - the voltage/current is proportional to the speed at which it cuts through the magnetic field, and as the field is unidirectional there will be times when the magnetic field is moving parallel to the winding (with no resulting current), times when it moves at 90* to the winding (when it produces maximum current) - if we illustrate this, we can see that if we pick a particular wire in the armature and track its height (the Sine of the angle, geometrically) as it spins it'll produce a smooth alternating wave, swinging above and below the zero point.

So, we have a nice smooth waveform, either 50Hz (most of the world) or 60Hz (USA, Canada and the Pacific rim).

Now to 3-phase...

Let's imagine there are now 3 sets of stationary windings in our generator, each set at 120* to the others - we can visualise these being swept by the magnetic field one after the other, and the waveforms trailing one another by 120*, and this is exactly what we see if we attach an oscilloscope or similar to the three phases.

If we look at the waveforms plotted out on a graph, they make a nice interlaced pattern - and interestingly we find that the areas under the waveforms referred to the zero line sum to a constant value! (this is one reason why 3-phase motors are smoother in operation, and 3-phase generators are a more even load than single-phase)
Note that we don't have to reference to the neutral (tied to ground) line any more, there's a voltage difference between the three phase terminals so we can connect our load between them and they'll deliver power, either by tying one end of each coil together (Star or Wye connection) or tying one end of each coil to the next coil around (delta connection).

So... we have 3 phase voltages chasing each other around and around a circle, and each coil generating (as an example) 240V AC. If we connect the phases in the star / wye configuration, we have two 240V sources 120* apart - draw this on a graph and measure the distance between the connection points, and we find it gives us 415V (root 3 times the 240V), if we connect in delta the 240V appears directly at the terminals - hence the 3-phase motors and generators can be swapped between the two voltages by swinging links 

If we look at the current in the three phases of a motor, they'll be applied to 3 coil windings at 120* to each other, and the magnetic field  will chase the maximum current in the coils, setting up a rotating magnetic field, which then drags the motor rotor around with it.

Now, if we swap two of the terminals over (whether star/wye or delta) we'll see the sequence of phases change - from ABCABCABC to ACBACBACB - one is the reverse of the other, so the rotating magnetic field reverses direction so our motor will reverse too!

Now I should start on motor theory - why the motor never runs exactly in step with the rotating field in normal induction motors.... But I think that's for another time!


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## pdentrem (Feb 5, 2012)

Good explanation, also believe or not the alternator in your car is also 3 phase, just lower voltage. That is why the diode pak has 3 diodes in it, which converts the AC voltage to DC.


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## racecar builder (Feb 5, 2012)

bob
those guys beat me to it!
i was going to say start out slow with:
110 & 220.
something about all houses have 3 wires going to them.

Have A Nice Day!


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## Hawkeye (Feb 5, 2012)

Okay, Race. Since you mentioned three wires... On this side of the pond, what we call single phase actually has two phases at 180* apart. This gives us our 110/220 (115/230, 120/240, depending on where you live). Connect from Neutral to one phase and you get the lower voltage. Connect phase-to-phase and you get the higher voltage.

When it comes to motors, 3-phase automatically has the rotating magnetic field. The motors are easier to build and cheaper than the equivalent single phase motor. If you remove the armature from a 3-phase motor and connect it to power, you can throw a ball bearing into it and it will race around inside the motor frame. Just don't do it for long as there won't be enough counter-EMF generated to keep the current down. The winding will heat up and eventually fail.

In a single phase motor, the rotating field has to be 'artificially' created. This is done by adding a second set of windings, sometimes with a capacitor added. That's the hump on the top of the motor. Once the motor is up to speed, the start winding is no longer needed. You'll hear a click and the winding will be disconnected.


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## racecar builder (Feb 6, 2012)

hawkeye
good stuff. let me get back to you.
bob
what are you doing to me?
i just brought this over to house. ( see pic )
Radio Shack Electronics Learning Lab.
i'm starting at the bottom.
hooking wires to little light bulb.
what did you have to bring this up for?

Have A Nice Day!


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## racecar builder (Feb 6, 2012)

bob
here's the sensible answer.
start here.
http://en.wikipedia.org/wiki/Single-phase_electric_power
link in second paragraph to 3 phase.
read both 3 times.like homework in school.
write down some questions.
that's how i'd do it.

Have A Nice Day!


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## Rbeckett (Feb 6, 2012)

racecar builder said:


> hawkeye
> good stuff. let me get back to you.
> bob
> what are you doing to me?
> ...



Because I too am deep in the learning curve for basic electronics.  I have been self studying the electronis field for about a year now and have learned quite a bit.  I am always on the look out for more knowledge.  So I try to get as much as I can from as many places as I can find.  I am currently learning to program microcontrollers in basic from a company called Picaxe, they are sort of simple and slow but will do nicely for the device I am building for my CNC Plasma table z axis.  Torchmate wants 3 grand and I just dont have that kind of cash anymore, so I have to look at alternatives and cheaper ways to do stuff.  There are a number of really good books by Forest Mims and others who make learning the basics and some advanced stuff pretty painless.  I also am watching a series on you tube  by a guy in india at the Madras school of technology.  A little hard to unnerstand but worth the time to learn quickly.  Have a great day and get busy studying...
Bob


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## arvidj (Feb 6, 2012)

Hawkeye said:


> Okay, Race. Since you mentioned three wires... On this side of the pond, what we call single phase actually has two phases at 180* apart. This gives us our 110/220 (115/230, 120/240, depending on where you live). Connect from Neutral to one phase and you get the lower voltage. Connect phase-to-phase and you get the higher voltage.
> 
> ...



What you are refering to is not actually a two phase system. It is a split-phase system. http://en.wikipedia.org/wiki/Split-phase_electric_power or as they mention, a *3-wire, single-phase, mid-point neutral*, which is more words than I care to remember.

Arvid


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## racecar builder (Feb 6, 2012)

arvid
i'm with you about more etc.
bob is switching directions on us.
i thought he wanted a concise answer so 
he could wire machine shop shed.
get in, get out.
now it turns out he is getting into electronics.
bob
i thought that was hawkeye's answer but it's yours.
i was going to say let's all go  over and 
experiment on bob's new shed!
i'm for hooking up all three wires to shed
and seeing what happens!

Have A Nice Day!


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## Alan Douglas (Feb 6, 2012)

I agree, it would be best to ask specific questions, as there is far too much to put into one discussion thread and cover everything.

The electrician's concept of "two phases" in a household supply is not the engineer's definition of phase, which causes confusion.  120/240V is single-phase.

Regarding motors, a single-phase stator (the stationary portion of the motor, as opposed to the rotor) also has a rotating field, just as a three-phase stator does.  That's why the rotor turns, trying to keep up with it.  The tricky part, is that it actually has two counter-rotating fields, and the rotor would run equally well in either direction, depending on how it's started.  The starting windings take care of that, by creating more pull in one direction than the other.  A three-phase motor is inherently self-starting.


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## racecar builder (Feb 6, 2012)

alan
any chance we can back up to DC motor and then take a run at this?
maybe 'winding' type of motor ( i saw about half dozen types of DC motor on wik )
plus leave electricians/engineer difference out for the time being?
i'm familiar with Kohler generator on side of motor.
windings bolted to block with little 'heads' on them and magnets in flywheeel.
i know it's GENERATOR not motor.

Have A Nice Day!


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## Alan Douglas (Feb 7, 2012)

Maybe start a new thread?  This one is going in various directions, and seems to be in two different places too.


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## racecar builder (Feb 7, 2012)

alan
like herding cats?
well i'll be busy dusting off my Electronics Lab and
looking at experiment left on it before i take it off
and start at Lesson #1.

Have A Nice Day!


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