'Add-a-Phase' Question

3 phase AC/DC machines are actually rare, the machines with AC capabilities are either single phase, or inverter based three phase.

For old transformer machines, a rule of thumb is that if it offers AC, it is single phase.

The answer to your question is that it is possible to do with a capacitor. A friend of mine did it at the age of 17.

i

************** I knew a electronics tech who was very smart. He said a cap delayed the changed the voltage 90 degrees, So two in series would give you a 180 degree change. I know a cap delays amps 90 degrees. For the life of me I don't understand why single phase is catorized as single phase, since either leg at the same time is 180 degrees different when taken to ground. thanks. . .

The only thing for sure is that, with a sinusoidal stimulus, the voltage lags the current by 90 degrees in a capacitor. In an inductor, the current lags the voltage by 90 degrees with a sine wave stimulus. You can't say anything more, if a capacitor and/or and inductor is involved, unless the whole circuit is described.

Two capacitors in series will never add more than 90 degrees of phase shift. The only thing that happens is that the reactance of the series string is higher than either one of the caps by itself. The total phase shift, again, depends on the rest of the circuit. You can hook up more than one RC network and get more than 90 degrees of shift, or you can have combinations of capacitors and inductors and get more than 90 degrees of shift, but any given cap or inductor (or series/parallel combinations) won't delay things any more than 90 degrees.

The only way to guarantee 180 degrees of phase shift, without active electronics, is to use a transformer and hook it up appropriately.

You're right about being confused about what is coming into to most houses. It really is two phases (one at 0degrees and the other at 180degrees - both with respect to the neutral), but it's been called single phase for so long that the name stuck. Three phase is really three phases. Oh well.

Bob

******************* Thanks for the insight. I even talked one time to a 'hot shot' elect eng grad and he said the 240 V. in the USA was single phase. . . . . A ham radio friend of mine hooked his 240 up to his dual trace oscilloscope and there were the two distinct sine waves, 180 degrees apart. You are the first to admit that our 240 v (120 to neutral) is 2 phase. . . . .With out searching for the plans I saved somewhere, to build an 'Add-a-phase', I don't remember any devices being in the circuitry besides condensors, relays, and another motor.

As I understand it, a more correct term for the US system is "split phase." The two 120 halves that make up the 240 are a (deliberate) artifact from simply center tapping the output of the step down from whatever single phase voltage the distribution system runs on. Purists will say that you can't create a phase by changing your reference point (from one end of the 240v sine wave to the middle) but that a proper 2-phase system actually has those phases 90 degrees apart, not 180, and can be used to run appropriately wound motors without needing start capacitors. Hmmm...apparently wiki has a page:

Clear as mud? --Glenn Lyford

Another newsgroup discussion revealed the delicate distinction between a 'phase delayed' waveform and an 'inverted' waveform.

Take a sawtooth for example.

Delay it by 180 degrees and add the delayed version to the original. You get a sawtooth of double voltage.

OTOH, that same sawtooth *inverted* and added to the original gives you a sawtooth of zero voltage.

--Winston

Pintlar, my suggestion to you would be to delay any electrical work, until you get some more knowledge of electricity.

i

[snip]

If you take any waveform and add it to its inverse then you get zero at all points of the waveform.

If you take any waveform and subtract it from its inverse then you get twice the value at all points of the waveform.

When you hook a voltmeter from one hot phase to the other hot phase then you are measuring the difference (subtraction) of the two waveforms. That's why you get 2*120V = 240V for normal house power. It don't matter if it's a sinusoid, a sawtooth, a squarewave, or a whatever wave - you always get 2x the volts.

Either way, be VERY careful around these voltages. It only takes a few milliamps through your heart to stop it forever.

Bob

My point was that when we phase shift any waveform by 180 degrees and add the shifted and non-shifted waveforms together, we are not always guaranteed a zero voltage output.

If there is a zero voltage output, the waveforms *could* be phase - shifted in relation to each other, or they might more accurately be described as 'inversions' of each other. We can get an easier visceral understanding if we improve the language we use.

If there isn't a zero voltage output, there almost certainly is some amount of phase shift, with or without an inversion.

In those instances it is more accurate to say that the waveforms in your example are 'inverted' rather than 'phase shifted' in relation to each other, yes?

My main point and conclusion: Calling 'split-phase A.C.' 'two phase' is not accurate or useful as a description.

Certainly.

--Winston

Winston wrote: ...

Bad example, 'cause it's not symmetrical about zero. One cycle is from

0 to max/0.

No, 180 degrees is 1/2 way up the slope. Adding that to the original gives a signal of twice the frequency, same peak-to-peak, but offset (DC component) of 1/2 peak.

Graph it, you'll see.

...

Bob

Blush.

You are right of course. Still, it is clearer to consider L1 and L2 to be inversions of each other rather than strictly 180 phase - displaced.

--Winston

I hope I'm not missing anything here or getting it mixed up!

I think the answer to "why bother" is that 55V is the electrical shock you'd get in any mishap with this "centre-tapped 110V" supply (two side leaking into say water would short-out). You the human are "Earth". So the "hottest" voltage with respect to Earth is of interest.

Maybe that I have never been shocked by one of these UK construction-site supplies (or never known of it!) is the point...

Using the "construction standard 110V supply" I've worked in water and rain, in all conditions. Cables have been cut, crushed and everything. Nothing - never known a single tingle.

OTOH in the rare conditions on "domestics" (putting up balconies and the like) where 240V tools have been used (and that's 240V "hot" to Ground), you are getting "bitten" by anything and everything when there's as much as a drizzle coming down. You are having to tap everything with the back of your hand to see if it is "live" - and not just metallics. Electricity seems to track through water films. Scary.

The open-circuit voltage (OCV) of many welding sets is around 55V - and you'd know what that feels like if on a wet day you are holding a piece of metal in place and someone inexperienced tries to first strike on the "loose" component you are holding. It tingles for sure and causes involuntary muscle contraction it is hard to willfully over-ride in this worst of circumstances where you are full-body part of an electrical circuit. Not recommended, but a familiar experience.

In the UK - 240V is totally forbidden on construction sites. The site operator usually abundantly provides multi-outlet "site-110V" transformers around the site. So no excuse and no access to any other form of power. You arrive with your extension leads and tools. Smaller jobs - you bring you transformer, which plugs into the 240V mains.

Other point:

That any single-phase induction motor works is because you can imagine a single phase as being two 180deg-apart voltage waves of half the peak voltage. Problem is the two-pole motor isn't self-starting and its torque is uneven during each revolution when running.

Hope I've got all this right!

Rich Smith

...

I think that you are missing what I said. Consider this: a 2:1 transformer that takes 240v in and outputs 120v. Not center tapped. And the secondary is not tied to ground in any way. Now what is the voltage to ground from the output? It is ZERO, not 120. The secondary is completely isolated from ground, and from everything else, for that matter.

If you grab one side of this secondary with one hand and ground with the other, what happens? Nothing. For something to happen there has to be a path back to the other side of the secondary, to complete the circuit. If the other side was grounded, you could complete a circuit, but it isn't.

I think the problem is that people assume that there is always a ground side of a power supply. That "ground" is some fundamental, inherent, aspect of power. But it's not. Power sources that have a ground side, have one ONLY because that side is TIED to ground, somewhere.

Another problem is of semantics - one side of a supply is often called "ground" even though is has nothing to do with being tied to earth. It ought to be called the "reference" side, or something. For example, one side of a car's battery is tied to the frame and considered the "ground" side. Even though it is completed isolated from earth-ground by the rubber tires.

Bob

Yep, the word "ground" is really misused. "Circuit common" is more appropriate, in most cases. Ground is only ground if it's ground.

Be aware - hopefully without sounding too pedantic - that a transformer does not offer true galvanic isolation from the primary side. There's always primary-to-secondary capacitance to deal with. That is, if you connect one lead of a sensitive current meter to one wire of the "isolated" secondary and its other lead to earth, you will measure some small current. However, at 60Hz, the resulting current could only be in the microamps back to the transformer's earth-referenced primary side.

Bob