Can "wattage" trip a GFCI?

[trader4]

Actually, both views are correct. Regardlesss of how the phase difference is generated, if you view the two legs of a 240V service on an oscilloscope the two voltages are in fact 180deg out of phase with each other. Note that the post that generated the discussion did not even call it a two phase system. It only said the two voltages in a 240V service are 180 deg out of phase with each other. That is correct. If they showed that picture on a EE test and asked "What is the phase difference between the two signals, what would you answer have been?"

[zek]

You can test a socket say, by plugging in another device. How about a heater less than 1500 watts.

You can also try measuring resistance to ground on either line input on the plug. It should be megohms or higher. Try also working the switch. Its possible to buy breakers with GFI's, but they are usually in the main household box.

[zek]

Heck, I bought a 500 watt single tripod lamp at my local hardware for $20. It has a two position switch for brightness. They get some nice deals. Also make sure ground is connected to the metal stand, but that's just a safety issue.

greg

[DerbyDad03]

FYI..in our latest exchange he said that he has already swapped out the GFCI and it didn't help.

His final answer was "I no longer do much work that requires a high wattage worklight, so I gave up chasing the problem. My underpowered incandescent rough duty bulb is enough."

What are you going to do...

[David Nebenzahl]

On 1/12/2011 7:22 PM Ralph Mowery spake thus:

No, I had never really known what they were until now. Just looked it up, and it's just as you say.

I'm wondering, though, just how effective or even useful 2-phase systems really were. If you graph the waveforms, you see that there's a hole in it, a "missing" phase, the one that would start at 180° that's present in a 3-phase system. So what you have is current pulses that go "bump-bump (pause)" instead of "bump-bump-bump", right?

Apparently that's one reason that 3-phase superseded 2-phase power. It's true that 2-phase was better than single-phase for running certain types of induction motors.

It's a matter of semantics, I know, but the 120+120=240 system we've been discussing actually is a 2-phase system, even though it's not really called that. One side is 180° out of phase with the other side, so by definition you have a 2-phase system.

Hopefully the previous poster who brought this up and was confused by this is less so now.

[Jeff Thies]

In particular starting torque. All motors need a push in the right direction to get them going. Often this is an artificial phase made by the starting cap. You really want 3 phase for the big motors. In fact the power company generates 3 phase power (sort of the reverse of a synchronous motor), barring electronic means it is hard to get otherwise.

It is indeed. The main advantage here is that you can combine the phases to get a higher voltage. Less current. A 120V dryer would take some hefty wiring.

Jeff

[Dean Hoffman]

I have no idea where I read that thing about electrons just jiggling in an AC circuit. Curiosity got the best of me so I snooped a bit. These popped up:

[krw]

Not hard to get. A rotary phase converter will do it. Many woodworkers use a three-phase induction motor as a rotary phase converter to power big tools (often bought at auction from commercial installations).

Technically, it's not two phase at all.

[bud--]

I haven't seen one. (Weren't the original Westinghouse/Tesla AC generators at Niagara falls

2-phase?)

I have seen Scott (or T connected) small 3 phase transformers that essentially convert 3-phase to 2-phase to 3-phase (2 transformers for

480/277 to 308/277).

Three phase probably uses significantly less copper to convey a given amount of power.

Three phase motors are probably cheaper than single phase starting at somewhere less than 1 HP.

You combine a 120V transformer winding with another 120V transformer winding that is in-phase to get 240V. In fact, as everyone knows, it is a single winding with a center tap.

You won't find an electrical engineer for power systems who will say

120/240V is not single phase. You are not likely to find an electrician that deals with 3-phase who says 120/240V is not single phase. Wikipedia is not likely to say 120/240 is 2-phase.

[krw]

A Scott-T needs two phases to get a third. One split phase won't do it.

Yes. Increased efficiency, too.

Simpler, but I doubt cheaper (volume).

Only because it's not. ;-)

[Jeff Thies]

I hadn't seen those before, using just a 3 phase idler motor. The Rotary Phase Converters I had see were more traditional, and more involved pieces of machinery (not to mention expensive). But it is not necessary for a woodworking motor to have perfectly balanced 3 phase. Clever and useful, nonetheless.

HVDC doesn't seem to have much impact in the US, but my rough understanding is that converting back is all solid state these days.

Call it what you want, but the two 120V lines are 180 out of phase relative to neutral. Certainly not useful for starting a motor. I'll not quibble over symantics.

Jeff

[krw]

No they, in fact, aren't. One is the negative of the other.

[dpb]

[e-mail] wrote: ...

Which is the same as a time phase shift of pi radians.

To see so (in Matlab)

The resulting plot is a complete sine wave over 0-2pi, the negative of that and the last (while starting at -pi) overlays the -v section from

0-pi identically (and will from there on out if extended the t2 axis.

There are two meanings of "phase" here which is the difficulty in common usage. The generation is indeed a single electrical phase; the two derived currents are out of phase (in time) with each other.

--

[krw]

Matlab is wrong.

[dpb]

...

Not unless

sin(pi?t) = sin t

cos(pi?t) = ?cos t

are no longer identities... :(

--

[dpb]

Above is missing one line that somehow didn't get pasted from the Matlab command window...the definition of t2--see above

[krw]

Ok, your understanding of what Matlab is telling you is wrong.

[dpb]

How thinketh thou so? That's what's happening in the transformer by the location of the two taps -- taking the voltage at two differing points along the (single) sinusoidal waveform at the same point in time is the same thing as a phase shift of one relative to the other.

Again, as noted above it's the confusion between the two meanings of "phase" -- the (single) electrical generation phase and the phase shift along that sinusoidal waveform for the two individual voltages.

--

[krw]

Math engineering

Nope. Define CT as zero. The signals at each end are the same but opposite sign.

THen why are you using Matlab as your source?

[Jeff Thies]

I think you mean: - sin t

That completely sums it up for me. And better put than I could. :-)

Jeff