Reply posted by Bryan Wright on March 31, 2001

That makes SO much more sense to me now... thank you VERY much. Your last excellent explanation has also cleared up the question of why putting sound on one leg and lights on another leg of the same distro is really not separating anything... it's all the same current flowing back and forth in-circuit between the hots and the neutral, right?

Now that I understand that concept, the problem of keeping lighting system noise out of my sound system and still having enough power to run the sound and lights together becomes even more perplexing, since a single distro will not solve this problem.

Anyone have any recommendations?


Reply posted by D. Parker on March 31, 2001

We are mildly off topic, but lighting buzz in the sound system should be on. I have never worked with high-end lighting gear, but the NSI and such most of us work with does buzz from time to time. Although there is no electrical explanation why, I have moved the lighting to the other side of my single phase distro and had the buzz go away.

Other times I have tried that and it didn't help. Sometimes the lights don't buzz. Sometimes they do. Sometimes you can fix it, sometimes you can't. I have done jobs where I used wall outlets and plugged the lights in 100 feet down a hall in another room and the lights buzzed. I've done others where everything was plugged in not too far and it didn't. When you have a buzz don't hesitate to move around where things are plugged in, sometimes it works. Sometimes just moving the cables around works. Sometimes not.

David


Reply posted by Chris Hindle on March 31, 2001

The hot legs (110V), additionally, are of opposite POLARITY to each other (NOT out of phase by 180 degrees). This is accomplished by the distribution transformer that knocked the voltage from distribution level (i.e. 12,470V hot-neutral or whatever the distribution level is in your area) down to the 110V per hot leg. The two secondary (low voltage) windings of the transformer are wound around the transformer core in opposite direction to each other so that when one voltage waveform is going negative (wrt neutral) the other is going positive.

In an AC system, this is phase. Polarity is relative, due to the phase relationship of the 2 hot lines. Please keep polarity issues to talk of DC, to avoid lots of confusion.
my .016 (Canadian, Eh)

(other than that, very nice writeup)


Reply posted by Doug Matthews on March 31, 2001 at 10:31:55

Chris,

Thanks for the compliment.

I must, however, respectfully disagree with your comments about the term polarity being incorrectly used, and further being an inappropriate term in AC discussions.

It is most definitely the correct term here, and certainly has a place in the AC world.

Phase represents the shift (or relation) of an AC waveform in time, or with respect to another waveform. Polarity represents the inversion of (in this case) an AC waveform with NO shift of the waveform in time. Change in polarity and 180 degree phase shifts of a periodic (i.e. sinusoidal) waveform are easy to confuse.

Let’s look at a couple of examples:

[wrt = with respect to]
[a “phasor” can be extremely loosely thought of as an arrow in a horizontal plane, pinned down at the tail, with a length corresponding to the magnitude of a periodic (sinusoidal) phenomena (i.e. voltage, current). It rotates 360 degrees around the origin of a cartesian coordinate system superimposed on the plane. It’s angle wrt the X axis is the angle we are feeding to our periodic (sinusoidal) system.]

Take a piece of electrical equipment, a toaster, with a two-prong AC plug (an older one without a polarized plug)(current toasters may still have un-polarized plugs - I haven't checked).