Sound and Electrics

[moderately_clueless]

So I work with all this stuff pretty often, but I'm still having trouble getting my mind wrapped around it all, so here's a few questions.

I'm still having trouble relating volts, watts and amps to regular theatre/sound use. I know that most household circuits have a 15/20 amp load and that they have a certain voltage limit. 110? 220? I don't remember. But I don't quite get what all that means. Particularly amps, I keep hearing them used, but I have yet to get a good explanation.

Also, I know that on a three phase power system (ie a generator), if it's not properly grounded you get that hum on the sound system. But I don't quite understand why the ground can affect the sound like that.

And lastly, on a three phase system, what is the purpose of the neutral wire? I know that the three phases each have 120? (amps?), and I know that the neutral usually carries a fair amount of electricity, but what does it do, exactly?

Thanks in advance, everyone. I know most of this is pretty basic, but I'm just seeking to feel a little less stupid at gigs =)

[Footer]

I'll tackle a few....
Edit: what icewolf said, its been a long day....
First... West Verginia... Watts*Volts=amps
Basically, amperage is a result of how many watts of electrcity is being pulled multiplied by the amount of voltage being supplied. A traditional (north american) household outlet is either 15 or 20 amps @ 120 volts, so they can supply a total of 2400 or 1800 watts.

Now the question for neutrals on 3 phase. You will hear the term "load balancing" thrown around by people. When a dimmer rack is hooked up to three phase power each dimmer is supplied by a different set of phases (so 8 8 and 8 in a 24 channel rack). Say you have 20 fixture on at full on the first 8 dimmers and nothing on the other 2 sets. You are pulling in a ton of power on two phases but nearly nothing on the third. That power has to go somewhere, so it gets put on the neutral to go back out in the world. You will notice that 3 phase motors have no neutral because they use all 3 phases equally all the time. You will also see company switches that have three phase, and ground, and two neutrals. This is extremely common for sound tie ins where the load will very rarely be balanced. If you put too much power accross the neutral you run the risk of overheating it and starting a fire, hence the reason that double neutrals are becoming more and more common.

[audioslavematt]

Originally Posted by moderately_clueless

So I work with all this stuff pretty often, but I'm still having trouble getting my mind wrapped around it all, so here's a few questions.
I'm still having trouble relating volts, watts and amps to regular theatre/sound use. I know that most household circuits have a 15/20 amp load and that they have a certain voltage limit. 110? 220? I don't remember. But I don't quite get what all that means. Particularly amps, I keep hearing them used, but I have yet to get a good explanation.

I like the analogy of the water pipe. The voltage is like the water pressure. The amperage (current) is how much water is actually in the pipe. The wattage (power) is how much water is actually being pulled through the pipe.

Also, I know that on a three phase power system (ie a generator), if it's not properly grounded you get that hum on the sound system.

It doesn't have to be three phase. This can happen in any electrical situation. Think of it like this. You have two runners running a one hundred meter race. One runner starts at the beginning and the other runner starts at the fifty meter line. Which runner is going to reach the finish line first? The one with the shortest distance to run.

[SHARYNF]

Minor correction it is watts divided by volts that is amps

Here is a simple trick to remember

W
V A if you cover the one you want for the answer you will have the formular SO w equals volts times ams, Volts is watts divided by amps, amps is watts divided by volts
I'll sit on the side lines of the discussion for a while ;-)

Sharyn

[icewolf08]

Originally Posted by Footer4321

First... West Verginia... Watts*Volts=amps
Basically, amperage is a result of how many watts of electrcity is being pulled multiplied by the amount of voltage being supplied. A traditional (north american) household outlet is either 15 or 20 amps @ 120 volts, so they can supply a total of 2400 or 1800 watts.

This is like half there and half not. West Virginia is W=VA, which is how you did the math at the end of the paragraph. So, you only loose 50 points...

For M-C, in the united states the standard supply voltage is between 110v and 130v. Generally, as you sort of mentioned, at the power generation plant they send out 3 phase power. The most common 3 phase system is 3 phase Y (wye), this setup has each phase to neutral at 120v and the neutral is tapped from the center. The voltage between any two phases is 208v. From the generation station the electricity is transformed to a very high voltage at a very low amperage, which is best for long distance transmission. The transformer on your street or in your theatre transforms the electricity back to a useable voltage.

After the transformer, you probably have some kind of power distribution system, this could be anything from one high amperage main breaker and sub breakers or a rack of high amperage breakers that feed other panels. It is common in theatre to see something like a 400 amp, 3 phase service. This means that on each phase you can load up to 400 amps for a total of 1200 amps on the service. In theatre terms, one source four with an HPL575 draws 4.8 amps so you could put 250 units on the 400 amp 3 phase service.

As for your question on what the neutral line is for, it completes the circuit. The hardest thing to fathom about neutral is that it doesn't run back to the generation station, it just runs into the ground and it works... Without completing a circuit from hot to neutral though, your devices wouldn't work, it would be like connecting a flashlight to only one end of the battery.

As for 60 cycle hum, this is most often caused, as you said by bad grounding. According to code, and best practice, every building is only supposed to have one path to ground. This means that there should only one connection point from the device ground (generally the green wire) to neutral, which is then connected to earth. In systems where there are multiple connections between device ground and neutral or where there are multiple earths this can produce a hum.

Hope that helps with your questions, and if I mis-spoke anything someone feel free to correct me.

[Van]

Here's a picture I made. It might help you visualize the diferent "phases" of electricity. The three colored line corespond to the three primary colors used to label the standard three phase power used most places. The second chart shows standard 110/220v 60hz power.

Why a neutral in a three phase system? Weeeelllll since most gear, dimmers and amps included, actually run off of 110/120 you need to allow for that provision. Having a neutral allows you to get 110 power out of even a 208 system. Notice that in a 110/208 three phase system the voltage potential between ground <earth> and any hot leg < phase A,B or C> is still 110volts.

Where does the hum comes from? wellllll notice the time line the voltage "flips" 60 times per second, another way to say "times per second" is Hertz, or Hz. So a poorly grounded system can allow sympathetic, wave forms to show up in the final output of the sound gear, and lights. you don't notice it in the lights because of "Persistence of Vision" , but your ears are attuned and can hear the resultant 60 hz signal comming out of the speakers.

I'll follow all this up with the statement Icewolf made, feel free to correct me, y'all.