explain 120/208v - Page 4 - Electrician Talk - Professional Electrical Contractors Forum

Tyha · 10-26-2009, 08:28 PM

A really great way to really undrstand exactly what is going on is to look at the sine wave graph in relation to a generator that is producing it. This is truly where the EMF (electromotive force - voltage) is being created. you will see the start points for each phase and there coralation between each other is due to the position of the outgoing conductor and the distance between it and the magnet that is spinning around creating an EMF. You remember the science experiment with the magnet and the lamp cord, well thats how most of our electricity is produced. If you put a little thinking into positioning diffferent magnets and some sort of other force to drive it you can create your own 3 phase generator.

sparky105 · 10-26-2009, 10:30 PM

Quote:

Originally Posted by InPhase277

Yes. What Toronto was saying is that most stoves aren't just 240 V. They are 120/240, and require a neutral. So if you have just a straight 240 V delta, then you couldn't use anything that required a neutral. But if you had a 120/240 V delta with a center tap providing a neutral, then it would be fine.

Specifically, the single phase 240 could be used for anything that required it. You will usually find a 120/240 V delta in heavy commercial buildings, like machine or cabinet shops. Very often, table saws are 240 V single phase. Many window unit and through-the-wall A/C units are 240 V single phase.

using delta for contant balanced loads eg single phase or 3 phase heaters more accurate element control. like heat trace or barrel heaters on plastic extrution machinery

BCSparkyGirl · 10-26-2009, 10:50 PM

Please dont get your info from wikipedia. It can be put up there by anyone, and that being said, some douche can put up any info they want nd have people believe it. I never look at that site.

Mr. Sparkle · 10-26-2009, 11:50 PM

Quote:

Originally Posted by BCSparkyGirl

Please dont get your info from wikipedia. It can be put up there by anyone, and that being said, some douche can put up any info they want nd have people believe it. I never look at that site.

Hahaha my brother was the drummer for The Who for a few days.

zgozvrm · 10-30-2009, 12:48 PM

Quote:

Originally Posted by InPhase277

Yes, it does. But it is just convention that when two of the three phases is used it is called single phase. And when using only two of the three phases, the resulting waveform is that of a single sine wave, a composite of the two.

There seems to be some confusion here. I think you're confusing a "leg" with a "phase." Each phase is produced by a single secondary transformer coil and thus, 2 wires per phase. The 3 secondary transformers are then connected in such a way that only 3 wires are connected to a breaker panel (in either a wye or delta configuration). Tapping off any 2 of the 3 wires is essentially the same as tapping off of only 1 of the 3 secondaries, and thus only a single phase. So the single phase sine wave is not a "composite of the two," rather it is simply just one of the 3 sine waves that make up the 3-phase voltage, depending on which wires were tapped from. It's not "just convention."

Remember that a sine wave (a phase) doesn't exist on a single wire (a leg) by itself. For instance, how do you measure a sine wave with an o-scope? You connect the probe to one point and the ground clip to another ... it takes 2 wires!

euroah · Yesterday, 12:53 PM

Quote:

Originally Posted by guitarboyled

The delta configuration with center tap concept seems harder to grasp.

If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?

So does this mean that with a 120/208v wye system you do not have any more than 208v single phase (A-B, B-C, C-A)? Thanks, Greg

zgozvrm · Yesterday, 02:45 PM

Yes, 208V is the most you can get.

Phase to phase voltages will all be 208V
Phase to neutral voltages will all be 120V