02-04-2019 10:29 AM - edited 02-04-2019 10:37 AM
As Tim points out, typical for U.S. homes is for the house to be fed from a medium voltage power distribution line via a step down transformer. This transformer has a center tapped secondary with the center tap forming a neutral between the two secondary winding halves. 240 volts is available across the entire secondary winding for large demand loads (air conditioning, electric clothes dryer, etc.) while 120 volts is available between either side and neutral for lighting and other appliances.
In this case the transformer is functioning as a phase splitter so that as the AC waveform is approaching a positive peak on one side of neutral, it is going towards a negative peak on the other side. The two phases are a creation of the transformer and stay perfectly 180 degrees out of sync due to the split transformer winding. Lower cost LED lighting can use a half wave rectifier so that it only is supplied voltage on half of the waveform and if two of these lights are on different halves of the 120 circuit then you will have them emitting peak brightness 180 degrees out of phase with each other creating some odd effects. Note that the full 240 volt available is a single phase and the split phase is purely a construct of the transformer reversing one side with respect to neutral.
Although you won't typically find it for a home; businesses, farms, and large power consumers will use tri-phase power with three "hot" lines all out of phase with one another. This is true multiple phase power unlike that "pseudo-split" created by the typical home transformer.
Quality LED lighting is fed regulated DC and this isn't a problem, the worst are stuff like cheap Christmas lights where the LED itself acts as a rectifier diode and flickering is very obvious to the human eye. Incandescent lighting has significant thermal lag so even though the power through it is changing throughout the AC cycle the human eye (and camera) don't perceive a difference in illumination unlike sources that respond quickly in response to applied voltage changes.
It is a little more complex than the UK but the U.S. system has some advantages and some difficulties. For best power utilization the house load should be as evenly distributed across each side of neutral because any current imbalance flows back through the neutral line. AND if there is a fault with the neutral line, bad things can occur because the voltage will no longer split evenly across the two 120 volt sides which can cause one side to rise to a damagingly high voltage while the other side drops to voltage low enough to damage motors. In terms of damage to electrical gear, the loss of neutral is about the worst in terms of carnage.
02-04-2019 10:39 AM
"... it is almost always a 220v line which is split into two 110v "phases"
I know of no instance where a USA house gets a single 220v line. It is always two 110v lines. Each is 180 degrees out of phase with the other. These two seperate lines a connected to seperate poles on our 200v receptibles. Breaker panels have two busses so there is two seperate legs each 110v.
02-04-2019 10:52 AM - edited 02-04-2019 10:52 AM
Nonsense. As said above, they all get a center-tapped 220 V signal that is split at the center tap into two 110 V circuits.
02-04-2019 11:08 AM
"Nonsense." I don't know what you consider nonsense but this is how it works in Kansas at least.
There are two hot wires and a neutral wire. The hot wires each carry 120 volts and are different phases. These phases are normally called "A" and "B" phases. The total voltage, when measured between them, is around 240 volts. Therefor two lines coming in not a single one. It has nothing to do with what happens before the transformer. That's it and this a a photography board not an electrical engineering board.
02-04-2019 11:19 AM
That's it and this a a photography board not an electrical engineering board.
That's true but it has answered my query, I am now much better informed, thankyou.
02-04-2019 11:26 AM
I didn't expect this to turn into a discussion on electrical systems, but there you go.
Anyway, this is a bit off topic ... but a YouTube channel I enjoy had an espisode that explained a bit about the UK's crazy bulky power plug and how, when you a buy a common appliance (such as a toaster) it doesn't actually include a plug, you have to wire the plug on yourself (is this still true in the UK?)
The story goes that while in the US, all the wiring in the house routes to the fuze box (now breaker box -- we haven't used fuses in years), that requries a lot of extra wiring to place recepticals around the home. During the war years, metal was at a premium and that excess wire was considred wasteful. So instead, the whole house got wired in one giant continous loop (do they still do this?). The problem is... now you don't have individual breakers for each section of the house. To solve that problem, the UK designed plugs with the fuse located in the individual plug (in the US, our plugs typically don't have fuses in them. The only exception I can think of is holiday lighting strings.)
Meanwhile back to the topic ... flickering tends to be based on the AC power cycling rate (if you have AC power at 60Hz then your flickering is probably happening 60 times per second. If you're on 50Hz then you're flickering is probably happening 50 times per second.)
02-04-2019 11:28 AM
I *am* an Electrical Engineer.
Where do you think the two phases come from? Do you have two transformers? If you really had two "separate" phases you would have 4 wires coming into the box.
Do you even know what "Center-tapped Transformer" means?