Why Is American 220v Not Considered 2 Phase?

Yes thats true for most areas of the country. Here even 1-2 family homes that have single phase service are getting two of three phases from the street, not split phase.
 
Not quite, or maybe I don't understand what you're saying. 208 three phase will always be a wye connected secondary with 120v phase to ground. If the voltages are the difference of 1.73 (√3) they are always wye connected. 240/120 is always delta connected as it is 1/2 ratio. phase to phase is 240 and phase to ground is 120, it's a 4 wire system. Phase angles are 120° apart in all instances. RMS has nothing to do with 208v systems.


Well, I have to get sort of technical here, so ask questions if this gets really confusing. 208 3 phase is 3 legs of 120 volts to neutral that are 120 degrees apart. Split phase or standard residential wiring is 2 legs of 120 that are 180 degrees apart. The reason that its not 240 3 phase is the phase angle difference. the 120 volts is the maximum plus and minus voltage that is referenced to neutral. Now becasue the 240 is 180 degrees out of phase as one leg hits the peak positive voltage, the other leg is reaching it's maximum negative voltage. The difference between the two legs at that point is 240 volts. Because 3 phase is 120 and not 180 degrees apart, only one leg at a time reaches it's max voltage. The 208 is the difference between the two legs at the point of maximum voltage of only one leg and a reduced voltage of the other leg because it's not at its maximum voltage at that same time.

The whole RMS vs Peak is another discussion.
 
Well, I have to get sort of technical here, so ask questions if this gets really confusing. 208 3 phase is 3 legs of 120 volts to neutral that are 120 degrees apart.
Agree, this is found in a wye connected secondary.
Split phase or standard residential wiring is 2 legs of 120 that are 180 degrees apart.
It is one 240V leg center tap split . There is no phase shift just opposite polarities. Same reason why it is not called two phase.
The reason that its not 240 3 phase is the phase angle difference. the 120 volts is the maximum plus and minus voltage that is referenced to neutral. Now becasue the 240 is 180 degrees out of phase as one leg hits the peak positive voltage, the other leg is reaching it's maximum negative voltage. The difference between the two legs at that point is 240 volts. Because 3 phase is 120 and not 180 degrees apart, only one leg at a time reaches it's max voltage. The 208 is the difference between the two legs at the point of maximum voltage of only one leg and a reduced voltage of the other leg because it's not at its maximum voltage at that same time.
√240²-120²=208 We don't even use the 208v on a delta secondary, do we? 208 is derived from one leg (240v) and the center tap (120v).

The whole RMS vs Peak is another discussion.
 
Ahh... 240v delta - probably responsible for burning up more equipment than any other voltage known to electricians. ;)
 
OK, now I've got a question. Probably staring me in the face, but atm, I can't see the answer. Might be the cold medicine. Since our secondary is a center tapped 240 volt winding, and we derive our 120V from basically what amounts to 2 windings secondary to the same primary (or are they?), how is is that they are out of phase 180° to get the 240V? Or is it that each 120V sine wave is above and below the neutral in entirety? How can that be? It would appear to me that they (the sine waves of 120V) should run in exactly the same time domain, and would operate more like parallel secondaries. If I think about it a while, I might realize how simple the answer is, but right now it's not coming to me. So someone 'splain it to me please.
 
Neutral isn't a voltage in relation to the other two hot legs, it is the center of the winding. If you consider the whole secondary as an autotransformer then as the winding ratio increases the voltage between the two legs will increase. The center tap - our usually grounded conductor - will be 120v to the bottom leg. The top leg - the other hot - will be 240v to the bottom hot leg with a 120v difference to the center tap. The phase of the sine wave is dependent on the magnetic flux "moving" in the core and transferring energy into the secondary. Think of the output legs pushing and pulling in sync with each other. This is why there are phasing dots shown on schematics when you have transformer coils to wind. If you could flip the connections in the secondary so that the phasing of the core is the same you would have parallel 120v connections without a 240v connection. Think of a buck-boost transformer connection where the polarity is additive or subtractive.

Here's one to think about - assume the power company didn't ground the center tap or you have a 480 to 120/240 transformer. You could ground either "hot" leg and connect it to the neutral bar in a panel. Center tap to the main breaker and the other "hot" leg to the main. Nothing would trip out and depending which phase you used it would give you 120v to neutral bar, 240v to the neutral bar and 120v between phases on a 2 pole breaker with no difference in shock hazard than in a properly connected panel. We only get shocked or sparks between hot and ground because of the ground connection we establish when we install the panel allows a complete circuit back to the transformer.
 
As I suspected, now with a clear head (sorta) I see it as no longer 2 separate windings in a secondary, but a single winding with the appropriate turns ratio to give the 240V. Grounding the center tap is immaterial to getting the 240V, but is part of how we use 240V safely.

Thanks Jim.
 
I'm wondering, (and I should know this) if, in the case of a motor wired for 220 volt, the connection point of the two run windings could be tied to neutral to stabilise the voltage that appears there and reduce the voltage spike/swing impressed on the start cap. The start winding, cap and centrifugal switch are usually across one of the run windings, and I bet the midpoint of the run windings thrashes around quite a bit when starting and stopping. Anybody?
Mark S.
 
I'm wondering, (and I should know this) if, in the case of a motor wired for 220 volt, the connection point of the two run windings could be tied to neutral to stabilise the voltage that appears there and reduce the voltage spike/swing impressed on the start cap. The start winding, cap and centrifugal switch are usually across one of the run windings, and I bet the midpoint of the run windings thrashes around quite a bit when starting and stopping. Anybody?
Mark S.
The motor wouldn't start. Each winding would be connected directly across 120VAC and and neither would be subjected to any phase shift. The capacitor would draw lots of current but it would not affect the motor.

Look at how a 120V capacitor-start motor is wired. The capacitor actually achieves its effect by being in *series* with one winding.
 
Well, to further complicate matters I have heard US power referred to as Edison 2 phase.
Now that makes little to no sense since Edison HATED AC and wanted DC in everyone's house with a small generator in a barn out back.
That's actually the exact reason AC became the standard! You see Edison did many demonstrations showing how dangerous AC current was in comparison to DC. He maintained that it would be the perfect thing for executions for example. Well the short version is that the government tried it out, and agreed it was good for the electric chair. Now the government needs to buy AC generators, and the rest is history. That's just from memory but you can find the info if you're interested.
 
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