208 sine wave

johnjohn89

I don’t understand that this 208 sine wave, has 180 degree different,but they run and peak by same time, it they running by same time there is no voltage potential difference,how this complete the circuit?  What I understand like three phase one peak other not peak, give u different time and different potential then complete circuit! Can anyone explain?

Jamie Hall

That graph is voltage referenced to the return wire -- usually ground. In a three phase circuit, the voltage peaks referenced to ground at 120 degrees out of phase with each other. There are several different ways to connect three phase power. 208 volt power is found phase to phase in wye connected systems where each individual phase is 120 volts to the neutral.

johnjohn89

Jamie Hall said:

That graph is voltage referenced to the return wire -- usually ground. In a three phase circuit, the voltage peaks referenced to ground at 120 degrees out of phase with each other. There are several different ways to connect three phase power. 208 volt power is found phase to phase in wye connected systems where each individual phase is 120 volts to the neutral.

I know that! But we read voltage only there is voltage potential difference , two leg peak by same time that means no opposite potential,
with same potential won’t read voltage 

ratio

Two legs don't peak together. With a center-tapped single phase (US residential) system, the two legs peak 180° apart = 240 volt leg to leg, like your graph above shows. With a center-tapped three phase (low voltage) system, each leg peaks 120° apart = ≈208 volts peak to peak.

109A_5

In this case 208 is just two legs of a typical three phase system. It's all in the Trigonometry, plenty of YouTube videos on the subject. There are many different 3 phase Voltages 120/208, 277/480 and many others depending where in the world you are and the age of the systems. I would be careful using the "center-tapped" terminology. A center tapped winding is when the whole winding is on the same core with the same phase and not two magnetically separate cores that is 120 degrees out of phase. "center-tapped" 3 phase windings are found in 6 phase systems with a 60 degree phase difference. Polyphase can get very exotic depending on the application.

ratio

Here's a youtube video that does a decent job of explaining it (including 3 phase wye & [wild leg] delta): https://youtu.be/xMZkKI5rleg

Most interstingly, he graphically shows how we get the line to line voltage on a wye-connected system and also the 208 on a wild leg.

Missing from his explanations are 277/480 volt systems, as well as obscure stuff like open delta and corner grounded delta.

EBEBRATT-Ed

277/480 Y 3 phase is basically the same as 120/208 3 phase just higher voltage used.

Plus, on 208 single phase you will have run and sometimes starting capacitors to get things moving. With 3 phase you don't need them

Jamie Hall

I might point out that if your service is 208 volts, not all appliances rated for 240 volts will be happy with it. Check the ratings. Things like heating appliances -- stoves, water heaters, clothes dryers and the like -- may work, but be rather anemic. Motors may be very unhappy.