AC sine waves

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Cleveland Apprentice

Senior Member
Location
Cleveland, Oh
Hello,

I am trying to better understand AC sine waves. What does "out of phase" mean? I am confused when I hear phrases like "60 degrees, or 180 degrees out of phase." Does it matter if something is "out of phase?" What are the advantages of "out of phase" as opposed to "in phase?" I have very little knowledge in this area and I'm trying to better myself. I am not expecting any lengthy responses. If there are helpful links you could direct me to that would be appreciated. Thanks again!
 
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ron

Senior Member
It depends on the context. It is the relative position of one electrical sine wave as compared to another.

For example, if you looked at three phase power, Phase A, B and C each would be seperated by 120 degrees. If phase A was seen at 0 degrees, then B would be 120 degrees and C 240 degrees. Keep in mind at 360 degrees you are back to the same place for phase A again.

http://science.howstuffworks.com/environmental/energy/power3.htm
 

Besoeker

Senior Member
Location
UK
Hello,

I am trying to better understand AC sine waves. What does "out of phase" mean? I am confused when I hear phrases like "60 degrees, or 180 degrees out of phase." Does it matter if something is "out of phase?" What are the advantages of "out of phase" as opposed to "in phase?" I have very little knowledge in this area and I'm trying to better myself. I am not expecting any lengthy responses. If there are helpful links you could direct me to that would be appreciated. Thanks again!
Two sine waves which are out of phase with each other cross zero at different times. If you looked at the current and voltage of say, a small motor you might get a picture like this. The current is out of phase with the voltage. This load has a lagging power factor because the current lags the voltage - in this case by 45degrees or a quarter of a cycle.
Does it matter that it's out of phase. Yes, to the extent that the lag means that the current is higher for a given power. Cables and switchgear may have to be larger and thus more expensive. Sometimes this gets corrected using power factor correction capacitors

Phase shift 45.jpg

The bigger picture, which I tried to erase, is the phase relationship of a three phase system.
 

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