Looking for definitive answer on AC power

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Old 11-28-07, 07:20 PM
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Looking for definitive answer on AC power

I have asked several electricians about AC and none have been able to give me a suitable answer. According to the definition of AC the current alternates in direction. That being said AC devices have to accomodate the current going in both directions. For NA the freq is 60hz which means the direction of the current for one second will change 60 times.

The best answer that was given to me is that in theory this is the case but in the real world the current only goes in one direction. For example, a house with a 100 amp service has two hot wires and one neutral from the street into the panel. The neutral is attached to the ground and each hot wire (120v) will receive only half the sine wave (ie. for 180 degrees current will flow and 2nd half of the cycle current will be at 0).

This seems reasonable if the current only goes in one direction but according to the books I have read AC does change direction so this explanation is not valid.

What is the real answer?

A.F.
 
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Old 11-28-07, 07:30 PM
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Old 11-28-07, 07:36 PM
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Actually, at 60 Hz, the current changes directions 120 times a second, since each cycle includes both directions. To the flow of electrons, 1/120 of a second is a long, long, long time. Zillions of electrons can flow in that amount of time. And yes, they absolutely do reverse direction. To many loads, such as a light bulb, the reversing doesn't matter. But to other loads, such as a motor or a clock, it matters a lot.

In theory, it is possible that the current on the neutral can be zero, but in practice in North America, it will never be absolutely zero as long as there are any 120-volt loads running.

The model that current only flows in one direction is clearly wrong, but in some circumstances, it is simpler to think of it this way. Like almost everything in real life, how something really works is much more complicated than how we normally think of it as working. There's no way our puny brains can hold it all, so we simplify things. Except for those in research and development, the simple model is close enough.
 
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Old 11-28-07, 08:13 PM
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Originally Posted by wdthrush View Post
The neutral is attached to the ground and each hot wire (120v) will receive only half the sine wave ...
Both hots have full sine waves. The are exactly 180 degrees out of phase with each other. When they cross zero, both are at zero, which is neutral. It's nothing more than a center-tapped transformer with neutral at the center.
 
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Old 11-29-07, 07:48 AM
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Old 11-30-07, 02:06 AM
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Originally Posted by Rick Johnston View Post
Both hots have full sine waves.

With single phase A/C power generation the coils are part of a single conductor and there is a single sine wave.
 
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Old 11-30-07, 05:44 AM
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The OP asked if the neutral chopped half the sine wave. It does not. There is no rectification in the AC power. If you put an o-scope between either one of the hots and neutral you'll see a full sine. Put it between the other hot and neutral and you'll also see a full sine, but it will be opposite in phase to the first.
 
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Old 11-30-07, 10:29 AM
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It is the same sine wave viewed from a different point.
 
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Old 11-30-07, 05:33 PM
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We're viewing the sine wave from the point of the homeowner, who has two hots and a neutral and wants to know (1) if the current really reverses direction and (2) does the single sine wave get cut in half (rectified) to remain at zero volts for a period of 1/2 the cycle.

(1) Yes. Alternating Current (AC) reverses direction twice in each cycle. Direct Current (DC) does not.

(2) There is a full sine wave on each of the three possible pairs of connections when viewed on an o-scope. Each will appear to be different.

wdthrush, you sound like you understand that DC is derived by inserting a rectifier into the AC circuit. The rectifier passes current in only one direction. Since the AC sine wave moves in both directions, the rectifier will pass only half of the current. But there is no rectifier in the neutral to cause it to pass current in only one direction.
 
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