I feel that there aren’t that many good sources that get straight into the basics of power. With that being said I won’t pretend to be an expert on the subject, but would like to post about AC Power being that that’s the type of power we encounter the most as gaffers.
AC stands for Alternating Current, and essentially what that means is that the current travels in two directions throughout it’s full cycle. If we were to break open a generator we would find a conductor that spins within a magnetic field (two magnets – north and south). As the conductor spins it creates an EMF (Electro-Magnetic Field) and creates electricity which is then punched out into your distro system.
We can see this rotation by looking at the electricities Sine Wave, which is a graph representation of the electricities motion.
At The beginning of the cycle we are at 0. The conductor hasn’t begun to spin and there for there is no electricity traveling in either direction. As the conductor begins to spin the sine wave starts to create it’s curve. 1/4 through the rotation some electricity (+) has been created and the Sine Wave is at 90°. This is the peak of the North magnet. 1/2 way through it’s north rotation (+) the conductor is at 180° and now has no charge. This finishes half of the cycle and begins it’s decent into the south magnet. The same thing happens during the south magnet except in the opposite direction (-). This is why AC power is alternating.
This cycle happens in the USA at 60Hz. The rest of the world runs on 50Hz.
This is how the power we use in our homes is created at the power plant miles away. The only factor that is different is that the voltage is extremely high leaving the power plants in order to fight against ohms law. The electricity needs to travel vast distances and with a higher voltage it is able to do that with little resistance (friction if it makes it easier to understand), which means less heat is generated. As the power comes closer to your home it travels through several step down transformers, which brings the voltage down to a manageable level for home use.
From there we get into the different types of power. What we encounter in the film world is either single phase and three phase power.
Single Phase breaks down as follows:
Often called 4 wire it consists of:
Ground(green), Neutral(white), Black, and Red.
This means that single phase has two hot legs to draw power from(black & red).
If you were to test the Voltage between the Ground and the neutral you should read zero volts or a number near zero. If you measure Neutral to Red the reading should be roughly 120V. Neutral to Black should also be roughly 120V. But if you read Red to Black the voltage should read 240V. This is how you tell that it is single phase power. Both legs are on the same phase and there fore share the same sine wave. There for the voltage add up 120v+120v=240v.
Here is the same break down if it’s easier to see this way.
G-N = 0v
N-B = 120v
N-R = 120v
B-R = 240v (both hot legs add together to create 240v)
Three Phase Power:
There is a fine difference between single phase and three phase power. On the outside it appears to be the same with the exception of having an extra hot leg (Blue). (Three Phase consists of Ground, Neutral, Black, Red, Blue) However, if you took meter readings you would get different results. Again, Neutral to each of the hot legs (Black, Red, and Blue) should read at roughly 120v. However, if you were to read between the hot legs you would get a reading of 208v. This signifies three phase power because of how their sine waves interact with each other (see the diagram below).
G-N = 0v
N-B = 120v
N-R = 120v
N-BLU = 120v
B-R,B-BLU, R-BLU = 208v
It is possible however to have three phase power with at 240v spike. This is where you read the voltage neutral to hot and come up with two 120v legs and one 220v leg. If you run into this make sure you’re running the proper distribution box/cable in order to deal with the voltage difference.
Hope this helps you have a better understanding how how power works. Keep an eye out. I’ll be posting on basic cable/distribution shortly.