I work with equipment that operates on three phase electrical circuits. I can’t get my head around the concept of “phases.”
In: 17
Do you remember learning about the sine function in math in your youth? It’s that wavy function that keeps repeating itself. Ultimately it is related to circular motion, so much like you can split a circle into 360 parts and call each a degree (360 degrees in a circle) you can also split that sine function up into 360 steps. The phase is just what step number, or degree, you are at if you are tracing the function.
The sine function starts at 0 at step zero, reaches a maximum at step 90, goes back to zero at step 180, goes to a minimum negative value at 270 and back to zero by step 360, which is also step zero for the next cycle. We
Alternating current electricity is described by this function, where at maximum (step 90, or 90 degrees) the current is going one way and at 270 it is going the opposite direction.
When it comes to producing electrical power, the direction of current doesn’t really matter, just that there is current. So in terms of our steps, or phase, at 0 we have no power, at 90 we have max power, at 180 we have zero power, and at 270 we again have max power.
It isn’t ideal to have our power be cycling up and down like that. We can add in more electrical current on another wire but have it at a different step, or phase of its cycle, so when one is at step 180 and producing no power is the other is at step 90 and producing max power. We would say these electrical currents are 90 degrees out of phase.
Having two is better than one, but having three has some additional benefits as the vector sum of the three balanced currents is zero.
So we have three phase power, which is three currents out of step, or phase, by 60 degrees, so when one is at zero the others are at 120 and 240. By the time the first gets to 120, the others are at 240 and 360, which is the same as zero, and the cycle repeats.
Go to [wolframalpha.com](https://wolframalpha.com) and paste the following into the input field:
plot(Sin(x*PI/180),Sin^2(x*PI/180),(x,0,720))
The line labeled Sin(x*PI/180) in the resulting graph is indicative of the current. It’s just a sine fumction. The one labeled Sin^(2)(x*PI/180) is indicative of the power. The Pi/180 is a conversion factor as wolframalpha uses 2Pi radians rather than 360 degrees for splitting a circle up. Notice the graph goes for two cycles or 720 steps since 720 = 2 cycles*360 steps per cycle.
Let’s look at three phase:
Paste the following into the input field:
plot(1/3*Sin(x*PI/180),1/3*Sin((x+120)*PI/180),1/3*Sin((x+240)*PI/180),(x,0,720))
This is the individual currents or phases of the electrical signal. Notice that each of them is 120 steps behind one and 120 steps in front of the other.
Paste the following into the input field
plot(1/3*Sin^2(x*PI/180),1/3*Sin^2((x+120)*PI/180),1/3*Sin^2((x+240)*PI/180),(x,0,720))
This is the individual power for each phase and you can see that as one peaks and starts to fall another is rising to its peak. This gives you constant power output throughout the cycle.
so 3-phase power refers to the fact that you are using 3 sinusoidal currents, each of them 120 degrees out of step or phase with the other two, leading one and trailing the other. Each of these currents is called a phase, which is kind of lazy and confusing as now “phase” is bother referring to the current in one of the wires and where that current is in its cycle from 0 to 360.
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