FSP0022 – Three-Phase Power – Facility Science Podcast #22

For me, the easiest way to understand alternating current has always been in comparison to direct current. If you think about a battery, you know it has a positive terminal and a negative terminal. By convention, the positive terminal is at a higher voltage than the negative terminal and we say that the charges flow from the positive terminal to the negative terminal. Voltage represents the relative energy level between two places, so as the charged particles travel from positive to negative they have to lose some energy. We arrange the devices we want to provide power to between the positive and negative terminals of the battery so that excess energy will be dropped off in our devices. The loop from the positive terminal, through the devices and back to the negative terminal is called a circuit. I have just described direct current. That is the case where the current is always traveling through the circuit in the same direction.

Alternating current, on the other hand, isn’t always traveling in the same direction. It alternates, first traveling one direction, and then the other (which you should note also means that the voltage is also changing polarity or direction. Most of our power transmission and distribution systems use alternating current and that is generally what is delivered to our buildings. There are various reasons for this, 2 notable ones are that, in some circumstances, alternating current can travel over long distances with less energy loss than direct current, and also that most of our power generating devices generate power by spinning magnets inside coils of wire which naturally creates alternating current.

Our alternating current looks like a sine wave (or a cosine if you prefer). That means it (to choose a starting point randomly) starts at zero current (no current flowing in either direction) then starts flowing in one direction with increasing magnitude until it reaches some maximum value, then from the maximum falls back down to zero. The current then starts flowing in the other direction, gradually increasing in magnitude until it reaches the same maximum values it reached while flowing in the other direction, then gradually falling back toward zero current. The current repeats this pattern over and over 50 or 60 (50 or 60 hz) times per second depending on where in the world you are.

In trigonometry and signal analysis there is a concept called phase shift or phase angle. When looking at a periodically oscillating signal (one that simply repeats the same pattern over and over like our alternating current sine wave) there isn’t really a starting or ending point of the oscillation. We can draw 2 or three on top of each other in a way that they line up exactly. We can also draw them on top of with each other so they don’t overlap, for instance we could have one sine wave signal at 0 and starting to go upward in one direction while another is already halfway up to its maximum. Both sine waves are doing exactly the same thing but they are doing it at slightly different times. When we have 2 identical but time-shifted signals like this, we call the difference between them the phase angle or phase shift.

So when alternating current to a device, sometimes we bring one AC wire the device (we call this the hot wire). If we want any current to actually flow through our devices we need another wire to complete the circuit, so we also commonly connect a grounded “neutral” wire to complete the circuit. We call this single-phase AC.

Other times, instead of bringing one AC wire and one grounded neutral wire to a device, we bring 3 AC wires to the device. Each AC wire is carrying alternating current of the same type (so the same frequency, 50 or 60 hertz and the same maximum current or voltage amplitude in each direction), but each is phase shifted a third of the way through the cycle relative to the others. This means that none of them are zero at the same time and none of them are at the maximum magnitude in either direction at the same time. And because of the magic of the way these current sinusoids combine with each other, the three wires can complete the circuit for each other making an extra neutral wire unnecessary in many cases. We call this arrangement 3-phase power or 3-phase alternating current. It’s 3-phase because sinusoidal current pattern on each wire is phase shifted relative to the other.

Generally we call the three phase wires “lines.” So a three phase system has 3 lines. We often label these A-phase, B-phase and C-phase or something similar to differentiate them. In a 3-phase system we talk about line voltage and phase voltage.

Phase rotation is a term you might hear electricians and engineers talking about when designing or connecting a 3 phase system. If you look at the oscillation of just one phase, it will look exactly like the other since they are all exactly the same repeating waveform. When you look at all of them at once, you can tell that they are shifted in time. If you look at any part of each sine wave (say we look at the peaks), one of the phases will reach the peak first, then another phase will reach the peak, then the third one and then everything repeats. In an isolated system, it doesn’t usually matter which order you connect them in, but when connecting 2 different systems together, it definitely does matter. The order of the phases (that’s basically which one we call A, which one we call B and which one we call C) is called phase rotation, and like I said, it’s important when connecting 2 different electrical systems together. For example if you are connecting a backup generator to a building’s power system, it’s important to connect the generator so that the generator’s phase rotation matches the utility phase rotation. If you don’t, many devices (particularly 3-phase motors) won’t function properly on generator power. This is something that generally only the engineers and the electricians actually designing and connecting power systems really need to understand, but when you hear it, you’ll know what it means.

Two other common terms you might hear around the topic of three phase power are Y and Delta. These have to do with how the circuit is connected.

If we distribute single phase, we use 2 wires. One line and one neutral to complete the circuit. The power is fed from the one line (meaning that wire is connected to the output of the generator) while no power is fed on the neutral wire. We have to provide the neutral to complete the circuit. So let’s say we are using 2 wires to deliver 1 unit of power. If instead we distribute 3-phase, we use 3 wires, one for each line. Power is delivered on all 3 lines (meaning all 3 are connected to the output of the generator) and each line uses the other 2 to complete the circuit. So let’s say we are using 3 wires to deliver 3 units of power. In the 3 phase case, we are delivering 3 times the power of the single phase case and only using 1.5 times as many wires.

Why 3 and not 2 or some number more than 3? Any number would technically work, but 3 was chosen for practical reasons. More than 3 would be more efficient purely from the perspective of the power delivered per conductor material. But more wires means more complexity in distribution system.

Generating it is as easy as generating single phase. A rotary generator can easily be made to generate 3-phase power just by adding more coils and magnets spaced around the rotating generator shaft

For many buildings, especially larger building, the electricity supplied to the building by the electric utility is 3-phase.

Often we use the three phases separately (we use the three phases plus a neutral as 3 separate single phase systems). Many, if not most, off our electrically powered devices are not 3-phase devices. They are single phase devices in that that they only want one “line voltage” or “mains voltage” AC wire and a neutral wire. This is totally fine in a 3-phase electrical system, although it is important to attempt to balance the load on the three phases, meaning that ideally we want to distribute the three phases among our single-phase devices so that we use approximately the same amount of power from each phase. I said before that three phase wires act to complete the circuit with the generators at the utility for each other. If we don’t balance the 3 phases, we get some extra current somewhere as a consequence of the way current flows among the three wires and the neutral. This details of this are outside the scope of this podcast, but this extra current is basically wasted energy and in some cases can cause problems other than energy waste. You might say that if you don’t balance the phases they will balance themselves, probably to your detriment.

Sometimes we use 2 of the three phases to get more power per amp. In the US, we commonly have a line voltage of 120V which gives us 208V when we use 2 of the phases. We typically use the 208V for higher power devices

Some loads are 3 phase loads.