FAQ on three-phase AC power: part 1

We’re familiar with single-phase AC at the outlet, but multiphase AC offers advantages and is necessary for higher-power installations.

For most engineering designs, the circuit’s DC operating power comes from one of two sources: either a battery or a basic AC line, which has been stepped down via a transformer, rectified to DC, and then regulated. In the case of AC-line source, that AC source delivers a nominal voltage value of 120 VAC at 60 Hz in North America or 240 VAC/50 Hz (Europe); other voltage values are also used, such as 100 V in Japan, at 50 or 60 Hz depending on the region of the country). Depending on location and other specifics, that AC line is rated to deliver up to 15 or 20 amps for a total power rating of several kilowatts (kW).

The AC voltage that appears at the wall socket is a simple, single-phase alternating (bipolar) voltage, and it certainly does the job, as demonstrated by the countless billions of loads that it powers. However, as the load power requirements go into the 10 KW and higher range, that single-phase AC line becomes inefficient at delivering power to the road. This is especially the case for loads such as motors, but the challenge also applies to other loads.

This FAQ will look at the advantages and attributes of three-phase AC, also called multiphase or polyphase AC.

Start with single-phase power

Figure 1. The basic 50- or 60-Hz sine wave for AC power is well known and widely used. (Image: Cyber Power Systems)

Q: What does the standard single-phase AC line look like?
A: The waveform is familiar to engineers and electricians, of course, with a sine wave oscillating at 60 Hz (or 50) Hz, Figure 1. The physical wiring is straightforward, with two active wires: the line (hot) side and a neutral (return). AC current flows back and forth between the two. Note that the third wire in most (but not all) installations is solely there as a safety ground and carries no current in normal operation.

Q: This seems to work, so what’s the problem?
A: The problem is twofold. First, as the required power level increases, the current needed to deliver that amount of power increases (P = VI). This, in turn, requires thicker, heavier-gauge wire to keep resistive voltage-drop losses (IR) and thermal-dissipation losses (I²R) to acceptably low values. These heavier wires are more costly in terms of basic copper and much more difficult to work with as well.

While satisfactory for most residential needs, a standard 15-A, 120-V branch circuit can only support electrical loads up to 1800 watts. A home, office, or factory with 100-A, 240-V electrical service is limited to loads up to 24 kW.

Q: What’s the other problem?
A: If you look at that single-phase AC waveform, it goes through zero volts as part of its cyclic oscillation. As it does so, less power is being delivered to the load, eventually reaching zero until the waveform magnitude ramps up again. This is not a problem for incandescent bulbs or heating elements since they have carry-over thermal momentum with a longer time constant.

However, it is a problem for rotating equipment such as motors, which rely on mechanical inertia to carry them through the low/zero-voltage one. As motors get larger and their loads increase, the momentum effect provided by this inertia is challenged, and the motor begins to stutter with a “chattering” effect. Obviously, this is not good for the motor’s bearings, performance, or load.

Further, many motors can’t start on power-up since the single-phase AC waveform does not induce the torque they need to get started. For that reason, even small motors need an out-of-phase starter circuit, often implemented with a capacitor.

The three-phase alternative

Q: What’s the solution to this power density and smoothness problem?
A: Instead of a single alternating waveform at 50/60 Hz, three-phase AC is used. As its name indicates, the single-phase AC waveform is replaced with three waveforms, each at 50/60 Hz, but located (phased) 120° apart, Figure 2. It is much more efficient and effective for power generation, transmission, and use at the load. It is sometimes written in “shorthand” as 3P compared to 1P.

Figure 2. A three-phase system uses 50/60 Hz for each of the three cycles, which are spaced (time-displaced) 120° apart. (Image: Cyber Power Systems)

Q: What about the “steadiness” of power delivery?
A: Unlike single-phase, 3-phase power produces three separate wave currents, so there is no temporary absence of power. It delivers power at a steady, constant rate and can reach higher voltage levels, up to 480V in the USA. This steady stream of power and ability to handle higher loads makes a 3-phase supply ideal for industrial and commercial operations.

Q: Is this a new development due to our increasing power needs?
A: No, not at all. The concept of three-phase AC and its use started around the 1880s (yes, the 19^th century). At that time, the only uses for electricity were resistive heating, illumination, and motors, with the latter being the big “new” area, as motive power was critical to the underway industry revolution. Electronics as we know it today was not even a concept, let alone any sort of reality.

These motors got larger and larger, into hundreds and even thousands of horsepower (one horsepower is about 750 watts), so you can see that the electric-power demands were impressively large. Therefore, their electrical power requirements were quite high, while smooth rotation was essential. These motors ran directly from the AC line as the various motor-related electronics and switchgear we now have were not in the picture.

Q: Who were the engineers and scientists involved in developing three-phase AC power?
A: There are many names you will likely recognize, including Nikolai Tesla, Goerge Westinghouse, Charles Proteus Steinmetz, as well as some you may not know, such as William Stanley, Jr. Another major step was the development of the three-phase induction motor by Mikhail Dolivo-Dobrovolsky in 1889, along with delta and wye transformers (more on these later). This AC motor was superior to the DC motor in efficiency, smoothness, and lack of problem-causing commutating brushes. The References link to some credible historical source sites.

Q: Can you illustrate the virtues of three-phase AC with some numbers?
A: Consider powering a 15-kW rack with single-phase power at 120 VAC and drawing 125 amps. The copper gauge needed to safely carry that current, AWG 4, is nearly a quarter-inch in diameter, is hard to work with, and expensive. Clearly, single-phase isn’t practical for such loads. However, in a three-phase system, each conductor only carries about 42 amps and needs only AWG 11-gauge wire at just 0.09-inch diameter. That’s a huge difference in copper mass and handling.

Q: Can you run 120 VAC loads from a three-phase connection?
A: Yes, by using just one phase. Of course, it’s a little more complicated than that, but using just one phase is the key. The challenge is to “balance” the loads across the two AC sides. Many residences now have three-phase power, with power “split” at the power panel, delivering one phase for half the load and the other for the other half.

The next part delves into wiring details.