Arkansas HVACR NewsMagazine September 2021

HVACR NewsMagazine September 2021

Tech News

240v single-phase and split-phase

We sometimes use the terms single-phase and split-phase interchangeably, and it can be confusing. Although they roughly mean the same thing, they sound a bit contradictory, so we’ll explain the names. “Phase” refers to the use of the original power from the utility. The power company offers three legs of power, so a single-phase system only derives its power from one of those legs. We also refer to single-phase power as split-phase power because they split into two separate, opposing phases. If you look at a waveform, you’ll notice that there are three lines: two opposite sines and one flat line. The flat line is the center tap, and the sines are the split phases. The split phases are 180° directly out of phase and always have directly opposing values. If you think about our earlier circle, 90° and 270° are 180° apart from each other, and they are the peak and trough of the wave, respectively. Therefore, when one of those split phases reaches its highest point, the other phase will bottom out. However, they cross once they reach the zero point (on the center tap). That’s because 0° and 180° are on the same horizontal line in a circle. You can see all of this in waveform in the next column. Single- and split-phase motors are usually 240v or 230v. Some split-phase power sources may be 220v, 120v, 115v, and so on, but higher-voltage applications are more common nowadays. You’ll probably see 240v the most often, but you may see some even higher voltages, like 245v or 246v.

So, where does the 208v come in?

As you could probably assume, power sources with 208 volts are NOT really single-phase. They require two legs of power from the power utility, whereas 240v motors and appliances only use one. You will commonly see 208v present in three- phase buildings when “single - phase” equipment (like an AC condenser) is wired to two legs of three-phase wye power. Here’s where some people get co nfused: if a split-phase motor splits a single power source into two 120v phases, then shouldn’t a motor that uses two legs of 3-phase power also equal 240v? Remember when I said that the sine waves of split-phase motors are 180° out of phase? The power legs of 208v motors are actually 120° out of phase. The distance between the sine waves isn’t totally opposite. They can’t reach their full potential if they don’t directly oppose each other, and the voltage is lower overall as a result. You can especially see this when you look at a graph. The sine waves look more like they wind around each other, not cross over a fixed center point. The No, and here’s why: