Arkansas HVACR NewsMagazine September 2021

HVACR NewsMagazine September 2021

Tech News

Visualizing voltage

What’s the Difference Between 208v and 240v?

Before we dive into the differences between voltage types, we should probably talk about the way we visualize voltage and how it relates to motor movement. As you know, electric motors rotate in a circular motion . We don’t view the corresponding voltage as a circle, though. We visualize and map voltage patterns as sines, which are wavy lines that rise and fall at a regular rate on a chart. A sine wave takes the rotation of a circle (in degrees) and compares it to unit time. Look at the picture below.

Emily Gutowski: HVAC School

The easy answer is 32v. Class dismissed.

I’m only joking, of course. Finding the difference between 208 and 240v power supplies may sound quite simple, but there are some pretty sharp fundamental differences. Apart from the obvious differences in overall voltage, 208 and 240v power supplies use the electrical company’s power differently and the motors see different sine waveforms. The potential across these waveforms varies, which is where we get the different voltages from. We also tend to use 240v power in residential applications, while we use 208v in light commercial applications. This article will go over the differences between single- or split-phase 240v motors and 208v ones. It will also discuss the applications of each power source type and why it may not be a good idea to use 208v motors for 240v applications.

As you can see, the sine wave “rolls out” the circle by plotting the changes in its degrees as time progresses. When the motor rotates 30° from a starting point of 0°, you can see that rise on the sinusoidal waveform. The sine wave peaks at 90°, as that’s the highest point on the rotation. It also bottoms out at 270°, which is the lowest point. You can see all of those on the left circle. Each power leg has its own sinusoidal waveform, and that’s where you will see the differences between 240v single- phase motors and 208v motors.