Electrical Myths - Single phase is Really 2-Phase - Short #157
HVAC School - For Techs, By Techs
Bryan Orr
4.9 • 1K Ratings
🗓️ 15 November 2022
⏱️ 9 minutes
🧾️ Download transcript
Summary
In this short podcast, Bryan busts the common electrical myth that single-phase 240v power is really two-phase power.
When power goes into a structure that runs 240v appliances, we may understand that two 120v sine waves are 180 degrees out of phase with each other, but that isn't 100% accurate. If we were to use an oscilloscope to watch the electrical sine waves, we would see two sine waves 180 degrees out of phase because the transformers are center-tapped. Center-tapping creates a neutral center point that becomes our reference.
The transformer has two sides: a primary and a secondary. The number of wraps on each side is proportional to the other, and the number of wraps also dictates whether a transformer steps the voltage up or down. However, when you use the center tap as a reference, that also makes the voltage appear to be halved.
In many residential structures, a single phase of power goes into the transformer from the power company. If you were to use the center tap as your reference on each side of that transformer, you would read 120v; the two 120v readings add up to 240v. However, if you were to use the other side as the reference (as in a corner-tapped transformer), you would read 240v.
On an oscilloscope, you would see the same thing; using the center tap as the reference, you would see two 120v sine waves completely opposite each other. If you were to measure completely across the transformer, however, you would see a single 240v wave, which is larger.
Remember: only one phase comes from the power company. We only appear to get two separate waves because of our available point of reference.
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Transcript
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| 0:00.0 | Hey, hey this is the HVAC school podcast. This is a quick episode. It is in our |
| 0:08.1 | electrical myth series. I will be covering all of these myths and a bunch of |
| 0:12.4 | other stuff at the National |
| 0:13.8 | HVC Excellence Conference on Tuesday, March 21st, 2023, 4 p.m. to 5.30 in Sonoma, |
| 0:21.1 | room B, as if you're going to remember that. |
| 0:23.7 | But I would encourage you to go to escogroup.org and look into getting a ticket to be there |
| 0:28.7 | because it's going to be a good class. |
| 0:29.7 | We're going to have a lot of fun to answer some questions, but this is basically just some snippets of what I'm |
| 0:34.3 | to be covering. Now one of the myths, common electrical myths and one that I actually have |
| 0:39.3 | perpetuated and that's why it's so important that I talk about it on this podcast because it's one that I have poorly explained in the past and in fact did not fully understand even myself |
| 0:50.0 | Is this idea of single phase 240 and the relationship that that has with 120 |
| 0:56.8 | volt sign waves and those being out of phase with each other. That's something I've |
| 0:59.7 | said a lot that they're 180 degrees out of phase and that's not quite right. So before we talk about that |
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