Motor Speed Facts - Short #143
HVAC School - For Techs, By Techs
Bryan Orr
4.9 • 1K Ratings
🗓️ 10 May 2022
⏱️ 10 minutes
🧾️ Download transcript
Summary
Bryan lays down some motor speed facts in about 10 minutes in this short podcast episode.
We can figure out how quickly a single-phase motor (PSC) will run if we understand how many cycles it will make per second. In the USA, the standard hertz is 60 Hz (60 rotations or magnetic alternations per second). Motors are inductive loads that create an electromagnetic field with a spinning rotor and stationary stator; the amount of poles on the stator determines how quickly the rotor spins (RPM).
In the RPM counts, there are some allowances for slip. Slip varies depending on the load, with excessive loads causing more slip. Some multi-tap blowers have additional winding resistance and decreased current (due to the extra taps), which increase the slip. The rated load RPM usually accounts for the RPM at high speed, not medium or low speed with added resistance.
On the other hand, variable-speed motors or ECMs are powered by a variable frequency (sometimes a variable frequency drive or VFD). The motor control takes the incoming electrical frequency and converts it into a new frequency (turning AC power to DC and controlling the cycle rate). These motors also tend to be more efficient as a result. The RPM is more variable on these motors with VFDs, whereas we could only manipulate the RPM of single-phase motors by changing the number of poles.
When replacing a motor, you can't use a replacement motor with a higher rated RPM than the original motor. The only way to change the RPM is to get a new motor with a different number of poles, increase slip to make it slower or decrease slip to bring it closer to synchronous speed, or adjust the frequency.
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Transcript
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| 0:00.0 | Hey, hey this is the HVAC school podcast. This is a short episode and we're talking about |
| 0:08.8 | the basics of motor speed, motor speed control. Just a little bit of how that works and focusing mostly on single phase. |
| 0:14.8 | So those of you who are super excited from the more commercial side of things, we're not going |
| 0:20.1 | to be doing a lot of that, but we'll talk about it a little bit. |
| 0:22.2 | But before we do that, we want to thank our great sponsors |
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| 1:35.0 | All right, so basic question, how do we know how fast a single phase motor will run? Let's start there. So a single phase |
... |
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