Timeline for Is there an ideal PWM frequency for DC brush motors?
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| Aug 3, 2023 at 3:57 | comment | added | tomnexus | This answer gives no indication whether it means the electrical time constant (1 ms?) or the mechanical one (200 ms?), but a comment indicates it's the latter. Any sort of PWM will work to make a motor slower - think of a bad driver managing speed in a car by pressing and releasing the gas - but only several kHz PWM will deliver essentially constant current to the motor. | |
| Mar 26, 2021 at 0:13 | comment | added | KJ7LNW | It would be nice to see an equation in this answer noting that τ=L/R! I couldn't find τ explained above, but this article does a good job: electronics-tutorials.ws/inductor/lr-circuits.html . Thus, a good PWM frequency is at least 1/τ = R/L if I understand correctly. This post says maybe about 10x more than that so the PWM pulse time is 10% of L/R: electronics.stackexchange.com/a/555234/256265 | |
| Mar 25, 2021 at 14:10 | history | edited | winny | CC BY-SA 4.0 |
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| Mar 25, 2021 at 13:04 | history | edited | Null♦ | CC BY-SA 4.0 |
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| May 27, 2019 at 23:22 | comment | added | Big6 | @PageDavid It's been a moment since I did this, but you can measure this experimentally by applying an input voltage to the motor and see how long it takes for the angular speed to reach 63.2% of it final value. You may have to iterate this a couple of times and find the avg (though it should be fairly close from measurement to measurement). For this you'd need the right equipment, like tachometers/other tools. Maybe this link will help: mech.utah.edu/~me3200/labs/motors.pdf or google "find dc motor time constant"—this is one of the most common experiments in intro controls course. | |
| May 27, 2019 at 1:23 | comment | added | Page David | Thanks for your informative answer. I’d like to ask where could I get the motor time constant? For example, the datasheet for GA12-N20 does not have an entry named motor constant. Should I measure it on myself? If so, how? | |
| Jun 23, 2016 at 22:27 | history | edited | Big6 | CC BY-SA 3.0 |
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| Jun 23, 2016 at 20:49 | comment | added | Big6 | Let us continue this discussion in chat. | |
| Jun 23, 2016 at 20:21 | comment | added | alain | That was a rhetorical question. The first two graphs were correct, the new graphs are wrong. From v(t) = L*(di/dt) it's clear that the rate of current decay di/dt depends only on L and v. Anyway, I'm much too late, and I think we should not start a long discussion here. Let's just agree to disagree :-) | |
| Jun 23, 2016 at 20:06 | comment | added | Big6 | @alain I will give you a couple of references to sites that explain it. It's kind of long to explain here. 1) homepages.which.net/~paul.hills/SpeedControl/… 2) eg.bucknell.edu/~wismer/ee491/note4/index.html | |
| Jun 23, 2016 at 20:03 | comment | added | alain | Why does the current decay faster on the right graph? | |
| Jun 23, 2016 at 17:30 | history | edited | Big6 | CC BY-SA 3.0 |
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| Jun 23, 2016 at 17:11 | history | edited | Big6 | CC BY-SA 3.0 |
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| Jun 23, 2016 at 16:32 | history | edited | Big6 | CC BY-SA 3.0 |
Clarified that the previous explanation still holds but I was using the wrong parameter to present the information. Added a new figure to show what I meant.
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| Jun 23, 2016 at 15:50 | comment | added | Big6 | @alain Thanks. You are right. This goes back to the first comment made here by _transistor. I plotted the speed in response to the input voltage. These plots should show the current in response to the PWM signal instead. Then we should linearity in terms of the current drawn and the PWM duty cycle. | |
| Jun 23, 2016 at 13:31 | comment | added | alain | The facts presented do not support the conclusion: Both graphs have an average of 0.5. I think this reflects reality, the linearity does not depend on the PWM frequency. The only compromise to be made is current/torque ripple and noise on the lower side, and eddy current and switching losses on the higher side. | |
| Jun 22, 2016 at 17:56 | comment | added | Big6 | @NateSan Thanks! As one of the answers, which are really good, the best you could do is start with frequencies in the KHz range, like 2KHz for example. There is no way to estimate the time constant based on the given information or at least I don't know. You can find it experimentally, but you're better off just trying different frequencies until you get close to what you want. | |
| Jun 22, 2016 at 17:43 | comment | added | Nate San | This was very informative! I am not an EE, so I'm not extremely educated in this. I will likely just try different frequencies until I get a response that I like across the spectrum I need to operate in. However, I will keep this in mind when doing that setup! . I do have one question though. You said these numbers were all very theoretical, but could you give a ball park of the expected time constant? It's a 24 V dc motor that draws at most 300 mA. | |
| Jun 22, 2016 at 17:27 | comment | added | Transistor | Good answer. You might clarify that in saying "the time it takes the motor to reach almost 100% its final value" that you mean final or full current value. Readers may confuse it with 100% speed or who-knows-what? | |
| Jun 22, 2016 at 17:23 | vote | accept | Nate San | ||
| Jun 22, 2016 at 16:23 | history | edited | Big6 | CC BY-SA 3.0 |
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| Jun 22, 2016 at 16:18 | history | answered | Big6 | CC BY-SA 3.0 |