H-Bridge protection from DC motor noise and regen - Page 1
Forum section: electronics / projects

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 01, 2025, 05:21:28 am
content: Hello.As a long term "couple of hours every weekend" side project, im making a 30V 50A (cont) H-Bridge PMDC motor driver. It's one thing to do 50A peak, another to do it forever, fun challengeI expect this motor driver to be very abused (think fighting robots - battlebots, Sumo, etc), so I am worried about when the motor is going full power and is suddenly jammed it would induce a massive backwards voltage surge on the H-bridge output (motor would go from load to generator in that instant, and it would produce a fast surge if I understand correctly).I am not too sure if the body diodes of the MOSFETs would just conduct the excess energy into my power source (which will be LiPo or lead acid) and adequately protect from this. So, I am considering protection strategies for this.Initially, I wanted to stick a massively beefy TVS diode and hope for the best - such events would be high power, non repeating. But a beefy motor with high inertia will pack a whole lot more than a couple joules, so this solution isnt right.The two other options I am considering are as follows, I would appreciate your opinions on this please:1. Short the motor with the bottom half of my H-bridge when a surge is detected, protecting the H-bridge (I think) but stressing the motor2. Use two active clamp devices (such as the attached circuit, seen in the following article:https://www.diodes.com/assets/Articles/Automotive-MOSFETs.pdf) going from each motor terminal to ground, with a floating "gate". If sized above my driver's max voltage, but below the avalanche threshold for my main MOSFETs, I think this will provide adequate protection with proper heatsinking.Kindly let me know your opinions, thanks!Edit: Clarified the issue.Edit 2: Changed title from "H-Bridge DC motor surge protection - Active clamp?" to "H-Bridge protection from DC motor noise and regen" to be more accurate to the discussion and useful for others.

author: Smokey
author_rank: Super Contributor
timestamp: April 01, 2025, 06:07:04 am
content: um. use current mode control with your motor drive and don't command current which will make it destroy itself?Also build the drive with enough design margin to withstand the highest expected currents.Plus, don't use DC motors for stuff like this. Use a proper brushless 3phase motor and controller. There are a ton of robust, small, inexpensive options on the market now from all the ebikes and stuff like that.

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 01, 2025, 07:40:21 am
content: Quote from: Smokey on April 01, 2025, 06:07:04 amSure, but that's beyond the point. I do have cycle by cycle over current protection, and I can implement current mode or just PWM control on my controller. That's not my issue. I could be driving a motor with 50A all day no problem, but I will be in trouble when that motor is suddenly seized (have you seen battlebots? high inertia gets emptied out at once), which rapidly changes the motor from a load to a generator, dumping a lot of energy back into my circuit in a hurry. This can cause my H bridge MOSFETs to fail - that much I gathered from reading about motor controller failure modes.QuoteSure, but this is an educational project for myself, and PMDC motors are a good starting point. I've learned a lot and got pretty far, and if I would give past me some advice, it would be to start with a single MOSFET design to prototype the protections and speed control before ever attempting a full H-bridge, let alone a 3-phase one.In Jordan, there is a large "Sumo robot" competition scene, in which almost everyone uses DC motors due to cost, simplicity, and ease of access, and if I do ever finish this project, i'd love to have some robot builders evaluate it. I dont know people using brushless motors in small robotics locally, and I dont want to make another ESC.

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 01, 2025, 07:43:20 am
content: Thanks Smokey, I reviewed my post and realized it was very unclear. I made an edit, please let me know if it is clearer now

author: moffy
author_rank: Super Contributor
timestamp: April 01, 2025, 08:02:17 am
content: The body diodes should conduct before the protection MOSFET switches on, but they can be slow so you might wish to bypass the H bridge MOSFETs with some fast diodes. Place a high speed capacitor or two right next to the H bridge, and when regenerating one needs some way to disipate the excess power, the battery might not be sufficient. If you monitor the main voltage rail and it goes too high you can switch in a power resistor, think dynamic brake resistor, to help disipate the power until the voltage rail falls within bounds then it turns off.

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 01, 2025, 08:35:32 am
content: Quote from: moffy on April 01, 2025, 08:02:17 amCool, thanks for the advice. In case of shorting the motor, my circuit allows that to be done in ~1.5 us, which yeah is slower than the body diodes (reverse recovery time 72 ns -https://www.infineon.com/dgdl/BSC010N04LS_rev1+1.pdf?folderId=db3a304313b8b5a60113cee8763b02d7&fileId=db3a3043353fdc16013552c1c63647c4)During that time, voltage will rise, but how bad is it, I dont know. Then the body diodes will start conducting.- Or the fast recovery diodes will start conducting first, and I will use that: the power rail absorbs the energy and I burn it using a resistor or let the battery absorb it (while not overcharging it)- Or I short the motor 1.5 us later: the motor itself acts as the load and burns off the energy- Or I use an active clamp which will kinda sorta maybe act like a dynamic brake resistor, burning off the excess energy in those two active clamps going from each motor terminal to ground (probably with a fast recovery diode each to complete the circuit): Energy is burned by those clampsI think all three of the above are potentially valid paths, and I am having trouble understanding pros and cons, traps, things to watch out for, where each may or may not be applicable, etc. I'd appreciate if you can give me some more information. Thanks!

author: moffy
author_rank: Super Contributor
timestamp: April 01, 2025, 08:55:33 am
content: The first thing is the very fast inductive transient, that should be handled by the fast diodes and the capacitor next to the H bridge. Make sure the capacitor is large enough to absorb this initial surge without forcing the MOSFETs into avalanche. Some RC snubbers are handy also. This fast transient is the most difficult to deal with and needs good layout of the H bridge and its fast protection devices. The current, will then flow through either the fast diodes or the body diodes depending on their relative voltage drop. Whether you dump the energy/heat into the motor or use regen braking is a design choice, but I prefer dumping the energy into a resistor, but I come from a railway background where that is quite common on diesel electrics.

author: Benta
author_rank: Super Contributor
timestamp: April 01, 2025, 11:08:37 am
content: PMDC motors do not generate high voltages when the armature is locked. But they pull a heck of a lot of current.The back EMF of a PMDC is directly proportional to RPM.The internal inductance of the motor might create a few spikes, but they are easy to catch with a couple of TVS.Don't forget to place caps directly at the motor terminal to dampen brush fire noise.100 nF across the terminals and 2 x 22 nF from each terminal to motor case usually works.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 01, 2025, 11:38:53 am
content: When a motor is suddenly stalled, it won't cause power to be sourced back into the driver. Quite the opposite, it will start drawing greater current (the stall current) from the driver (unless the driver acts to limit this). Basically what Benta said above.The only circumstance where you could see net power being sourced from the motor to your H bridge is when a net torque is applied to the motor which is opposite to the direction the H bridge is trying to drive it. This net torque could come from the robot's own inertia as it decelerates quickly (i.e. regenerative breaking), but in this case the magnitude of this regenerated current/power should be benign compared to what the driver had to produce to accelerate the robot in the first place.That is, of course, assuming the regenerated power is allowed to flow back to the power source (a battery, I assume). If, for example, there were a diode in between the battery and H bridge (maybe for reverse polarity protection), then any regenerated current may cause the voltage across the bridge to shoot quickly, potentially damaging the bridge. There are many ways to deal with this, such as using large capacitors or clamp circuits to absorb the power, or you could control the H bridge to brake the motor (this would prevent the regenerative braking, but may also result in very large currents in the bridge, potentially harming the MOSFETs).

author: SteveThackery
author_rank: Super Contributor
timestamp: April 01, 2025, 12:13:57 pm
content: Exactly what I was thinking - the premise of the question is faulty.Jamming the motor won't produce a back EMF "voltage surge". It doesn't become a generator because it isn't rotating; it becomes a resistive/inductive load, so the current in it will rise rapidly (at a rate affected by the inductance) until the resistance limits the current.

author: H.O
author_rank: Frequent Contributor
timestamp: April 02, 2025, 02:55:13 pm
content: Quote from: Benta on April 01, 2025, 11:08:37 amAre those caps, especially the one across the terminals, really a good idea with a PWM-type controller? Wouldn't it present itself as a rather low(ish) impedance shunt to the PWM signal placed across it?

author: johansen
author_rank: Super Contributor
timestamp: April 02, 2025, 05:30:38 pm
content: The issue that people just simply don't understand is that what the motor does with the H bridge is not the problem. the mosfet turns off, current returns through the body diode to the battery.its the inductance from the battery to the H bridge that blows up the mosfets due to avalanch damage.the fix is that you put large enough low indutance capacitors at the H bridge, or you put some kind of voltage clamp that can handle the peak currents in the motor at the time the mosfet shuts off.if the motor shorts out and on a nominally 50 amp motor, you have 500 amps flowing.. what happens to the battery?if the BMS open circuits, where does that current go? well it continues flowing into whatever it can find a way into until the energy is dissipated either as heat or stored charge on the capacitors.

author: Benta
author_rank: Super Contributor
timestamp: April 02, 2025, 06:26:31 pm
content: Quote from: H.O on April 02, 2025, 02:55:13 pmIt basically depends on the PWM carrier frequency, but is usually negligible. At 1 kHz, 100 nF is around 1.6 kohms.That being said, I'd not go much over 3 kHz PWM with the cheap canned PMDC motors; the iron losses in the armature (and other losses) become significant and result in bad speed regulation and poor torque.

author: SteveThackery
author_rank: Super Contributor
timestamp: April 02, 2025, 06:35:55 pm
content: Quote from: johansen on April 02, 2025, 05:30:38 pmFascinating! I've learned something new today. So just to check: when the motor jams, all it does is draw loads more current, right? The higher current blows a fuse or a track, and at that moment the inductance of the track creates a high enough voltage to blow the MOSFETs. Is that correct?I'm amazed that an inch or three of track has enough inductance to create that much damage. And won't the track's capacitance with the ground plane or adjacent conductors cancel out most of the inductance?By the way, the premise of the OP's question is still wrong: jamming the motor will not, of itself, create "a massive backwards voltage surge".

author: johansen
author_rank: Super Contributor
timestamp: April 02, 2025, 06:40:34 pm
content: Quote from: Benta on April 02, 2025, 06:26:31 pmHigher is better.Its the ac component of the ripple current that causes heating.

author: johansen
author_rank: Super Contributor
timestamp: April 02, 2025, 06:42:25 pm
content: Quote from: SteveThackery on April 02, 2025, 06:35:55 pmYou dont have to open the circuit for the problem to occur.the energy stored in the inductance of the battery is proportional to the amps squared. This is why so many failures arise from breadboard prototypes that work at 10 amps fail at 100.In addition, you have to factor in the ESL and the ESR of the capacitors at the H bridge as well when estimating what is the actual drain source voltage at turn off under short circuit conditions.

author: Benta
author_rank: Super Contributor
timestamp: April 02, 2025, 06:50:23 pm
content: Quote from: johansen on April 02, 2025, 06:40:34 pmSo please explain why high.grade ferrites are used for other switching applications,According to your hypothesis, laminated iron would be just as good.Believe me, been there done that. The cheap canned PMDC motors (eg, Mabuchi, Johnson Electric etc.) do not like high frequencies. At full speed it's no issue, as 100% duty cycle = DC voltage. But at partial speed regulation and torque is horrible.

author: johansen
author_rank: Super Contributor
timestamp: April 02, 2025, 06:53:23 pm
content: Quote from: Benta on April 02, 2025, 06:50:23 pmYou likely have other problems such as minimum on and minimum off times screwing up your control loop.

author: Benta
author_rank: Super Contributor
timestamp: April 02, 2025, 08:26:29 pm
content: Quote from: johansen on April 02, 2025, 06:53:23 pmCompletely irrelevant. Control loops are not even in the picture here.I suggest you do the following:Connect a PMDC motor to a lab supply and vary the voltage. RPM will be proportional to voltage.Now connect the motor to a 0...100% PWM source and see what happens when varying D.C.If the carrier frequency is high enouch, the motor won't even start.Now, a control loop can fix that if you have tacho (or other) feedback. But the RPM/voltage proportionality is gone.Try it in practice, please, you'll see.

author: johansen
author_rank: Super Contributor
timestamp: April 03, 2025, 03:02:46 pm
content: Quote from: Benta on April 02, 2025, 08:26:29 pmNonsense..

author: H.O
author_rank: Frequent Contributor
timestamp: April 03, 2025, 05:15:14 pm
content: Quote from: Benta on April 02, 2025, 06:26:31 pmOK, my experience is more towards to the PMDC servo motor category than the cheap canned motors. In my own design (and many others I've seen and used) 20kHz seems to be the norm. I've got some drives from Maxon and they use 50kHz.Is it possible that the comment about the motor not starting with a high enough carrier frequency is indeed because the recommended cap is across the terminals ;-)

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 06, 2025, 06:39:02 am
content: Hi all! Thanks for the constructive discussion, I did not get the notifications after my latest replyThere are multiple topics discussed here, let's start with my question.Quote from: Benta on April 01, 2025, 11:08:37 amQuote from: mtwieg on April 01, 2025, 11:38:53 amQuote from: SteveThackery on April 01, 2025, 12:13:57 pmWhen I started this project, I extensively researched driver and MOSFET failures. I went back to my sources, and I believe that I misunderstood the "Jammed (or blocked) motor" section of the following:http://4qd.co.uk/docs/mosfet-failure-mechanisms/It's not the act of stopping the motor suddenly that causes the issue (as I thought), but rather the bounce/vibration causing high dv/dt noise. If I manage this high speed noise (with the capacitor on the motor terminals, good low inductance layout, and high speed capacitors - probably a peppering of ceramics along and Al Poly caps), my driver could survive. While I dont understand the issue fully, there exists a dynamic issue that I think cannot be simplified to 0 RPM = 0 back EMF, but its not the issue I thought. Thanks for helping me understand the issue better.Quote from: johansen on April 02, 2025, 05:30:38 pmYeah, I agree, I've seen robots with long, separated battery wires, and I've seen relays implemented on power rails to switch batteries from parallel to series for an extra push in some competitions. I have a beefy TVS on the power input for such cases - energy stored in the wires should not be too large, but it will need to be dissipated or absorbed into the capacitors quickly. I understand that having a TVS can be a bad strategy in some situations, but I think it is probably fine on the battery input alongside some capacitors.All in all, I think I have a better understanding of the issue I was worried about, it does not seem to be a high frequency and high energy issue together in one, but rather that I may get a lot of high speed spikes that I need to deal with. A small capacitor on the motor terminals will absorb some of those spikes, then fast recovery diodes should be used to shunt those spikes into capacitors. The capacitors and layout should be conducive to dealing with high frequency spikes. This is a different issue than regen and managing that energy.As these spikes are continuously repeating, using a TVS diode across the motor may not be a wise strategy - they are not made for continuous dissipation. I should understand the common characteristics of these spikes and design accordingly, instead of speculating.The next topic is slightly exciting for me - the PWM frequency. My driver is designed for 25 kHz operation to push the noise to an inaudible range. I've seen young kids in such competitions, and I dont want this to be a constant torture for themQuote from: Benta on April 02, 2025, 06:26:31 pmQuote from: Benta on April 02, 2025, 06:50:23 pmQuote from: Benta on April 02, 2025, 08:26:29 pmI think both Benta and johansen are partially correct here, this was one of the things I found out early on. This is a learning project, but it did not start off as a Silicon H-bridge. I initially wanted to make a buck/boost GaN converter for motor drive while I was still pursuing my bachelors -> too complex for my skillset, so I pivoted to H-bridge. There are multiple drive modes for an H bridge such as sign magnitude drive and lock anti-phase drive. Below is a simplified figure I created that illustrates both drive modes:Lock anti-phase has its advantages, but it requires considerable capacitance, so I pivoted to a high switching frequency GaN H-bridge for DC motor drive, thinking I would be able to reduce capacitor size considerably. Theoretically, the inductance of the motor would reject the switching, and a DC current would flow into the motor, but that's not the case. Probably due to motor internal parasitic capacitance.I asked a question on stack exchange about this (question here:https://electronics.stackexchange.com/questions/635373/how-would-a-brushed-dc-motor-react-to-high-pwm-frequency), and got recommended the following excellent article which addresses this. I see that they added more testing to it since then, and I do recommend that you have a look at their data:https://learn.adafruit.com/improve-brushed-dc-motor-performance?view=allHere's a snippet from that link. You can see that the motor has slightly different characteristics depending on the drive frequency, but also the decay mode (slow decay is when the motor is shorted during PWM "off" time, fast decay is when the FETs are turned off during that time). You can also see that the same motor model -but with a heavier gearbox- fails to start before the duty cycle is pretty high. This supports the argument that high frequency drive reduces torque.One potential solution I found at the time was to detect motor stall (including startup), start the motor at 100% duty and then throttle it as needed. But I do not know if the high frequency necessarily reduces running torque, or if it just affects startup. Once those motors are started, reducing the PWM duty cycle works as expected. I decided to not go down that path, and chose the 25 kHz frequency.While I think that you dont want your drive frequency to be too low or too high, 3 kHz sounds incredibly low to me even for a cheap motor, so I would investigate that further. But I think this does also show that control loops are relevant - slow decay vs fast decay, sign magnitude vs lock anti-phase will all produce a very different result here. As frequency goes up, dead time quickly becomes an issue in Si systems, turning your control loop into fast decay by virtue of the dead time becoming significant timing wise.

author: johansen
author_rank: Super Contributor
timestamp: April 06, 2025, 01:00:48 pm
content: I dont see how any change in the h bridge control topology changes the amount of capacitance needed across the H bridge to manage the supply inductance. even if you can guarantee the ripple current is cut in half by a free wheeling topology, it may not actually change anything. The caps have to be 10 times larger than the theoretical minimum anyways, to handle the esr Esl requirements and short circuit conditions.Also if emi is not a problem then no need for a capacitor (which should have a resistor in series) across the motor.Motors are very lossy inductors and there may be some benifit to placing an inductor in series with the motor. If you do this, you can then place a large value capacitor across the motor...Note that now you have a buck converter and your motor rpm should better match the pwm percentage and will behave exactly like a variable dc voltage source.Be careful drawing conclusions from software pwm and adfruit motor controllers. 27 khz is not so high that dead time should take out half the 0 to 5 volt range?!Its interesting to me how strictly linear the 27khz volts vs rpm is on the upper right graph, but the 16khz is not, until the last 10%. Something very screwy is going on there

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 07, 2025, 02:12:45 pm
content: Quote from: AnasMalas on April 06, 2025, 06:39:02 amWell that section of the article is just word salad IMO, so anyone would be forgiven for misunderstanding it. I can't even tell if it's trying to describe a real problem or not.QuoteI'm assuming that in their "fast decay" mode they are disabling the driver during the PWM "off" time, rather than enabling the driver with opposite polarity (equivalent to locked antiphase pwm). In the former case, it's not surprising that there will be some observed nonlinearity of speed/torque with respect to PWM duty cycle and frequency.If the nonlinearity is a problem, then just use sign-magnitude or locked-antiphase drive.Quote from: johansen on April 06, 2025, 01:00:48 pmI'm guessing they're thinking just about PWM ripple current, not dumping all the stored energy into the DC bus at once. Obviously in the latter case the PWM control method and frequency doesn't matter.

author: johansen
author_rank: Super Contributor
timestamp: April 07, 2025, 04:34:15 pm
content: Quote from: mtwieg on April 07, 2025, 02:12:45 pmPure genius.

author: Picuino
author_rank: Super Contributor
timestamp: April 07, 2025, 04:47:35 pm
content: I join the thread.

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 08, 2025, 02:28:27 pm
content: Quote from: Picuino on April 07, 2025, 04:47:35 pmWelcome! Anything to add?Quote from: johansen on April 06, 2025, 01:00:48 pmThanks for your reply, you did expose to me that I may have misunderstood the energy demands of a motor. I did think that managing ripple currents and noise was the primary job of the on board capacitors - lots of capacitance needed for locked-antiphase drive (I calculated it at 667 uF min for my board), not as much for slow decay sign magnitude (where you push some energy into the motor, then short its leads during off time. Calculation came up at 16.7 uF min). Source:https://www.modularcircuits.com/blog/articles/h-bridge-secretsMy "mental model" for this was that a motor's inductance would be much higher than a LiPo's inductance - so energy demands other than ripple can be directly fed from the LiPo. I should read a bit more about this, or maybe I should probably just send the board out and test it - 3 years after starting this hobby project, maybe ill learn more that way.One thing I dont understand is why would I care about source inductance in case of a short circuit? Can you please elaborate on that? If anything, seems like a high source inductance and less onboard capacitance would be better there.QuoteI dont plan on selling this board, so I probably wouldnt care too much about emi. I do really want it to work well though - does that cap not also offer some protection to the circuit from the spikes (from commutation or else)?Quote from: mtwieg on April 07, 2025, 02:12:45 pmYeah - one curse of going into something you dont know much about with no mentorship is the quality of sources. I consider this to be one of the "better" ones I found, it's not easy to follow, but couldnt find better:https://www.modularcircuits.com/blog/articles/h-bridge-secrets/sign-magnitude-drive/.Do you happen to have good sources at hand? I would appreciate some, thanks!

author: johansen
author_rank: Super Contributor
timestamp: April 08, 2025, 06:29:17 pm
content: Quote from: AnasMalas on April 08, 2025, 02:28:27 pmthe energy stored in the source inductance follows the current squared.lets say the motor shorts out and the gate controllers all turn off. what happens?well its pretty simple.. the current keeps flowing.. where does that energy go? -into the dc capacitors at the H bridge.what happens when there is too much inductance between the mosfets and the capacitors? well they release the magic smoke.you don't need anywhere near 600uf for a 50 amp h bridge. the battery can handle the ripple current, but you need something more like 10uF worth of low inductance capacitors located right next to the mosfets. 50 amps in 1uH source inductance is 1.25 mJ. that energy will make 10uf increase from 0 to 15 volts. lets say you have a 500 amp short circuit current. now that 10uF h bridge cap gets charged up to .125 joules which will push it from 0 to 158 volts. are your 60 volt mosfets big enough to handle .125 joules of avalanche current? probably not.here is some construction ideas:https://highvoltageforum.net/index.php?topic=1324.0

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 09, 2025, 05:04:57 pm
content: Quote from: johansen on April 08, 2025, 06:29:17 pmHuh, I hadnt considered that. Thanks for bringing it to my attention.If im running at 30V 50A, and my output is shorted, my circuit takes ~1.5 us to turn off (in which it shorts the motor before deciding to try the next cycle, or to shut off). If my source inductance is 1 uH, and im already at my 50A limit when the short happens, then the max current I can get to on my input is 95A, which now must go into my capacitors. That's about 4.5 mJ, and my circuit currently contains 29 uF of ceramic capacitors (after DC bias) which will charge from 30V (assuming worst case) to 35V, and I do have a TVS diode on the input too.I double checked. While 1 uH would likely be fine, a smaller input inductance is worse. For example, at 100 nH, you get 500A, 12.5 mJ, increasing my capacitors to 42V. At 10 nH - even worse, 4550A, 104 mJ, 90V. I added a TVS on the battery input to protect it from external spikes, funny that the TVS will protect the driver from itself... protecting itself.My TVS (MDD SMCJ30A) seems to be able to take a pulse of ~1-2 J, which corresponds to around 0.5-1 nH as my minimum, but a TVS cant sink 500A, and the entire situation would be pretty dynamic... I need to do more simulation on this topic, but it seems like an input inductance of lower than 0.5 nH would be problematic. That's fun.QuoteThat's very cool! My layout is looking like I would need multiple parallel conductors to support the 50A, with ground and all, im looking at a 6 layer PCB currently. The PCB itself will be pretty low inductance. Every layer will reference a ground plane in a [SP G SP S G SP] stackup :DI get why it doesnt exist (dissimilar thermal expansion), but imagine if you could buy SMD busbars from digikey and just plop them down with a pick and place and solder them. Would make high current design easier.Anyway, even with 6 layers I am managing to stay under $30 BoM cost (at 1k qty) including PCB, heatsink, and fan - was part of the self imposed challenge of this project :)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 10, 2025, 06:09:13 am
content: Quote from: AnasMalas on April 09, 2025, 05:04:57 pmNote that reasonable gauge wire in free air, is 10-15nH/inch ballparks.https://coppermountaintech.com/what-is-the-inductance-of-a-straight-wire-in-free-space/

author: MathWizard
author_rank: Super Contributor
timestamp: April 10, 2025, 01:26:53 pm
content: So if you use a brushed DC motor to lift a mass, and then want to hold it it place, your motor would have to be not turning, and just acting as an electromaget, with potentially a huge DC current.I have a couple of big brushed DC motors I want to gear up, and see what's possible to lift with them. Lately I'm brushing up on linear systems of ODE's, and I remember trying the ODE's for motors before, so it should be way easier to absorb all that again.But yeah I'm guessing the max Fg the motor can balance against, will be greatly limited by the max safe DC current.

author: AnasMalas
author_rank: Regular Contributor
timestamp: April 10, 2025, 03:46:25 pm
content: Oh hey oh hey, look, two wizards appear in quick succession! How coolQuote from: PCB.Wiz on April 10, 2025, 06:09:13 amVery nice! I went and checked some other scenarios, like a tightly twisted pair, but it seems like 1 nH is not very realistic, and that ignores the battery's own parasitic inductance, and the connectors, etc. Phew!I will need to come up with an educated guess for minimum realistic inductance and make sure my board that survive that.Quote from: MathWizard on April 10, 2025, 01:26:53 pmThat's an unexpected use case for a board like mine! With cycle by cycle over current protection, you can start by overloading the motors quite a bit and the current would be pegged to the max by the driver. by removing mass, you can find out how much current carries how much weight. I guess the configurable current limit (0 A up to ~62 A) would also come in useful here ;)I do wonder how long the motors would survive under such conditions

author: MathWizard
author_rank: Super Contributor
timestamp: April 10, 2025, 04:14:21 pm
content: I wish there were robot demolition wars around here.The 2 motors I have, were from an adjustable bed, like a hospital bed. And they have a controller box. I'm sure their controller turns them very slowly, but with a lot of torque, against a worm gear or something. And friction alone or something locks it all in place, with the motor off of course.But yeah I better carry on with the equations before going and testing the motor/controller. Making any gears, would be a whole other problem. And paying a local metal shop, would cost a fortune too.