DC motor constant speed regardless of load - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: August 13, 2024, 09:44:35 pm
content: Hi,I don't have much experience in DC motors other than basic speed control using PWM but I would like to build a very simple small coil-winding machine consisting using this simple lathe from Amazon:https://www.amazon.co.uk/gp/product/B07HKZPDDD/ref=ox_sc_act_title_1?smid=AJPBV39SXHGIV&psc=1and I am going to guide the wire by hand.It uses a 12V DC motor and ideally I would like it to:- run at very low RPM, maybe even as low as 30 to 60 RPM- maintain constant speed regardless of load (a very small range in load). The load is just my finger which presses against the coil while it is being woundFrom past experience, especially at extremely low RPM, the smallest change in load made the DC motor behave a little erraticaly.Are there simple low cost solutions/controllers that can achieve both of the two points mentioned above?Perhaps some back EMF controller or some other technology/system/solution?I could not find any on Amazon (but perhaps I was searching for the wrong keywords)?Thank you

author: IanB
author_rank: Super Contributor
timestamp: August 13, 2024, 10:38:03 pm
content: Have you looked this other thread on a similar topic? There were various ideas posted there:https://www.eevblog.com/forum/projects/brushed-dc-motor-closed-loop-speed-control/

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2024, 10:51:49 pm
content: Not worth the trouble, and you'll never get it down to that speed... well, you might, but it'll be so jerky that it's useless. 60 rpm? That's one rev per second with a normal 3-winding, 2-magnet DC motor. No way.Make a stand for your battery screw machine instead.

author: IanB
author_rank: Super Contributor
timestamp: August 13, 2024, 11:01:14 pm
content: Well, yes, that machine is designed to run at 5000 rpm. To get low speeds, you are going to want a gearbox or other reduction gear. With a reduction gear the torque will be so high there will be no significant speed variation with load.As Benta suggested, look at using a cordless drill or screwdriver with speed control on the hand trigger.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 13, 2024, 11:23:52 pm
content: Thank you all,yes it looks like the cordless screwdriver is a much better solution.Any suggestion on a practical way to connect a chuck or other coil-supporting means to the hex screwdriver head hole/slot?I thought of a chuck with an hex shaft but have only found relatively small chucks (max 6.5 mm drill bit).Thank you

author: IanB
author_rank: Super Contributor
timestamp: August 13, 2024, 11:52:46 pm
content: Well, cordlessdrillscome with a chuck. Cordlessscrewdriversnot so much.Just get a cheap cordless drill from Lidl or Aldi or some such. Or get a more expensive one if you also want to use it for drilling.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 14, 2024, 12:24:29 am
content: Sorry... in the rush I wrote drill but meant cordless screwdriver (I now corrected it).And when I read the previous post in my mind I "read" screwdriver instead of drill.I think it might be time for me to have a break and relax a bit...Yes a low speed drill would work perfectly. Disregard my previous silly post...!!!Thank you all again!

author: Siwastaja
author_rank: Super Contributor
timestamp: August 14, 2024, 06:37:16 am
content: Position sensing e.g. by optical encoder, fast motor current control loop, and very finely tuned speed control feedback loop. And still getting it run stable at such low RPM is very difficult. Hence +1 to gearing the motor down. Making it run smoothly at 300rpm is so much easier than at 30rpm.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2024, 06:54:11 am
content: Why not use a small stepper motor? Then the RPM can be controlled by a function generator.

author: ricko_uk
author_rank: Super Contributor
timestamp: August 14, 2024, 03:41:42 pm
content: Yes, all good alternatives.Thank you both

author: Siwastaja
author_rank: Super Contributor
timestamp: August 14, 2024, 06:06:15 pm
content: Stepper is not bad - open-loop of a motor which is built as very high pole count (thus low fundamental RPM) side-steps all control difficulties. The downsides are much worse torque density than similar geared DC motor - asmallstepper might not cut it - and poor efficiency. Neither is probably a problem here: use a tad larger stepper and let it heat up. But do get a micro-stepping controller, otherwise the vibration from full-stepping drives you mad.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 14, 2024, 06:17:23 pm
content: Quote from: Siwastaja on August 14, 2024, 06:06:15 pmSeconded!Also, the torsional vibration might stress the wire. The last coil I wound had wire not much thicker than a hair. I had to build a very soft mechanical "buffer" (a pulley wheel on a very long, very low-rate spring) just to avoid breakages arising from the wire tugging on the reel.I think I'd go for a geared motor myself, with a nice, smooth speed control.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2024, 08:45:42 pm
content: Quote from: Siwastaja on August 14, 2024, 06:06:15 pm20 years ago we were using stepper controllers which worked great, and had one external resistor for current programming. But the current crop of Chinese steppers which are available do not seem to work nearly as well. My experience is that microstepping on a cheap Chinese controller was *worse* than full or half steps.Quote from: SteveThackery on August 14, 2024, 06:17:23 pmMaybe add a pulley reduction with a thinner rubber belt?

author: ricko_uk
author_rank: Super Contributor
timestamp: August 15, 2024, 03:57:39 am
content: Thank you all for the additional suggestions, all valid and interesting

author: SteveThackery
author_rank: Super Contributor
timestamp: August 15, 2024, 11:33:54 am
content: Quote from: David Hess on August 14, 2024, 08:45:42 pmYeah, maybe. Myself, I just don't think a stepper motor is right for this application. A geared motor with a speed controller is just easier and better. Especially if it could be lifted whole from a cheap cordless screwdriver.

author: BennoG
author_rank: Frequent Contributor
timestamp: August 15, 2024, 06:31:22 pm
content: Quote from: David Hess on August 14, 2024, 08:45:42 pmYou can still get these gecko drivers.Benno

author: David Hess
author_rank: Super Contributor
timestamp: August 15, 2024, 10:52:51 pm
content: Quote from: BennoG on August 15, 2024, 06:31:22 pmI will check them out.I tried to find the company that I used 20+ years ago, but could not remember the name, something like "SMC", and searches turned up nothing to jog my memory. Maybe they no longer exist.The Chinese stuff all seems to rely on the TB6600. Maybe the DM556 is better.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2024, 11:01:10 pm
content: Just buy something like this. Can't get much cheaper:https://www.amazon.de/JGY-370-4632-370-reducer-motor-self-locking/dp/B0CQPHG8GC

author: SteveThackery
author_rank: Super Contributor
timestamp: August 16, 2024, 08:27:47 am
content: Quote from: Benta on August 15, 2024, 11:01:10 pmAlthough a speed controller would be required as well.

author: EPAIII
author_rank: Super Contributor
timestamp: August 16, 2024, 09:33:33 am
content: The trick to slow speed, DC motors is pulse width modulation. I have had good results with using a 555 timer circuit, followed with a current booster. It provides full rated torque (well to a degree) at very slow speeds. That high torque will help provide a constant speed over a reasonable range of loads. If that is not good enough, then a closed loop servo can be employed.A stepper motor is another way. It also employs the rated torque (also with limits) on each step so there is, in theory, no lower limit on RPM. But it will be in steps. A high belt or gear ratio can help make the steps too small to notice. And it will also provide more torque.I would choose the DC motor with pulse width modulation if the requirements are not too severe. The project would be easier to finish.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2024, 11:13:44 am
content: DC or PWM or whatever. It will never solve the problem of commutator cogging.

author: Circlotron
author_rank: Super Contributor
timestamp: August 16, 2024, 12:14:35 pm
content: Automatic sliding door motors come in PMSM or DC with x pulses per rev tachometer. And typically 15:1 worm gear reduction. Dunkermotoren DC ones in particular are very smooth. Motor size in the pictures is typically about the size of a drink can.https://www.google.com.au/search?q=automatic+sliding+door+motor&sca_esv=560d0aa0f230caa6&sca_upv=1&udm=2&biw=1309&bih=957&ei=ckG_Zq-gBtCdseMPldm4iQI&ved=0ahUKEwivof2ivfmHAxXQTmwGHZUsLiEQ4dUDCA8&uact=5&oq=automatic+sliding+door+motor&gs_lp=Egxnd3Mtd2l6LXNlcnAiHGF1dG9tYXRpYyBzbGlkaW5nIGRvb3IgbW90b3IyBRAAGIAEMgUQABiABDIGEAAYCBgeMgcQABiABBgYSJguUKwRWJ8ocAF4AJABAJgBuAGgAZIMqgEEMC4xMLgBA8gBAPgBAZgCC6AC1QzCAgoQABiABBhDGIoFwgIGEAAYBxgemAMAiAYBkgcEMS4xMKAHthM&sclient=gws-wiz-serp

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2024, 01:53:04 pm
content: Quote from: EPAIII on August 16, 2024, 09:33:33 amMicrostepping reduces the step size so much that it might be ignored, however my own experience is that some microstep drivers are much better than others. They should all be the same because it is easy to implement, so I have no idea why results vary so much.Analog quadrature signals can also drive stepper motors without cogging, however I think the sine drive has to be corrected (arctan?) because the rotor position is a non-linear function of the magnetic flux. Digital microstepping should be much easier because the correction can be built in, but see above.Quote from: Benta on August 16, 2024, 11:13:44 amLow duty cycle PWM really does help though, and my own observations is that the commutator cogging becomes insignificant. Model trains take great advantage of this to operate at very low speeds without stalling.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2024, 02:57:24 pm
content: Quote from: David Hess on August 16, 2024, 01:53:04 pmHave you seen the gearbox on model trains? Not exactly simple. Also, the motors have a 5-pole armature, not just 3-pole.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 16, 2024, 03:41:23 pm
content: As a very simple alternative, you can use a brushed DC motor, combined with an adjustable constant current power supply. (A lab power supply in CC mode works) Current determines torque for a DC motor. The goal is to set the current high enough to have a bit of torque, and then you can just feed in the wire with whatever speed you like. You can even hold the wire to stop it or pull on the wire to unwind it a bit if you overlapped windings.I have done this a few times and it worked quite well. Of course it depends on the motor size, wire thickness and such.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2024, 03:44:29 pm
content: Quote from: Benta on August 16, 2024, 02:57:24 pmBut you can compare the difference in operation between constant low DC voltage and high voltage with a low duty cycle. The later can operate at much lower speeds without getting stuck.A long time ago, model train power supplies used to have a switch to allow half-wave rectification which had a similar advantage at lower speeds. Low duty cycle PWM is even better.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2024, 05:02:29 pm
content: You keep flogging a dead horse. You cannot ignore commutation cogging at low speeds. Period.

author: David Hess
author_rank: Super Contributor
timestamp: August 16, 2024, 06:44:12 pm
content: Quote from: Benta on August 16, 2024, 05:02:29 pmSure you can. I gave an example where that is exactly what was done, and the only change was DC to low duty cycle PWM.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 16, 2024, 07:31:47 pm
content: Stuck record here:1/ Don't use a stepper motor. Too much noise, too much torsional vibration, and it needs a special controller.2/ Use a brushed DC motor with a gearbox and a straightforward PWM speed controller.The gearbox is essential - it allows the motor to run at a speed where cogging is irrelevant. Something like the innards of a cheap cordless screwdriver might be ideal. Possibly a bit slow. If so, check out Benta's recommendation for a cheap geared motor. If you've got a variable voltage bench PSU, you can run that motor off that. It's what I do.