Switching from gear motor to direct drive - Page 1
Forum section: electronics / beginners

author: Peabody
author_rank: Super Contributor
timestamp: April 12, 2021, 06:24:17 pm
content: Up front - I know essentially nothing about motors.I've used this little motor in a hobby project:https://www.amazon.com/gp/product/B08NRFQRJP/It's spinning a rod about five inches long, weighing 6 ounces, with a bearing at the other end. There's no load as such other than what's needed to spin the rod. But the motor has failed pretty quickly, and taking it apart reveals it's the gears that are messed up, which is no surprise given how tiny and flimsy they appear to be. So I would like to replace this motor with a direct drive motor.The motor needs to be an inch or less in diameter with a 3mm shaft, spin at about 600 rpm and doesn't need to change direction. DC powered at 5-6V. Length doesn't matter. So the requirements are pretty simple.Based on a few Youtube videos, it seems that to get a motor to spin that slowly, it will need to be brushless, and if that's the case I will also need some kind of speed controller that's more involved than just varying the voltage. Is that correct, or have I misunderstood?Where would I find a direct drive motor like that, and what would be the proper search terms? Also, will I find something like an Arduino-based speed controller for it, hopefully with a library?And of course if there's a link that answers all of these questions, that would be great too.

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2021, 07:01:34 pm
content: I can't think of any reason why the motor would have failed other than the gears being made from cheese in the first place (or very long run time).I'm curious about a metal rod that spins without any load, presumably is does have some purpose though?600 rpm is probably fast enough that you could use a brushed motor, as long as it is a 'coreless' design, eg. a CD / DVD spindle motor (smaller shaft diameter though) to prevent cogging at low rpm.The speed is very low for a normal cheap model aircraft type brushless motor + ESC. You would probably end up with something rather more expensive.You don't mention what sort of speed stability you need.Edit: A reference on cored vs coreless brushed motors:https://www.motioncontroltips.com/what-are-coreless-dc-motors/

author: wizard69
author_rank: Super Contributor
timestamp: April 13, 2021, 01:55:10 am
content: Quote from: Peabody on April 12, 2021, 06:24:17 pmProbably wouldn't hurt to have more info here. You are not spinning the rod for the hell of it I imagine.QuoteYou really need to know the torque requirements.QuoteI'm not sure where that idea comes from. With a proper motor controller you should have no problem with a brushed motor. Even a stepper might work. It really depends upon many other factors.QuoteDid you try "DC motor" maybe add "miniature".As for suppliers:https://www.portescap.com/https://www.linengineering.com/https://www.dunkermotoren.com/en/(AMETEK)https://www.orientalmotor.com/index.htmlhttps://www.faulhaber.com/en/products/dc-motors/https://www.maxongroup.us/maxon/view/content/indexhttps://www.haydonkerkpittman.com/At one time or another we (at work) have used small motors from vendors like these.QuoteWhy do you need a library to run a motor at a fixed speed?QuoteGoogle is your friend. I'd suggest looking for the engineering documents some of the above vendors have which may help with sizing the correct motor. As for a gear box that may or may not be required, depending upon the performance your need to achieve.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2021, 06:16:20 am
content: $7.99 is ridiculously cheap for a gearhead motor, especially when you consider that Amazon sellers typically double or more the price from China for the same item. Rather than trying to go with direct drive, which is going to be difficult at that speed range, I would suggest looking for a high quality gearhead motor. A properly made gearbox will outlast the motor driving it.

author: Peabody
author_rank: Super Contributor
timestamp: April 13, 2021, 03:14:04 pm
content: Thanks for the replies. I need to follow up on the provided links, but it looks like I misunderstood about what's needed to get a motor to run at low speed. I thought a brushed motor would run very rough at low speed, but a brushless motor could avoid that. But it seems that could be a problem for any motor. Looks like I also need to research coreless motors.Anyway, since I now have the original motor without its gear box, I can experiment with it and see what happens when I try to slow it down by reducing the voltage. I have an LM317 that I can use at least temporarily for that.I also found another geared motor that's the same price but looks to be more substantial, and I may give that one a try:https://www.ebay.com/itm/JGA25-370-DC3V-6V-12V-24V-Reduction-Gear-DC-Motor-with-Gearbox-for-Robots-Cars/331101502778I apologize for not being more forthcoming about the device. I'm involved with others on something that might actually be a product some day, and just can't say more right now.

author: Gyro
author_rank: Super Contributor
timestamp: April 13, 2021, 03:38:14 pm
content: Quote from: Peabody on April 13, 2021, 03:14:04 pmLooking at the pictures in your original Amazon link, the motor appears to be a 7 or 8 pole iron cored motor (assuming it actually matches the picture). A reasonable number of poles but not coreless. Just something to be aware of when experimenting at low speed, it may be too 'coggy' or it may be fine for your needs.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2021, 03:49:36 pm
content: Either way PWM is the way to go, not reducing the voltage. With a DC motor like this, reducing the voltage will reduce the torque, and any load will greatly affect the speed, especially at the lowest end. This is the reason cordless tools with brushed motors universally use PWM.

author: Peabody
author_rank: Super Contributor
timestamp: April 13, 2021, 09:00:31 pm
content: Quote from: Gyro on April 13, 2021, 03:38:14 pmI've played with it a bit, and it's no good at any speed much lower than it runs at 6V. The speed wanders. A lot. I think whatever messed up the gears also messed up the motor. Anyway, for now I'm going to try the alternate gear motor and see if that holds up.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2021, 09:33:53 pm
content: Quote from: Peabody on April 13, 2021, 09:00:31 pmRead my previous post.It's entirely possible that the motor is a piece of crap that is already worn out, but even so, reducing the voltage is not the appropriate way of controlling the speed of a motor. PWM controllers can be bought for only a few dollars, or you can build one yourself with a microcontroller, or a comparator and ramp generator, or a 555, or a number of other ways. Use that to drive a transistor and there you go.I think you mentioned you had an Arduino already, whip that out and load the PWM sample sketch, there's one that demonstrates PWM using a pot to vary the duty cycle, that's perfect for controlling a DC motor.

author: Gyro
author_rank: Super Contributor
timestamp: April 14, 2021, 08:54:23 am
content: I can say that coreless motors behave much better than cored in terms of speed control by varying DC. They are far more efficient and their back-emf tracks speed very closely so their inherent speed regulation if far better. I once built a belt drive turntable using a Portescap coreless motor, which provided excellent speed regulation (stylus drag etc) with just an LM317 regulator. CD players too, just use a varying DC voltage (servo'd) to control the spindle motor rather than PWM. Such motors will track rotation vs voltage down to a couple of rpm. Torque at such low speed, is of course, almost non-existent as winding resistance becomes dominant rather than back-emf, but a few hundred rpm should be no challenge.Such experiments with a cored motor (sounds like damaged or poor quality) are doomed to failure. Even PWM (without speed feedback) probably won't help speed stability if the commutator and brushes don't provide consistent performance.

author: james_s
author_rank: Super Contributor
timestamp: April 14, 2021, 07:07:16 pm
content: Varying voltage works just fine if you have closed loop control of the speed as is the case with a CD player. The voltage will vary, actively modulating torque to maintain speed. It is with open loop control that PWM offers a substantial advantage.

author: wizard69
author_rank: Super Contributor
timestamp: April 15, 2021, 01:58:06 am
content: QuoteThis makes it hard to suggest anything at all. However a gear box should not be causing you problems if in fact there is no significant load on the motor. I just reread your post and just realized that I might have assumed that the spinning rod was some sort of drive shaft (inline with the motor). If it is mounted perpendicular to the motor or gear box shaft, you will have a very unbalanced load which will lead to all sorts of issues.At work we run a little Oriental AC motor and gear box, driving a conveyor, for years with no problem. The motor is rated (1)ONEwatt and being single phase AC only runs at one speed (geared down of course). If sized correctly and just as important installed correctly, a high quality motor will run for years. So you need to understand your load.What bothers me here is that you say the RPM varies widely which it shouldn't do with a stable power supply and an even load. If you are getting significant speed variations I suspect that you have not considered the load being driven, it might not be as trivial as you think. This comes back to the issue of a gear box, sometimes you need a gear box to better match inertia. IF this is going to be a product for sale you might want to seriously consider hiring a consultant or cranking through the numbers yourself. A brushed motor may or may not be the right solution here and frankly a gear box may be required no mater the motor technology used.

author: Peabody
author_rank: Super Contributor
timestamp: April 15, 2021, 03:49:02 am
content: Quote from: wizard69 on April 15, 2021, 01:58:06 amNo, you were right the first time. The rod is inline with the motor. However, even so, it turns out to be heavier along one side, so there is a balance problem I need to deal with. I've mounted it at the very center of two flanges, but that assembly always rolls to one position, and is clearly unbalanced. I can add weight to make it statically balanced, but I think it will not remain balanced when spinning - because the added weights, which stick out farther from the center of rotation, would be relatively heavier when it spins up. Something about moment of inertia, r-squared, and such, which I don't understand.Or I can mount it off-center on the flanges, but so that it's statically balanced at rest, but again, when it spins up I think it will be out of balance again because the mass isn't evenly distributed around the center of mass and center of rotation. It seems to me that there must be a mounting position on the flanges that would produce a balanced assembly when spinning at 10 Hz (600 rpm), but I don't know how to find that position. But at this point I don't know if the out-of-balance condition is typical, or just a bad copy.QuoteThe speed variation was after it broke and I removed the gear box. So it was completely unloaded at that point, and I was just trying to see how slow I could make it run by adjusting the voltage alone. But below about 4V, it would speed up and slow down with no change in voltage at all. So it seems the brushes are not right, and whatever broke the gears also messed up the motor.I hope the new, larger geared motor will work well enough. It certainly would be more convenient to just turn on the power and not have to add a controller circuit. But the rod-and-flanges assembly imbalance may be a deal breaker for the whole thing.

author: james_s
author_rank: Super Contributor
timestamp: April 15, 2021, 04:59:15 am
content: The motors you're working with are so cheap, buy 2 or 3 of each type you're playing with so you can tell if one isn't performing properly. Geared or not, if you buy a cheap motor there's a good chance it won't last very long. A lot of motors meant for toys are only rated for a few tens of hours of operation. A good quality motor with ball bearings and carbon brushes can run for many thousands of hours.