Help finding DC motor - Page 1
Forum section: electronics / beginners

author: SenSeS
author_rank: Contributor
timestamp: June 02, 2015, 09:04:35 am
content: Hi,I'm looking for a DC motor + gearbox that is small, operates at 3V, has at least 1.5Nm stall torque and can do about 0.5Nm@60RPM (output). I've spent quite some time searching for this and it seems that there are very little off the shelf solutions for this. It looks like I will need to buy a DC motor and gearbox separate. However there doesn't seem to be a standard way to put a gearbox on a DC motor. So my question is, how is this usually done? Does the DC motor manufacturer provide the possibility to add a custom gearbox onto it? Do I need to find a place that makes custom gearboxes? When I find the right one, I'll probably order about 100 or so. Anything that can help me in my search is greatly appreciated!

author: H.O
author_rank: Frequent Contributor
timestamp: June 02, 2015, 10:49:15 am
content: Hi,You're not saying anything about what physical constraints you have (just that you need one that is "small"), what type of gearbox (planetary, spur gear etc) you want/need/prefer or if price is of any considerable importance so this will be quite general.1.5Nm at 60rpm is roughly 10W mechanical power on the output shaft of the gearbox. You're obviously looking at quite a bit more electrical power going in at the other end due to losses in the motor and the gearbox.I've got a couple of old(ish) MAXON and Faulhaber catalogs here, both are manufacturing premium quality motors and gearheads. Neither of them have anything close to 15-20W with a 3V winding.Just to give you an example of something I sort of thought COULD work:A Maxon RE25, 20W motor with a 7.2V winding combined with a GP26B planetary gearbox with a reduction ratio of 76:1.The motor has a speed constant of 1590rpm/V so at 3V the motor will turn 4770rpm which is well within the maximum input speed of the gearhead. 4770/76=62rpm which just meets your requirement.The torque constant of the motor is 5.99mNm/A and the maxiumum continous current is 3.27A giving a motor torque of 19.6mNm. The gearbox ratio is 76 and the efficency is 59% so you'll get 0.0196*72*0.59=0.88Nm - not even close to your 1.5Nm specification. Besides - the gearbox is rated for 1.3Nm continous so it's on the small side anyway.....This motor/gearbox combination is 25/26mm in diameter and ~86mm long to give you something to visualize.Another "test":Maxon AMax32, 20W with 6V winding. (No go since you've only got 3V which cuts the motor power in half but lets give it a try).Speed constant 825rpm/V * 3V = 2475rpm2475/60rpm gives a nominal reduction ratio of 41.25Closest ratio I can find is 33:1 gicing you a no load speed of 2475/33=75rpm.Motor torque constant is 11.6mN/A and the max continous current is 3.43A giving you a motor torque of 0.0398NmThe gearhead efficiency is 75% so the outpur torque is 0.0398*33*0.75=0.98NmIf you can live with a lower speed then the above gear reducer is available with a ratio of 51:1 (efficiency is 70%) so you'd get 2475/51=48rpm and a torque of 0.0398*51*0.70=1.42Nm....To slow and to weak....66:1 ratio (efficency 70%) gives 37.5rpm but 1.8Nm...enough torque but way to slow.I'd suggest you lookup your local sales rep for Maxon, Faulhaber and Dunkermotoren, give them a call and talk to them. My experience is that they are very good at "picking out" suitable motor/gearbox combinations from their product ranges.

author: shanky887614
author_rank: Contributor
timestamp: June 02, 2015, 11:06:36 am
content: Quote from: SenSeS on June 02, 2015, 09:04:35 amdoes it have to be 3v? what is the max input watts?

author: SenSeS
author_rank: Contributor
timestamp: June 02, 2015, 12:03:41 pm
content: Thanks for the replies! 3V (for now) strict yes. However, I realized after reading your posts that I formulated my question a bit wrong. I need probably about 0.5Nm at 60RPM (not very strict, just as a guidance) and the stall torque needs to be at least 1.5Nm. Unlike what I said (1.5Nm @ 60RPM). Sorry for this, ll change it in topic.

author: H.O
author_rank: Frequent Contributor
timestamp: June 02, 2015, 06:43:30 pm
content: Well, that makes it a little easier.Maxon:AMax 32, 15W, 6V, P/N:236643Speed constant 996rpm/V = 2988rpm @ 3V, no load.Max torque (continous) 36.5mNmStall torque 127mNmPlanterary gearhead GP32A, 33:1, P/N:166163Efficiency: 75%Max continous torque 2.25NmMax intermittent torque 3.4NmThis combination would give you a no load speed of around 90rpm and max continous torque of around of 0.9Nm.At 0.5Nm torque output the motor pulls around 2.1A. (0.5Nm/33/0.75/0.00958)It's armature resistance is 0.454ohms so you'll get a voltage drop of almost 1V across the armature.That leaves you with about 2V for the actual "motor" which makes it spin at 2*996/33=60.36rpm.The stall torque of the motor is, as stated, 127mNm giving you a stall torque at the gearhead output of roughly 3Nm which is slightly over the limit so you may want to current limit the drive electronics.Lacking a motor with a 3V winding I think the above is a pretty damn good match.It's going to cost you north of $300 though (single quantities).And to get it out of the way:No, I don't work for Maxon or any other motor manufacturer or distributor or retailer or anything like that ;-)