Hello! I’m trying to see if anyone has an idea for sourcing low KV motors. I know it’s a tricky situation, because basically the best way (until you reach efficiency losses from bearings or rotor lamination) for an electrical motor is to run at high speeds and geared down to achieve the torque you need. But in my application, I want to run relatively low RPM, and somewhat high torque. The reason I want low RPM is that I’m sick of all the gear noise.
I am presently running the 25D motor, at 172:1 output. I am using 9v. So my output RPM is about 50. At 6v, 2.2A stall, I should expect a max torque of 170oz/in, however, I am currently (heh, pun), limited to 1.5A on my driver. So I expect I’m only getting 115oz/in max torque, and that’s enough for my application. I will consider changing my driver, so I can get the higher current that I’ll need if I want more torque with less gears.
I have a couple of options. I would outline two tasks, and the subcategories of options.
(1) I could try to reduce the noise of the gear. I’m making the text smaller here, because I’m not really actively pursuing this method. [size=90]
I’m new to this. I don’t know if the 172:1 gearbox would be much quieter than the 4.4:1 gearbox. Even though the 172:1 has the same input rpm (of the 25D motor), I’m assuming it’s louder than the 4.4:1 gearbox. I could be wrong about this.. In the past, I’ve had a hell of a time making anything like a gearbox “quieter.” Since I don’t need continuous use out of my motor, I could possibly stuff a lot of noise insulation around it. This has two downsides. 1) Bulk. 2) I don’t know if it will work very well. One side of the insulation (the output shaft) will always be exposed, so most of the sound I assume will escape.[/size]
(2) I could try a different motor. Target is 50rpm, 115oz/in.
(a) [size=150]Could I use a stepper motor? [/size]
Sort of. Pololu has a NEMA17 that offers 44oz/in. If I over-current it by 60%, and if stepper motors respond similarly to dc motors & brushless, then I would expect about 70oz/in. That’s not horrible, but not quite enough. I could deal with this in a few ways. 1) Make my pulley a little smaller, so I get a bit of a “gear” into it (I would effectively have a gear ratio of current pulley diameter:new pulley diameter. my current pulley is ~1", and I could probably get away with half of that. 2) find a deeper NEMA17 that can supply more torque.
Killer: A Nema17 is 43mmx43mm. That sucks, it’s much wider than the 25D of the current motor I use. I can do with a bit of depth, but NEMA14 is basically the largest I could do in diameter of motor, and even that isn’t awesome. And NEMA14 motors I’ve seen are much weaker.
(b) [size=150]Could I run the 25D at a lower voltage, higher current?[/size]
Sort of. Presumably the 25D has a 2.7 ohm winding resistance, because at 6V stall, it draws 2.2A. So I could run it at 4.5V (if I found some driver that could tolerate 4.5V) with half the gearing (remember I’m running at 9v right now, so 4.5v is half), but I would be stuck because it would only pull 1.65A. With half the gearing, that’s half the output torque of what I have right now. I would need a bigger motor, that has a lower winding resistance, in order to get the equivalent torque.
© [size=150]Could I run the 37D motor, at a lower voltage?
Sort of. The 37D motor has a 2.4ohm winding resistance. At 4.5V it would pull 1.875A. It would run 3.5k rpm, which I would need 1:70 gearing. I would expect 74 oz/in out of this setup (based on the 67 geared motor getting 200oz/in at 12v, and *4.5v/12v or 1.875A/5A, depending how you look at it).
Basically, it seems like I’m looking for a lower KV motor in the size range of the 37D motor, but with a lower KV. because I’m close to making it with the 37D motor, but I don’t really make it. The problem is, as I understand it, a lower KV motor has more wraps on the windings. That means that it has a higher winding resistance, and so I will STILL have a problem getting the torque out of it, because I have the SAME limitation with not being able to get the current into the motor at lower voltages.
So basically, that brings us to the end of my thread. I’m geeking out over my limited understanding of DC motors (mostly making some assumptions, based on what I understand about brushless DC motors), and struggling to find a solution. I thought someone here might actually understand the physics, and not just be a spinning in circles kid like me.
Regards. Thanks for any tips!