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I am designing a small metal lathe for home use, and I am having trouble sourcing a motor for it. Disclaimer: I am aware that such lathes are available for purchase, and anything home-built will be nowhere close to the quality and repeatability of commercially available ones. It is being made for fun and education purposes.

The requirements for the motor are:

I will be running the lathe in my apartment, so the motor must be as quiet as possible. Brushed motors are out of the question. Brushless (three phase) would be my first choice.

It must be compact in diameter. The motor will be housed inside the lathe, and I can fit a motor only as large as 60mm in diameter and 90mm in length.

It must be powerful enough. I am hoping to get something close to 1 HP (750W). 500W is probably bare minimum.

I will be designing reduction gearing, so RPM doesn't matter much, but I'd prefer somewhere around 1000 RPM to make things easier. Definitely no more than 4000 RPM.

It must be able to run for a prolonged time without overheating.

I am located in Lithuania, and the local parts supply is really scarce over here. I do most of my hardware/electronics purchasing from Aliexpress/Ebay.

There are plenty of BLDC motors on Ali that fit the size and power requirements, such as this one, or a much more powerful this one, but while they are incredibly powerful for their size, I seriously doubt they would hold for long before overheating in full enclosure, since they seem to be made for quadcopters. I could add a cooling fan, but that would cause way too much noise.

I have considered reusing a motor from a scooter (like Segway), since these motors are powerful and designed to run in full enclosure. Problem is, I can't find one that is small enough, and their stator is typically in the axis (and the wires run through the inside of that axis), which would make it very difficult to adapt for a lathe.

Perhaps I should be looking for a more traditional three-phase motor (perhaps with VFD controller), but I can find nothing of sufficient power that would fit into the frame of my design.

I have been searching for 3 days now, but the difficulty comes from the fact that sellers and manufacturers of these generic motors rarely provide info on the overheating characteristics of their motors. Which is why I'm posting here, hoping that maybe someone could suggest something from experience. Specific models of motors would be best if you happen to know what fits these requirements, but even general comments on the types and characteristics I should be looking for would be very helpful.

Thanks!

Justin8051
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    Recommendations for specific products are off-topic here, but product categories or classifications is not. By the way, a BLDC is a 3-phase synchronous motor which is almost identical to a PMAC motor. The difference is the BLDC has a trapezoidal BEMF while the PMAC has a sinusoidal EMF which makes them more suited for square wave drive vs sinusoidal drive. But some make no distinction in the terminology. Then there's the 3-phase induction motor and that's the "traditional" one but you are correct in that you won't find a small one. – DKNguyen Feb 07 '22 at 22:18
  • You're also not going to get around cooling. I think your priorities are a bit off with wanting to be quiet since the the gear train is going to make noise anyways and you're worried about the noise from a fan? Unless you feel like just circulating water around the motor for water cooling but since no fan = no radiator it's just going to be a huge reservoir of water that hopefully heats up slower than the amount you use the machine. I am also curious how you managed to size the motor to fit inside the lathe already without knowing the gearing. – DKNguyen Feb 07 '22 at 22:26
  • I will be using belts instead of gears, and I already worked that part out - belt drive is extremely quiet. Thank you for the explanation. So what kind of motor should I be looking for? That BLDC from the second link looks very promising to me (because it is so powerful and small, and likely quiet), but it will probably overheat. What kind of motors do similar small lathes (7x10, 7x12, etc.) use? Perhaps I'm re-inventing the wheel here... – Justin8051 Feb 07 '22 at 22:27
  • As far as small sizes you probably are stuck with a BLDC motor of one kind or another. I'm still not sure how loud you are envisioning the cooling fan to be. Something like an electric airplane propellers of the size you are talking about is lot quieter than something like a vacuum cleaner. – DKNguyen Feb 07 '22 at 22:28
  • Well, I don't know, since I have no idea how much cooling a BLDC motor actually needs for a given voltage and current... But if I imagine a drone propeller - these can get pretty damned loud. That is the reason why I'm asking this question - I don't know how to estimate these things, and buying a motor just to try it out is very expensive... – Justin8051 Feb 07 '22 at 22:36
  • For a gut feel for the cooling required, imagine the motors found on an RC airplane. They overheat when run for too long in static testing on the ground because they are not flying through the air constantly moving air past them. So minimum flying speed of an RC airplane? 20-30km/h which you can get a feel for with driving speeds and sticking your hand out the window. – DKNguyen Feb 07 '22 at 22:37
  • Is it realistic to get that kind of cooling for a stationary lathe? Or do I need to choose a different kind of motor? That one in the second link in my question is actually way too powerful... Anything around 750W that is below 60mm in diameter and 90mm in length would do, as long as it is quiet... – Justin8051 Feb 07 '22 at 22:45
  • I don't see why it wouldn't be. Define what makes a motor "too powerful" when the physical dimensions are appropriate. A more powerful motor isn't automatically going to be significantly louder. Small lathes are already fraught with torque issues. Have you had any experience on a lathe? There are lots of issues people have online on smaller lathes because they can't cut anything properly unless the tools are really sharp because the machine bogs down. – DKNguyen Feb 07 '22 at 22:48
  • You make a valid point. The only reason I am afraid to use a "too powerful" motor is because it might require more cooling to work at it's intended RPM (I think most brushless motors have issues if run too slowly, as they can't produce good torque). Perhaps I'm wrong, or maybe it wouldn't be an issue here... – Justin8051 Feb 07 '22 at 23:05
  • A more powerful motor will run cooler since it's underloaded. Most brushless motors do have issues running too slowly and a larger motor, all things being equal will have an easier time running more slowly. However that's why you have reduction. – DKNguyen Feb 07 '22 at 23:45
  • Okay, I see. Thank you very much for the extensive explanations. I will try my luck with that 6354 motor and try to come up with some kind of heat sink for it. – Justin8051 Feb 07 '22 at 23:50
  • @DKNguyen, if you don't mind, I have one more question - I looked up how to power that BLDC motor. As I understand it, I need an ESC capable of providing the amperage (for a 24V 1500W BLDC, that would be ~62A, right?). Problem is, how do I get 1500W 24V DC from the mains (220V)? A AC-DC converter capable of such wattage is 3 times more expensive than the motor itself. Surely there must be a simpler way to get that motor running off the mains? – Justin8051 Feb 08 '22 at 02:19
  • There isn't. That's what happens when try to run something designed to run off batteries off of the mains. Radio control airplane guys run into the same problem on how to power their battery chargers, designed to run off car batteries at the field, from the mains at home. We all have to spend a few hundred dollars on the a massive AC-DC supply. I suppose you could buy a step-down transformer and make a DC-rectifier with very large smoothing caps. Not particularly difficult, but dangerous if you have zero electrical experience. 1500W 10:1 stepdown transformers aren't cheap either. – DKNguyen Feb 08 '22 at 03:26
  • Maybe you can find a 300VDC BLDC motor. That will run off a 220Vrms VFD. Because 220Vrms converted to peak is ~300V (which is what the VFD does inside itself same as the rectifier+smoothing capacitor thing I was talking about). Those seems to span a few hundred watts all the way up to kW ranges. Difficult part is finding a cheap one. – DKNguyen Feb 08 '22 at 03:29
  • There definitely are brushed DC motors in that voltage range as well of the size you want. I have one at work that is quite a bit larger. Maybe 20cm diameter and 40cm in length. It is actually very quiet, at least when not under load. Under load I don't know because the load it is connected to makes a lot of noise so I can't tell if the motor is loud or not under load. – DKNguyen Feb 08 '22 at 03:34
  • I have looked up VFDs. Most I can find are VFD controllers paired with milling machine spindle motors, such as this one: https://www.aliexpress.com/item/1005001821647790.html. Problem is, they are very expensive, very large (both the motor and the VFD itself), and these motors are running at very high RPM, which will make reduction a pain in the butt... What about these brushed motors you mentioned? Do you know of any such motor (1kW+) that fits my size requirements? Do they need some kind of big and expensive controller/transformer? – Justin8051 Feb 08 '22 at 11:45
  • Let us continue this discussion in chat. – DKNguyen Feb 08 '22 at 14:19

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Here in the states there was a popular fad of buying electric treadmills for indoor exercise a few years ago. These had permanent magnet DC motors in them which were nice units. Within a few years most of those treadmills wound up in junkshops or the trash when their motor drive boards blew up, and the hacker community here sprang into action and starting tearing the treadmills apart to have the motors out of them, and inventing more robust driver boards for them.

These motors are now very popular for driving full-size milling machines when coupled with variable speed control boards which can be bought or built using plans off the web. They are rated at about one horsepower and are not noisy as they are capable of driving the cutter spindle directly at low speeds. These salvaged motors are bigger than what you originally wanted, but there's a whole community of treadmill hackers out there ready to support you in your endeavors!

niels nielsen
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