brushless DC motors for mini lathes

[darryl]

I see where most of the controllers for brushless rc motors are sensorless. This is suitable for airplanes because the loading and rpm ranges are within parameters that allow the controller to work well. To get decent performance at low rpms- or let's say to get a wide range of speed control with widely variable loading, these controllers don't work so well. With sensors- hall sensors usually- you have good control.

As far as the motors themselves, the larger they are the larger the bearings tend to be, and the more lifetime you could probably get with a lesser maintenance. The larger motors are also much more power capable- but that doesn't mean you have to run them that 'hot'. Some of those motors are about the size of a beer can- one such motor was rated at 15 hp. But of course it would never produce that if the power supply wasn't capable of that, so there's an automatic limit. But such a motor would also give you much higher torque at low rpms, so that alone would make it more suitable for an application where you want to be able to run at low rpms, yet still have 'guts'.

Something else in this same vein is that such motors also want more voltage in order to produce that power without the current draw becoming ludicrously high. Ten amps at 60 volts is easier to get in a power supply than 50 amps at 12 volts for instance. The main drawback to using one of these larger brushless model motors is their cost.

And that brings me around to the other idea- there's really nothing inherently wrong with using brush type motors. I've been using one on my lathe for a long time now (a treadmill motor) and have never had a problem with brush wear. There are several applications where a brush type PM motor is used in appliances, and their power ratings are in the 80 to 200 watt range. Power heads for vacuum cleaners is one- drink mixers is another one. Electric knives and paper shredders is another, but most of those use a universal motor. Those are not suitable for powering a 'micro' lathe.

What else- how about a corded electric lawn mower motor. These are similar to treadmill motors. More power than you need? Probably, but the capability of lower speed torque is there- which is something you do need.

[Mcgyver]

Yes. Thats why I have a watchmakers lathe. (I also have 3 other sized lathes and a second watchmakers one... Latheaholic )

Nice work. may just be personal preference then, but it would drive me nuts not to have the foot control. I do think its preferred by a huge majority of watchmakers, but agree that doesn't mean its the only way.

PS. hope I didn't come across as snarky, wasn't the intent, just find for those who haven't turned up close with graver and loupe there's not the same sense as to what working on these lathes is about so figured if you didn't have a foot pedal you might not have been there

[PStechPaul]

Would a series wound motor like a starter work well for low speed high torque applications? Or possibly a motor from a battery powered drill. Might even be able to make use of the planetary reduction gears.

[darryl]

I've played with a lot of motors in my time- I'm not talking about induction motors, but PM motors with brushes for the vast majority. Some are quite coggy at low rpms, while others are actually pretty smooth. In general, the higher the armature slot count, the smoother it will be- but that's not always the case. What is almost always the case is that you can drive those motors with a simple variable voltage power supply, and it will maintain an rpm within a few tens of % from no load to near-full load. I've found that method to be very satisfactory on my lathe, the tool post machine, the drilling attachment machine, and the other two lathes that have pm motors. One is the Unimat- the other is one still under construction. I like that I can tell how much loading I'm giving it by the rpm drop- I find that very intuitive. It has never mattered to me to keep some exact rpm regardless of loading. The type of power supply I use mainly is the selectable tap transformer/rectifier/filter- the 'old school brute force' power supply. I wind them myself to get all the taps I want. No electronics per se- just the taps on a secondary winding to select the input voltage to the bridge rectifier. Pretty crude- but reliable, and quite enough to satisfy me in doing all the machining operations I've done to date.

Of course, that involves building a power supply. About the simplest way I can see to go is buy a vfd- that is its own power supply. Then find a suitable motor that will run from it. Our edging machine at work uses two motors to drive polishing wheels- those would be about the right size for you. Visit a woodworking machine repair service- they might have just what you need.

[J Tiers]

Series motor have horrible speed regulation, which makes them particularly unsuitable for machine tools other than maybe grinders. Many toolpost grinders use universal type motors, which are of course series wound.

The field strength in a series motor varies with motor current, so loading changes the field. They are great for traction applications, because their "base speed" changes with load/speed.... the slower they go, the more current, and so the more field, which means a lower base speed. That same characteristic makes them have lousy speed regulation.

The torque can vary, reaching often 300% of normal. Often, the torque (which changes with current) is limited only by the saturation of the iron stator structure, and of course by heating due to excess high current.

But just the back gears on a normal lathe usually reduce speed and increase torque by around 5 or 6 times. That is larger than the series motor max torque percent, and that does not include other pulley ratio changes, etc.

A lathe having a 50 rpm minimum rpm, and using a 1750 rpm motor with mechanical speed reduction, will have a torque at 50 RPM that is about 35 times higher than the motor torque itself. Power, of course, will be about the same as at any other speed. That is far higher than the increase that a series motor ever has.

[dian]

scanning through the thread, it seems everything has been proposed except a "regular" dc motor. e.g. this type, they come in 350, 500 or 1000w. whats wrong with them? this one seems to have a controller of some kind, but that less than $10 usually.

https://www.ebay.com/itm/500-W-36V-DC-electric-motor-1020-kit-w-base-control-box-f-scooter-gokart-or-DIY/162133598280?_trkparms=ispr=1&hash=item25bfea6048:g:AFgAAOSw9CpatqYF&amdata=enc:AQAFAAACcBaobrjLl8XobRIiIML1V4Imu%252Fn%252BzU5L90Z278x5ickkfOCvCjTOBWK8pwriaolq5nuTdbRrcBm3GN0MzgfwWu95bEP2pc%252F35wwQJ5TUW4VGxEFdhfxYYrXj%252F6tIw2dAATVIyhMebLorqeeQNJ65Xz65AT%252FtiSohgKmeQLyr6W%252BTvaXyjURIOwa0E%252FRiaHpaH5OmYbKvSefzA75GqAkUSFJciYBor64w5gDUMKBa5S%252FFC9Z%252BA%252BFg7DQsw3mHDm2K7u%252FjfybIzaPzvjPmWXZKMMLp7ikRvxhREbjTgFRhLCuPkHo7O%252BUe0ynqOCxrBSQZN9a4b217dUqUoPibS0XC09dwYSxIwraSoOpoE3ykaa0UEASEXfSmE5qqCWL7Ctp%252B1rnQqqfMKaXJo4KYDHTpDlvp5P0LqG2wcbv5CmAjw0G8ok9tXljNxak%252BVwwgAvkZsARmR3KGpCh5bvdDwJ6hrXPg9MLxphlgg4byKSlUFw5k%252FAWY%252B8AnW7KvXt22bCL%252Fl6lHb8gAsdO%252BjASO6JRK%252FM5F%252F%252BHY3mzEENG7FVB2yLW1Eugfz%252FDKUlEX7S%252FedvOVZEjqMMK32xI%252BxjaBU6KoLLUWv6g79A3QHxlumUxCQSBdcD6wi2P%252BK4gtyI20wTUXhCMUnPw5mxVLeZpU5254FdlCSOSzNHSg6INRzS6IF%252BXZ9AVzrIndGnu01IhRNKeniXXVmeRLegWPOn5vsGylBaZdtAOGxQgqBzQcJG4alelPG7x5yC%252F9j1mE5Lxharm3WJDcr6Ci7qVcuH2rniT8npYbXIPPqq5I0D%252F61rWxUgqTjIKyuCWIqEX8B2GATdRvKQ%253D%253D%7Ccksum:162133598280778aaec04ac74f209d22d6e0613f25f2%7Campid:PL_CLK%7Cclp:2334524

[BCRider]

Looking at dian's motor and not seeing any 350 watt options in the side bar I did a search for "350W motor" and THIS BRUSHLESS 350W COMBO turned up. For under $100 you get a motor driver and at least a throttle pot within the throttle grip assembly to work with. Adapt as desired.

And of course we're still after a 24v 20 amp supply.... Another quick search and I find a 24V 30A SWITCHING SUPPLY for another $50.

And really since the 350 watts is way over what would ever be needed for a watchmaker's lathe even given the start up surge I'm thinking that the 24v 15A supply from that same seller at $27 would more than likely fit the use here just fine.

The speed of 2750 RPM off the motor would certainly call for a smaller or at least wider range motor step pully setup though. A bigger size step or two for any overhead work might be nice along with one or two small sizes to run the lathe since you're finding that the setup you have is too fast.

[dian]

then there is this. maybe you find one without collets for even less.

https://www.ebay.com/itm/AC110-220V-CNC-Air-Cooling-500W-Spindle-Motor-Speed-Governor-ER11-Spring-Clamp/323809331018?_trkparms=aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=20201210111314&meid=9376cc141b3b4fcdad8a06b2d623c886&pid=101195&rk=1&rkt=12&mehot=co&sd=222596355910&itm=323809331018&pmt=1&noa=0&pg=2047675&algv=SimplAMLv9PairwiseUnbiasedWeb&_trksid=p2047675.c101195.m1851

[BCRider]

then there is this. maybe you find one without collets for even less.

https://www.ebay.com/itm/AC110-220V-....c101195.m1851

That was my first thought too. But Henry already has issues with the motor being too fast at 3K. Otherwise I think the package could be a good match.

[Paul Alciatore]

The thing about sewing machine motors that bothers me is that a sewing machine motor does two things:

1. It pushes a needle into the cloth and pulls it back out.

2. It operates the mechanisms that move the cloth the distance of one stitch and that passes the thimble below the deck through a loop of thread when the needle is at it's lowest point.

OK, some sewing machines do have mechanisms to produce patterns, like button holes. But that is not much more than the items I mentioned in #2 above.

That's all that a sewing machine motor has to do. So just how much torque or power do they need, even at slow speeds? Not much. My grandmother made a living sewing draperies. The motors on standard sewing machines were so weak that she could not use them on many of the fabrics she encountered. She had to have an industrial quality sewing machine which had, among other things, a larger motor. But even then, just how much larger did it need to be just to handle heavier fabrics?

And keep in mind that a sewing machine is heavily geared DOWN. Things at the working end of the machine are using an RPM that is a lot slower than the motor's RPM. Most of the motors will be running at a higher shaft speed.

Are sewing machine motors that much better today? And if some of them are, how do you find that fraction of them while avoiding the vast majority that are designed to a price point to sell low end sewing machines?

I'm rather thinking that if a foot pedal was used for the speed control that controlling the speed would become pretty automatic.

Henry, what have you used up to now? I'm seeing a lot of 100 to 150 watt rated regular sewing machine motors that come as a kit with a matched speed control. Granted they are not brushless but any time I'm using my old black Singer one stitch fits all for something it seems pretty easy to modulate. And when the belt is off the motor it runs very smoothly. Any shake, rattle and roll is either in the machine or the operator.

[Paul Alciatore]

If you have ever searched among the power regulator ICs. you know that most of the modern and even many of the decades old ICs used for this have current limiting built in to them. Most modern, switching supplies will incorporate one of these ICs and will have current limiting. The thing to be aware of is just how that current limiting works. You do not want a fold-back circuit that needs to be reset if the current exceeds the limit. Laboratory type power supplies simply have meters and front panel controls so you can set the point where current limiting occurs while supplies packaged to be buried inside another device will have those controls on the PC board (set once and forget) or may just have fixed resistors setting it at a fixed point that can not be changed.

I have often thought that a stepper motor, which has full torque even at it's slowest speeds, combined with a motor driver board that has microstepping and a simple control would provide excellent slow speed operation. I would also try to gear it down by a significant amount. Such a stepper motor could provide a very wide range of speeds if combined with several gear or belt selected ranges.

I may be wrong on this but if the setup uses the hall sensors then it will push whatever current is needed to try to keep the set RPM. So no way to "feel" or hear the change due to the load on the motor.

Running it "open circuit" without the sensor being hooked up, provided the motor driver is a sensorless operation type, would make it more like the old AC motor but with more speed options.

Henry, do you have another lathe which could be used to make a new smaller motor pulley? That would be one possible fix.

As for cost don't forget that you'll also need a suitable high current DC supply. That Allelectronics motor does not list the watts or max current. And those are pretty small wires. But assuming it's good for a something like 200 watts at 12v that means you'll also need a power supply that can provide at least 200/12= 17 amps. So call it a 20 amp supply. You won't load it down to the full current all the time but it has to be there for when it's needed or the supply will be overloaded and fail on you. Or it needs to be a supply that can self limit itself such as a voltage and current limited test bench supply.

[darryl]

Nobody has mentioned this yet, so let me put my neck on the chopping block and do so- there's another method of speed control that can be used- a mechanical governor. It used to be that the motor current went through a set of contacts that would open when something akin to a flyball governor opened the contacts. These days you would pass the motor current through a power transistor and the contacts would control the transistor (or mosfet). In this case the contacts would only carry a fraction of the current or voltage, so could be quite simple. This could be an HSM project, and could be based on an existing design. Mixmasters used to use this method, and some other motors did too. Provided you can come up with a governor, and a way of pushing against it to control the speed, the electronics needed would be very minimal.

Ok- chop away-

[RB211]

I've been using an Emesco 90N 22,000 RPM ac motor and it is much to fast and has low torque at low RPMs. And I've recently learned that many of the commercial sewing machine motors are brushless DC servo motors but all I found are 3/4 HP. But there are many companies making brushless DC servo motors from less than 1/10 hp to very high horsepower and all, but China made, very are expensive.

I recognize that motor from my childhood. My dad was a dentist and he had that in his lab, along with the rest of it, with pulleys and drive belt going to a handpiece for working on impressions and what not.

[MaxHeadRoom]

Nobody has mentioned this yet, so let me put my neck on the chopping block and do so- there's another method of speed control that can be used- a mechanical governor. It used to be that the motor current went through a set of contacts that would open when something akin to a flyball governor opened the contacts.
Ok- chop away-

The modern version would be a speed pot input to 8 pin micro and using a slot opto-interrupter on the motor for RPM feedback, and PWM control of a DC/BLDC motor.
Someone on the CNCforum designed one for using router motors in a CNC application, due to the nature of the Universal motor & Triac control, the bottom end speed is 5krpm.
It uses a retro-reflector sensor for RPM feedback.
It can be found marketed now under SuperPID controller.

[MattiJ]

T

Are sewing machine motors that much better today? And if some of them are, how do you find that fraction of them while avoiding the vast majority that are designed to a price point to sell low end sewing machines?

Low end sewing machines are not any better today but when you actually pay more for the motor&controls than entire low-end sewing machine you'll probably get something else.
Industrial sewing machines are typically quite beefy and built for high-speed sewing 24hours per day.

Old "home use" sewing machine motor might be something like 50 to 70 watts, industrial machines 500 to 750W

Home use Singer and industrial 3-phase Juki are same as 7x14 chinese lathe vs Monarch 10EE.