# DC Motor Selection

Joined Dec 6, 2015
12
I'm looking for some input on what type of DC motor runs with the lowest amount on vibration. I'm gonna use a DC motor to drive a disc, called a lap, for polishing gemstones. Max speed will never exceed 1500rpm, and a huge amount of torque isn't really necessary. I originally built the machine using a v-belt pulley drive, but this setup had way to much wobble and vibration for it to be of use. I know it's possible to engineer a extremely smooth running drive train using pulleys, but that's not the way I'm tackling this project.
So now I'm trying to decide on which motor I want to use in a direct drive. As I'm set on using a DC motor(for a number of reasons), I've narrowed my choice between a couple different motors. Right now I'm leaning towards brushless permanent magnet motor because of size vs power and efficiency. The only negative is price and controls for the BLDC.
Second idea was for a stepper motor, nema 23 or 34. I've seen people use steppers in continuous rotation, but it's not something I've done before, only ever used them for stepping. Since my speed requirement in fairly low, I've been toying with the idea of using a stepper. Will running a stepper continuously cause excessive heat to the driver or motor? Would the motor run more smoothly than a basic brushed permanent magnet DC motor?
The third option, if all else fails, would be a basic pm DC motor, which are a good deal cheaper and easier to control.
So I'm looking for some suggestions as to which of these will best suit my needs. I don't have a huge budget to spend on a motor, but I believe that I can find a good fit for under $100. #### MaxHeadRoom Joined Jul 18, 2013 19,747 Of the motors you have mentioned, the DC brushed will give you the smoothest operation, the BLDC used in a velocity mode (open loop) exhibits 'cogging' at the lower rpm's, the stepper the same to some degree when used without gearing. The stepper motor runs at a constant torque (current) so this produces heat, even when stationary. The heavier armature of the DC brushed also helps provide some constant inertia. However most DC motor over a certain size are usually limited to around 2.5krpm. You could use one of the AMC etc servo drives to run it, which provides some nice ready made control. That would be my 2¢ Max. #### GopherT Joined Nov 23, 2012 8,012 I'm looking for some input on what type of DC motor runs with the lowest amount on vibration. I'm gonna use a DC motor to drive a disc, called a lap, for polishing gemstones. Max speed will never exceed 1500rpm, and a huge amount of torque isn't really necessary. I originally built the machine using a v-belt pulley drive, but this setup had way to much wobble and vibration for it to be of use. I know it's possible to engineer a extremely smooth running drive train using pulleys, but that's not the way I'm tackling this project. So now I'm trying to decide on which motor I want to use in a direct drive. As I'm set on using a DC motor(for a number of reasons), I've narrowed my choice between a couple different motors. Right now I'm leaning towards brushless permanent magnet motor because of size vs power and efficiency. The only negative is price and controls for the BLDC. Second idea was for a stepper motor, nema 23 or 34. I've seen people use steppers in continuous rotation, but it's not something I've done before, only ever used them for stepping. Since my speed requirement in fairly low, I've been toying with the idea of using a stepper. Will running a stepper continuously cause excessive heat to the driver or motor? Would the motor run more smoothly than a basic brushed permanent magnet DC motor? The third option, if all else fails, would be a basic pm DC motor, which are a good deal cheaper and easier to control. So I'm looking for some suggestions as to which of these will best suit my needs. I don't have a huge budget to spend on a motor, but I believe that I can find a good fit for under$100.
Cutting, polishing and lapping smoothly rely on, as you said, minimizing vibrations. The key to minimal vibrations, whether it is a table saw, lathe or phonograph turntable, the best way to minimize vibration is high quality bearings of the appropriate design for the load (transverse, lateral, thrust, ...). Unfortunately bearings and the bearing races in nearly every motor are designed for simple loads that would be in plane with a pulley and not designed for a deflection of a pulley (as you will have with a lapping disc). A lapping disc will need a thrust bearing and motors with thrust bearings are rare and expensive.

You will be much better off with an optimized version of your original setup (with belt). To minimize vibration in a pulley system, use machined pulleys (not die cast pulleys), use belts with links, not continuous v-belts and you need the proper bearings at the proximal race and distal race.

Good luck.

Joined Dec 6, 2015
12
Max: To be honest, I had never heard of "cogging" before. I assumed(wrongfully so) that standard BLDC motors would run the smoothest because there is no brushes to wear down or cause friction and lead to vibration. I'm really glad I read more about the subject, as I was fixin to buy a bldc thinking it was what I wanted.
After reading these suggestions, as well as a couple other sources, I end up buying a PM motor. I found a new-old-stock motor on eBay for a really good deal. It's a disc or flat armature motor made by Yaskawa. The printed rotor is supposed to prevent cogging. Plus the P-series motor is a quite compact pancake style motor. I just powered it up to check its function, but I was really impressed by how little vibration is produced.
Now I'm on to my next step, machining an platen to hold my laps, that will fit my new motor.
Thanks for the replies everyone.