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I am designing a motor system that has to run in a vacuum chamber (sub mbar). I am a little bit worried about the heat that would not dissipate at all in the vacuum. Therefore, I want the least amount of energy wasted to heat(high efficiency). Some people advised me to get a big motor that is much more capable so that it can withstand the heat generated under low load.

The rpm I need ranges from 30 rpm to 600 rpm. So I need PWM on top of gearbox+motor. PWM and speed input will be controlled using Arduino. Also I realised that there are brushless and brushed motors. I figured that brushless motors are a bit more efficient which means less wasted as heat.

So in conclusion, I want a setup that is, big brushless motor + gearbox + PWM. Am I thinking correct? I have never gotten a chance to do any electrical engineering so I might be missing obvious flaws. Please let me know if there are any better ways to achieve this.

Additional info) I chose dc motor because the important part is to run continuously and smoothly at a set constant speed without bumps or jittering. One session of running time would be ~10 minutes. Torque is not so much of a problem, probably maximum of 1000gcm needed I guess.

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  • \$\begingroup\$ I would try and go with a big outrunner instead of a gearbox. Gearboxes are expensive. For low speed smoothness, you want a sensored drive. Maybe even a sensored sine drive so you can avoid the gearbox but 1kg-cm may not allow that. \$\endgroup\$
    – DKNguyen
    Commented Aug 12, 2019 at 15:02
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    \$\begingroup\$ You want a brushless motor because brushes and their lubricant will almost certainly contaminate your vacuum, and because if the vacuum is high enough the brushes will vacuum-weld to the commutator (unless you use graphite brushes, in which case the dust will really contaminate your vacuum). Gearboxes have similar problems in a vacuum, so you should seriously consider direct drive. \$\endgroup\$
    – TimWescott
    Commented Aug 12, 2019 at 15:09
  • \$\begingroup\$ @DKNguyen Thank you for your kind reply. Budget is less of a problem as the project is funded but I will definitely have a look into this outrunner thing. I am very new to any motors so need a little bit of time :) \$\endgroup\$
    – Seung
    Commented Aug 12, 2019 at 15:09
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    \$\begingroup\$ I forgot to mention bearings -- you need bearings that are vacuum-rated, too, for the same reasons you need to worry about brushes and gears. \$\endgroup\$
    – TimWescott
    Commented Aug 12, 2019 at 15:10
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    \$\begingroup\$ An outrunner is probably not what you want, as there's no ready way to get the heat out of the coils. Consider something like a stepper (probably with low steps/revolution) or else a conventional inrunner, and then wrap malleable tubing around the outside and pipe a coolant through that. If you are going to use an inrunner, at least machine a custom center/mount from aluminum and heatsink it to something like an aluminum base of the chamber, with the goal being to maintain a large cross section. This question isn't really on topic, you do not yet even know if you really have a problem. \$\endgroup\$
    – Chris Stratton
    Commented Aug 12, 2019 at 15:23

2 Answers 2

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Interesting question!

Your worst case is that you have a perfect vacuum and no heat path out of your chamber. 600 rpm at 1000 gcm is about 6 watts of mechanical power. I wouldn't choose a big motor because it would be less efficient; rather, choose a "right-sized" brushless motor and gearbox for your speed and load. You should be able to achieve at least 50% efficiency, meaning that 6 watts will be going into your load and 6 watts will be lost as heat. If you run for 10 minutes (or 600 seconds), 6 watts will produce 3600 joules or 860 calories. You only need to attach a heat sink capable of absorbing 860 calories. Aluminum has a specific heat of 0.215 calories/gram-Kelvin. Do the math, and 200 grams of aluminum heat sink will have a temperature rise of 20 degrees K. So attach a heat sink and you can play around with the time, losses, and thermal mass until you get what you need. Of course, you must cool it after each run by allowing air in for convection or some other heat path out of your chamber. Otherwise the temperature will continue to rise with each run.

Good luck!

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  • \$\begingroup\$ Ahh... I could not have asked for more! Fantastic guideline and thank you. \$\endgroup\$
    – Seung
    Commented Aug 12, 2019 at 20:36
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I recommend putting the motor outside of the vacuum and use a magnet-coupled drive through the container wall.

Another option is using an AC induction motor and putting the coil outside of the vaccum for better heat transfer. This is done for high-power X-ray tubes to rotate the anode. Again, the reason is better heat transfer.

https://en.wikipedia.org/wiki/File:Rotating_anode_x-ray_tube_(labeled).jpg https://en.wikipedia.org/wiki/File:Rotating_anode_x-ray_tube_(labeled).jpg

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  • \$\begingroup\$ Thank you for your answer. Unfortunately the chamber is fairly big and it would be hard to make a way to deliver the force from outside. \$\endgroup\$
    – Seung
    Commented Aug 12, 2019 at 20:43

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