I have been trying to find a High Vacuum Compatible DC motor that runs around 200 RPMs and have not had any luck. The ones that we have been trying to use have been failing at a high rate and we are actually trying a modified RC Car/Hobby motor but is is running at close to 5 amps and 6-7 volts. We run at around E-8 mTorr to E-9 mTorr range.
Google is only giving me vacuum compatible actuators or E-6 compatible motors.
Does anyone know if there is a reason that E-9 mTorr DC motors are hard to find? Are there physical limitations that prevent them from being made? We have a brushless motor on order because it seems that the brushes are the failure point. Would that make a difference in a High Vacuum environment?
It is easier for electrons and ions to go zipping around in a high vacuum. Air is a good insulator. Maybe that’s the problem. Can you see the motor, or get a look at one as an experiment, while it runs? Maybe there’s aggressive plasma all around the commutator.
Unfortunately, it would be very difficult to see the motor since the view ports are located on the top of a long vacuum chamber and the motor is surrounded by the housing. Although the rear of the motor (where the electrical contacts are) is exposed but on (for the sake of argument) the right side of the cart. There is actually plasma already being produced via magnetron sputtering. The targets are located on the “left” side of the chamber.
Chamber diagram from top:
====Target=======Target======= LEFT SIDE of Chamber
----->—>—Path of travel------>---->------
=========================== RIGHT SIDE of Chamber
Although, I may see about suggesting a mock run with the targets off and see if there is any glow from the area of the motor.
I guess when I said E-8 mTorr that was the pressure without the targets lit and ramping up to running speed. The motors that move the cart are external with feedthroughs. The motor that I am asking about is fully in the chamber and powered by rails.
*Magnetron Sputtering a.k.a. Sputter deposition
“Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering, that is ejecting, material from a “target,” that is source, which then deposits onto a “substrate,” such as a silicon wafer.”
I would think the biggest problem would be outgassing from the bearing lubrication. So, you will need to find one with solid lubricant or teflon bearings.
The windings are another issue. Lots of surface area and crevices to hold volatiles and create virtual leaks. Often the windings are dipped in varnish, which is volatile in itself and seals in any bubbles well enough that they take a really long time to outgas.
Is there any way you can put the motor at atmospheric and drive the load via a magnetic coupling through the wall of the vacuum chamber?
I was going to reply suggesting brushless motors - but see you’re already on the right track there.
Regular brush DC motors fail rapidly in vacuum because the graphite brushes are no longer slippery - closer to being “pumice like” in their action since the slipperiness depends on a small amount of absorbed moisture. Well known phenomenon in spacecraft design. They are still used in cases where operation only needs to be a few minutes or so.
See if you can use a stepper motor if this is some kind of instrumentation gadget that doesn’t need much power. Or as another poster suggested drive through a sealed shaft & cup seal (or magnetic coupling) and leave the motor outside the chamber.
Thanks for the information.
Unfortunetly, we discovered that the position of the motor does not allow us to see if there is a plasma generated on the motor itself. Also, this motor is mounted on the moving cart and we would not be able to mount it through a feed through.
The RC Car/Hobby motor seems to be holding up pretty well, considering that it was not designed with vacuum in mind. The theory is that since it is designed for a lot higher RPMs than we are running, it is more durable and not wearing out as fast. Funny that a $5 hobby motor is out lasting a $150 vacuum compatible motor. I was informed that until we get the brushless one, we will use the multi-purpose one for not. They just put 10 on order and will replace them if/when they fail. It has already lasted 3 runs longer than the expensive ones.
we are trying to improve a vacuum deposition system. this means to make the substrate rotate during evaporation. but we are working with 10E-6 torr and the rotate speed can be as low as 20 rpm. I wonder if you could share where you find the vacuum compatible motor, such as the one working with e-6 torr.