# Choosing a proper BLDC motor for lab centrifuge

I'm trying to make a swing rotor centrifuge and find a proper BLDC motor to reach 6000G with capacity of 4x50ml Falcon tubes. I did the calculations roughly but I'm not so sure about them and also couldn't find a proper motor for my calculations.

My rotor will be something like this;

Well for my calculations;

• Mass of the system : 250gr (50gr for body and 4x50gr for tubes)
• Radius of the body : 6.5cm (I'm trying to minimise it to minimise the inertia)
• RCF : 6000
• Ramp-up from 0 to 6000G : 10s

And probably my supply voltage will be 12V or 15V and my custom ESC can deliver up to 20A.

And from those inputs I get;

• Inertia : 0.000528125 kg*m^2
• Angular Acc. : 95.1056763541775 rad/s^2
• RPM : 9100
• Torque (with %20 safety margin) : 60.27mNm
• Power : 57.4W

But for example when I go and search for a motor such as 200W High Power Brushless Outrunner DC Motor Bldc Motor Price

It seems the torque is well within specs. But to get 9100RPM I need to supply 10.1V which may cause decrease in torque (?).

Also taking Torque Constant into account I get; 5.7A and it seems too low to me. I mean can I really make this thing spin from 0 to 9100RPM in just 10 seconds with just 10V and 5A?

And there is also Back EMF which I still didn't quiet understand. For the motor in the link, BEMF constant is 0.0011V/rpm and when I calculate torque producing voltage I get 0.6V, does this mean that I actually drive this with 0.6V? If so, than I need 95A to spin it with 9100RPM? But with terminal resistance involved, my available current is 0.6/0.1 = 6A, so I can't drive this with 95A, because of terminal resistance?

As you can see, I'm really confused and I'm really new at this things, so any help would be great.

• Why not just buy a centrifuge? It bound to be cheaper. Feb 3 '19 at 23:10
• Well, thats not the point, and also I don't think that will be cheaper, the mechanical parts will be 3d printed, controller is less than 50\$, only thing left is BLDC, and I don't think that will add cost more than the commercial centrifuges. Feb 4 '19 at 21:44
• 3D printed parts tend to weak. Badly designed centrifuges can be dangerous. Feb 4 '19 at 22:13
• You are planning to spin 3d printed parts at 9000+ rpm. Follow @EricShain 's advice and buy a centrifuge. Feb 5 '19 at 2:45
• I don't think this is a solution to this problem. How about aluminium? Feb 5 '19 at 7:20

## 1 Answer

This is not an answer to your question, per se, but I suggest you consider purchasing a centrifuge rather than trying to make your own. Centrifuges are highly engineered products. Rotors especially are designed carefully because a rotor failure can be catastrophic. A decent centrifuge will have a containment housing designed to keep a rotor failure from injuring users. There will be programmed ramps up to and down from speed and safety interlocks. All of these features are necessary. Commercially available centrifuges are also tested thoroughly.

While a new centrifuge may seem expensive, it is probably cheaper than designing and building an equivalent device yourself, especially if you value your time at all. As of the writing of this answer, there are currently 9,729 listings for centrifuges on eBay, 4,247 of which are used.

• You didn't understand, this is not a DIY project, I'm doing a freelance project as a software developer, and mechanical part of it is poor, and I asked this question to get a decent answer, buying a centrifuge is not an option Feb 5 '19 at 17:04
• @ArifBalik I'm providing the best guidance I can. The danger of an exploding centrifuge is so great that I doubt anyone would take on the liability of advising you. If I were you I would simply decline to help. It is that dangerous. Feb 5 '19 at 21:50