Choosing a proper BLDC motor for lab centrifuge

Question

I'm trying to make a swing rotor centrifuge and find a proper BLDC motor to reach 6000G with capacity of 4x50 ml 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 : 250 g (50 g for body and 4x50 g for tubes)
• Radius of the body : 6.5 cm (I'm trying to minimise it to minimise the inertia)
• RCF : 6000
• Ramp-up from 0 to 6000G : 10 s

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

And from those inputs I get;

• Inertia : 0.000528125 kgm²
• Angular Acc. : 95.1056763541775 rad/s²
• RPM : 9100
• Torque (with 20% safety margin) : 60.27 mNm
• Power : 57.4 W

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

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

Also taking Torque Constant into account I get; 5.7 A 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 10 V and 5 A?

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

Answer 1

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.

Answer 2

Picking a BLDC motor for the application of a Centrifuge is a bit more difficult than selecting an Universal Motor. While small size BLDC motors such as the one you initially selected can achieve the high RPM your looking for, they typically are not of the size or design that would match the requirements of a centrifuge.

While it is true that Centrifuge manufactures have switched over to brush-less designs this is mainly due to increase efficiency and reduce maintenance at the trade off of higher cost for the motor. So if cost is a major factor I would reconsider the use of a Universal motor instead.

But to answer your question. In your application the torque available at a given motor frame size is a designer’s limiting factor. You can add a gearbox, but the torque gained comes at a cost to footprint compactness, precision, efficiency, and responsiveness.

For example the ElectroCraft, the RPX22 gets you close to the requirements you mention, where as RPX32-150 would probably be a bit overkill. These go for as low as $100 on Ebay.

If you look at the size of this motor compared to your initial selection it would seem you would need 4+ of the other motors to even get close to the torque and power requirements.

https://www.electrocraft.com/products/bldc/RPX32/