# modifying the speed of a brushless AC motor

#### SgtWookie

Joined Jul 17, 2007
22,221
Between a rock and a hard place for sure!

PC HDD's spin fast enough and run on DC which could be regulated with PWM down to 717 RMPs, but you would have the time of your life balancing your sample. This, of course, is a bit of humor. Do not attempt this at home!!!
PC HDDs' have BLDC motors (Brushless DC), which are not really PWM friendly; they basically have a 3-phase bridge driving the windings, and the drive electronics monitors the speed of the platters.

PC fans also have BLDC motors, but they frequently also have PWM inputs as well as outputs that give feedback as to the motors' speed.

One of the things about a centrifuge is that the rotor is a relatively large mass; the samples are fairly insignificant in comparison. This is important as the rotor comes up to speed; if there was a large relative imbalance, the motor bearings would be damaged to to the unequal force applied, and the centrifuge could start "walking" around; sort of like a washing machine slamming around during the spin cycle, but at a much higher speed.

I had that hunch too, as I'm pretty sure I made a couple false assumptions with that math...it was more of an estimate. I would expect it to be less than 3.2 minutes realistically. Lets give it a fudge-factor of ±50%
I think you need a bigger fudge factor; the standard RPM of that centrifuge results in about 22 times as many G-forces as you want!

Care to share what the samples are that you're intending to spin?

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#### staratsx

Joined Sep 30, 2011
16
Care to share what the samples are that you're intending to spin?
Surely. It is blood - for cPRP preparation. Traditionally lower G's are used in the first spin cycle to procure the platelet-rich region. Like I was saying though, I'm unsure as to whether or not the resolution would be significantly affected if I just adjusted the spin time accordingly for my unit. I know it would certianly be lower, but I might still get workable results.

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#### SgtWookie

Joined Jul 17, 2007
22,221
I'm afraid that I can't offer you any advice about the lower spin time; my use of centrifuges has been minimal and had nothing to do with the field of medicine.

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#### staratsx

Joined Sep 30, 2011
16
thanks again for all the input

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#### staratsx

Joined Sep 30, 2011
16

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#### staratsx

Joined Sep 30, 2011
16
I asked them for reference what the cost of a new/refusbished unit would be without my trade-in. Just got an email back and a new is $970, refurb is$500.

Sounds like a pretty sweet deal to me. they're willing to eat my unit for $300, and sell me a better one for$200!

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#### staratsx

Joined Sep 30, 2011
16
Hm...i've been asked for a purchase order number. I wonder if they really need that, or if I can just make one up lol.

#### SgtWookie

Joined Jul 17, 2007
22,221
If you are purchasing it as an individual, just make one up.

If you are purchasing it as an employee of a company, you should get a purchase order number from your accounting/finance department.

http://en.wikipedia.org/wiki/Purchase_order
Issuing a P.O. basically means that you have a contract with the seller to exchange money for products or services.

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#### staratsx

Joined Sep 30, 2011
16
very good, that's what I figured.