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% :D
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?
 
Last edited:

Thread Starter

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.
 
Last edited:

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.
 
Last edited:

Thread Starter

staratsx

Joined Sep 30, 2011
16
Well, The Drucker Co. got back to me and it looks like I can trade my 642E in for a 642VES (variable speed), but it would cost me 200$. So I have that to think about...
 

SgtWookie

Joined Jul 17, 2007
22,221
Did they say whether the 642VES can be made to run as slow as 718 RPM?
The specs in the manual say 1,000 to ~3400 RPM, so that's ~128g-140g's; still about twice what you wanted.
 

iONic

Joined Nov 16, 2007
1,653
Well, at least your sample resolution would be much closer to what you wanted. You might be able to spin for 9 - 10min as apposed to 3min 11sec. And at least your not out the full cost of the centrifuge you have now. That's a big +.
 

Thread Starter

staratsx

Joined Sep 30, 2011
16
Yeah it goes down to around 145 G's or something, but thats close enough for govt. work. I think I've even read a couple articles where they go as high as 300 G's for a first spin
 

SgtWookie

Joined Jul 17, 2007
22,221
$200 for the upgrade sounds like a heck of a deal. That's a drop in the test tube (pun intended) as far as medical costs go.
 

Thread Starter

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! :D
 

Thread Starter

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.
 
Top