Electric Motor Query For Small Scale Stirrer

Bench trials will determine the suitability of motors and stirrers for any application.

<...the stirrers need to operate with the same speed or very close for each stirrer...> Nonsense, as it takes no account of the diameter of each stirrer. The agitation within each vessel will be the same if the Power Numbers for each vessel are the same.

https://en.wikipedia.org/wiki/Power_number

However, an obvious way of having all the stirrers operating at the same speed is to connect them together mecahnically, so they are all driven by one motor.

Thank you for your reply and suggestions PW

The reason for the post was to try to narrow the field down a bit before buying any motors for bench trials.

To clarify the speed issue, fluids and stirrer blades will be identical in each cell. I am just not experienced enough to know if motor or component tolerances would be a significant factor and what the simplest suitable controller would be.

If you use AC motors, the frequency will pretty much determine the speed and you could drive them with a VFD. Synchronous motors, of course, would be identical speeds, but induction motors would be pretty close provided they are loaded close to the same.

If you use DC motors, the voltage determines the speed and the current determines the torque, so you can wire them in parallel and drive with a variable DC voltage supply.

<...wire them in parallel and drive with a variable DC voltage supply...> That's fine until the gloop in one of the containers is gloopier than the gloop in another, at which point the gloopier gloop's agitator will slow down in comparison with the less gloopy one(s), which ain't what the original poster wanted.

_Definitions:

• _{"Gloop" ≈ undefined fluid of unknown and possibly variable physical, chemical and biological attributes.}
• _{"Gloopy" and its derivatives ≈ Kinematic viscosity = density divided by absolute viscosity, both contributions varying with temperature, among other factors.}

That's fine until the gloop in one of the containers is gloopier than the gloop in another, at which point the gloopier gloop's agitator will slow down in comparison with the less gloopy one(s), which ain't what the original poster wanted.

A DC motor accelerates until it's back emf (plus a little copper IR loss) equals the applied voltage.

So, say if motor A slows down due to more gloop, its back emf will be less, and it will start drawing more current and generating more torque, which will cause it to speed up again. The speeds will tend to even out with all motors generating the same back EMF but motor A drawing more current.

The speed equalization won't be perfect because a small part of the motor voltage is due to the slightly larger IR drop in the copper in motor A, and there might be slight differences between motors. More power will be dissipated in the gloopier cell, which may or may not be a good thing, but the OP asked for equal or almost equal speed.

<...drawing more current and generating more torque, which will cause it to speed up again...> Um, ah, it actually moves to a different place on its current/speed operating line at constant applied voltage. It cannot speed up, as speed and back-EMF are proportional to each other, and it will slow down (try it by applying a brake, simulating "gloopier" in agitation terms, to any DC motor shaft.

You are correct. The unloaded motor speed is proportional to voltage. With a constant voltage, if you put a physical load on the motor it slows down and draws more current. Parallel motors is not a good idea. Thanks.

https://circuitglobe.com/servo-motor.html

What was the definition for, " Gloopier " ?

It's a derivative of "Gloopy", meaning "more Gloopy".

Here's a digital controller and motor from ColeParmer which also reads system torque. Also pricey. There is also a model with higher RPM and lower torque.

MK-50008-00 MIXER SERVODYNE CONTROLER 115V $1,092.00

SI-50008-20 SERVODYNE MOTOR 20-900RPM 115V $1,076.000

Maybe a different technology? Use one of these magnetic stirrers rather than a blade.

Cole-Parmer Five-Position Stirring Hot Plate, 120 VAC

The key to precision is feedback...you need to know the exact speed and current draw on each stirrer, with an adjustable speed control on each stirrer...this will tell you change in viscosity....

Maybe a setup like this...

Brushed vs brushless motors....

http://platinumasi.com/Brushlless-VS-Brushed.pdf

https://www.catscientific.com/overhead-stirrers/r100s-d-overhead-stirrer/

http://www.orientalmotor.com/company/pr/press-releases-06-21-16-bmu-series-200w-400w.html

Can't add much to the previous comments but you might consider that if you have one motor driving all of the stirrers & it fails, all of the stirrers stop until it is replaced. If you have 6 motors or some intermediate number, 2 motors driving 3 stirrers each or 3 motors driving 2 each, you have some continuation whilst repairing.

Would it be possible to use an electric razor motor, several types spin the blades at 1400-1600 cycles per minute.

Plenty of small 12 volt/24 volt motors on eBay set them up with pwm controllers and set speeds with a laser RPM reader.

I would suggest arranging 6 planetary gears each directly driving a stirrer arranged round a single driving sun gear with twice the number of teeth to give your 2000rpm stirrer speed from a 1000rpm motor. Mod 1.5 Delrin gears 12mm wide would operate at a more than adiquate maximum torque of 2.4Nm (based on 50 tooth gear at 1000rpm, derate for higher speed and smaller size) Typically a 40 tooth sun running 6x 20 tooth planets would mean that your vials were equally spaced on a 90mm pcd circle. 20 tooth Mod 1.5 Delrin spur gears cost about £6 each and a 40 tooth spur is about £15. Throw in 6x 8mm stirrer spindles and a 12mm drive spindle with a sliding disc motor coupling (Oldham coupling), a pair of gear carrier plates with spacers and legs and the whole set up could be constructed for about £100 plus motor and controller. Guaranteed matched stirrer speeds. A 1000 rpm motor is about 70% the cost of a 2000 rpm motor.

Best way to get exactly equal speeds would be to mechanically tie all 6 to the same motor with a system of equal gears or a toothed belt. You can buy such systems off the shelf, but they are pricey. I assume this is why you want to design in house.

http://www.biotage.com/product-page/biotage-endeavor