DIY Magnetic Stirrer

At 3600 rpm, if you got it started.

A rotating magnetic field is better, but needs two coils, 4 magnetic poles, and quadrature electrical drive.

I guess you need a central ferrous thing to keep the magnet centered, too. It's an interesting stability problem.

John

I should mention... There's also 2 types of drop in magnets for magnetic stirrers..

One, just drop in a bar magnet. It spins around in the liquid.

The other, the magnet spins at the end of the rod.

I'd probably do the magnet on a rod way so I don't get into chaotic magnetic motion..

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Hint: By cutting the core apart, you have introduced a _large_air gap into the core. It will saturate at a much lower voltage than the original transformer's rating.

You are not going to want to run this thing at line frequency. As John pointed out, that's 3600 RPM. In fact, you might want to implement a variable speed setup. Also, as John pointed out, you will need more than two poles to actually cause anything to spin. 3 would work. 4 might make for some simpler control logic.

You could try for an open loop system, where the electromagnets are switched regardless of the magnet's position and hope that it just 'follows' the rotating field. That should work about as well as the motor-driven magnet types. A more advanced design might incorporate a Hall effect sensor to detect the magnet's position and optimize the drive timing to maximize the coupling between the stator and the rotor.

Are you sure? If a pair of poles lay un-energized with a magnet between them, and they get energized in the wrong polarity for the magnet, it will flip. If you flip the current when it reaches the opposite orientation, it will continue in the same direction and flip again. It might not be as efficient, but it will spin. It will tend to stay centered over the coil, too.

Most of the little "build it yourself" demonstration motor kits made for kids only have two poles, and they seem to work alright.

Clifford Heath.

You ccould use the un-driven phase to detect the magnet position (iff it's moving).

Best regards, Spehro Pefhany

Bummer... :( The idea seemed worthwhile but.. only if one transformer needs to be hacked.

D from BC

This looks like it

Geez...this thing looks scrappy.. :) I don't think junior is gonna like this pile of junk for Christmas.. :)

A little neater looking toy motor kit.

Super simple motor..

D from BC

This actually works quite well if you can synchronize the pole commutation with the rotor. That's how a PM DC motor works. and they work quite well.

An open loop system might work if it is equipped with a speed control and the operator brings it up slowly (or a ramp function could be built in to the controller).

The ones I've seen are commutated DC motors.

It will keep spinning if you externally spin it up to close to 3600 rpm. But it won't start by itself; at best it will vibrate a little.

I'm not sure about the centering. It would probably prefer to be over one of the poles.

They have brushes.

John

What about a stripped down stepper motor guts just under the beaker, would it have enough field to rotate a magnet 3-4 mm away?

Seems possible.. I suspect the field pattern is best inside the stepper. I think the poles need to point toward the beaker for more field strength on the stirrer magnet. I've never taken a stepper apart.. Maybe I could bent the poles toward the beaker???

Some educational stepper motor animation on

If the stirrer magnet is out of range, I can still use the same control electronics if I make poles.

D from BC

Don't know if it helps - years ago I bought some cheap heated stirrers for the place I was working at. I vaguely remember inside was a small ac motor turntable / exhaust fan (universal ????) motor + magnet. pretty sure speed control was via a cheap incandescent light dimmer module.

Or one of these very flat motors that are used as spindle motors in floppy drives or capstan motors in VCR's.

Wim

Ohh yeah!.... :) I've taken those apart.. Very cool looking. I think that'll work.. Great idea.. I forgot about those things. Floppy drives and VCR's are landfill material these days. I should be able to get either for next to free. If easy, I might be able to use the drive electronics too. I'll go for the VCR motor.. I think it handles more power than a floppy motor.

Thanks D from BC

Neat... That's just like my current homebrew magnetic stirrer. Bar magnet + computer fan + variable supply. I bumped into a website not long ago where somebody did the same idea. There's nice photos too..

I'm surprised a mass production type existed with that construction.

D from BC

Clifford Heath snipped-for-privacy@spam.please.net posted to sci.electronics.design:

Most of those are brush and commutator motors.

D from BC snipped-for-privacy@comic.com posted to sci.electronics.design:

Not all is lost. Look up shaded pole motors.

They have no starting torque, OR they have two pole stators and three pole (or more) rotors.

The no-moving-parts idea is a bad one, because 60 Hz fixed excitation will mean it has to start at full speed. The motorized stirrers have weak motors that lag badly when starting, and go 'way slower than a synchronous motor to start (even if they go full speed after startup).

If you were to modify the scheme to use variable-magnetic-frequency and derive magnetization current efficiently from switchmode modulators, it'd be more trouble than a cheap shaded-pole motor that slips.

Yes, Line frequency is useless. It has to start from zero to some usefull revs.

Rene