DIY Magnetic Stirrer

[D from BC]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

Let's say I sawed off the core at the end of a Hammond transformer as
seen on
http://www.hammondmfg.com/pdf/5c0013.pdf
Approx cost: $30.00

Now it's an electromagnet.
Plugged in, the field alternates at line frequency. (60Hz here.)

Would it spin a bar magnet?

If so, what winding specs should I look at so I don't have transformer
melt down..???

Any quick hints?


D from BC

 

[John Larkin]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

Let's say I sawed off the core at the end of a Hammond transformer as
seen on
http://www.hammondmfg.com/pdf/5c0013.pdf
Approx cost: $30.00

Now it's an electromagnet.
Plugged in, the field alternates at line frequency. (60Hz here.)

Would it spin a bar magnet?

At 3600 rpm, if you got it started.

If so, what winding specs should I look at so I don't have transformer
melt down..???

Any quick hints?

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

 

[D from BC]

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..

----------- <top
| |
| | <rod
| |
| |
/\/\| |\/\/\/ <liquid
| |
| |
| |
| |
-----|------
-----|------ <spinning magnet
^
---------------
((((((())))))) <field generator



D from BC

 

[Paul Hovnanian P.E.]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

Let's say I sawed off the core at the end of a Hammond transformer as
seen on
http://www.hammondmfg.com/pdf/5c0013.pdf
Approx cost: $30.00

Now it's an electromagnet.
Plugged in, the field alternates at line frequency. (60Hz here.)

Would it spin a bar magnet?

If so, what winding specs should I look at so I don't have transformer
melt down..???

Any quick hints?

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.

 

[Clifford Heath]

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.

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.

 

[Spehro Pefhany]

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.

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



Best regards,
Spehro Pefhany

 

[D from BC]

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.

Bummer...
The idea seemed worthwhile but.. only if one transformer needs to be
hacked.
4 poles is too much work and hacking up 2 xformers is costly compared
to buying a used magnetic stirrer.


D from BC

 

[D from BC]

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.

This looks like it
http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=10135
Toy motor kit..

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.
http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=10130

Super simple motor..
http://scientificsonline.com/product.asp?pn=3052874&bhcd2=1197172872
Should hold a kid's attention for about a minute in this video game
era.


D from BC

 

[Paul Hovnanian P.E.]

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.

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).

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

The ones I've seen are commutated DC motors.

 

[John Larkin]

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.

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.

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

They have brushes.

John

 

[Barry Lennox]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

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?

 

[D from BC]

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
http://elect.awardspace.com/stepper/

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


D from BC

 

[Robbo]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

Let's say I sawed off the core at the end of a Hammond transformer as
seen on
http://www.hammondmfg.com/pdf/5c0013.pdf
Approx cost: $30.00

Now it's an electromagnet.
Plugged in, the field alternates at line frequency. (60Hz here.)

Would it spin a bar magnet?

If so, what winding specs should I look at so I don't have transformer
melt down..???

Any quick hints?


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.

 

[Wim Ton]

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
http://elect.awardspace.com/stepper/

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

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

Wim

 

[D from BC]

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

 

[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.

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

 

[JosephKK]

Clifford Heath [email protected] posted to sci.electronics.design:

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.

Most of those are brush and commutator motors.

 

[JosephKK]

D from BC [email protected] posted to sci.electronics.design:

Bummer...
The idea seemed worthwhile but.. only if one transformer needs to be
hacked.
4 poles is too much work and hacking up 2 xformers is costly
compared to buying a used magnetic stirrer.


D from BC

Not all is lost. Look up shaded pole motors.

 

[whit3rd]

They have brushes.

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.

 

[Rene Tschaggelar]

There are 2 types of magnet stirrers.
One that spins a magnet on a motor.
The other is done electromagnetically.

I have an idea to make the electromagnetic version.

Let's say I sawed off the core at the end of a Hammond transformer as
seen on
http://www.hammondmfg.com/pdf/5c0013.pdf
Approx cost: $30.00

Now it's an electromagnet.
Plugged in, the field alternates at line frequency. (60Hz here.)

Would it spin a bar magnet?

If so, what winding specs should I look at so I don't have transformer
melt down..???

Any quick hints?

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

Rene