why are stepper motors so hard to turn?

I took apart an old inkjet printer and was surprised the stepper motor has such resistance to it, if it's just an iron gear rotor. What gives?

Reply to
mrdarrett
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Never mind, I think I figured it out. It's got a permanent magnet for a rotor. (?!)

This is the exact one.

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Michael

Reply to
mrdarrett

yeah, they're steppers, not switched reluctance.

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Reply to
Jasen Betts

has such resistance to it, if it's just an iron gear rotor. What gives?

a rotor. (?!)

The search for a cheap reluctance motor continues. :p

What's the difference between variable and switched reluctance, anyway?

Michael

Reply to
mrdarrett

It's even harder if you short the leads.

They make pretty good AC generators and speed sensors. Or, with a cap, an AC-powered synchronous motor.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

There are/were some VR steppers, but not as common as PM types.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Oh nice!

And to think, I thought the resistance was due to precision micrometer spacing between the (reluctance) rotor and stator, such that air was having a hard time getting out of the way :p

Michael

Reply to
mrdarrett

just me using the wrong word :)

If you can disassemble and reassemble them easily, possilby you could put a laser-cut steel rotor in there to make a variable reluctance motor.

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Reply to
Jasen Betts

PM steppers actually do have variable reluctance magnetic paths -- that's why they have favored positions when they're powered down.

Basically, when you have a magnetic path, the magnetic attraction is in the direction that reduces the reluctance of the path and takes the most energy out of the magnetic circuit. With a magnetic circuit "powered" by a permanent magnet, this energy comes out of the magnet (and is returned when you pull things apart). With a variable reluctance motor, this energy comes out of motor's source of current.

If you just want to do a science experiment with reluctance motors, you could make one by hand -- a soft iron bar or ferrite on a shaft and a pair of coils should give you something that will spin if you treat it right.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

Oops -- I think you need at least three independent coils if there's not a magnet involved. But you may know more than me at this point.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

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Ahh, ok.

But, if precise motion is not required, a brushless DC motor would be more efficient than a stepper, right?

Yep, it looks like I'll have to wind my own reluctance motor. I just wante d to study how a real one is made. Why would they laminate the iron rotor. .? Is laminating the stator coils recommended too?

Thanks,

Michael

Reply to
mrdarrett

t
,

On that note, is this a reasonable low-power 3-phase circuit?

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Maybe I'll replace the transistors with PN2222s which can take about 1 amp.

Thanks,

Michael

Reply to
mrdarrett

In an inkjet printer?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Why, increased cost?

Reply to
mrdarrett

every two steps the flux path through the rotor reverses (WRT rotor) if it wasn't laminated there would be large eddy currents getting in the way of the flux, and producing extra heat..

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Reply to
Jasen Betts

There was an interesting application in "Nut's and Volts" years ago where the author used one as a rotary encoder and to read out the position on an up-down counter. One phase was input to the clock and the other to the U/D pin.

I did that for a kind of selsyn. One stepper generates the signal to turn the slave (driving a potentiometer in my application). I used op-amps with protection diodes with their full open loop gain of ~200K (with protection diodes to limit the voltage on the inputs). I couldn't turn the input stepper so slow that it wouldn't make the slave move...

No holding torque (but that's the way I wanted it). Add some flip flops and I could have also had holding torque.

Reply to
default

Yes, I took a random stepper motor and added some diodes and a capacitor, and I got a white LED to light. I needed a bigger capacitor, but the concept was there. I didn't find that stepper motor difficult to turn. Yes, it was't like a cheap DC motor out of a toy, but it wasn't "hard" to turn.

Michael

Reply to
Michael Black

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