stepper motor with ultra low torque at rest?

[andrewmiri]

I am looking for a stepper motor, but I want it to have as little resting torque as possible when no voltage is applied, like a computer fan.

The stepper motors I find for sale aren't rated in terms of their torque at 0 V.

Do anyone know where I might find such a motor?

Thanks.

 

[Gryd3]

I am looking for a stepper motor, but I want it to have as little resting torque as possible when no voltage is applied, like a computer fan.

The stepper motors I find for sale aren't rated in terms of their torque at 0 V.

Do anyone know where I might find such a motor?

Thanks.

This will also depend greatly on what you plan to drive it with...
Most stepper drivers will apply 'breaks' by tying the phase wires to ground. You would need to leave the leads to each phase disconnected or left floating to accomplish this.
Why are you attempting to find such a free spinning stepper?

 

[andrewmiri]

Thanks for the reply. For my application, I have a narrow shaft (1/8") that I want to drive to a specific position, but then once it reaches that position, I need the shaft to be able to spin freely.

I worry that most steppers will also have rotors with too much inertia as well, preventing them from spinning freely.

 

[Gryd3]

Thanks for the reply. For my application, I have a narrow shaft (1/8") that I want to drive to a specific position, but then once it reaches that position, I need the shaft to be able to spin freely.

I worry that most steppers will also have rotors with too much inertia as well, preventing them from spinning freely.

Perhaps you should look into using a small motor and an encoder...
Why does it need to spin freely though? and what else might be spinning the shaft?

 

[andrewmiri]

Thanks, I have an absolute encoder I can easily mount on the shaft.

Do you think I could implement a control scheme that would allow reasonable control of a computer fan position using a voltage command? The position accuracy needed is not extreme - 0.1 - 1 degree of rotation should be fine. I just have never tried to control a computer fan - they spin so fast at full voltage!

The shaft will be rotated by air currents against blade on the shaft - the air will generate little torque.

 

[Gryd3]

Thanks, I have an absolute encoder I can easily mount on the shaft.

Do you think I could implement a control scheme that would allow reasonable control of a computer fan position using a voltage command? The position accuracy needed is not extreme - 0.1 - 1 degree of rotation should be fine. I just have never tried to control a computer fan - they spin so fast at full voltage!

The shaft will be rotated by air currents against blade on the shaft - the air will generate little torque.

Typical control schemes for motors are:
Using it like a servo... This requires some kind of position feedback, and a PID loop to rotate the motor forward and backward to rest at the desired location.
Using it for positional information only. This is typically done in engines, and POV displays where something is triggered based on the position of the shaft. The shaft itself is never accurately positioned, but the reactions must be.
How quickly do you need to set the initial position of the shaft? How much force must it be able to overcome if the attached fan is attempting to move it elsewhere?
Can you please share more information about the project? I honestly can't think of any need to accurately position a shaft to 1 degree if it will be free to turn on it's own moments later..

 

[andrewmiri]

A control scheme like those you describe would be easy to implement - I have experience with that. The position of the shaft would not have to be set very quickly. The position corrections will be a small fraction of a rotation - maybe 1/8th of a rotation at most, and I'd have about a second for the correction.

Do they make bi-directional motors that spin as freely as computer fans? Do you know what motor types I should look into?

Sorry to be vague about the application - it's a bit of a science experiment. The shaft holds a propeller-like blade that will be moved by air currents, but the blades will periodically need to be returned to a set position.

 

[Merlin3189]

It may be worth looking up a Permanent Magnet Synchronous Motor.
Many years ago I used one of these as a stepper motor and it had to remain energised to hold it's position. When not energised there was no detent at all I think.
I can't remember how many poles it had, not many, 6 I think, but I believe some do have more poles. It was a 3 phase drive and I think I used a bipolar drive keeping all phases energised with the odd one out rotating (+ - - , - + - , - - +, etc.)
Bear in mind that I did not know much about these motors in those days (and don't know a lot more now!)

Edit: PS. Just noticed this thread which seems to be talking about something v.similar under the name of BLDC motors.

 

[Arouse1973]

You can operate a BLCD motor in what's called micro stepping mode same as used for some stepper motors. How well it will work I don't know. Both motors are designed for different applications the coils maybe too far apart for this to work well. You may find the resolution unacceptable. @Fish4Fun may have the answer, he know a lot about motors.
Adam