Electronics > Beginners

Do two steps per revolution stepper motors exist (aka help me identify this)?

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ivaylo:
Came across the pictured device. The rotor is a solid cylindrical permanent magnet. The stator consists of two parts:

* a coil with just two wires coming out (and that goes straight out of the device)
* a permanent magnet with similar shape to the coil sitting on the same axis as the coil (the rotor goes through both)When I turn it by hand (not powered in any way) it has one distinct positions it prefers to stay in (the rotor and stator magnets just act with each other). There are no distinct teeth, or poles on neither the stator nor the rotor. Unfortunately I assembled it back before taking pictures, but the whole thing is super simple.

The part number is C35MT02A12, by Portescap. Looking at the part numbers for steppers of that company - https://www.portescap.com/products/can-stack-motors the "02" part seems to be corresponding to 'steps per revolution'. I've messed with stepper motors, taken them apart, etc, but haven't seen anything this simple (two magnets, two wires and a coil).

Reached out the manufacturer, Portescap. The first response I got was "this part doesn't exists". I sent them the pictures and got a "This part is obsolete and proprietary" back.

I went the complete "for dummies" way and tried to see what is the minimum steps per revolution for which steppers exist. The minimum selection at Digikey is 20 - https://www.digikey.com/products/en/motors-solenoids-driver-boards-modules/stepper-motors/179. This article says it could be 4 - https://learn.adafruit.com/all-about-stepper-motors/types-of-steppers

Any help appreciated... Could this be some very low steps per revolution stepper, or is it some sort of an actuator/servo/whatever? And how do you think it's driven?

james_s:
I had a similar motor when I was a kid, IIRC it also had an optical encoder on the back. I don't know for sure, but I believe it was a type of servo motor which would be driven by a closed loop analog circuit, a bit like the voicecoil head positioning used in hard drives.

emece67:
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T3sl4co1l:
Two wires, a stepper it ain't.

You need two axes (in three or more wires) to provide directional information.  Otherwise it's just pulsing on and off, and some lucky timing, or contrivances, can make a net rotation, but not anything in the same general way that a multiphase motor does.

Could be "single phase" BLDC?

Or it's for angle, not rotation, and it's an oversized d'Arsonval movement, say?

On multiphase:
Single phase induction motors normally start up thanks to a "start" or "run" winding that is electrically phase shifted.  The phase shift sets up a rotating component to the magnetic field, which drags the rotor along and makes it spin.  The electrical means is usually an L/R or LC time constant: hence the shorting bar in "shaded-pole" motors (L/R), or the "run" capacitor in split phase motors, some induction motors, and rotary phase converters.  These are frequency-dependent elements, so they only work over a modest frequency range, and the starting torque drops very quickly as frequency falls.  Whereas a multiphase induction motor can run down to DC with maximum torque proportional to frequency (using a VFD to drive it), and a synchronous (permanent magnet) motor can run down to DC period -- a true "four steps per revolution" stepper, as such.

You can't have less than 4 steps, because the two phases are driven in quadrature, and each step is 90 degrees, both electrically and mechanically.  You can always have more steps, by dividing the magnet and coil into more poles.

ivaylo:
Thanks, guys! I’ll look into those movements. If there was no stator magnet I can perfectly see how you could measure RPM or phase with this. The device actually looks exactly assembled from the body, front coil and rotor from this picture of the Portescap steppers https://www.portescap.com/products/can-stack-motors/understanding-can-stack-motors Just add another ring magnet on top of the stator coil and throw away everything else on the picture. If this isn’t a stepper (and it sounds like it probably isn’t) it would be impressive how they figured out how to sell a completely different product reusing the majority of parts from their steppers...

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