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Stepper motor slipping

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Sceadwian

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I have a unipolar stepper motor that I shorted all it's leads together to see what kind of torque was required to make it move and found that if I turned it past a certain speed it would start slipping and move easily. How exactly is this occurring? It's definitely not the gears slipping.
 

Pommie

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Once you get past the cogging forces then it is a closed system and no energy is being taken out bar I²R losses. Think of it as one phase powering the others.

Mike.
 

Sceadwian

Banned
Could you explain that in more detail Pommie? I'm really a dunce when it comes to motor's.
 

Pommie

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Steppers contain a rotor that has permanent magnets that like to align themselves with the stator cores. This manifests itself as a tendency to prefer certain positions and is known as cogging as it resembles a stiff cog. If you spin it fast enough then the tug of the next cog overcomes the previous one and hence it spins freely. I'm assuming that the I²R losses are not as significant as the cogging forces and hence your experience. I would guess that at low speed the cogging forces are the same whether you short the windings or not.

Thinking about it further my comment about 1 phase powering the others is bunkum.:D

Mike.
 

Sceadwian

Banned
No, I don't think that explains what I'm noticing. It's like breaking in a DC motor, if you short the + and - leads together the motor will drag as it's generating it's own breaking voltage. That's what I was trying to test with the steppers with all the phases and commons tied together. What I expected to happen did at first, the stepper was simply harder to turn (independent of cogging) I'm not sure of the RPM as it's geared, but once it was more than a handful of revolutions per second all of a sudden it was like all electric load on the stepper was completely removed like I'd disconnected the wires and the stepper became dramatically easy to turn.
 

Pommie

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In that case could it be that the frequency of the induced voltage became high enough that the impeadance reduced the current?

Mike.
 

Sceadwian

Banned
hmm, that actually sounds plausible, maybe the frequency hit's some reactive resonant frequency with the inductance/capacitance of the motor windings and the phase shift aligns somehow. I'd like to understand it better, which each phase being separately rectified into a common ground it may not occur on real load.
 
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Mike - K8LH

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Try connecting the leads from two stepper motors in parallel... Turn one motor by hand and the other motor moves from the voltages generated by the first motor...
 

Sceadwian

Banned
One motor doesn't seem to generate enough current to step the other unless I assist it but again these are pretty heavily geared steppers, if I spin it faster the same slipping sensation is felt, and all torque on the 2nd motor is lost again. I have the like colored wires all tied together between the two steppers. So I still don't get what's going on here.
 
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