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A gentle guide to choosing a stepper motor?

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New Member
I have built a machine that drags a (20lb, say) camera up some rails and just need to pick a suitable stepper motor. This is something that engineers must do frequently, but I can't work out a basic set of guidelines (other than torque requirements). I've browsed manufacturers' specs and the variety is wild and bewildering.

Desired specs:
Size: NEMA-23
Drive voltage: 12v battery
Current: I have some IRFZ20 MOSFETs for a simple driver so current not an issue(?)
Torque > 120 Oz-in. I have a 180:1 gearbox installed so massive torques aren't necessary.
max RPM @ 12v: ideally >100, but not critical.
Unipolar - I'm doing the switching from a uC. I know how to do basic full- half-stepping etc.

What I have
Currently I'm using this (cheap) stepper, and am not entirely impressed by it;


and was thinking of replacing it with a motor from rows 4 to 6 of this selection,

but don't know how to pick one, and whether I could achieve those currents (>1Amp) with a constant 12 voltage (as opposed to a fancy constant current-type driver).

The following just reflects tidbits of my understanding, please speak up if I'm horribly wrong,

Inductance: for a given voltage, a low inductance motor allows you to drive it at higher RPM, because the current in each coil can rise faster towards it's final value.

Reactance: By adding a resistor, and cranking up the voltage - provided one respects the max current rating - the overall impedance (resistance and reactance) becomes more resistive and less inductive -> higher rpm possible.

Voltage: the current and resistance ratings of steppers sometimes suggest they're rated for v.low voltages ~few volts. Fancy drivers achieve high rpm by exceeding this voltage during the current-rise time, then dropping it once current approaches max value.
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Well-Known Member
you really want to use a current sourcing driver. Pick your motor based on the torque that you need at the lowest possible voltage within the current range of a reasonably priced drive chip and drive it with as much voltage as you can.

The "rated voltage" is a fallacy and is simply the voltage that will drive the rated current through the winding resistance.

The "rated torque" is a fallacy. In reality this number is the maximum continuous torque.

The "rated speed" is also a fallacy. It is simply the maximum speed you can get when applying the "rated voltage" with out losing the "rated torque".

In short normal data sheet numbers are for continuous operation under simple full step controllers.

At the "rated speed" the exponential R-L charge curve just reaches the "rated current" before the next step.

Because of the exponential charge curve 4x the drive voltage through a current source motor drive will give you around 15x the speed at the same torque. For intermittent duty you can get twice as much torque at lower efficiency sine you can let the motor cool down in between.



Is there any particular reason you need a stepper? A regular DC motor seems like it would be a little simpler to chose, would just need to be geared properly.
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