Rotor inertia

briskspirit

Hi there!

I am choosing stepper for my Y axis. Now it uses 34mm long 22N/cm 1.8 degree stepper and it seems enough. I plan to switch to 0.9 degree steppers and choosing right now between 22N/cm 34mm long and 36N/cm 40mm long. As it is Y axis - I don't care about weight of a stepper. But what about rotor inertia? 34mm - 35g/cm, 40mm - 54g/cm. Is it a big difference? Is it critical?

Thanks

Catalin_RO

@briskspirit I was going to ask you for the source of those small and extremely high torque motors... Are you sure the torque is Nm and not Ncm? I have 86mm long NEMA23 steppers and those are barely 2.4Nm!!!

briskspirit

Sorry, mistyped, corrected.

RCarlyle

You probably won’t notice the rotor inertia difference. A couple reasons:

Torque increases along with rotor inertia at about the same linear rate with motor length, so you don’t really have to worry about the motor moving itself... you should always gain load-moving ability with a longer motor of similar specs (though the same is not necessarily true of jumping to a larger frame size eg NEMA 23; they might accelerate slower than NEMA 17 despite higher torque.)
Most 3D printers have a drivetrain load reflected inertia a few times higher than the rotor inertia, because no gearbox is used. So the motor is working a lot more to move the load than to move the rotor.

Note 1: the motor can deliver maximal acceleration to the load, or have largest safety factor for accelerating the load, when the reflected inertia of the drivetrain equals the rotor inertia. For optimal acceleration, you set the gearbox ratio to match the inertias. But you may not be able to reach desired speeds at that point. Any reflected inertia between 1x-5x the rotor inertia is good in practice, and higher ratios than 5x can work just fine with open-loop steppers if you’re gentle.)

Note 2: motor torque and rotor inertia are both roughly proportional to the length of the black laminated section between the end-caps.

briskspirit

@rcarlyle said in Rotor inertia:

Note 2: motor torque and rotor inertia are both roughly proportional to the length of the black laminated section between the end-caps.

Strange thing: disassembled 2 steppers, both 34mm length. One of them has shorter rotor than other. Shorter is rated for 1.2A, longer - 0.4A. Torque by datasheet is the same.

Dougal1957

@briskspirit the low current one will have a higher inductance

briskspirit

@dougal1957 Ahh, right...

wilriker

@briskspirit said in Rotor inertia:

But what about rotor inertia? 34mm - 35g/cm, 40mm - 54g/cm. Is it a big difference? Is it critical?

To answer this initial question: If you are e.g. using 16tooth GT2 pulleys on the rotor this will add an artificial mass of of 130 or 210g respectively to the mass of your Y axis and will slightly reduce your maximum acceleration. Unless you have a featherweight Y axis these extra masses due to rotor inertia don't have much influence.

For details see Maximum Acceleration Calculator.