Confused about stepper sizing

[Jeff]

Yeah that was where I was going wrong :). The 4Nm motors from cnc4you will give me cutting speeds of around 3000 mm/min and rapids around 10000 mm/min (which is as high as I can go due to the critical speed of the ballscrew), on a 48v supply with a 3x margin. 68V is way above the ideal voltage for these motors (3.0mH inductance) so I think I'll stick with 48V rather than turn my motors into workshop heaters :)

I'll probably get the CWD556 DSP drivers from there too.

[Clive S]

Yeah that was where I was going wrong :). The 4Nm motors from cnc4you will give me cutting speeds of around 3000 mm/min and rapids around 10000 mm/min (which is as high as I can go due to the critical speed of the ballscrew), on a 48v supply with a 3x margin. 68V is way above the ideal voltage for these motors (3.0mH inductance) so I think I'll stick with 48V rather than turn my motors into workshop heaters :)

I'll probably get the CWD556 DSP drivers from there too.

Jeff It is the norm to use 68V with the motors from cn4you 3.1Nm you won't have any trouble with them. You will stiffen the machine with only 48V. Your choice though>

[Ger21]

68V is way above the ideal voltage for these motors

The "ideal" voltage is the voltage that makes them perform the way you need them to.

If you'll be cutting wood, you'll quickly find out that 3m/min is quite slow.

[Jeff]

I was basing the "ideal" voltage on the 32 x Square Root of inductance which, in the case of the cnc4you steppers would be 55v. The information on the Gecko website seems to infer that voltages above this "ideal" will just produce more heat through iron losses with negligeable torque gains. If you guys are using 68v without issue that would suggest the mathematical "ideal" and the real "idea" are not one and the same thing :)

[Neale]

It's more important to get the max current right and not worry too much about the voltage. The higher voltage gets the motor current up quicker on each pulse which helps keep the torque up. I'm running 3Nm motors on 68V with no problems, like a lot of people. I'm running 2 X axis motors, 3Nm, each driving 2005 ballscrews. I'm getting critical speed on the ballscrews, very similar size to yours, which is around 5m/min rapids. My gantry is probably heavier than yours as well. When you do the sums, don't forget the inertia of the ballscrew as well - when you get to this size, that value can be as much as the gantry.

Clive'a rule of thumb advice is on the money - do the sums, analyse as much as you like, and the answer's pretty well always 3Nm (4Nm), 68V! No point in going smaller (doesn't save much money anyway) and bigger gives worse performance.

[Ger21]

5-10 years ago, the owner of Gecko used to post on the forums quite a bit. He used to say that the ideal voltage, was the lowest voltage required to provide the performance that you need. And any excess voltage is wasted energy and heat.

They then came up with a formula that was the theoretical maximum (not "ideal") voltage. As all motors are different, this formula isn't a firm rule, even though their website will lead you to believe that.

Here's the main issue.
In an industrial application, a stepper motor would be sized to have 30%-50% more power than the application required, to insure the motor will never lose steps.

In the DIY CNC world, a motor is selected, and then the user tries to run that motor as fast as he possibly can. This requires throwing more and more voltage at it. With better drives, like AM882's or EM806's, the current reduction, and other features, can allow you to run at much higher voltages, without the motor overheating. Mounting your motor to a big block of aluminum also helps keep the motors cool, by acting like a large heatsink.

[Jeff]

Thanks guys, really great insights.

I guess that the lazy ass part of me thought that 48v was good because I could get away with an off the shelf PSU instead of building a 70v 12A linear :)