Using Stepper Motors

[healthyhome]

Hi All, I can never seem to find any information regarding stepper motors and there use as to there ratings. I expect to be using a controller which has stepper drivers rated at 2 Amps and are software controlled (no trimpots). I have steppers as follows: Nema 17 1.8 deg 4.5V 1A 6 wire 350mNm (50 oz.in) to be used on the x and Y axis. I have NEMA 23 motors rated 1.8 degree 3.7 VDC, 4.2 Amps per phase, to be used on the Z axis (2 motors). My question has always been What determines the min or max ratings for the motors to be sued. Is it the controller rated amp per driver or the ratings per motor and power supply, etc, etc. I can't find anything specific on use of motors and if these will even work for my 3d printer build. Any help would be appreciated.

[awerby]

We were talking about this yesterday, in a different thread. You need to find drivers that will handle 2 amps to drive the NEMA 17 motors, and the one that drives the NEMA 23 will have to handle 4.2 amps. The power supply that feeds them will have to be able to supply the sum of all the amperage that any motors running at the same time will draw.

The nameplate voltages don't have much to do with the voltages that they need to run optimally. That's determined by the inductance of the motors, which you haven't told us about. The square root of their inductance times 32 is the voltage you want to supply your motors. If that's higher than your drivers will tolerate, you either need to get different drivers, rethink your choice of motors, or resign yourself to speeds that fall short of the motors' true capability.

Andrew Werby
www.computersculpture.com

[healthyhome]

Thanks Awerby. As I feared I do not know of any 3d controller than can handle 4.2 amps. The smoothie is limited to 2 amps and the ST Electronics Eval board is limited to 3 amps. So I can only presume that high power motors are not to be used in 3d printing, unless you want to use a high power CNC controller. As for the Voltage, the inductance on the NEMA 23's is 3.8 MH. Using your equation at square root of 3.8 x 32 is over 62 Volts!! I don't think that would work on a motor rated to run at 3.78 VDC. Is my math wrong? Thanks for the help.

[awerby]

Your math is correct. The 3.78 vdc nameplate rating doesn't mean it will run well on 3.78 volts; that depends on the inductance. For 3D printing, you want motors that will run fast, but you don't need a lot of torque, since there usually isn't much resistance to motion. Amperage is related to torque, voltage to speed. If you want to use the Smoothie board, then look for motors rated at 2 amps or less. Pay attention to its maximum voltage spec, and look for motors with low enough inductance to run fast at that voltage.

[printbus]

As I feared I do not know of any 3d controller than can handle 4.2 amps. The smoothie is limited to 2 amps...

That is, of course, using the on-board stepper driver. The smoothieboard is however already set up with connection pads for external motor drivers. I know I've heard people in the smoothieware IRC channel talking about this, but I haven't paid it enough attention to offer what others have done for high current motors.

EDIT: Although I admit I don't know how you'd do software control of the current limit for the external drivers in this case.

...The square root of their inductance times 32 is the voltage you want to supply your motors.

Can you provide a source reference for this?

[awerby]

If you want to use those high-current motors, I'd say forget about the Smoothieboard and get something that will handle their amperage. Drivers like that are pretty common in the CNC world, although most 3D printing machines use motors with lower current requirements. Geckodrive makes some drivers that will handle the load: http://www.geckodrive.com/geckodrive...or-drives.html

Elsewhere on their site, you'll find the formula for calculating motor voltage (I didn't make it up) among some other useful information on steppers: http://www.geckodrive.com/support.html/ (they used a capital V for a square root sign, but that's what they meant).

[printbus]

...Elsewhere on their site, you'll find the formula for calculating motor voltage (I didn't make it up) among some other useful information on steppers: http://www.geckodrive.com/support.html/ (they used a capital V for a square root sign, but that's what they meant).

Yeah, the equation shows up on a few sources. I was sort of hoping you had one that explains it. The Geckodrive document doesn't do that. While I admit I wasn't a motor designer, the equation doesn't pass a sanity check from my engineering background. To do that I'd want to understand the units and terms that translate us from henries to volts, what yet millihenries to volts. Don't get me wrong - the equation may very well provide a reasonable result - but I look at it as more urban legend than fact.

The "squares" aspect suggests the equation is based on power. In fact, that is implied in this reference - http://www.piclist.com/techref/io/st...inductance.htm wraps up with a statement that reads "Max supply voltage is 32 times the square-root of inductance in mH. It's an equation we derived for NEMA-23 motors without a heatsink where the motor temperature rise will be 85C above ambient." Some key points here is that it is the *max* voltage a motor can handle before likely overheating, and that the equation is for NEMA-23 motors. This doesn't mean the result would be the best voltage to use, and the equation would/should be different for NEMA-17 or any other motor with power characteristics that differ from a NEMA-23. The same page lists max power dissipation of a NEMA-23 as 125W, and how that changes to 32 for the equation isn't clear. Maybe it's just some derating included in their "derivation". The page also leaves me questioning why the equation is in millihenries and not the henries value native to inductance.

EDIT: Sort of reminds me of the engineering joke that goes through a lot of equation derivation, states "and then a miracle happens", and then it gives you the final result.

[awerby]

Marriss Friemanis, the fellow who designs those Geckodrives, is an experienced engineer; he does a good job designing stepper drivers and has done so for some time. I suppose it's possible that 17-frame motors are entirely different in their voltage requirements, but if you want to quibble about the derivation of that formula, I'd suggest you take it up with him; I'm content to follow his advice without calling it insane, or legendary.

Yes, you can run under that max voltage, and you'll get a maximum speed that's a little lower but still acceptable. More powerful motors will handle more current at the same voltage. Basically, the idea is to run as much electricity through the thing as you can without burning it up, although stepper motors can operate comfortably at fairly high temperatures.