Newbie- Importance of over-voltage?

[crochambeau]

Hello,

I'm in the midst of selecting a driver for my stepper and read the "Step Motor Basics" pdf document downloaded from the Gecko site.

According to that document, I want to hunt out a power supply 3-25 times greater than my rated stepper motor.

Here is what I have in terms of stepper for X,Y & Z:
Pacific Scientific PowerMax II M22NRXA-LNF-NS-00
Bipolar
24 volt
Parallel = 6.5 amp / 0.7 mH / 0.21 ohms per phase
Series = 3.3 amp / 2.8 mH / 0.84 ohms per phase

According to the PacSci spec sheet, either configuration will make 230 oz-in holding torque, I gain from the aforementioned stepper selection guide that lower inductance translates to better power. I'm with it so far..

I have a 24 volt 25 amp industrial battery charger linear power supply slated for stepper supply, one of those over 100 pound behemoths. Am I to expect sub-par performance by providing the motor with close to its rated voltage?

I have yet to decide on a driver, but I'm leaning toward paying up for a domestic drive like the Gecko G201, something that will support the 6.5 amp parallel load.

The router table (still in process as well) is slated to be ~ 24x36x6 or 12 and the primary purpose is cutting PCBs and holes in aluminum enclosures/faceplates, though the occasional wood project will be shoved through. Nothing super taxing and not trying to win any races.

What sort of performance/headache will I face with these mentioned parts?

[irving2008]

First point is that the voltage rating on a stepper motor is usually of little interest as its rarely meaningful - for your motor the voltage is not 24v but the product of the current and the resistance = 6.5*.21 = 1.37v parallel or 2.8v series which suggests a supply between 6 and 67v. The voltage stated is likely to be some recommended upper limit of drive voltage to avoid insulation breakdown.

Current = torque, so what is of primary interest is the current you can drive it with. Where the power supply voltage comes into the equation is where the top end of the torque is in terms of speed. A stepper motor has roughly fixed torque up to its corner speed, which is determined by the coil inductance and the supply voltage. Above the corner speed it is a constant power motor, power = torque x revs, so as the revs go up the torque drops off. Ideally you want the operating speed of the motor to be at or just below the corner speed.

A rough rule of thumb suggests the driver voltage should be 32 * sqrt(L) where L is motor inductance which for your motor = 32*sqrt(.7) = 27v in parallel (53v in series). So 24v is perfectly sound for that motor when operated in parallel mode. In fact the corner speed (in parallel) is around 1000rpm (490rpm series) so as long as you're running the motor below that speed you'll be getting the best out of it and there would be little value in going to a higher voltage.

As you can see, parallel mode gives the best torque v speed and is the recommended mode of use.

[crochambeau]

Excellent! I had something of a "you've got to be kidding me" moment when I read the stepper motor primer, thanks for putting my mind to ease.

[dilbert0815]

Originally Posted by crochambeau Here is what I have in terms of stepper for X,Y & Z: Pacific Scientific PowerMax II M22NRXA-LNF-NS-00 Bipolar 24 volt Parallel = 6.5 amp / 0.7 mH / 0.21 ohms per phase Series = 3.3 amp / 2.8 mH / 0.84 ohms per phase

Shure that 24V rating is read correct ?
Maybe it is 2.4V and the dot weathered away.

I have actually never heared of a motor rated for the rule ot thumb driver voltage. The rated voltage is always the one for static operation like irving2008 pointed out.
Were did that 24V rating come from ?
A spec paper or a label on the motor ?

By the way 24V as insulation rating seems a little low.
40..50V would make more sense as this is the typical border were insulation becomes safety critical.

[irving2008]

yes, you could be right, I did wonder about the 24v being low for insulation breakdown, in my experience they are typically 85v. 2.4 being read as 24 makes more sense...

[Pandinus]

Another equation thrown around for the power supply feeding the driver is V=32*SQRT(L) where L is the inductance in miliHenrys

So 32*SQRT(0.7)=26.77Volts which is close to your rated 24volts (but like mentioned above, if you hooked DC directly to the motor it would be 1.37 volts)

This has to do with the fact that a lower inductance allows the current/magnetic field to build faster... which means you don't need as high a voltage to offset the AC resistance of the coil.

your steppers should spin up real fast with that low of an inductance (more usable high speed torque)

I have a G540 from gecko and am very happy with it... I'd go with the 201X

[crochambeau]

Originally Posted by dilbert0815 Were did that 24V rating come from ? A spec paper or a label on the motor ? By the way 24V as insulation rating seems a little low.

Label on the motor. This is the most beat up label of the 3, and while I expect the distance between 2 and 4 would be larger with a decimal point, stranger things have happened. Pic follows:

http://i7.photobucket.com/albums/y27...uilds/pmx2.jpg

I agree that 24 volts sounds way too low to be an insulation breakdown voltage.

Originally Posted by dilbert0815 I have actually never heared of a motor rated for the rule ot thumb driver voltage.

A thought just occured to me, perhaps this is an OEM motor, and they print 24 volts on it because the Pac-Sci drivers are delivering the optimal 24 volts? Motors were sourced gray market on eBay, so anything goes really..

[crochambeau]

Originally Posted by Pandinus I'd go with the 201X

That one has indeed risen to the top of my want list, if it holds its position until payday it will be be finding a new home.