Need Help!- Real power supply requirements

[Walky]

Hi

I've been reading this forum for a year or more, and it's been very, very helpful and seems like a nice community, so I decided to join.

I'm a self learner, and at first designed a small Dremel based wood router with a controller based on tip31 transistors and a PIC (code written from scratch), using printer steppers and 1/4" all-thread; which worked quite well for a first try. Then I wanted to go for something bigger, so I can start making some money out of it (besides making some parts for myself... I love building things!).

Some months ago I got a kit from Probotix including one 280 oz-in, two 400oz-in steppers, 3 unipolar drivers, a BOB and a 24V, 6.5A switching power supply.

After realizing I screwed up by getting the 400 oz-in steppers, I decided to buy a 36V, 9.7A switching power supply on ebay (best I can afford at this moment). It should arrive soon I hope.

I read that the amperage needed can be calculated by adding the rated amperage of my motors (3+3+3) and multiplying it by .6 (.67 for safety).
What I'm not entirely sure is if that calculation can be applied to any controller (more specifically, the Probotix ProboStep VX unipolar driver), or does it only apply to the Geckos?

If it applies to the ProboStep, that would mean I can take the voltage up to 40V (according to the datasheet, the power supply I'm getting can be adjusted between 32 and 40V; I'm pretty sure that isn't exactly accurate, but still very close to the actual adjustment range). The supply is rated at ~350W, so 350W/40V=8.75A, which is still higher than 9*.67=6.03.

The ProboStep can take up to 42V, So I guess 38-39V would be safe to work with, wouldn't it? (I don't want to risk my drivers, since I can't spend more money on replacements and, even if I could, can't wait a month or so for them to arrive).

I'm stuck with 3/8" all-thread (16 TPI) for a while, so any speed improvement is welcome. Do you think 3/4" (10 TPI) would be an improvement, or would the extra weight make it not worth it?. Considering I'm using my "half-done" machine to build parts for my new one (I'll be using linear bearings and mounted rails from ebay, which should be here in 2-3 weeks I hope), I think I can keep whipping to a minimum even with my 3/8" screws, so that shouldn't be a problem.

BTW, don't worry about the BOB and cooling fan voltage, I have that figured out so I won't fry them.

Greetings from Chile, and thanks in advance for your help!

[keebler303]

If your drives are full current all the time, you need a power supply of the rated current. Geckos (and some others) have idle current reduction when they are not moving. When they are moving at a decent speed, the current drops due to the inductance of the motor. So they are at reduced current at standstill and when moving quickly, which is probably most of the time on a CNC machine. I prefer to use an oversized power supply as it should increase reliability a bit by reducing the stress on components.

Running a voltage close to the rated maximum also decreases reliability. If you need it to be reliable, give yourself plenty of leeway on both voltage and current (less voltage, more current capacity).

Matt

[ger21]

I wouldn't go over 36V if you want it to last.

I'd stick with the 3/8". I doubt you'd see much improvement (if any) switching to the 3/4". It might even be slower.

[Walky]

Thanks for your answers

It's a chopper driver, with no idling or other fancy features (maybe a few ), so maybe it's still feasible? But there might still be simultaneous PWM peaks on the 3 axis, so that makes me worry. I have no idea about how a power supply behaves under a driver's PWM load.

Here're the driver's specifications:
http://www.probotix.com/manuals/ProboStepVX.pdf

The new power supply alleguedly has overcurrent protection, but I still want to do it the safe way instead of just rising the supply's voltage until it turns off. It would surely be a very small improvement on speed, but every extra IPM counts

Anyway, maybe I'm worrying too soon and 36V will give a satisfying speed. Time will tell

[Walky]

Originally Posted by ger21 I wouldn't go over 36V if you want it to last. I'd stick with the 3/8". I doubt you'd see much improvement (if any) switching to the 3/4". It might even be slower.

The same seller has identical power supplies, at the same price, rated at the same wattage but at different V/A proportions, which makes me think the only difference might be the potentiometer, so I guess it's probably safe to take the voltage up as long as I don't overload the supply. It has a built-in cooling fan, which is a nice thing.

I guess you're right, it's probably a good idea to keep the 3/8". Another reason is that it fits my couplings nicely, and changing to 3/4" would imply having to improvise a turning jig to be able to turn the screws' ends, making me lose valuable time (and if my cnc build happens to go slower after that, curses would follow).

I'm tempted by either cheap ballscrews from Ebay or sprockets from BuildYourCNC so I can forget about all-thread once and for all

[Walky]

My power supply arrived today! I'll tell you how it works as soon as I have the time to install it.

[Walky]

Well, the drivers get quite hot (probably because the motors weren't moving so often on the tests), even after lowering the current closer to the 2A mark (my steppers are rated at 3A unipolar). I guess the higher voltage have a lot to do with it (38V), since at 24V the drivers only got warm, but not hot even near 2A. Even when lowering the current, I'm happy with my speed improvement so far, but won't tell an official number until I get the real thing done (my build's crappyness is limiting my speeds a lot). I'll surely post some pictures of my router once I get my rails and build it using cnc-made parts.

I'm going to set the supply back on 36V and check the temps.