so i guess i need more volts to go past 300rpm

[mike10]

i posted a tread about a 300 rpm barier on my motors
the best answer i got was that i need more volts
im runing 12v now if i run 24v will i need new resistors or will they remain the same 160watts 2 ohms?
the resistors cost me alot of money


i have two 12v power supply's can i wire them positive to negative and use the other positive and negative to produce 24v
(same prinsible as using 1.5v batteries to get 3v)?.

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[chrisw765]

i had a simullar problam,but it has nothing to do with volts persay,i was running my board at 1/8 micro drive and 1/4-20 leads so=32000 steps per inch when i speed the motors up they ramp up and stop and start sping on ramp down,tuned out it was to many steps for my p233 to keep up with,unless i slowed it way down,my fix was put them in 1/4 micro stepping and that cut the steps per inch in half and i was able to go full out on my motors,also a pc power supply is a switching/pwm setup and im not sure you can run thows in series due to the frquncy output but ther cheap try it.

[Dr.H]

dont buy other resistors. if you put 24v with 4 ohm resistor its the same thing as 12v 2 ohm

you can wire your power supply like batteries but dont do it unless you are near 300 rpm or you could burn your motor. try do add a switch or something to activate your 2nd power supply when you wat to go faster. if you have a amp meter you can use it to make sure you dont go past the amp rating of your motor

[fyffe555]

Mike, If you increase the voltage you will need to change (increase) the power resistors. If you increase the voltage then you have to increase the restance to maintain the same max motor amps. And no they're not cheap. It could be as simple as doubling up the reistors you have though, Theres a price breakpoint where chopper drivers become cheaper than resistor limited L/R and its a lot lower than you'd think.

Balsaman put out a free resistor calculator a while back called steppercalc (?) it's a very nice little calculator. Do a search or drop balsaman a PM and see if you can get a copy.

You need to be careful using two 12 volt supplies to produce 24v. Its possible but theres a few issues.

Putting them in series will double the voltage but the combined supply will have a max Amp rating of the lowest rated supply and you'll need to limit the current drawn by the driver to that level .

Two supplies in parallel to double amps but same volts isn't a good idea even with 'identical' supplies since one will drive the other, especially bad with switched supplies, notrmally you need voltage tracking supplies to do this with switchers.

For series linked supplies you have to watch the ground, usually supplies are wired to use house ground and if the seperate supplies have the same ground it shouldn't work. You'd need to isolate the supply grounds and take the use precautions you'd expect with ungrounded equipment.

Its usually better, more reliable and usually cheaper to find a single supply rated for your needs. A linear supply is better than a stepper for our use and are usually cheaper.

Best solution is often a transformer, rectifier and smoothing cap and thats it.

hth

Andrew

[fyffe555]

dont buy other resistors. if you put 24v with 4 ohm resistor its the same thing as 12v 2 ohm

The resistors will need to change if you change supply voltage - doubling them, in series, as suggested would work if the voltage is approximately doubled too. Either way you will need more resistors thou'

you can wire your power supply like batteries but dont do it unless you are near 300 rpm or you could burn your motor. try do add a switch or something to activate your 2nd power supply when you wat to go faster. if you have a amp meter you can use it to make sure you dont go past the amp rating of your motor

Ah, I don't think that will work. If you 'switch in' a supply then you're either increasing amp capacity or voltage. Amps are not required here but that could blow the supplies depending on type and config. Switching in more volts requires you to switch in more resistance too or the driver/motors will draw more amps and blow the motors. If you have the supplys and the resistors then you might as well leave it all in place from the start.

Andrew

[H500]

The resistors are expensive, the power supply needs to be bigger, a LOT of heat is produced, the performance is inferior, resonance. Those are the disadvantage of the resistor type drive.

Are you sure you don't want to go with a chopper drive instead? If you do it yourself, it will cost $10-$20 per axis.

[Evodyne]

Mike,

Hi. You'll find that there are two different kinds of stepper drives.

The first are the simple unis that use resistors for current limiting. The resistors are sized to deliver the motors rated current for a given supply voltage. You can't really do much with this to speed up your steppers-re sizing the resistors to give more current will "cook" your motor as you've exceeded its design ratings.

The second family of drives are the chopper types. These you intentionally run with a higher supply voltage-much higher than the motors ratings. The electronics monitors the current through each winding and modulates or "chops" the supply voltage (like in a switching power supply) to maintain the current at the desired value.

If you need better high end performance, you really ought to be using a chopper type drive. Geckodrive and Rutex drives fit into this category.

[Ringo42]

I guess I've got eh same problem. I just firsed up my CNC for the first time and I'm only using a 5V supply. I had to run it very slowly or it would miss steps. I looked at teh gecko drives and they are $114 per axis, too expensive for me. Anybody know where to get less expensive chopper drives, or better yet schematics so I can make my own?
Thanks
Ringo

[ger21]

http://www.hobbycnc.com
http://www.xylotex.com
http://www.embeddedtronics.com

And I think in the open source forum here there may be some info