g251 questions

[Jack000]

I'm currently powering my 3 drives using 36v but I'm considering hooking up my pc power supply to get 48v, however this will also reduce my max current from 20a to 10a. I would like to know the max current required by the geckos (is it just the current-set resistor value?)

my current per phase is 2.8a
inductance/phase 6.8mH (P) 27.2mH (S)

also, using the 32*root(mH) formula gives me 80 volts (parallel) and 160 volts (series) for the power supply voltage. This doesn't seem right.. am I doing something wrong here?

[Crevice Reamer]

Originally Posted by Jack000 I'm currently powering my 3 drives using 36v but I'm considering hooking up my pc power supply to get 48v, however this will also reduce my max current from 20a to 10a. I would like to know the max current required by the geckos (is it just the current-set resistor value?) my current per phase is 2.8a inductance/phase 6.8mH (P) 27.2mH (S) also, using the 32*root(mH) formula gives me 80 volts (parallel) and 160 volts (series) for the power supply voltage. This doesn't seem right.. am I doing something wrong here?

Hello Jack!

The G251s can only handle Max 50V and 3.5A.

It sounds like you are running 425 oz steppers. You probably got caught in the most common beginner trap--Thinking BIGGER torque is always better.

YEP! They will need 83V for best performance. Moving up to 48V will help though. Oh and you DO want to run them Bipolar Parallel.

Are you wiring two Computer PSUs in Series?

Formula for necessary PSU Amperage is total motor amps X .6 or for safety factor X .67. Three times 2.8 is 8.4A and times .67 is 5.62A. Ten Amps should be more than enough to run FOUR of those motors.

what equipment are you powering with these motors?

CR.

[Jack000]

thanks your help!

I'm using 3 computer psus (4 including the pc psu), hooking together the 12v rails. Each psu gives 20 amps, but for the pc psu the cpu/mobo/harddrive takes about 10 amps, which will limit the total output.

I'm just moving a 60 pound or so gantry, including a 20 pound router.

I tested it first with 2 psus for 24v, and it stalled at around 120 in/s with no load (loud buzzing + no movement). I hope that with 48v I can cut reliably at 120 in/s.

[ger21]

What kind of screws are you using?

[Crevice Reamer]

What kind of drive system? Acme screws? Ball screws? R & P Etc? What are axis dimensions?

CR.

[Jack000]

1/2" 10 tpi acme screws.

the screws are 1' z, 2' y and 4' x

[ger21]

Multiple start screws would probably double or triple your speed. Those motors just can't spin fast enough to get good speed out of them. You need 1000 rpm to get 100 ipm, and they just won't do it, at least not with any power left.

[Crevice Reamer]

There are a lot of factors here, and they all contribute to low speed. The motors are too large and very undervolted. Some Keling 270s on X and Y and a 387 on Z would run faster at 48V.

The PSU is too small--Frankly, I would recommend that you scrap all those hinky PSUs and get a $60 48V 7.3A Keling K 350. At least that will bring SOME stability. It's adjustable--so crank it up to 50V.

http://kelinginc.net/

You bought bare G251s. Bare 251s don't have the protection circuitry of the G540, although they cost nearly as much by the time you buy a BOB.

Definitely get the KL-350 PSU. That will increase speed some. Either replacing the X and Y motors with $39 KL23H276-28-4B 270s (Or the new 318s) or changing your screws to multi-start (OR BOTH) should increase your rapid speeds considerably more.

CR.

[ger21]

Uh, the G251's should have the morphing. Why wouldn't they? Just checked the website, and yes, they do.

[Crevice Reamer]

Originally Posted by ger21 Uh, the G251's should have the morphing. Why wouldn't they? Just checked the website, and yes, they do.

Whoops! You're RIGHT Gerry! I guess that lets out ONE possible cause of low rapid speed.

CR.

[Jack000]

hey guys, thanks for the tips!

unfortunately the motors and screws were the most expensive parts of my build, and I'm already really over budget!

I think I'll see if 46v will work, and if it doesn't I'll have to look for a local supplier of steppers (shipping + duty killed me last time).

I'll definitely keep this in mind for my next build, which I think will be very soon