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View Full Version : Need a higher rated PS, but will it make a lot of difference?



alexccmeister
09-08-2008, 02:10 AM
Hi all,

I am looking to upgrade my PS with this one (KL-600-48 48V/12.5A $139.95 115V /230V) from a 24V PS which I found out is not enough to power my 495oz.in stepper motor. I have the one with 3/8" shaft but now Keling is selling 1/4" shaft. Are the spec the same for both as far as voltage and inductance are concern?

Anyway, the inductance for the 495 motor is 7mH. Hence using the formula for square root of inductance x 32 will give the power supply needed for optimum use of the motor. Mine worked out to be 84V. My KL5056 driver only accept maximum 50VDC. Hence using an 84V supply will surely kill the driver.

Since I am using a 24VDC PS currently, will the motor have more torque if I switch to the 48V/12.5A PS? Thanks.

Alex

PS, John, if you happen to be browsing, please comment if you can? Would like to put in an order soon for the PS. Thanks.

Riceburner98
09-08-2008, 08:06 AM
I'm not dealing with Keling, but using IMS483 drivers on my machine with 269oz motors. I upgraded from 24v -> 48v supply, and while I don't think my torque changed much (if at all), I was able to increase my motor speed 5x without stalling. The higher voltage allows the motor coils to power up faster, which gives you the speed. Same deal at work with a stepper-powered turntable, we doubled the speed with the same motor by swapping in a 48v supply. Now if the motors aren't reaching maximum power because the phase changes are happening quicker than the coils can be fully charged, then you would gain torque. Unfortunately my motors now run 2x hotter as a result of the increased voltage...

alexccmeister
09-08-2008, 10:22 AM
Hi Riceburner,

Thanks for your post. Wow 5 times faster than previous? That's a lot of speed. One way to test I guess is to hold down the ballscrew with your hand (provided the ballscrew is not installed onto the table) and turn your stepper at various speeds.

With my current setup, at 24V I can stop the stepper with my hand at speed of about 40ipm. At 20ipm I couldn't stop it. Any higher than 40ipm, not much torque at all. I can easily stop the motor. Now if by changing from 48V and if I can't stop the motor at say 80ipm, I would be very happy.

I am guessing since your speed goes up without stalling, the torque must have increase as well although you can't feel it. For my setup, if I increase the speed, all of the time, the table binds up.

Which PSU you are using? I am going for this (http://cgi.ebay.com/500W-48V-10-4A-Switching-Power-Supply-for-Radio_W0QQitemZ360085037624QQihZ023QQcategoryZ20589QQcmdZViewItemQQ_trksidZp1742.m153.l1262)one. What do you think?

Alex

Riceburner98
09-08-2008, 10:58 AM
Actually, I almost bought that power supply.. In the end I guess I was a little put off that it was from Hong Kong.. I bought this (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=140254202209) one instead for $30 US and while it was a good deal, unfortunately it has a temperature problem where it won't turn back on after it's warmed up. It'll run all day when warm, just can't turn it off then back on. Or, it's something in my driver box that's not letting it turn on, need to check into that.. I did buy this (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=330262372549) 312watt one for my personal machine (I'm selling my current machine to where I work..) which looks of much better quality. (sells for $450 on Digikey, and still had the Digikey sticker on it. :) ) The one you posted kind of scares me in that they get "500W" out of the same form factor, kind of like when a car stereo amplifier claims "1000W" for only $50.. Not saying it is bad, but I scare easily.

As for my current machine, it's a Taig (with 20TPI thread lead screws, ugh..) which due to various things I was only getting rapids at 30IPM max, cutting at 18IPM max, if I touched the table it would stall. With the 48v supply it cruises along at 90IPM rapids, and I've been cutting at 45IPM. 5x was a bit of an exaggeration I guess doing the math, but I have been able to crank it up to 120IPM rapids, though not 100% stable. (those motors are whipping really fast!) And with a 6"x12" envelope, you run out of room pretty quick travelling that fast.

I have a feeling you'll be happy with the 48v supply. Do you have a bench power supply anywhere that you could test it out with before dropping the $ on the eBay one?

alexccmeister
09-08-2008, 11:45 AM
Oops! too late. Just clicked BIN a few minutes ago. Their feedback looked impressive except for a few -ve. I am quite happy with that. I am confident it will get here safely in the next couple of weeks. Can't wait to install and test it out.

I will use the 24V for my lathe in the near future. Don't need a high rating for that.

30ipm seems low for a taig. I hear Taig is a well made product and should be more precise than an X2. BTW I got my maths wrong. I am currently cutting at 60ipm. That's roughly 1.5m/min. I would like to rapid at 3m/min (120ipm) or if possible 5m/min (200ipm). Will cut at around 2m/min (80ipm).

Alex

Riceburner98
09-08-2008, 02:29 PM
I think 30 is definitely low for the Taig, I really need to take apart, clean, and tweak everything one of these days.. Then it'll likely go even faster. The problem is, at 100IPM with the Taig's 20 IPM leadscrew the motor is doing 2,000RPM, and if you're microstepping it (I'm doing 4 steps / step) that's 480k pulses / second into the driver (x 3 axis) if my math is right.. If I bump the kernel speed of Mach up to the next level my computer goes all sorts of crazy. :) Not sure what the steppers are rated for as well, 2k RPM seems (and sounds) a bit fast..

That's why the next one I'm building from scratch is using ballscrews, and 5TPI ones at that. I can microstep it down to get good resolution, and you can't go wrong with next to no friction. :) I had those whipping at 600IPM with quite a bit of weight on them using the same drivers and motors.

Cutting at 60ipm sounds good for the 24v, hopefully you'll get the speed you need with the 48. :)

lclore
09-08-2008, 04:57 PM
Hi,
Been reading thru this thread- There are some general rules about step motors related to speed and torque-
voltage is speed- the higher the voltage the higher the speed potential, however, step motors are typically 48 poles (way more than servo & induction motors). With more poles, they generate something called back emf (a reverse voltage created by the motor poles that knocks down the desirable field currents, especially at higher speeds). A cheap step motor is good up to ~600 rpm before the affects of back emf are realized. A better step motor is good up to ~1000 rpm before torque is reduced by the affects of back emf. Use of motors with rare earth magnets helps (neodynium) from mfg's like Superior Electric (KLM Series).
120V Powered Drives will get you a bus of 160Vdc and typically maximize speed/torque capacity of the motor. Speed Torque curves are handy for reference and usually available on the website of the motor mfg. I've had a lot of success with Copley Controls StepNets (48Vdc External supply or 120Vac Line). Great Drive with some nice optimizing features. Not cheapest, but good performance.

Thought I'd offer that up as just a little background- hope it helps.

LC
Onyx Industries

Riceburner98
09-08-2008, 05:04 PM
Woah, that's not good.. What kind of back EMF do you get at 2000 RPM?? Does that mean it's frying the driver? Or just de-rating the torque of the motors?

lclore
09-08-2008, 10:24 PM
That's an interesting question.... I've never seen or heard of back emf damaging an amplifier or causing an amplifier failure. Basically it just drastically reduces the usable torque at the motor shaft. 2k rpm on the shaft will not likely allow aggressive accelerations or very dynamic loading. There is probably a speed/torque curve for the motor on the web in order to get an idea on capability.

It is possible to 'slip' a step motor shaft if torque available doesn't meet torque demand. I've proven this the hard way- and the negative of that in an open loop system (no encoder feedback) is loss of position accuracy. That risk just goes up as available torque goes down.

Many indexing servo amps allow for step & direction command signal for drop into existing stepper systems. Would 2k rpm requirement justify a servo knowing the servo could run up to 4-6k rpm without torque reduction?