PS and steppers

I'm obviously new to CNC and am trying to figure out the relationships between the PS, drivers, stepper motors Amp, Voltage, and inductance. I am using the gecko 251's which max driver limits are 3.5 amps and 50 volts. So if I get a 48 volt power supply, my drivers are at their limit. I know the PS amps should be the motors combined amps x .67 and any motor over the drivers 3.5 amp limit will have there oz-in rating adjusted by oz-in x 3.5/(motors amps). So the steppers amp rating should be 3.5 or less to maintain their oz-in rating (or adjust the size to for the rating loss). The question I have is about the relationship between mH and voltage. I understand that using mH ( sqrt(mH) x 32) you can calculate the peak voltage for each motor (for a 48v PS that would be 2.25mH). Now my quandary, should I look at the motors I'm thinking of using and use the lowest mH rating for the PS voltage rating, use the highest (keeping in mind the 50v limit) and resisters on the lower mH rated motors, or something in the middle.

Does voltage just control acceleration? Because I intend to use it for wax carving and therefore don't need a lot of acceleration or IPMs.

scottluther,
If I were in your position, I would look at the motors and run the numbers.
1. What amps are they rated for.
2. What is the inductance of these motors.

analysis 1.
IF the motors are rated for much more than 3.5 amps, then the 251 is probably not satisfactory for this project unless you want to seriously derate the motor.
analysis 2.
If you run the 32*sqrt of inductance and the number comes up seriously greater than ~50V then again, this driver is not a good choice.

There are all kinds of tradeoffs that you have to look at. You are correct that the holding power is ~to the current and the speed is ~to voltage as I understand it.
Never having milled wax, I can't say for sure, but I would think that you want a lot a acceleration and speed, but agree that it will not take a lot of power.

Hopefully someone with more knowledge than I will make more sense.

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Does voltage just control acceleration?

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Not really acceleration. Rpm is proportional to voltage. The higher the voltage, the higher the maximum usable rpm will be. The downside, is that higher voltage create heat. Ideally you want to use the lowest possible voltage to give you the performance that you need. Any voltage above that will just create heat. The problem is that it's hard to determine how much is enough.

Are you using motors with different inductance ratings? If so, then you probably want to go with the lowest, if they are similar. If they are very different, you might be better off with separate supplies for each motor.

The question came because I was trying to optimize my different stepper motors to the gecko's limits 3.5 and 50v. When I was looking at my guesstimating for the oz-in sizes I would use I obviously saw some wide ranges of amp and mH in the same oz-in range and was wondering what the impact would be. My goal was to get the oz-in range with motors close to or below the 3.5 and 2.25 mH range. I came up with these motors:

Nema 17 62 oz-in. 2.8v. 1.7a. 2.5mH
Nema 23 177 oz-in 2.5v. 2.83a. 2.5mH
Nema 23 270 oz-in 3.2v. 2.8a. 3.6mH

These were as close to the oz-in that I thought I would need and the specs. I am trying to set up a Taig with a small trunnion table for 5 axis. A, X, and Y would have the 177 oz-in steppers, the Z would have the 270 oz-in stepper, and the B axis (3in rotary) would have the 62 oz-in stepper.

Do I need to worry about heat if I'm under their max voltage ratings? These are all above the 2.25mH number for a 48v power supply.

Those motors are going to be screaming at 48 volts. I would recommend starting with a (MUCH less expensive) 12 volt supply. e.g. from a PC power supply:

Converting a PC Power Supply Converting a standard ATX PC power supply into a bench supply.

Getting a Compaq HP DL580 Proliant Server Power Supply to Work - RC Groups Converting hot swap PC power supplies into bench supplies.

For some information on how fast a motor will probably spin given a certain power supply, etc.. see:
techref.massmind.org/techref/io/steppers.htm#Estimating under "Estimating Maximum Speed"

a 12v powersupply will NOT work with a gecko 540, It requires a supply between 18 and 50V.

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Originally Posted by Bubba

a 12v powersupply will NOT work with a gecko 540, It requires a supply between 18 and 50V.

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Doh. My bad. Still... start with something cheap and smaller and go up if you need to. 24 volts maybe.

Does anyone else agree with the 24v comment? I only ask because someone else commented that I would be disappointed with that low of voltage. I was hoping for consensus ;). That hope has obviously bit the proverbial dust. Anyone else want to throw their weight on one side or the other of the scale?

Just to be clear, I recommended starting smaller because those motors are tiny, and not terribly powerful, and your little Taig shouldn't need that much especially cutting wax. My GUESS is that 24 or less will move them around ok, but I could be wrong.

HOWEVER: I've just now realized that you may be confusing the HOLDING torque of the motors with the actual DYNAMIC torque required to operating the axis. Just to make sure: The torque rating most often supplied with motors is the torque required to turn the shaft against the holding ability of the motor. That means nothing when your goal is for the motor to turn something. You want dynamic or pull out torque when measuring how much is going to be required to turn the axis on the Mill. Actually, torque readings are only useful AT SPEED, and so unless your motor mfgr supplies torque curves, it really doesn't mean much and isn't necessary to design a drive system.

Here are the steps:

1. If you have the actual hardware, measure the actual torque required to turn the shaft where the motor will be connected. Measure with a torque wrench, or a weight at a distance like you use to measure motors. Here is an example: techref.massmind.org/techref/io/stepper/measure-torque-rb.htm

If you don't have it, estimate the weight of the items that the axis screw will be moving.

2. Figure out what speed you want your machine to be able to move at, as a top speed. More speed = more money.

3. Use the formula or calculator at:
techref.massmind.org/techref/io/steppers.htm#Estimating to figure out how many WATTS of drive you need to get that speed. Watts = IPM * Lbs / 531 or Watts = torque (in in-oz) * RPM / 1351. These are "rules of thumb" from Geckodrive. Add a little extra for error and to be certain.

4. Watts = amps * volts (or more usefully: amps = watts / volts) so you need a current and voltage rating that, when multiplied, reach more than your minimal required watts. For example, if you need 200 watts of drive power, and you are using a 24 volt supply, then you would need more than an 8 amp motor... 200/24=8.333 NOT reasonable! So then you bump up to 36 volts and that would require a 5.5 amp motor. At 48 volts, you would need a 4 amp motor. That seems doable. Again, this is an /example/! You may not need 200 watts of drive. Use step #3 to find out what you really need! A small Tiag with those little motors shouldn't need more than 50 watts for very reasonable speed.

5. Given an amperage rating, and a voltage, find a driver that will manage that combination. Your Geckodrive will do what was it? 3.5 and 50? So hopefully you don't need more than 175 watts. If you only need 50 watts, your 2.8 amp motors will do just fine at 24 volts. If you need more drive, you will need a higher power supply... it all comes down to knowing what you need. Again, I'm guessing your setup won't need that much.

6. Once you have a driver, and know the amps and volts, find motors that are rated for that. Those little guys are probably fine. MAX voltage is 32 * the mHs or 80 in that case, but you certainly don't need to run them at 80 volts! LOL...

7. Finally, pick the lowest supply voltage you need for the drive system. The lower the voltage, the cooler the motors will run and the less ringing and mid band resonance issues you will have.

That is the COMPLETE process for designing a drive system. Not a guess... I KNOW that works: I learned it from Mariss (of geckodrive) and Roman (of Linistepper and SLAm), and they certainly have it right.

Ok James, I bow to your expertise ;)

Not just the knowledge, but you put it out there in a way that was easy to understand and that I could use to do the calculations. It was extremely helpful and gave me confidence in the choice. Using the 24v number and my motors amp rating to calculate watts and then working backward with the lbs*IPM/531= watts formula because I don't have the exact lbs number or the IPM. I could then substitute different numbers in there.

So it went something like this 24v * 2.8 amp motor = 67.2 watts for the proposed PS and my motors combination

Then 67.2 * 531 = 35,683 = lbs * IPM

Even if I substituted the entire weight of the Taig for lbs, I would still need to exceed 419 IPM for 24v to be under powered.

So in conclusion... "YOU DA MAN!"

Since I have you on the hook, one last question (for now), does it matter whether I use a regulated or unregulated power supply? In my situation, I'm not going to get any back power generated from the motors.

You NEED to use an UNregulated supply. Motors and motor drivers draw massive current when a coil turns on and release large spikes when the coil is released. A poor little semiconductor trying to keep that regulated is just going to have a nervous breakdown!

I'm a little weird on this, but I really like car batteries with chargers as power supplies. You put them in a battery box, and NEVER overcharge them (cheap intelligent chargers are everywhere) so they are pretty safe, especially in a shop or garage. And then you can't possible not have enough current handy... Cold cranking amps right? The only trick is you have to stack them for 24, 36, 48 volts and that means the chargers have to play nice when stacked (not a given) or you have to find a charger for that voltage of battery which can be done, but isn't as cheap. It's a bit of a pain, but there can be no finer power supply!

Other than that, some people have been able to stack PC power supplies... that's scary... but cheap.

Let us know what you find for a supply? I'd like to collect some good recommendations for the page at:
techref.massmind.org/techref/io/stepper/power.htm

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Even if I substituted the entire weight of the Taig for lbs, I would still need to exceed 419 IPM for 24v to be under powered.

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On 24V, you're maximum rpm from your steppers will probably be about 1000 best case. With a 20tpi leadscrew, that means you probably won't see more than 50 ipm.

While the formulas may tell you how many watts you need, it can be far more complicated than that. There are numerous threads here from people who have much larger motors and drives than they need, yet they get poorer performance than others using smaller, less powerful motors and drives.

To get the best performance from steppers, everything must work together, and compliment each other. That includes the motor, drive, power supply and leadscrew.

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You NEED to use an UNregulated supply.

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And yet 95% of people are using cheap, regulated switching supplies with no problems. :)

Gerry is right about the top RPM, but that can be compensated with a different TPI lead screw. If your lead screw is hard to change (and it probably is about impossible considering that it's a mill) then you do need to consider that as another limiting factor. Not sure why 50 IPM is so horrible? Seems plenty to me... especially for a small mill.

And yes, lots of people use switching supplies. And they do regulate, but switcher are special and can handle it where other types of regulated supplies may not. I was concerned that he might purchase a non-switching regulated power supply.

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Not sure why 50 IPM is so horrible?

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I guess I was trying to say that while he thinks that 24V will provide enough power to go 400+ipm, in reality, it's not enough power to break 100ipm.

Regardless of the motors and drives used, I would think that it's unrealistic to try to spin any leadscrew much over 1500rpm, let alone 8000 rpm.

Well, I would be fine with 50 IPM, but my curiosity is killing me. How do you figure out how many RPMs you can get from your motor at a given voltage? Do I look at the motors torque curve graph and just look at the speed at a specific torque, then use that number divided by TPI to get the IPM for that torque level? Will the motors move at the fastest possible speed (if not limited in Mach) all the times? And if I limit it in Mach (for discussion) to say 5 IPMs and have a 48v supply, is it still creating full 48v heating?

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How do you figure out how many RPMs you can get from your motor at a given voltage?

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That's the tricky part. :)

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Do I look at the motors torque curve graph and just look at the speed at a specific torque, then use that number divided by TPI to get the IPM for that torque level?

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Torque curves aren't always an accurate representation, because they are only accurate for the specific driver and power supply used for the chart.

But, they can give you a rough idea.

But you're going about it backwards. You don't want to start with "how many rpm can I get". What you want to ask, is, "How many rpm do I need.
Then you answer these questions:
How fast do I want to go? This tells you what rpm you need.
How fast do I want to accelerate to that speed. You use this number to calculate how much torque you need.

Then you look at your torque curve, and see if you'll have enough torque at your target rpm. If not, more voltage may help.

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Will the motors move at the fastest possible speed (if not limited in Mach) all the times?

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The drives don't know what the fastest possible speed is, so Mach3 has to limit it to a maximum that you set. This is your rapid speed.

The 48V causes just as much heat whether it's spinning at full speed, or not spinning st all.

On a small machine like that, 24V may be perfectly adequate, with the right motors.
I use a Xylotex drive, with 24V, and 250 oz motors wired bipolar series. My router is 30x45.
I can get up to 190 ipm, using 1/2-8 2 start screws, 4 turns per inch.

My Z axis uses 1/2-10 acme and can only go 55 ipm. If I had 20 tpi screws, I may only get 20ipm

Thanks everyone for the help.

I was originally thinking that around 20-25 IPM was a reasonable number for carving wax on a Taig, the working area being only 12" x 5.5" x 6". So I took a look at the torque graph for my motor and for the torque I would operate at and got the speed (the obnoxious engineers did the graph in N-m and PPS :) ). Then found a formula for converting PPS into RPMs ( rpm = PPS * step ° /6 ). (not sure why the divid by 6). I took the RPMs / TPI * (24v/30v) and got 48 IPMs.

I added the (24v/30v) myself because the graph was made at 30v.

What shocks me the most, because I'm assuming that no one commenting actually looked up any numbers or did any math, is that you guys guessed it would be around 50 IPMs. I think you guys need to get out a little more ;) That is just scary.

I realize the graphs are for a specific motor-driver combo, so it is really just a useful estimation tool. Yet it gives me an idea if my IPMs were in range with target at 24v.

Between this and the previous calculations, it seems that 24v will be enough.

Thanks again everyone. I've been reading the forum quite a bit and so many people are trying to maximize their setup for speed, I just assumed that was the way to good. Then again, most of the threads I've read are CNC routers. I'm now thinking, and I'm going out on a limb here, that maybe my requirements for carving wax may be somewhat different than theirs :). But that's just a guess.

(Thanks to the guy who said I was going about this backwards.)

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Originally Posted by Scottluther

Well, I would be fine with 50 IPM, but my curiosity is killing me. How do you figure out how many RPMs you can get from your motor at a given voltage? Do I look at the motors torque curve graph and just look at the speed at a specific torque, then use that number divided by TPI to get the IPM for that torque level? Will the motors move at the fastest possible speed (if not limited in Mach) all the times? And if I limit it in Mach (for discussion) to say 5 IPMs and have a 48v supply, is it still creating full 48v heating?

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It's all math... math based on physics. If you really want to learn this stuff, MIT has a great physics course on YouTube. The prof is a hoot! Fun experiments and all... does require some pretty heavy math, however.

There is a calculator that will give you a pretty good idea of how fast a motor is going to spin at techref.massmind.org/techref/io/steppers.htm#Estimating... Scroll down a bit.

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Then found a formula for converting PPS into RPMs ( rpm = PPS * step ° /6 ). (not sure why the divid by 6)

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Multiply PPS *60 for Pulses per Minute, then divide by steps per rev. Most charts I've seen use half stepping, which = 400 steps/rev.

So with half stepping, RPM = PPS*60/400

I used both formulas and the RPMs came out the same:

PPS * .9 (1/2 step °) /6

And

PPS * 60 / 400 (1/2 step per rev)

I have another question:

I was given a nice power supply board which you connect to a separate transformer. The guy was using it with a 4 axis setup using Mach3 and gecko drives. He was using a transformer to get 50vdc and 14amps. I've been reading up and it seems that I can use the board with a different transformer to get a lower voltage and amp output.

Is that true?

I read that the bridge rectifier will work as long as you don't over power the diodes. Money is the only advantage to a smaller powered BR.

In the same vain, the filtering capacitors are similar, as long as you don't exceed their voltage rating and the microF rating, they will work as well. Just store power for longer and are more expensive.

The relays on the board won't be over powered.

The only other thing that would need to be changed is the fuses.

And as an unregulated power supply, I don't have to worry about a voltage regulating electronics.