speed vs torque graph for stepper?

I've seen speed vs torque graphs on some stepper/driver combos but I can't find that on the ones I just ordered from automationtechnology. I ordered the motors and drive from this kit. https://www.automationtechnologiesin...-3-axis-kit-1/

I emailed their tech support and they said if it's not on their website they don't have it! I put a question back on them to disclose the manufacturer so I can request it from them.

This is going on my Seig X2 conversion and I'd like to have this info to select pulley ratios to drive the screws. I'd like to know when the torque really starts to drop off.

Hi,
isn't it a bit late to ask about that AFTER you've ordered and paid for them?

What they do publish is the specs, particularly the inductance...which they claim to be 2.5mH. That's not great but not the
worst I've seen either. With 23 size steppers you should aim for 1mH or less and 2mH or less in 34 size steppers.

Inductance determines how quickly the torque degrades as the speed increases, the lower the inductance the better.

For instance with a 23 size stepper of 1mH you might expect 25-40% of rated holding torque at 1000 rpm. A 23 size stepper
of 2.5mH might have about 10% of rated holding torque at 1000 rpm.

As you can see the torque at speed is VERY sensitive to the inductance. First time buyers tend to buy on holding torque alone
overlooking the inductance spec. Its often the case that a stepper that has lower holding torque but also having lower inductance may
retain more torque at speed than another unit that has high holding torque but commensurately high inductance and thus the lower torque
unit perform better overall.

Many manufacturers don't publish speed/torque curves because they know that their products perform modestly typically because they
have selected a high inductance design which produces great holding torque but very low torque at speed.

Craig

A simple way is just to look at the amps draw.

If you have a 425oz and it runs 3A it is high inductance and will be weak at high speeds unless you hit it with 80V.

If your Nema 23 is 5A then it will be faster.

All the stepper and driver behave the same way. They loose power at high speed. I have found that you almost always you want the higher amp steppers for speed.

Luckily with most setups and slower spindles you don't need really fast steppers.

Hi,
its generally true that low resistance steppers have low inductance, but its not an exact rule.

High current usually means fewer turns of larger diameter wire which also happens to be low inductance. The current limit is set by the amount of heating,
thus a manufacturer may allow his motor to get very hot just in order to allow the most current, therefore the most torque and therefore be most saleable
in a competitive market. Another manufacturer may not be so sales driven and therefore rate their motor at a lower current so that it runs cooler despite
both motors being near identical..

The true determinant is called the time constant. Any coil has inductance by virtue of having turns, almost certainly on an iron core, but also a resistance of
the wire, When voltage is applied to such a coil the current does not increase instantly but builds up slowly. This is the effect of inductance. the rate at
which the current builds up is the time constant:
time constant= R(esistance) X L (inductance)

With your steppers:
time constant= 0.6 x 2.5mH
=15ms

This means that the current will build up to 67% of its ultimate value in 15ms.
For example if you applied 1V the coil it will eventually have a current of 1 / 0.6= 1.666A after several seconds.
The current would be 1.666 X 0.67 = 1.17A after 15ms

The shorter the time constant, ie the lower the resistance and/or the lower the inductance, means the current builds up quickly. This is absolutely required
to make a motor run fast.

One way to counteract the effect of inductance is to apply a higher voltage, the higher voltage 'forces' the current through the coil faster. You need
some means (electronic switching) to limit the current to the rated maximum otherwise the high voltage and unrestricted current it produces would
destroy the motor. At the current time the highest voltage drivers on the market are capable of 80V. In your case you have already bought drivers
and power supply so can't do anything about that choice.

Craig

Just as a reference I have a Sieg X3 mill and I used the 381oz-in motors K23H2100-35-48 which are 2.8mH and I have had no problems with them at all. I am running them at 48 volts using Gecko G203V drives.

I would not have bought it as a kit.
I personally would have got different drivers.
With 2.5mh inductance 48v will be fine.
The issue for me is the 5.6A limit on the drivers.

When microstepping you need to increase the current setting for optimum performance.
I can't remember the site right now but is is said that you need to increase the driver current setting by 1.414 times the motor current.
This would mean 5A x 1.414 = 7.07A. So for full motor capability when microstepping the maximum 5.6A of not enough as far as I'm concerned.

Quote:

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Originally Posted by dazp1976

... So for full motor capability when microstepping the maximum 5.6A of not enough as far as I'm concerned.

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Well, the OP is converting a Sieg X2. I highly doubt that either the max speed or the max torque will ever be needed unless its being programmed for self destruct mode.

Hansw3 - you are best off direct driving the ball screws - that will keep the stepper motor in its happy place of lower RPM, which means it can develop max torque.
Also, a typical ballscrew with 0.2" lead (~5mm) coupled to that stepper motor will make enough force to break stuff if it jams and at 750rpm will still move at 150"/min which will be crazy fast on a small machine if you are just starting out.
Lastly, torque drop off at high speeds isn't that big a deal - you're not usually cutting at those speeds so no need for big torque. With direct drive you'll have lots of torque in the velocity range where it matters most, for having high accelerations.
Mike

Hi,

Quote:

---

When microstepping you need to increase the current setting for optimum performance.
I can't remember the site right now but is is said that you need to increase the driver current setting by 1.414 times the motor current.
This would mean 5A x 1.414 = 7.07A. So for full motor capability when microstepping the maximum 5.6A of not enough as far as I'm concerned.

---

This is incorrect. The current limit is effectively set by thermal considerations, once you reach the thermal limit you can't increase the current without
overheat damage.

That figure you mention is the square root of 2 and occurs in calculations involving 1/2 stepping. The sine of 45⁰ is 1/1.414
What the previous poster is referring to is an artificial means to maintain average current in a 1/2 stepping situation. That is so last century.
Modern stepper drives have way more sophisticated current management than that. Do not increase the current beyond the rated limit, in fact
setting the current at 2/3 to 3/4 of rated current will go a long way to having the motors run cooler.

Craig

The Sieg X2 was my first dabble into cnc stuff. The BIGGEST mistake I made is using thrust bearings on the ballscrews.
AC bearings are the best thing to use by a mile.
The bearings don't sit right on the chinese ebay C7 specials and caused my screws to run bendy. I had to set the machine really tight to compensate.

I have 425oz steppers on mine running 36v and they work well. I only get 1400mm/min rapids because of it being so tight though.
I'll be re-building it with AC bearing carriers using my new machine to make the parts which will make a massive difference.

With the steppers you have I would just direct drive it, no pulleys etc.