[FlyingElectron]

Can anyone give any advice on which configuration will give higher torque at high RPMS?

For example, with this motor

http://www.kelinginc.net/KL23H276-30-8A.pdf

it can be wired a variety of different ways.

If the voltage is fixed at 42V and the max current the power supply can give out is 3A, would unipolar or bipolar driver provide more torque at say 1200RPM?

Right now I am running the motor unipolar 42V @ 3A and can reliabily get the motor up to 600 RPM before there is not enough torque to move the load. Anyone think that changing to a bipolar driver would dramatically increase the max RPM I can reach with the same load?

Thanks!

[pminmo]

It's not quite that simple and depends on the driver. But in general the simplest way to maximize that motor for higher rpm's is running it bipolar parallel.

Using Keling's posted torque curves for this motor this is a force plot of three different screws to show the performance when you take screw pitch, starts, efficiency and rpm.

If you want to get into the 1200 rpm region, get a Gecko G203V to run that motor. Just make sure you have enough PC to run the 10 microstep at 1200 rpm which would be 40khz step rate.

[BobF]

Step motors have higher torque at lower speeds. As speed goes up, torque goes down. Most will not operate at much over 1000rpm if they can even reach that. I doubt you can reach the 1200 with no load, and you will certainly have a lot less torque no matter the driver.

Here are some applicable quotes from a pdf I found on the net.

Inductance reduces a step motor’s high speed torque
performance. Inductance is the reason all motors
eventually lose torque at higher speeds. Each motor
winding has a certain value of inductance and resistance.

In summary, the current and the number of coil turns in
the windings determine a motor’s maximum torque
output, while the voltage applied to the motor and the
inductance of its windings will affect the speed at
which a given amount of torque can be generated.

[FlyingElectron]

Thanks to both of you for your responses, they are helpful. With that info, I think I understand the problem more.

From my understanding so far, the amount of torque available is related (not sure if it's proportional or exponential?) to the current flowing through the coils and the inductance of the coils. So to increase torque on a given stepper motor either the current needs to be increased or the stepper needs to be wired in such a way as to give more inductance per coil.

But when the stepper driver switches on the current does not flow instantly through the coil, it takes time to build the magnetic field in the coil which allows the current to flow. As the RPMS increase, there is less and less time that the driver remains on for the coil and there reaches a point where the time interval is not enough to fully charge the coil and allow the maximum current to flow, and this is where the torque drop off begins?

The amount of time needed to charge the coil is dependent on the inductance of the coil. For a given current, say 3A, a lower inductance coil causes a shorter charge time but produces less holding torque then a higher inductance coil which would have a larger charge time.

I think if I look hard enough there should be an equation to calculate the amount of torque produced by a stepper motor as a percentage of maximum holding torque based on current, and from there maybe it is possible to calculate what kind of benefit switching from a unipolar driver to a bipolar parallel driver would provide.

I'm pretty sure that the bipolar parallel will be better, but it's a big investment to switch over 4 drivers without any idea of what kind of benefit it would bring.

[pminmo]

Inductance is the high speed enemy. Torque is the product of to amp-turns and is the basis as to why a bipolar driver produces more torque than unipolar. Bipolar is utilizing the full winding where unipolar is only using half the winding. However, where the windings are series, the inductance increase is counterproductive to high speed torque. See this video: http://pminmo.com/which-stepper-motor.

You’re over simplifying a drive design. There are techniques that maximize the power as you increase in rpms. For example the G203V changes step methods as rpm's increase to maximize produced motor power. It essentially is running a motor at full step at higher rpms to maximize motor power.

BTW, I'm not affiliated with Gecko, just trying to point you to a solution pre your request. The cheaper way to get where you want maybe to change the mechanical ratio's you need to require less rpm's. There are other benefits to reducing rpms as well.

[Mariss Freimanis]

1) A NEMA-23 motor driven hard can deliver 100W mechanical or more.
2) 100W is 112 in-oz at 1200 RPM, 56 in-oz at 2400 RPM, etc. (in-oz = Watts * 1351 / RPM).

Mariss

[amplexus]

Bobf
You have it partly right but steppers will run at way more than 1000 rpm both open loop as Phil and Mariss eloquently explain, and at much higher rpm if run closed loop. I think Mariss has stepper/servo runing at 15,000 rpm or something like that, think of them as a high pole count bldc motors.
Amplexus Ender

[johnmac]

But, would it have enough power to do anything?

John

[stevespo]

I noticed on the Oriental motor site the other day that they claim their AlphaStep (ASX series, closed loop stepper motors) can be run at 5000 RPM. Torque does drop off to about 20% at 5000 RPM, but the performance is reasonably linear. Interesting product. Clearly the 1000 RPM stepper limit that people talk about is not a hard and fast rule.

ASX series AlphaStep motors

Steve

[amplexus]

Closed loop steppers have way more torque than open loop and no resonance. I suspect you will see a cool new closed loop gecko drive one day soon, and like Phil I have no affiliation with gecko, but I do appreciate great design.
Amplexus, Ender

[FlyingElectron]

Pminmo: Great video! I think your video answered my question by showing that for a given motor a unipolar driver and a bipolar driver seem to max out the IPM at the same place, so in my situation there may not be any benefit for me to switch to bipolar from unipolar.

The machine I am working on only does positioning so barely any torque is needed, just need to move quickly, but on any future machines, I think there is benefit to just starting out with bipolar drivers and motors since they give better torque in the low and mid range.

The AlphaStep motors look really cool. I'm guessing they are servo motors with electronics to make them interface with step dir signals?

I've got a silly idea, we could organize a stepper motor driver shootout in the future one day. Buy a stepper motor and mail it around to different people with different driver setups to compare the performance of each driver. I was really confused when I started out trying to figure out what to buy and I still am confused. If there was a chart that just listed the relative performance of the drivers, then it may have been easier.

[amplexus]

I think the advantage would be the higher torque from bipolor will allow you to run faster screws 2tpi would give faster transits.
Amplexus Ender

[pminmo]

Pminmo: Great video! I think your video answered my question by showing that for a given motor a unipolar driver and a bipolar driver seem to max out the IPM at the same place, so in my situation there may not be any benefit for me to switch to bipolar from unipolar.

Actually that wasn't the intention and it's an incorrect assumption. Since the video was done with a six wire motor, there are three ways to wire it, bipolar series, bipoar half coil and unipolar. If that was an eight wire motor, the bipolar parallel would have won all things equal.

[chicof]

Hi,

As a newbie to stepper motor wiring I would like to ask a question in regards to Unipolar x half coil...

Is it the same thing? If not, do you get a similar torque curve if you have a motor wired unipolar or half coil?

thanks,
Francis

[pminmo]

No, there not the same. In bipolar half coil you are reversing polarity of the same half of the coil as you go through the stepper translation. In unipolar the polirity change comes in switching the other half. Torque curve isn't necessarily similar, it depends on the specific driver.