[jseiler]

I'd like to drive a stepper at >1000rpm, so I am looking for a very low inductance stepper motor. It needs to be less than 2A. I have a keling that has reasonably low inductance driven half coil, but I has hoping to do better. I searched the forum and can't find anything related to lower current, lower inductance motors? Any suggestions?

John

[ger21]

Low inductance motors tend to be low voltage high current. You'll have a hard time spinning a low current motor at 1000 rpm unless you can supply very high voltage to it.

[jseiler]

I realize I'm fighting some physics here. I've got 44V, so that should get me in the realm of 20X spec voltage to overcome whatever inductance I see. Right now I've got this motor running half coil. Not too bad at about 4.8mH, but I suspect I can do better. I can spin this at about 1200 rpm, but the torque out there is pretty low.

http://www.kelinginc.net/KL23H256-21-8B.pdf

I haven't tried it yet, but would there be an speed advantage to running parallel on this motor for reduced inductance, even if I can only run 2A out of 3A spec'ed?

John

[Oldmanandhistoy]

Hi John,

Looking at the spec’s of the motor I would run it parallel at around 2.1Amps for highest speed and max torque if micro stepping.

If you are using a micro stepping drive it will be RMS rated not peak. The stepper motor spec sheet will very likely be rated for peak but you would need to check.

3Amps peak = 2.1Amps RMS

If you are using a full step drive then use the peak 3Amps or in your case as near as possible.

I am working form memory so double checking my info would be wise.

John

[jeffs555]

I haven't tried it yet, but would there be an speed advantage to running parallel on this motor for reduced inductance, even if I can only run 2A out of 3A spec'ed?

There would be no difference in inductance between running bipolar half coil and bipolar parallel. The only advantage of parallel over half coil is that it allows you to use higher current. This gives more low speed torque but won't make a difference in torque at high speeds.

PS According to the spec sheet it is 2.4mH not 4.8

[Zappautomation]

Its the other way round.
If it is microstepping use Peak.
A full step driver is a square wave so peak is RMS.

Hi John,

Looking at the spec’s of the motor I would run it parallel at around 2.1Amps for highest speed and max torque if micro stepping.

If you are using a micro stepping drive it will be RMS rated not peak. The stepper motor spec sheet will very likely be rated for peak but you would need to check.

3Amps peak = 2.1Amps RMS

If you are using a full step drive then use the peak 3Amps or in your case as near as possible.

I am working form memory so double checking my info would be wise.

John

[jseiler]

There would be no difference in inductance between running bipolar half coil and bipolar parallel. The only advantage of parallel over half coil is that it allows you to use higher current. This gives more low speed torque but won't make a difference in torque at high speeds.

PS According to the spec sheet it is 2.4mH not 4.8

I don't think this is right, but I may be wrong. When you put two coils in series, as long as they aren't coupling, they should add in inductance. When you put two coils in parallel, they should sum 1/L=1/L1 + 1/L2, which would divide the parrallel inductance in half. The half coil inductance should be 2x the parallel inductance...or....4.8mH. Did I do something wrong here?

I found a couple of motors that are lower inductance within my current restrictions, but nothing too significant. Sometime real soon now, I'll try the current starved parallel and see if that helps over the half coil config.

John

[jeffs555]

I don't think this is right, but I may be wrong. When you put two coils in series, as long as they aren't coupling, they should add in inductance.

Your only mistake is assuming the coils are not coupled, they are. The coils are bifilar wound, in other words they take two parallel wires and wind them at the same time around the same core. If you connect the two coils in parallel, the only effect is to decrease the resistance, the inductance doesn't change. If you connect the two coils in series, it doubles the number of turns in the coil, so the inductance goes up 4 times(inductance is proportional to the square of the turns).

PS Sometimes you will see unipolar steppers refered to as bifilar steppers.

[jseiler]

Thanks Jeff. That looks right and I found a few motor manufacturer's that quote inductance in half coil, and they agree with you.

Another thing I found is that higher speed is possible in half coil over bipolar series, not because of inductance, but back EMF. I think I might be OK with the motors I have, but I'd still like to locate a lower inductance 2A motor to experiment with.

John

[kreutz]

Check this video:

[ame="http://www.youtube.com/watch?v=y6BS4B3yT54"]http://www.youtube.com/watch?v=y6BS4B3yT54[/ame]

The drive is a Mardus-Kreutz DIY drive. The motor is a high inductance motor from Keling Technology. The New Kreutz-4 Bipolar drives get even more impressive speeds.

Regards,

Kreutz.

[pminmo]

I'd like to drive a stepper at >1000rpm, so I am looking for a very low inductance stepper motor. It needs to be less than 2A. I have a keling that has reasonably low inductance driven half coil, but I has hoping to do better. I searched the forum and can't find anything related to lower current, lower inductance motors? Any suggestions?

John

The question I would ask is what is the requirement mechanically. It's one thing to spin a stepper at such a rate, another as to the mechanical forces and loads encountered. If youre using leadsrews, you need to consider wip. If your moving any weight, you need to consider starting and stopping forces.

[jseiler]

The question I would ask is what is the requirement mechanically. It's one thing to spin a stepper at such a rate, another as to the mechanical forces and loads encountered. If youre using leadsrews, you need to consider wip. If your moving any weight, you need to consider starting and stopping forces.

Indeed. I've ordered a small desktop cnc with .1" lead screws. Not expecting whip as the largest possible unsupported length would be less than 14". I'm very limited by the choice of control box, limited to 5-8K pps (don't ask ). This means that 1000rpm = 100 ipm on this particular machine. Lets pretend I can get 120 ipm or 2 ips. 2 in/sec * 200 steps/rev * 10 rev/inch * 2 pulses/step = 8000 pulses/sec. The x axis (one containing z carriage) is the only one that needs high speed and high acceleration. The Y axis can be slower and have low acceleration (due to the application). The Y axis is the one with the highest load as it needs to move both the x axis gantry and the z axis. Force is limiting to my acceleration. Acceleration is limiting my ultimate speed because of the short gantry. I won't be sure how much everything weights and how much screw friction I have until I get the machine. Once I get everything spinning and working right, I'l likely ditch the slow control system and get something with a lot more current and pps capability.

Still, any suggestions on low inductance, low current motors would be appreciated.

John

[Mariss Freimanis]

Low inductance and low current current equals tiny motors (NEMA 17).

Mariss

[pminmo]

Indeed. I've ordered a small desktop cnc with .1" lead screws. Not expecting whip as the largest possible unsupported length would be less than 14". I'm very limited by the choice of control box, limited to 5-8K pps (don't ask ). This means that 1000rpm = 100 ipm on this particular machine. Lets pretend I can get 120 ipm or 2 ips. 2 in/sec * 200 steps/rev * 10 rev/inch * 2 pulses/step = 8000 pulses/sec. The x axis (one containing z carriage) is the only one that needs high speed and high acceleration. The Y axis can be slower and have low acceleration (due to the application). The Y axis is the one with the highest load as it needs to move both the x axis gantry and the z axis. Force is limiting to my acceleration. Acceleration is limiting my ultimate speed because of the short gantry. I won't be sure how much everything weights and how much screw friction I have until I get the machine. Once I get everything spinning and working right, I'l likely ditch the slow control system and get something with a lot more current and pps capability.

Still, any suggestions on low inductance, low current motors would be appreciated.

John

Usually people want rpms to traverse long spans, then mechanics become a serious factor.

FYI, I'm getting around over 14xx rpm on a small machine similar in dimension with a keling KL23H51-24-8B running unipolar at 42V. Leadscrew is 12tpi acme. Quarter step, Allegro SLA7078. Over 120 ipm reliably. Their 282oz in motor has lower inductance and coil voltage rating.

[jseiler]

Ok, thanks guys. Now for the followup question:

I am limited to 2A drive current. Am I better off using half coil or parrallel on this motor (bipolar drive)?

http://www.kelinginc.net/KL23H256-21-8B.pdf

[pminmo]

I would think you would get more power at 2A with parallel even though its only 66% of the rated current. You are doubling the number of coil turns at the same current.

[jseiler]

At low speed, I definately agree, but I don't have any torque issues there. At high speed though? Hmmm. I've not had much luck finding information about torque speed curves running half coil. Is it exactly the same as running unipolar? If so, I can compare pretty reasonably (unipolar vs scaled down bipolar).

John

[jeffs555]

I am limited to 2A drive current. Am I better off using half coil or parrallel on this motor (bipolar drive)?

You are doubling the number of coil turns at the same current.

Going from half coil to parallel does not change the number of turns. The coils are wound with the two wires in parallel (ie bifilar), and connecting the the two coils in parallel is equivalent to just increasing the wire size. The only effect is to decrease the resistance of the coil.

For the same current, bipolar half coil, bipolar parallel, and unipolar will all have the same torque vs speed characteristics. If you use the unipolar current, you can use the unipolar curves for all three connections. The motors would run cooler with the parallel connection because of the lower I^2 R losses.

[pminmo]

Jeff's right and wrong, but the result he is correct.

I wasn't thinking...(too much grey hair) The current is split between the two windings, so the effective result is the same. So 2A thru half coil should produce the same magnetic effect as 1A through two windings (bifilar). However you would have a net effect of a lower coil voltage so the parallel winding should still be better as inductance would be the same.

[jseiler]

Got it. I think it finally sunk in. Thanks guys.