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Interpreting Stepper Specs: Inductance, Etc.

Posted by UncleSas 
Interpreting Stepper Specs: Inductance, Etc.
August 12, 2016 01:54PM
I've got a few newbie questions about how to read stepper specs. I found what may be a very good fit for a Kossel, and I don't quite understand what I'm reading. Here are the specs for the motor:

Frame Size: NEMA 17
Number of Lead Wires: 6
Lead Wire Configuration: flying leads, no connector
Lead Wire/Cable Length: 300 mm
Unipolar Holding Torque: 23 oz-in
Bipolar Holding Torque: 32 oz-in
Step Angle: 1.8 deg
Bipolar Series Current: 0.67 A/phase
Bipolar Series Resistance: 7.8 Ohms/phase
Bipolar Series Inductance: 11.2 mH/phase
Unipolar Current: 0.95 A/phase
Unipolar Resistance: 3.9 Ohms/phase
Unipolar Inductance: 2.8 mH/phase
Rotor Inertia: 5.38E-04 oz-in-sec2
Insulation Class: Class B (130 ÂșC)
Shaft Run Out: 0.002 inch T.I.R. max
Radial Play: 0.0008 inch max w/ 1.1 lb load
End Play: 0.003 inch max w/ 1.1 lb load
Perpendicularity: 0.003 inches
Concentricity: 0.003 inches
Rated Voltage: 3.71 V

I recall reading in this forum and on the wiki that inductance between 2-5 mH is ideal to ensure rapid acceleration. Unipolar Inductance is at the lower end of that range, which would make this motor pretty good. On the other hand, the Bipolar Series Inductance is way out of range for this application if that's the thing I should be concerned with.

So, my question: What's more important? The Unipolar Inductance or the Bipolar Series Inductance?

Thanks,
Uncle Sasquatch
Re: Interpreting Stepper Specs: Inductance, Etc.
August 12, 2016 04:59PM
All standard controller boards drive the motors in bipolar mode, so it is the bipolar specifications that matter.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Interpreting Stepper Specs: Inductance, Etc.
August 12, 2016 05:17PM
Thanks!

That would make that motor very bad for a Kossel; as the acceleration would be terrible.
Re: Interpreting Stepper Specs: Inductance, Etc.
August 12, 2016 10:19PM
After a bit more googling, I came across this:
[dovermotion.com]

and this:
[www.quora.com]

If I understand what I'm reading,I could and probably should wire the motors like the one I'm looking at for bipolar half coil operation for applications like the Kossel I want to put it in. That's because bipolar parallel wiring gives a broad, flat torque curve where bipolar series gives a high torque curve at low speeds that drops off sharply. I didn't think I could drastically change the performance curve of the motor so drastically by changing the wiring up, as I'm used to doing fairly simple things with much larger motors. (Rotary phase converters...)

Does my limited understanding of this info seem on target, or am I really out to sea in a leaky canoe?
Re: Interpreting Stepper Specs: Inductance, Etc.
August 27, 2016 09:49AM
The first link last part of document, it talks specifically about motors with 8 leads, which u do not have.

You have 6 leads, so you can only have the classic bipolar mode, which is also called "series". In other words you have one long coil with a center tap, which equates to two smaller coils connected in series.

To have bipolar "parralel" mode wiring, you need 8 wires, schematic just as before with 6 wires but with center tap made by two wires, difference being the end of first small coil and the start of the second half coil are NOT connected. In other words, if you connect the two middle wires, you get a longer coil, and exactly same situation above, the "series" connection of 6 wires. BUT alternativelly you can also connect both of these small coils in parallel, e.g. the start of the first coil with the start of the second coil, and also the endings together. This makes an overall very different "L/R" constant, and basically makes the motor be able to get "energized" faster. Theoretically better at higher speed, but also other considerents. But you need a motor with 8 wires to try this. Your link has a motor with 6 wires, and you can not wire them as bipolar "parallel" unless you have 8 wires.

Edited 1 time(s). Last edit at 08/27/2016 09:51AM by NoobMan.
Re: Interpreting Stepper Specs: Inductance, Etc.
August 27, 2016 02:42PM
If you are determined to use those motors, you have a few possibilities:

1. If your electronics is 24V-capable, then you should get reasonable performance out of them using a 24V supply.

2. Use half of each coil only, i.e. one end and the centre tap. The torque will be halved, but the inductance will be reduced by a factor of 4.

3. Take the motor apart. You will probably find that there are two wires going to each centre tap connection, in which case you can rewire them as bipolar-parallel.

4. Or just use them as they are, and upgrade to more suitable motors when you need higher speeds.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Interpreting Stepper Specs: Inductance, Etc.
August 28, 2016 10:58PM
Quote
dc42
If you are determined to use those motors, you have a few possibilities:

1. If your electronics is 24V-capable, then you should get reasonable performance out of them using a 24V supply.

2. Use half of each coil only, i.e. one end and the centre tap. The torque will be halved, but the inductance will be reduced by a factor of 4.

3. Take the motor apart. You will probably find that there are two wires going to each centre tap connection, in which case you can rewire them as bipolar-parallel.

4. Or just use them as they are, and upgrade to more suitable motors when you need higher speeds.

Thanks!

I was thinking of upgrading in two phases:
First would be hooking them up half coil, as suggested in #2. The manufacturer provides documentation for doing that. I'd do the wiring in such a way as to not foreclose changing over to bipolar series wiring.
The second would be upgrading my electronics to 24v, probably going to 32 bit processing at the same time.

I ordered a spare, and may take it apart and and try rewiring it as bipolar parallel. That seems like it would give me the most bang for my buck, and the cost of the experiment wouldn't be that high if I make a hash of it.

The motors are here: [www.applied-motion.com]
and here: [www.applied-motion.com]

I'm hoping that they're nicer than the usual offerings on eBay...
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