6 wire unipolar stepper performance

So I just bought a bunch of these

[www.mpja.com]

for $1 each!

They are 12V, 48 Ohm unipolar motors with six wires and a rated torque of 42 oz/in.

I am currently playing around with them. Wiring them up as a bipolar motor and leaving out the center taps gives me motion but very low torque at 12V. This is expected since the resistance is double what it would be under a unipolar wiring scheme. So I'll need a 24V supply at least for these motors. My question is what can I expect from performance?

By my naive Ohms law calculations a 24V supply should bring current to the levels of the original 12V unipolar drive conditions (about 0.25 A), while doubling power dissipation. My questions are as follows.

1. What happens to torque since both sides of each coil are on at the same time? (Rated torque is 42 oz/in)

2. Will I need to limit the current to less than 0.25A due to heat issues? (I'm thinking twice the power dissipated might end up overheating the motor or blowing coils. )

3. What about top speed?

... I'm driving stepper motors rated to 3Volts @ 2Amps with drivers capable of 70 Volts @ 6 Amps (and others with 325Volts @ 2 Amps) without overheating or killing them - you only have to adjust the current according to the motors (and optional used coolers), so they will heat to maybe 50 centigrades max. in continuous use.

The current defines the torque and 'generated' heat, (much) higher voltages are needed for faster charging/switching the coils, so you can try with drivers supporting until 48Volts with adjustable current setting ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Yes I do understand about current limiting drivers. I'm asking mostly about what the difference is between wiring as a unipolar vs. bipolar wiring. The main difference seems to me to be that the full coil is energized at a time when wired bipolar rather than half coil at a time for unipolar. So if we keep the current the same (.25A) whether through voltage or current limiting, the power doubles. All of this is under stalled conditions.

I think maybe "current defines torque" would apply for various current settings with the same wiring scheme but not necessarily true across wiring schemes? So torque would scale with current when wired as unipolar or bipolar but .25 A unipolar might not equal .25A bipolar. That was kind of the gist of the question. I'll be doing some qualitative tests anyway, just wondered if anyone had a more theoretical understanding of what goes on.

The genesis of the question was the Stepper motor wiki page which simply states that you can wire 6 wire unipolar motors to bipolar drivers by ignoring the center taps. So then I wondered what else happens when you do that.

... with unipolar wiring you have 4 separate coils you can drive solo for fullstep and separate pairs for halfstep, so the current/torque is depending on the count of activated coils, where either one or two coils are active over time.

With bipolar wiring you have only two 'coils' (with doubled wirings/resistance) which you'll drive as single or both with [+] or [-] polarity with doubled voltage for the same current through the wires. - here you'll have two or 4 coils active over time, compared to the unipolar setup ... so with same current per coil you'll need doubled voltage and energy but you'll get doubled torque too ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

u are getting 50degC in cont use? ... hmmm so id be overdriving mine if it 60-70 unloaded then? yes?

... yes, if my motors heats over 60degC, then something went wrong and I'll check the current settings and/or apply a cooler.

AFAIK most steppers will degrade (lose magnetisation) when over 80degC, so be carefull with overtemp!

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]