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4hv.org :: Forums :: General Science and Electronics
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6-Wire Stepper Motor seems wrong
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Maxwell
Sat Jan 04 2014, 04:11AM Print
Maxwell Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
 		For a project I'm working with I have a stepper motor with 6 leads - no big deal, the two wires should be centertaps, one for each winding pair?

However when I took an ohm meter to the windings I found the following wire resistances:

Wire 1-2 1.37 kOhm
Wire 1-3 268.3 Ohm
Wire 2-3 272.5 Ohm

Wire 4-5 1.33 kOhm
Wire 4-6 264.7 Ohm
Wire 5-6 267.6 Ohm

What am I missing that 260+260 ≠ 1.3 kOhm for the windings?

There is infinite resistance between the 1-3 winding and 4-6 winding sets.

Assistance requested,

Thanks, -Max.
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Ash Small
Sat Jan 04 2014, 05:12AM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
 		Just an idea, but is it new, or second-hand?

I have some 6 wire steppers, I'll try and find them and take some measurements tomorrow.

EDIT: You can usually open them up fairly easily, and look inside. They usually go back together without problems.
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Maxwell
Sat Jan 04 2014, 03:11PM
Maxwell Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
 		This is a used stepper which I saw working in its previous mechanical fitment.

After more research it appears that the center tap could have its own resistance - but I highly doubt that it would be upwards of the other coils.
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Dr. Slack
Sat Jan 04 2014, 04:55PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
wrote ...
the two wires should be centertaps, one for each winding pair?

almost certainly.

If you draw up the three terminals 1, 2 and 3, and put in linear resistors of the values you have measured, it is impossible. Current flowing 1 to 3 to 2 will have a maximum resistance of 540ish ohms, and you measure 1370.

So, other possibilities, like strike out the word 'linear'

Is your meter battery OK, are its resistance measurements badly non-linear?
Does the stepper motor have integral catch diodes across the windings, and they are reducing the apparent resistance for the single winding measurements?
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johnf
Sat Jan 04 2014, 07:36PM
johnf Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
 		Do not take steppers apart if you want them to have the same torque. Dismantling partialy demagnetises the rotor.

Stepper rotors are magnetised after assembly using capacitor discharge into the windings
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Ash Small
Sat Jan 04 2014, 09:51PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
johnf wrote ...

Do not take steppers apart if you want them to have the same torque. Dismantling partialy demagnetises the rotor.

Stepper rotors are magnetised after assembly using capacitor discharge into the windings

Thanks for that John. The two I have are for a coil winding machine I'm building, and they are much bigger than required, so should still be ok, but do you have any relevant links, or suggestions regarding re-magnetising them?

I was reading a book written in 1904, and linked to by PM in another thread recently, regarding magnetising steel using a pulsed DC supply, with a coil wrapped around it. I'll re-read that chapter, but it basically suggested you just keep repeatedly saturating the core, in the case in the book it was done by running one wire along a coarse file to repeatedly make and break the circuit. I guess as long as I don't burn out the windings I shouldn't have any problems?
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