rewinding 12V/0.4A steppers?

jens55

@o_lampe, as far as I know, as soon as you separate the rotor and stator you will cause substantial demagnetization of the permanent magnets which would give you a very weak stepper.
I have not tried this myself but this is what I have read.

o_lampe

@jens55
I must admit, although I've build hundreds of BLDC motors with neodym magnets, I never touched the coils of a stepper motor before.
It's the first time I heard of the demagnetisation thing and will read up on this.
When it is so easy to ruin a steppers magnet field, how do they withstand the abuse of loosing steps, sensorless homing a.s.o.?

jens55

@o_lampe, I have no clue what actually happens to demagnetize things.
While I applaud your idea of upcycling I wonder if it is a fools errand for a device that is probably less than $10 for a very small stepper.
Keep us posted if you find out anything!

o_lampe

@jens55 I agree it's not worth it, given the time it takes. It's more like a challenge for me.
I opened the backside of one of them, secretly hoping to see the coils were drenched in resin, but they are not.

dc42

@o_lampe said in rewinding 12V/0.4A steppers?:

My idea is to unwind the coils, take the wire double and rewind it. That would double the cross-section and reduce the inductivity and resistance to 50%.
If my theory is right, they will behave like 6V/0.8A steppers.

You would quarter the resistance and inductance, so they will behave like 3V 1.6A steppers.

I am far from convinced that demagnetisation would be a problem. Good luck!

o_lampe

@dc42 said in rewinding 12V/0.4A steppers?:

You would quarter the resistance and inductance, so they will behave like 3V 1.6A steppers.

Right, twice the cross-section but half the length... I'm getting old

mendenmh

The issue of demagnetization is one I brought up a long time go, but I now believe it isn't a big issue. I was working from badly outdated personal experience.

It was a big problem with AlNiCo magnet motors of 30 years (or more) ago. These magnets were typically oversaturated and if you pulled out the rotor from a motor, and put it back in, the motor would be MUCH weaker. I tried it (probably in the 1970s ) on some DC motors, and they were useless afterwards.

I think modern SmCo and NdFeB magnets have such a high coercivity, they don't need to be prepared this way to make them strong enough. I have seen plenty of reports of people disassembling and reassembling steppers, with no harm.

mrehorstdmd

If the motors demagnetized just by disassembling them, how would they have been assembled in the first place? Hmmm....

fcwilt

@mrehorstdmd said in rewinding 12V/0.4A steppers?:

If the motors demagnetized just by disassembling them, how would they have been assembled in the first place? Hmmm....

Magic.

mendenmh

@mrehorstdmd I think they were magnetized by a capacitor discharge, with the flux going through an armature which could be pushed out at the same time the real rotor was inserted, so the flux path never got a chance to open open. AlNiCo was sensitive to this. High-strength AlNiCo magnets required 'keepers' (flux shorts) to not lose strength when stored.

It's worth realizing just how much the performance of permanent-magnet motors has improved in 30 or 40 years, due to rare-earth magnets. They were really wimpy before, even if not accidentally demagnetized.

jens55

@mrehorstdmd said in rewinding 12V/0.4A steppers?:

If the motors demagnetized just by disassembling them, how would they have been assembled in the first place? Hmmm....

That's easy ... the same way the magic smoke gets stuffed into all the chips ....

o_lampe

I ran the unchanged steppers with 0.35A and they were very weak, so I guugeld the model number and found almost nothing. One seller listed them as 1.2A steppers, so I wrote a 1.2A torture test and checked for excessive noise or temps, but they were all right.
Seems, I owe Sintron an apology for all the trouble I had with their I3 clone (mainly bad extrusion and misaligned Z-axis from skipped steps).
I might have misread their stepper datasheet and ran them with 0.4A all the time...

Rewinding them is no option anymore.

dc42

@o_lampe if you measure the resistance of one phase, then you can get an idea of the rated current by comparing it with the data for other stepper motors of the same size (e.g. Nema 17) and similar length.