Hard drive head motor question

it is likely aluminum wire, to cut mass for faster seeking. Seek time is a published parameter, and all the drive makers try to make them go faster.

Jon

Hmm. Doesn't account for all the difference -- but I can see why they may do that.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

To expand: Copper has a resistivity of 1.7 * 10^-6 ohm-cm. Aluminum has a resistivity of 2.7 * 10^-6 ohm-cm.

So you'd expect an aluminum winding to have 50% more resistance, not 10.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Well, the wire drawing and winding process may wreck the conductivity. Aluminum as cast is really porous and brittle, then they roll it which crushes the grains together and makes it more ductile as well as have better conductivity. When pulling it into thin wire, it may go up in resistance.

But, aluminum is not a great conductor when hot. Possibly they actually used something else that has (nearly) constant resistance over temperature, like nichrome? (Sounds really crazy, but you never know what a clever engineer will come up with when thinking outside the box.)

Jon

Your data more-or-less matches mine (~11 ohms ~4900mm). The random hits returned by a search also more-or-less matches our data.

My hard drive voice coil actuator contains only 4900mm of wire (no hidden components). It seems that the resisitivity of the copper in the voice coil differs from the standard value. FWIW, wikipedia says that a long thin copper wire has a much larger resistance. [1]

All copper wires, irrespective of their shape and size, have approximately the same resistivity, but a long, thin copper wire has a much larger resistance than a thick, short copper wire.

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Don Kuenz KB7RPU

15 Ohms sounds wrong. Those heads need to move REALLY fast and 12V doesn't put much power into 15 Ohms. 15 Ohms might be right for the head. Maybe it's the wrong wires?

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Could the metallic wire be much thinner than you think because the enamel thickness is much greater than usual?

piglet

Great hypothesis.

Or maybe the wire isn't copper, for some reason.

S.

Don't confuse resistivity with resistance.

A single strand of voice coil actuator wire from my drive measures 35 AWG. 5X magnification reveals an unknown amount of residual enamel on the wire. The length of the wire is ~4900 mm with a resistance of ~11 ohm.

The resistance of 35 AWG copper is 1.08 Ohm/m [1]. The theoretical resistance of 4900 mm of 35 AWG copper is ~5.3 ohms, or about half of the measured resistance.

A reverse lookup of the measured resistance of ~2.2 Ohm/m yields ~38 AWG from the table. The possible use of copper-clad aluminum magnet wire [2] may also explain part the difference.

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Don Kuenz KB7RPU

I don't suppose it's like those transformer winding where they use a heavier gage for the external leads, with a much smaller gage just under the paper skin?

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To expand:

R = rho*length/conductor area (R=rho*l/a) where rho is resistivity, l is length, and a is the area. So, for a given resistivity, it is patently obvious that a long, thin conductor has higher resistance than a short, thick one.

The referenced comment is trash.

Those are the wires whose voltage wiggles when I move the coil in the magnetic field.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
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Don't think so. It's an amazing little piece of manufacturing engineering, by the by.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

A new coil, using about 20 feet of #34 wire (I calculate about 160 turns, but did not count), generates about twice the signal (60mV peak-peak at

1Hz) with a 4-ohm coil -- and it doesn't fill as much space with wire as they used. So I should be able to improve on their performance.

Whatever was in the drive, they weren't optimizing for conductivity.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

That's not terribly surprising. The head movements are very fast and the currents are in pulses which are likely not resistance limited.

Don Kuenz said his coil wire was larger gauge than estimated by resistance and suggested it might be copper clad aluminum. Given the skin effect on fast pulses and the need for low weight, that is not an unreasonable suggestion. Have you tried scraping or cutting the old coil wire to see if it was solid copper?

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Rick C

I may eventually do that, but at the moment the original coil is the only usable one I have -- My shop is slightly more capable than the usual home shop, but the one coil I've built so far is a hand-made atrocity that is unusable. Hopefully the second try will be usable, with more bulk of wire, maybe in a smaller gauge, and wound more neatly on the form.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Is the winding star coupled, so you are in fact measuring two in series?

Cheers

Klaus

Den mandag den 9. maj 2016 kl. 21.50.02 UTC+2 skrev Klaus Kragelund:

that was my first thought but I think he is talking about the "voice coil" for the head arm, not the three phase spindle motor

-Lasse