# Hard drive head motor question

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I know that someone who works, or has worked, on hard drives lurks here.

I'm trying to re-purpose a hard drive head motor into a science demo.  It
appears to have 64 turns of #30 or #31 (or metric equivalent -- 9 turns
of enameled wire fits into .09", at any rate), with a circumference of
around 1.4".  That works out to a total length of 90", and a resistance
of 1 ohm or so.

Yet the measured resistance of the coil is 15 ohms.

Might there be some reason to intentionally add resistance to the coil,
(buried where I can't see it), or is my math just cracked today?

Also -- some control guys swear by embedding a shorted coil in their
torquer motors.  Is that common practice in the disk drive world?

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Tim Wescott
Wescott Design Services
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Re: Hard drive head motor question
Tim Wescott wrote:

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

Re: Hard drive head motor question
On Sun, 08 May 2016 18:01:35 -0500, Jon Elson wrote:

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
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Re: Hard drive head motor question
On Sun, 08 May 2016 18:22:33 -0500, Tim Wescott wrote:

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
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Re: Hard drive head motor question
Tim Wescott wrote:

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

Re: Hard drive head motor question

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.

Note.

1. https://en.wikipedia.org/wiki/Resistivity

--
Don Kuenz KB7RPU

Re: Hard drive head motor question
On 5/8/2016 10:23 PM, Don Kuenz wrote:

Don't confuse resistivity with resistance.

Re: Hard drive head motor question
On 5/9/2016 5:42 AM, John S wrote:

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.

Re: Hard drive head motor question

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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Re: Hard drive head motor question
On Sun, 08 May 2016 22:58:17 -0700, Kevin McMurtrie wrote:

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

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Tim Wescott
Control systems, embedded software and circuit design
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Re: Hard drive head motor question
On 08/05/2016 22:52, Tim Wescott wrote:

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

piglet

Re: Hard drive head motor question

Great hypothesis.

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

S.

Re: Hard drive head motor question

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.

Note.

1. http://www.daycounter.com/Calculators/AWG.phtml
2. http://preview.tinyurl.com/zujeg4o

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Don Kuenz KB7RPU

Re: Hard drive head motor question
Is the winding star coupled, so you are in fact measuring two in series?

Cheers

Klaus

Re: Hard drive head motor question
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

Re: Hard drive head motor question
On Mon, 09 May 2016 12:55:38 -0700, Lasse Langwadt Christensen wrote:

Indeed I am -- sorry about the unclear title.  I only posted about the
part that mattered to me, not the part I threw away :).

--
Tim Wescott
Control systems, embedded software and circuit design
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Re: Hard drive head motor question
On Sun, 08 May 2016 16:52:52 -0500, Tim Wescott

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?

Best regards,

Bob Masta

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Re: Hard drive head motor question
On Mon, 09 May 2016 13:33:20 +0000, Bob Masta wrote:

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
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Re: Hard drive head motor question
On Sun, 08 May 2016 16:52:52 -0500, Tim Wescott wrote:

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.

--

Tim Wescott
Wescott Design Services
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Re: Hard drive head motor question
On 5/9/2016 12:52 PM, Tim Wescott wrote:

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?

--

Rick C