Hard disk motor - DC or stepper?

Hi there. I am knocking up a prototype of something for a friend (yeah, I couldn't be more vague then that, hey!)... anyway, I somehow need to get the motor in an old hard drive to run. That's all I need to do for now. All I know about the motor is that it has 3 terminals.

Does anyone have any experience or knowledge when it comes to HDD motors? Are they normally DC or steppers? If it is a stepper, how would the coils be likely to be arranged (there's three terminals on it).

Any thoughts or wisdom would be much appreciated.

I think in the end I'll have to pull out ye olde scope and see what happens when I spin the spindle, but I don't have the time right now, so I figured if anyone has any pointers in the right direction that'd be a good start.

Thanks folks.

Which motor? A Hard Disk has two. The Head Motor is a Stepper. The Spindle Motor isn't... it's a conventional DC motor.

It's the spindle motor we're talking about here.

So would the third terminal be some sort of encoder output to regulate a constant speed?

Any guesses as to what voltage it would run on? HDDs have 12V and 5V connections to a computer....

Well, if it's a 2.5in Drive, it probably doesn't use the 12V connection.

As for the (physically) larger sizes, 12V ceratinly was for the spindle motor when I played with drives (no saying that they don't have a regulator to drop the 12v to something else nowadays - check your PCB). Going back to when I was a teenager and I used to pick up old 5.25in HDDs at the junk store for a few cents, they would have a jumper (or switches) to stagger the power-up time of the spindle motor at initial switch-on, so as not to overload the 12v line if you had multiple drives in a system.

It's a 3.5" drive, and I think it's about 10 years old, but I only have the skeleton (one part of the mounting chassis + spindle motor) to work on right now.

I think I'll rig up a variable regulator. Shouldn't be too hard.... I'll grab a 7805 and figure something out.

Any clues on the encoder output? Could it be a hall effect sensor or something? I have no idea what sort of sensor would be used in this type of DC servo motor. It's not THAT important that I get the feedback working, it's more so I can work out which terminal is which. I guess if I plug it into my PC scope and spin the spindle, two of the terminals should show a (relatively) constant voltage, which will hopefully be the two motor drive terminals.

Thanks for the info, Melanie.

Double post.....

I have tried applying 9V across each of the two terminals.... but it seems to act like a stepper motor!? When any particular pair of terminals has current through them, the spindle will jump to a certain position and lock there... was there any stage in HDD history when stepper motors were used in the spindles?

Hi,

Does the motor have 3 wires or 4 wires? (You say 3 terminals in your first post).

* * *

Are the wires coloured?
I the wires are not coloured, put a label on each wire with A,B,C.

What is the resistance in ohm between the wires?


Example:

The 3 wires are RED, GREEN, BACK.

Between RED and GREEN I have 10 ohm.
Between RED and BACK I have 20 ohm.
Between GREEN and BACK I have 10 ohm.
The RED wire is insulated from the metal case of the motor.
The GREEN wire is insulated from the metal case of the motor.
The BACK wire is insulated from the metal case of the motor.

* * *

Can you post pictures where we can see the the drive and
a close-up of the motor and wires?

Best regards,

Luciano

Man I feel stupid for not even getting out my multimeter in the first place... it's been a long week/month....

The 3 terminals are unmarked metal tabs poking out of a black plastic block set into the back of the spindle moter. There is 4.7ohms between any given pair of terminals... would this make it a 3-phase stepper motor? All 3 terminals are insulated from the case. Sorry, I don't have a camera here at the moment so I can't be more descriptive.


The part number is KP(might be an F not a P)1013SDAPV4. Neither of them came up with anything on Google (although this is an old drive so I didn't expect it to, really)

If it IS a 3-phase stepper, how are these normally driven? Are they driven bi-directionally to get 6 'steps'?

I always thought harddrives used brushless dc motors. From what you say i still beleive it to be so. Microchip have appnotes on how to drive them, check there. If you want to take the easy way out you could buy a suitable driver chip. There are a gazillion diffrent to choose from.

Hi,

See these links.

AN INTRODUCTION TO SENSORLESS BRUSHLESS DC MOTOR
DRIVE APPLICATIONS WITH THE ST72141
http://www.st.com/stonline/books/pdf/docs/7209.pdf

Sensorless BLDC Control:
http://ww1.microchip.com/downloads/e...C%2000901a.pdf

Best regards,

Luciano

Thanks for the info folks. Hmmm I'm wondering, why are Brushless DC motors called that when they clearly require AC to operate? I know that you would nearly always source that AC signal from a DC supply, but still it seems a bit strange.

Ah well.

It looks like a bit of a PITA to setup a fully featured DC brushless controller... is there any way to shortcut this? Or is it going to be a whole chunk of work no matter what.

What's likely to happen if I just make up some sort of 'fixed' sequence to send to the motors which includes a startup routine? I know that if the motor got stopped, it may not start without assistance, but would the motor be likely to actually move properly? For now I just want to get it spinning, and it has a very low load on it, which is static, so once it's running there shouldn't be too many synch issues.

Hi,

Brushless, sensorless motors with three connections are in
fact, not DC motors at all. They are actually permanent
magnet synchronous AC, 3-phase motors.
The commutation is done electronically. Three distinct
semi sinusoidal waveforms (not pure sinewave AC) that come in
at different times (or degrees) will causes the rotor to
rotate with the changing (alternating) magnetic fields of
the stator.

See page 7 of this PDF:
("some sort of 'fixed' sequence to send to the motor")

AN INTRODUCTION TO SENSORLESS BRUSHLESS DC MOTOR
DRIVE APPLICATIONS WITH THE ST72141
http://www.st.com/stonline/books/pdf/docs/7209.pdf

* * *

Read the documents below to understand the problem.

AVR444: Sensorless control of 3-phase brushless DC motors
http://www.atmel.com/dyn/resources/p...ts/doc8012.pdf


ATAVRMC100
http://www.atmel.com/dyn/resources/p...ts/doc7551.pdf

The ATAVRMC100 is an evaluation kit dedicated to brushless DC motor control, for
both Hall effect sensor control and sensorless control using Back ElectroMotive Force.
The kit includes an evaluation board, a 3-phase BLDC motor and a demonstration software.
It allows users to quickly evaluate the capability of the AVR® microcontroller
AT90PWM3 to control high speed brushless DC motor applications.
The kit can also serve as a development platform. Low cost AVR development tools
make debugging easier, and source codes, written in C, can be easily re-used by developers
for their own motor control applications.

* * *

Why do you need this motor?
What is the final application for this motor?

Best regards,

Luciano

I can't actually say why I need it due to a NDA :P

It's for a prototype of something which needs a fast (3000rpm or higher) motor with a reliably constant speed. The motor will have almost no load, it just needs to spin fast, and constantly. For what I'm trying to do, a hard disk chassis is actually a good platform to build on, which is why a hard disk spindle motor seemed the obvious choice.

Hi,

Why not use a ball bearing computer cooling fan with speed signal output?
(A PC cooling fan with tachometric signal output).

* * *

Fan Speed Control is Cool!
http://www.maxim-ic.com/appnotes.cfm...te_number/1784


If your PIC is too busy, use a I2C fan controller.

MAX6650 I2C fan controller
http://pdfserv.maxim-ic.com/en/ds/MAX6650-MAX6651.pdf

Best regards,


Luciano

http://www.elektor-electronics.co.uk/


Magazine - Elektor Electronics February 2006

- Brushless Motor Controller


Best regards,


Luciano

You could use a brushless motor controller from any RC model shop