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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Hard drive motor question
Date: Thu, 17 Oct 2002 05:06:15 GMT
NNTP-Posting-Date: Wed, 16 Oct 2002 22:06:15 PDT
> How can I get a motor salvaged from a hard drive to run? All the ones I
> have salvaged so far are from 3 inch drives and all have 4 connections which
> go to the motor, one of which is a ground. I haven't been able to tell much
> from tracing things out on the PCB except that I now know that the motor
> does not run from the 5 volt or 12 volt power supply directly.
> That's all I want to do is get the motor turning at something close to it's
> original speed.
> I guess that there must be some fast switching going on amoung the 3
> remaining connections to the motor, and if this is so, is there some way to
> duplicate this that isn't too complicated?? I made a wrong assumption that
> since a hard drive motor is always turning when the computer power is on,
> that it was running directly from the power supply. So now I need some help
> in figuring things out.
Actually spin is controlled by a micro or DSP in a drive. However. the
motor needs some explanation.
Typically, the motor is a three phase brushless DC motor but it does not
have commutation sensors. Usually a DC brushless motor has some sort of
commutation sensors or encoder. A 3 phase motor using linear hall effect
sensors has 11 wires, 3 coil, 2 hall bias and 3 differential pair hall
outputs. These motors are found in many CD and DVD players. A 3 phase
motor using hall switches will usually have 8 wires. 3 coil, 2 for hall
switch power and ground and 3 hall switch logic signals. With linear
sensors, the motor driver has three differential comparators to convert
the hall voltages to logic levels. If hall switches are used, the logic
from them goes directly to a part such as the TI/Unitrode UC3625 which
produces the correct signals for the coil drive transistors.
Hard disk drives use a different approach. The motors have only the
three coil wires. To start the motor, the MCU or DSP blindly attempts to
start rotation by operating it as a stepper motor. A back EMF detector
will start working when the motor is spining fast enough. When the BEMF
detector starts to work, the MCU or DSP detects that event and uses the
BEMF detection from one phase to "dead reckon" optimum commutation based
on a timer. The motor is then accelerated to desired speed.
There are methods of optimal starting covered in various patents but
this is needless detail since the blind stepper start does work and has
been used in many drives.
Running a hard drive motor will require adding a commutation sensor or
using an MCU as mentioned above. It is not all that trivial.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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