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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
References: <31pr9.4557$46.3661@fe01> <3DAE6BDA.255FB8C9@imit.kth.se>
Date: Thu, 17 Oct 2002 12:35:07 GMT
NNTP-Posting-Date: Thu, 17 Oct 2002 05:35:07 PDT
Uwe Zimmermann wrote:
> Jim wrote:
> > 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??
> These motor contain three coils (if it has 4 wires) which have to be
> switched in sequence in order to get the rotor rotating in the
> rotating magnetic field created by the coils - much licke a stepper
> For a simple test you could use a three stage decimal counter and a
> set of power transistors. The switching frequency will directly
> determine the rotation speed, i.e. for 7200rpm you'd have to switch
> each single coil 120 times per second.
> The problem is that the motor will probably not start moving, unless
> you ramp up the switching frequency slowly from 0Hz until you reach
> the desired rpm.
Ken Taylor provided a link to a TI note on reducing motor noise which
shows driving wave forms for essentially half step drive fro a 3 phase
DC brushless motor. An easy way to get all of the required switch states
is to use a 3 stage Johnson counter which is 3 D-flops with common clock
wired Q to D exept the third which is wired !Q to D on the first D-flop.
All states can be decoded with two input gates.
The motors are never used this way except when starting because there is
a resonance of the rotor in the magnetic field. The resonance is well
known to designers of stepper drives. With a large inertial load (such
as disks), it may be impossible to pass through the speed that excites
this resonance. A hard drive motor and load have this resonance at only
a few Hz.
Another reason that the motors are not normally run as steppers is that
they are very inefficient that way. For optimum efficiency, the phase
switching needs to take place where the superimposed BEMF curves of the
3 windings intersect. This maximizes torque of the motor thus reducing
the current drive required to maintain constant speed. (See my post that
explains commutation sensors and how the motors are normally run.)
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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