Re: [CAD_CAM_EDM_DRO] Differences between AC & DC brushless

Roy J. Tellason wrote:

>On Thursday 25 March 2004 12:44 pm, Jon Elson wrote:
>
>
>
>>To be strictly correct, the only motor that can be called DC brushless are
>>the ones on computer fans and the like, where the commutation function is
>>built INTO the motor, and it has only two wires coming out.
>>
>>
>
>I've not taken one of these that far apart, but am wondering if they werk the
>same way as the direct-drive stuff that started showing up on floppy drives
>after the belt-driven stuff went away...
>

Yes. mostly. The difference is floppy and hard drive motors are speed
controlled,
so they often have all the drive electronics integrated on the drive's
circuit board.
This is especially true of hard drives. Most 8", many 5" and a lot of
3" drives
have integrated motors that require just +12 V and a "go" signal. These are
easily salvaged and it usually isn't too hard to figure out which wire
is power
and which is control. I have a 5" floppy drive motor used as the power
source for
a fuel pump on my lawn tractor. I just hooked the +12 V terminal to the
switched
battery supply, and the control terminal is connected to a float switch in a
tiny header tank above the carburetor. I am amazed that floppy motor
has survived
10 years of electrical abuse on the tractor's battery bus, as well as
tremendous
vibration, without any problem!

Anyway, what I was trying to draw a distinction on was that to be called
a "DC"
brushless motor, it needs to have 2 wires ONLY, and the commutation is built
into the motor, as a single unit. It will spin when connected to a DC
battery
of the proper voltage. Brushless servo motors normally have 3 wires,
and need
to be fed essentially AC signals of the right polarity at the right time
(as controlled
by the hall effect sensors or commutation signals from the encoder) to spin.

>>The other general types are (1) permanent magnet brushless motors, where the
>>rotor poles are always in the exact same spot on the rotor, hence the
>>encoder provides commutation info to the drive.
>>
>>
>
>Speaking of fans they sure seem to have permanent magnets in there someplace
>as on the CPU fan of the test board that's sitting here. I'm not sure I see
>the distinction you're trying to make here.
>
>
>

These are usually "inside out" motors, with a laminated stator on the
inside, and
an inward-facing ceramic magnet as the hub of the fan blade. There is a
board with
hall effect sensors and power transistors to commutate the motor windings.
Most newer small fans are 2-phase motors, not 3 phase. That saves
transistors.

Jon