Re: [CAD_CAM_EDM_DRO] B: Brushless motor

Whilst I agree with the setiments about brushed motors; is the problem to do
with the atmosphere they are being run in? The biggest killer for brushed
motors in my experience is dust when it can enter the brushgear. Wear,
arcing, noise and destruction can in extreme cases often follow. The other
cause of brush wear is operating brushed motors outside their ratings for
extended periods or poorly timed brushgear.

Any views Michael ?

Regards Peter

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, October 05, 2002 8:22 AM
Subject: Re: [CAD_CAM_EDM_DRO] B: Brushless motor


> Michael Holm wrote:
>
> > > Why? Is it against the law to use brush motors there? Why does it
have
> > to
> > > be brushless? The only reason I can think of is for operation above,
say,
> > > 40,000 RPM. As for controllers, unless you need variable speed under
> > > computer control, an adjustable DC power supply will probably work.
> > > A variac, transformer, rectifier and capacitor will vary the speed
with
> > only
> > > slight response to line voltage variations, for this purpose.
> >
> > I prefer a brushless motor for several resons.
> >
> > 1) I already burned 2 brush motors, and I hear brushless are more
stirdy.
> > 2) I hope a brushless will make less noise
> > 3) A brushless motor will run fast and slow with equal strength
> >
> > I brushless motor works more or less like a stepper. The idea is to have
the
> > coils fixed around a magtnetic core, so there's no physical contact
between
> > the core and the power supply. This takes away some of the noise, and
allows
> > for faster speeds, because there won't be a 'lighting' around the
contacts
> > where the coal touch the cobber on a brush motor.
> >
> > For the same reson - the fixed coils - the motor needs a controller,
just
> > applying DC will hold it, like a stepper - not make it turn.
>
> I'm quite aware of the design differences between brushless and brushed
motors.
> But, I really think you are making assumptions that are unwarranted. the
brushes,
> themselves, don't make noise except perhaps in toy motors. There are
design
> changes used in high-quality servo motors and other motors designed for
> quiet operation. One of the most important is the shape of the armature
or stator slots. Poorly
> designed slots (or slots designed to make assembly easy at the cost of
proper
> magnetic operation) will produce what is known as reluctance ripple, due
to the
> changing amount of iron in the pole magnetic circuits. this can cause
flexing of
> the stator in either type of motor, producing a great deal of noise from
the motor.
> One trick to recuce the noise is to twist the armature or stator
laminations about
> one full slot width along the length of the motor. This allows the
magnetic circuit
> to always have the same amount of iron lined up with the poles at all
rotor positions.
>
> The small amount of sparking at the brushes is totally silent on the servo
motors
> I've worked on.
>
> I have brush servo motors on my mill. They are about 40 years old, and
were used
> quite heavily in their earlier life in a production shop. The only brush
motors I have
> burned up were consumer-grade motors in cheaply-built home appliances. If
you
> take a motor designed for 15 minutes a week use in a vacuum cleaner, and
use it
> at full load in a machine tool spindle for 15 hours straight, it will be
no surprise when
> it burns up. Industrial-grade brush motors are built to a much tougher
standard,
> and should be quiet and reliable. (15,000+ RPM motors directly cooled by
integral
> fans are going to be quite noisy, however. the fix is to remove the fan,
or get a motor
> designed for external forced air cooling, and supply a quiet cooling fan.)
>
> Anyway, AC induction motors and a flux vector drive can do anything a
brushless
> servo motor can.
>
> Jon
>
>
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