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