Re: Rotor inertia.

Kim,

Cool question (no pun intended). Eddy current and hysterisis can only
occur in the surrounding iron, not in the inductor.

Think of it as a tennis match; the ball goes back and forth, so do
1,000 pairs of eyes, right to left, right to left.

The "eyes" are the magnetic dipoles in the iron, the ball is the
current in the winding. The magnetic dipoles swiveling back and forth
generate a "friction" that causes heat in the iron. Actually it is a
current that creates I^2*R heating but it's not as easily visualized.

The actual heat generated in ironless armature motors is not iron
losses (eddy current) but rather plain old resistive (I^2 * R)
heating you get anytime current passes thru a resistor (armature
resistance).

Iron losses are very small because there is so little iron in the
flux path; it can be neglected in fact. Now it's only 10 pairs of
eyes instead of a 1,000.

On the otherhand, the armature is still copper windings, iron or no
iron. Large currents passing thru it will generate large amounts of
heat either way.

The difference is very little inductance is present to limit these
peak to peak excursions.

Mariss

P.S. I really love this stuff!

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "kimvellore" <kim@b...> wrote:
> Mariss,
> My 2C on this subject. The heat generated in a inductor is due to
> the resistance or due to eddie current. For the low-inertia both
> would be small, hence the heat generated due to the increased
current
> should in theory be less then in regular motors... what do you
think?
>
> Kim
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@y...> wrote:
> > Be careful with "low-inertia" motors for another reason entirely.
> > Oftentimes they are unsuitable for use with switching type drives.
> >
> > Motor inertia is reduced by eliminating the laminated steel
> armature,
> > leaving only the windings, which are wound in such a way as to be
> > self-supporting.
> >
> > The absence of iron also reduces motor inductance dramatically
and
> > that is bad if the motor is used with a switching type drive.
> Almost
> > all high power drives are switching type (PWM).
> >
> > Compare two motors:
> >
> > One is a low-inertia with an armature inductance of 100uH. The
> other
> > is a standard motor having 5mH of inductance. Both have similar
> specs
> > otherwise and will be run at 48VDC using a 25kHz switching drive.
> >
> > The 25kHz ripple current thru the standard motor will be:
> >
> > I = V / (2L * f) = 48V / (2 * .005 H * 25,000 Hz) = 0.192A
> >
> > The low inertia motor:
> >
> > I = 48V / (2 * .0001 H * 25,000 Hz) = 9.6A!
> >
> > The standard motor will stay nice and cool while the "low-
inertia"
> > motor will probably burn down with nearly 10A of current passing
> thru
> > it continuously.
> >
> > It would require large inductors in series with the motor in
order
> to
> > use it.
> >
> > Mariss
> >