Re: Rotor inertia.

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
>