Re: [CAD_CAM_EDM_DRO] Re: Rotor inertia.

Mariss Freimanis 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.
>
>

Wait a minute. Just because the iron doesn't MOVE doesn't mean the
iron isn't there! The motor still needs the iron to complete the magnetic
circuit. (Yes, there are often twice as many air gaps in an "ironless
rotor"
motor, and often the inductance is designed to be very low, to keep the
electrical time constant short. Some of these motors are designed to
accelerate to thousands of RPM in a few mS.) A number of bobbin-style
motors I've seen the innards of use a solid iron slug inside the bobbin
to complete the magnetic circuit. the magnets are outside, as usual, but
the iron inside the windings is fixed to the back of the motor, and the
windings are a thin basket or bobbin between the slug and the field.
There are also some that put the magnets inside, and the outside is just a
magnetic field conductor. The printed disc motors have really low
inductance as the complete winding between brushes may be only 20 turns
or so.

>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).
>
>

That's why most commercial servo amps have some inductance on board
to prevent large AC currents from reaching the motor. I have 200 mH
per each output of the bridge in a balanced 2 pole filter. The ripple
current
the transistors see can be around 3 A, but the ripple current, even into
a short
is only a couple hundred mA. I run a 100 KHz PWM frequency to make the
filter smaller and cheaper. Of course, this filter drives up the cost of
the servo amp. It also allows you to run an AM radio right on top of the
CNC control, even though the motor cables are not shielded. (I prefer
FM, however.) I don't think a commercial product could be built with
any Gecko drive, servo or stepper, without adding filters external to the
gecko drive. No WAY could it pass even FCC class A certification!

Jon