N. T. wrote:
>> You shouldn't need that. A motor can be its own filter. You can chop
>> the motor from substantially higher voltage than what it is intended
>> to run at, just make really really sure the duty cycle is low enough
>> so that the average is right. Put another way, you need a buck
>> converter, but the coils in the motor can be the inductor and the
>> final load at the same time.
>
> I am not sure what duty cycle are you talking about. The motor coils
> operate in continuous current mode.

Sortof.  The motor's ability to see current changes goes down as the
frequency goes up.  At a sufficiently high frequency, only the average
current matters from the motor's point of view.  The inductance of the moto=
r
coils will smooth out the current to a good extent, and the mechanical
inertia can't respond to torque changes in a meaningful way usual over a fe=
w
100 Hz.  The biggest problem is usually the force on individual wires of th=
e
motor coils.  These can vibrate a lot faster than the motor can move, which
can stress them and cause annoying audible whine.  With a switching
frequency of a few 10s of KHz, these effects are usually reduced to the
point of irrelevance.

> To do so you need to switch a coil
> from 0 to 300V to let it "suck" extra current from ground.

Yes, that is the essence of a buck converter.  But note that this coil can
just as well be a motor winding than a deliberate external inductor in a
separate buck converter.  The former will be more efficient.

> The specs
> may feel bad about high frequency 300V signal on 50V DC motor coils
> (isolation).

This may be true.  It depends on the motor, of course.  Most motors can be
chopped at substantially higher than their normal operating voltages, but o=
f
course you have to check.  In this case we're asking for 6x insulation
capability over normal operation.  Probably that's OK, but again, you have
to check the specs for your specific motors.  I never meant that you should
just assume your motor can handle this in any particular case.


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