Re: What makes a motor a servo?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Mariss Freimanis wrote:
>
> >Peak torque has limited utility. Assume the previous example; peak
> >torque is 5 times the continuous rating. Motor power dissipation
is I
> >squared R and current is proportional to torque.
> >
> >The motor would draw 5 times the rated current at stall and
dissipate
> >25 times its safe heat dissipation. Taking the reciprocal of 25
gets
> >you a 4% duty cycle using peak torque. For this 4% you need a
power
> >supply 5 times as big costing 5 times as much yet you cannot use
the
> >excess 96% of the time.
> >
> >
> Agreed, you better not use it much. On a home shop machine, the
motor has
> plenty of time to cool while the user changes tools manually,
thinks
> what button
> to press, etc. On a production machine, especially a router, there
are
> lots of
> long accelerations, and the peak torque has to be used very
sparingly,
> if at all.
>
> The bulk DC power supplies also have a peak rating, usually 2 x
continuous
> rated current. Also, it would be unusual in most cases to use the
peak
> torque
> near maximum rated speed, so the switching action of the drive
would allow
> peak current to be delivered without drawing matching current from
the
> supply.
>
> But, you are right, it is important to consider the long-term power
> dissipation
> in the motor, and not exceed the rating. However, one can trade
off the
> continuous
> against the peak. If you normally use less than 1/2 the continuous
> rating, that can
> allow you to use the peak up to 4% in your example. If you already
use
> the full continuous
> rating nearly 100% of the time, you have no reserve to use the peak
at all!
>
> Jon