Re: [CAD_CAM_EDM_DRO] Re: What makes a motor a servo?

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