Re: What makes a motor a servo?

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.

Motor power is listed at rated continuous torque, 266 Watts in the
example. That same motor generates a peak power of 416 Watts (not
listed) at half of stall current. It would generate 6.25 times more
heat than it can safely dissipate while doing so.

The same motor's load torque vs. power output vs. motor heat:

000_in-oz 7500_RPM 000_Watts_Output 0000_Watts_Heat (1)
030_in-oz 6750_RPM 150_Watts_Output 0017_Watts_Heat
060_in-oz 6000_RPM 266_Watts_Output 0066_Watts_Heat (2)
090_in-oz 5250_RPM 350_Watts_Output 0150_Watts_Heat
120_in-oz 4500_RPM 400_Watts_Output 0266_Watts_Heat
150_in-oz 3750_RPM 416_Watts_Output 0416_Watts_Heat (3)
180_in-oz 3000_RPM 400_Watts_Output 0600_Watts_Heat
210_in-oz 2250_RPM 350_Watts_Output 0816_Watts_Heat
240_in-oz 1500_RPM 266_Watts_Output 1066_Watts_Heat
270_in-oz 0750_RPM 150_Watts_Output 1349_Watts_Heat
300_in-oz 0000_RPM 000_Watts_Output 1665_Watts_Heat (4)

(1) No-load. Lots of RPM, no torque, zero power output, zero heat.

(2) Motor's max rated torque. The motor's theoretical efficiency is
80% and it is generating 66W of heat, the most it can safely
dissipate.

(3) Motor's peak power output. The theoretical efficiency is 50% and
it dissipates a Watt for every Watt it delivers as mechanical power.

(4) Motor stalled. Lots of torque, no RPM, zero power output. Lots of
heat though, it'll take a 1,665W power supply to properly cook this
baby!

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Mariss Freimanis wrote:
>
> >The picture is not quite as rosy as painted in favor of DC brush-
type
> >PM servo motors. Consider this comparison between a NEMA-34
stepper
> >and a NEMA-23 servo motor:
> >
> >1) Servo: 6,000 RPM rated load speed, 60 in-oz continuous rated
> >torque. The max continuous torque is just that; no to be exceeded.
> >
> Why do manufacturers give a peak torque, if the max continuous must
never
> be exceeded? I know your servo drives do not have an RMS limit
circuit,
> and a lot of others (like mine) don't, either. So, there would be
a certain
> kind of "going out on a limb" to set the current limit above the
max
> continuous
> rating. but, most servo motors have a peak torque (and current)
that is
> 4 to 8 times greater than the max continuous rating.
>
> If you insist on never exceeding the max continuous rating, you
can't
> burn up
> the motors, but it will require you to use MUCH larger motors. For
> instance,
> the people using the ubiquitous Ametek motors (Camtronics, etc.)
generally
> cannot even break the static friction on the machine at the
continuous
> current
> rating of the motor. IIRC, the continuous rating is 2 A, the peak
is 12 A.
>
> Jon