Re: servo Motors

Hi,

Wally, it might work with brushless DC motors but I haven't taken the
time to derive the equations yet. I am working on a DC brushless STEP
and DIRECTION input servodrive, so I'm sure it will come up
eventually.

If you are interested, here's the background of the equation for DC
brush motors:

1) Maximum electrical power you can put in a DC motor is according to
ohm's law; P=V squared/R

2) DC motor's power vs. torque curve is a parabola, peaking at 1/2
stall torque, (also 1/2 no-load speed).

3) At peak power the motor is impedance matched,(1/2 input power to
load, 1/2 input power as heat)

4) At peak power, power in is V/2R, (Vsupply times stall current/2)

5) According to (3) efficiency is 50%, so Watts delivered to load
becomes (V squared / 4R)

6) Torque times speed (power in Watts) is derived from the
identity "1HP = 550 ft-lbs/sec" and 1HP = 746 Watts

7) The normalizing constant (easily derived) is 1351 for torque in oz-
in and speed in RPM. I can show it if you like.

8) Since peak power is at 1/2 stall torque (2), and knowing the
motor's measured no-load RPM, you can use (5) and (7) to set up:

((T times S)/1351)/2 = (V squared)/4R or,

T = (1351 times V squared)/2RS

The above stuff is paricularly useful to predict peak motor power for
a given power supply voltage, (4) and predicticting available torque
at any given speed (7).

Mariss
Geckodrive Inc.

--- In CAD_CAM_EDM_DRO@egroups.com, "Wally K" <cnc4me@u...> wrote:
> Mariss, That torque calc is a good trick. Does it work with any
> other kinds of motors.
>
> Wally K.