# Servo watts calculation

According to R18:

PWM COTS servos with a maximum power rating of 4W each at 6VDC
Per the Servo Industry, Servo Max Power Rating = (Stall Torque) X (No Load Speed)

I’m wanting to do the calculation for the HS-422 servo. Can someone verify my work?

Assumes 6v power at PWM port on roboRIO.

Torque T = 4.1 kg-cm = .4 N-m
Angular speed w = .16sec @ 60 degrees

Watts W = N-M / s = .4 / .16 = 2.5

You can compare to this spreadsheet. Looks like the only difference is due to rounding. http://www.chiefdelphi.com/media/papers/2751

It’s worth noting that what the servo industry calls “power”, knowledgeable people call “wrong”. Divide that false rating by 4 to get the theoretical maximum power of the servo, which you can use for mechanism design.

(It’s false because there exists no condition1 where a DC brush motor is simultaneously spinning at its free speed and drawing its stall current.)

1 Assuming that the motor is connected only to an electrical power source at steady state.