VEX Servos Legal in 2013?

Technical Motors
1

I am trying to find a data sheet that will give the wattage of a VEX servo and am having no luck. Does anyone have this documentation? (Not sure of the wattage or if it is in the allowable range.) Thanks, Meredith

2

Does thischart have what you are looking for?
The VEX servos are the VEX parts that begin with “276”

3

That info is for the 2-wire 393 motor, not the 3-wire servo motor (276-2162)

4

This work?

5

Thanks, Bryan. I did look at that earlier. It doesn’t say anything about wattage. Maybe it has enough information. This is what happens when you ask the business plan mentor to research servo data sheets…

6

Actually, looking at how FRC has you determine power it is,

Servo Max Power Rating = (Stall Torque) X (No Load Speed)

And I am not seeing a speed rating on Vex’s site. So it seems impossible to determine the power rating given the specs.

7

Well, it was certainly impossible for me! Thanks for making me feel not so dumb.

8

The best I can find you is an Internet Archive link to the 3-Wire Motor Inventor’s Guide page. Since I believe the motor and servo are identical, with the motor just modified for continuous rotation, you should be able to calculate power with that.

9

Thanks, Chris. Since we would be running them at 6.5 volts, they cannot exceed .615 Amps and be within the 4 watt limit?

10

That would make sense, using P=V*I, but for whatever reason FIRST defines servos likes this.

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)

Following this steps from this post and this and from the spec sheet above.

Free Speed 100 RPM @ 7.5 Volts
Stall Torque 6.5 in*lbs (approximately)

So Torque = 6.5inlbs=0.7344Nm (According to online converter)

100*.7344*0.1046=7.68Watts

So if all my math is right and I am converting everything proper, it does not seem that servo is under 4watts.

11

I didn’t think it could work out to 4 watts or less, but didn’t know enough to prove it. So I have to say “no,” but I have an explanation. Thank you!

12

Since those values are at 7.5 volts and on the robot they will run at 6 volts, you can derate both the speed and the torque by 6/7.5. Unfortunately, that only gets you to 4.9 watts.

13

This math is correct. The VEX Servo rated at 6V is 4.9 W.
We will post this rating on our product page to eliminate any confusion.

-John

14

Thanks, John. I was on a quest for an answer. Perhaps no one but me was confused.

Maybe you could provide a legal one for next year. :slight_smile:

15

Using the fake rating and calling it power will just add to the confusion.

It would be best for IFI to list the point on the motor curve where the product of speed and torque is maximized as “Maximum Power: P[sub]max[/sub] = 1.2 W” and then list the fake maximum power rating as “Servo Industry Maximum Rating: R[sub]max[/sub] = 4.9 W”. Don’t even use the word “power” to lend credence to that meaningless number.

Also, I don’t see the 100 rev/min figure (implicit in the figures above) on the spec sheet. It’s in some other documentation shared with the continuous rotation motors. Is it correct for both? (The servos do say 100° rotation.)