VEX Robotics Motor Data - BaneBots RS-775 18V
Note: Published Specifications were taken from the BaneBots website and are current as of December 10, 2015. Experimental tests were performed using new motors that were purchased during the summer of 2015. Specifications were scaled down from nominal 18V to 12V (VEX tests were performed at 12V).
|Published Specification||Experimental Data|
|Free Speed (RPM)||13000||13050|
|Free Current (A)||1.8||2.7|
|Maximum Power (W)||265||246|
|Stall Torque (N · m)||0.78||0.72|
|Stall Current (A)||86.7||97|
The "Down-Up" Dyno Test
VEX Robotics motor curves were developed experimentally using a “down-up” dyno test.
1. A motor is spun at free speed
2. A brake is slowly applied (linearly increasing in torque over time), bringing the motor down to a predetermined RPM
3. The brake is slowly released and the motor is allowed to return to its free speed
A variety of data, such as output speed, output torque, current draw, and power input/output, is taken throughout this test. The “down” (brake applied) side is then averaged with the “up” (brake released) side.
The mechanics of a dyno test are crucial to developing and publishing accurate motor specifications. When a motor is spinning at free speed while attached to a dyno drum, the system contains a high amount of rotational inertia. This inertia complements the motor’s own output, creating a false reading for peak output power that can be higher than the motor’s actual performance.
However, manufacturers do not always adjust for inertia cancellation when measuring their motor’s performance, and spec sheets rarely detail the circumstances under which their information was derived.
By testing and averaging both the “down” (inertia helping the motor) and “up” (inertia resisting the motor) sides, this method is the best way to represent a motor’s true capacity.