VEX Robotics DC Motor Testing

DC motors are an integral part of hobbyist and competition robotics, and proper motor selection is essential to successful robot design. To help understand this key relationship, VEX has developed an educational guide that explains the four key characteristics of DC motors and how they can be used to select the ideal motor for your application.

 

Learn more about using DC motors and motor curves to design robots

 

How we test: Why VEX provides the most accurate motor data possible

 

Furthermore, to ensure that users have the data needed to make such a selection, VEX will also be testing popular motors using industry-standard methods and publishing the results. Click on any of the motors below to view more details, including a complete motor curve. If you have any questions, feel free to contact prosupport@vex.com.

    Free Speed
(RPM)
Free Current
(A)
Maximum Power (W) Stall Torque
(N · m)
Stall Current
(A)
CIM Motor 5330 2.7 337 2.41 131
Mini CIM Motor 5840 3 215 1.41 89
BAG Motor 13180 1.8 149 0.43 53
775pro 18730 0.7 347 0.71 134
AndyMark RS775-125 5800 1.6 43 0.28 18
BaneBots RS-775 18V 13050 2.7 246 0.72 97
AndyMark 9015 14270 3.7 134 0.36 71
BaneBots RS-550 19000 0.4 190 0.38 84

VEX Testing Methods

Motor Curves: 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.

Why "Down-Up"?

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.

3 Minute Peak Power Test

Peak power data was acquired experimentally using the following test:

1. A motor is spun at free speed
2. A brake is quickly applied to bring the motor to 1/2 free speed (theoretical max power)
3. The motor is held at this RPM for 3 minutes and power output is recorded

Note: If you know the efficiency of the motor, you can also determine how much heat is being absorbed by the motor during this test. This is also useful for learning how fast a motor conducts heat to the exterior.

Locked Rotor Stall Test

The locked rotor curves were created by running the following test at multiple voltages:

1. A motor is spun at free speed
2. A brake is quickly applied to stall the motor (0 RPM)
3. The motor is held locked for 5min or until the motor fails, and torque data is recorded

This test helps to gauge the "durability" of the motor, experimentally determine stall torque, and observe how it handles rapid heat buildup.