VEX Motors
A VEX Motor is the primary actuator for a VEX robot.
A VEX motor is a DC Motor that converts stored electrical energy from the VEX battery into mechanical motion, and is controlled by the V5 Brain.
Maximum power is 11W continuous and maximum torque is 2.1 Nm. Free speed is software-limited by the motor’s processor to keep consistent performance motor-to-motor and to allow top speed under loads. Metal gears are used in all high torque locations for strength. Plastic gears are used in the low load, high speed locations for smooth and efficient operation. An internal gear cartridge is user changeable for output gear ratios of 6:1, 18:1, and 36:1. The motor’s internal circuit board has a full H-Bridge and its own Cortex M0 microcontroller to measure position, speed, direction, voltage, current and temperature. The microcontroller runs its own PID with velocity control, position control, torque control, feedforward gain, and motion planning. PID is internally calculated at a 10 millisecond rate. The motor’s PID values are pre-tuned by VEX for performance across all operating conditions.
Stall current is limited to 2.5A (typically, see undefined) to keep heat under control without affecting peak power output. Limiting stall current eliminates the need for automatic resetting fuses (PTC devices) in the motor, which can cause unintended motor outages. The 2.5A limit essentially removes the undesirable region of the motor’s performance curve, ensuring users do not unintentionally create stall situations. Finally, to make sure the motor lasts, the internal temperature is monitored. If a motor is approaching an unsafe temperature, the user gets a warning. If the motor reaches its temperature limit, performance is automatically reduced to ensure no damage occurs.

Current Limiting

The Vex V5 Motors have a variable current limit that is determined by VexOS. Robots that use 8 or fewer motors, the competition-legal max in VRC, will have a default current limit of 2.5A set for each motor. Robots using more than 8 motors, typical with Vex U teams, will have a lower default current limit per motor than 2.5A. That current limit is determined in Vex OS by a calculation accounting for the number of motors plugged in, and the user's manually set current limits. That calculation is described in a Vex Forum post by James Pearman and used in the GUI below.
The V5 Motor Current Calculator embedded below will display the Vex OS current limit for each motor based on that calculation. Entering a value for the Current Limit field on any of the motors will limit that motor's current and recalculate the limits for the other motors.

Specs

V5 Smart Motor Specifications
Speed
Approximately 100, 200 or 600 rpm
Peak Power
11 W
Continuous Power
11 W
Stall Torque (with 100 RPM cartridge)
2.1 Nm
Low Battery Performance
100% Power Output
Feedback
Position Velocity (calculated) Current Voltage Power Torque (calculated) Efficiency (calculated) Temperature
Encoder
1800 ticks/rev with 36:1 gears 900 ticks/rev with 18:1 gears 300 ticks/rev with 6:1 gears
Dimensions
2.26” W x 2.82” L x 1.30” H 57.3 mm W x 71.6 mm L x 33.0 mm H
Weight
0.342 lbs 155 grams

Troubleshooting

One on the main issues that arises with the VEX V5 motors are dead motors. Many times this means that a motor stops working often without any notice. Sometimes the led in port compartment of a motor may flash red. This could be sign that the motor is dead or there is a loose wire connection. A good idea to make sure a motor is dead is to test it with another wire and, if possible, test it with another V5 Brain. If these steps are unsuccessful there is as of right now no reproducible method of fixing a dead motor.
Static buildup in motors could be a reason for them dying suddenly. When this static buildup discharges (commonly referred to as electrostatic discharge, or ESD), the connected port on the V5 Brain may be permanently damaged. Please consider using our ESD protection boards to help ensure that you do not lose any ports on the V5 Brain.

Old Vex Cortex Motors

This section refers to the old VEX Cortex Motors which are no longer competitive.
Two-wire motors such as the 393 can be connected directly to the VEX Cortex by using one of the 2-wire motor connection ports. These motors can also be connected to a 3-wire port on either the Cortex or the VEX PIC Microcontroller by using a VEX Motor Controller. They also include internal resettable fuses that replace the now discontinued clutch that was needed to protect a 3-Wire Motor during a stall.
While one might be tempted to design a mechanism based on the rated "stall" torque or free speed of the motor, these are not actually realistic specifications. For the most commonly used motors, the torque and speed at the points of maximum efficiency, power, and torque are listed. For the longest life and least power loss, motors should operate between the maximum efficiency and maximum power points during typical circumstances, with anything exceptional detected quickly by operators or autonomous code. While running motors at the "maximum torque" operating point should not cause lasting damage, it wastes power and risks performance loss or a stall during routine operation.
Unfortunately, due to their nature, all VEX motor types are known to both cause and be affected by Line Noise.
The main workhorse of the VEX motion lineup, the 2-Wire Motor 393 is larger and has approximately 60% more torque than the visually similar 2-Wire Motor 269. These motors are primarily used in drive trains or heavy lifting scenarios. Only 4 motors of this type were allowed per robot in and before VEX Gateway, but in VEX Sack Attack and later, no restrictions are placed on the usage of this motor. Its high Power Consumption under load may cause problems if several 2-wire Motor 393s are used at the same time.
Despite the higher current consumption, these motors are more efficient and robust than the 2-Wire Motor 269 and actually consume less power for a given torque output than any similar motor. Care must only be taken that a stall is detected quickly.
The stock internal gears of the 2-Wire Motor 393 can be replaced with another set of "high speed" gears included with the motor. This reduces the output torque to the equivalent of a 2-Wire Motor 269, but increases the speed to 160 rpm, making it the fastest motor currently in production. Sets of replacement gears for both the low-speed and high-speed options are available separately as well.
Operating Point
Speed
Torque
Current
Free (no load)
100 rpm (160 rpm)*
0 in∗lbs
0.15 A
Maximum Efficiency
75 rpm (120 rpm)*
3.7 in∗lbs (2.25 in∗lbs)*
1.4 A
Maximum Power
50 rpm (80 rpm)*
6.7 in∗lbs (3.5 in∗lbs)*
2.5 A
Maximum Torque
40 rpm (64 rpm)*
9.0 in∗lbs (4.3 in∗lbs)*
3.0 A (fuse trips eventually)
Stall
0 rpm
13.5 in∗lbs (8.4 in∗lbs)*
4.8 A (fuse trips quickly)
* Indicates specifications when using the "high speed" internal gearing option
All motor specifications are at 7.2 volts. Actual motor specifications can fall within 20% of the values above.

Teams Contributed to this Article:

  • BLRS (Purdue SIGBots)