New coaches may feel a bit at sea with VEX’s different motor options. First things first, if you have a competition robot, you only want a 2-wire motor 393 (or 3-wire servo, but I’m just discussing motors here). If you see some websites mentioning a 3-wire motor, that’s an old, discontinued product that you don’t want.
On the VEX motors product page, you’ll see that they list the 2-Wire Motor 393 ($15), then toward the bottom of the section is another product called a Motor Controller 29 ($10). Some resellers offer a package bundle of a motor and a controller together; on the VEX website, they are listed separately. If you’re buying new motors for your team, be sure to purchase (or have on hand) one motor controller for each motor to be used on the robot.
So what is the motor controller thing and when do you need one? On the VEX cortex, there are 10 slots in which to plug motors. Slots 1 and 10 have the motor controller built into the cortex; slots 2-9 do not, and require external motor controllers (see image below).
So what does this motor controller do, anyway? Well, the motor controller makes use of what’s called PWM to control the speed of the motor. PWM stands for Pulse Width Modulation, and it turns the circuit (battery-to-cortex-to-motor) on and off in rapid succession. It’s a very fast version of what your microwave does on 50% power. The microwave is not actually outputting half of the power of “High”, but rather it’s cycling its one fixed level of power on and off about half the time. Motor controllers do the same thing to control the speed of a VEX motor; setting a VEX motor to power 64 (half of the maximum 127), means that the controller is sending power to the motor about half the time, but it happens in such rapid modulation that there’s nothing noticeable going on to the outside observer.
VEX motors have a plug on the end with 2 wires: red and black. The motor controller has one end with a 2-wire plug that connects to the motor, and the other end has 3 pins that plug into the cortex (red, black, and white). (See my 2018 post for important information about 2-wire and 3-wire extensions and where the motor controller needs to be wired in relation to the cortex.)
For slots 1 and 10, that whole 3-to-2 goes on inside the cortex. Annoyingly, the motor controllers built into the cortex function better than the external controllers; if you want the details, please read this VEX forum post. or my post “Motor Ports 1 & 10 — Way Different!” The upshot is that it’s unwise to have multiple motors on your robot that must work in unison plugged into a combination of internal and external controllers.
More on motors and wiring in future posts.