A motor is an electromechanical device that converts electrical energy to mechanical energy.
An electric DC motor spins very fast, often in the range of 4000-6000 RPM and has very little torque. This isn't very useful for a robot. As an example, if the motor output speed were 4000 RPM and it was turning a 10" tire, the robot would go 119 mph. In reality though, the motor would likely not have enough torque to turn the wheel.
Luckily, we can use a gearbox to slow the motor down and generate more torque. The more the motor is geared down, the more torque it will have. There are many different types of motors out there but, for the most part, we use brushed DC gear motors. They come with a gearbox already attached so that it has enough torque and runs at a more appropriate speed for a robot. We categorize the motors first by their size and then we specify the further by their speed.
Your car uses a transmission and starts in 1st gear (most reduced) so you have the low-end torque to start moving. Then, as you speed up, you change gears to get more speed. Most robots, however, cannot change gears. The maximum speed and torque are fixed. You have to find the right balance so you have enough torque to get started from zero speed, but then enough top-end speed. If you gear it down too much you will have plenty of power, but never get any speed. If it's not geared enough it will probably be plenty fast but it will have trouble accelerating, driving up hills, and turning. You need to find a balance where it works well for both. The big electric motors will help because they will muscle through it if you don't have it geared just right, but if it works too hard you will be drawing a ton of current and burning up your batteries!
- Servos - Very small robots < 5 lbs
- Micro Metal Gear Motors - Very small robots < 5 lbs
- 32mm Gear Motors - Small to medium robots 5-35 lbs
- 42mm Gear Motors - Small to medium-large robots 5-50 lbs
- 52mm Gear Motors - Medium to large robots 25-150 lbs
- 90mm Gear Motors - Large robots up to 250 lbs
- Wheelchair Motors - Large robots up to 250 lbs