Motors (Brushed and Brushless)
How do electric motors work?
Before deciding which electrical motors you need, we should spend a moment to cover how they work.
- The purpose of most motors is to convert electrical energy (AC or DC) into kinetic energy (movement). This is accomplished by using the unique relationship between electricity and magnetism; often referred to as electromagnetism.
- All of the motors we will be discussing are DC or Direct Current. AC or Alternating current is what comes out of the outlet in your home. DC current is what comes out of a battery.
- There are 2 kinds of DC motors used in robot sports: Brushed and Brushless
How a DC motor turns: We won’t dive into the detailed physics here but the diagram below provides a simple picture of how a basic brushed DC electric motor uses direct current to create temporary magnetic fields. By turning these fields on and off very quickly, the motor is able to pull/rotate the inner shaft in one direction.
Quick Tip: You can reverse the direction of rotation of a brushed DC motor simply by reversing the positive and negative wire connections from the power supply to the motor + / – terminals. Some brushed DC motor speed controllers actually can do this for you. This is how they allow you to quickly reverse one or more wheels to go backwards or turn left/right.
Analysis: Drive Motor RPM’s vs Wheel Speed
Once you start experimenting with different types of motors, you may ask yourself, “How fast am I going with a 100 rpm vs a 1,000 rpm motor?” (RPM stands for “rotations per minute”) Speed is, of course, an important piece to consider when designing your bot.
Other than the overall size of your robot (1 lb vs 250 lbs!), there are a lot of factors that contribute to how fast your robot can actually travel in a competition. Below is a sample list of common things to keep in mind:
Things in your control
- Motor Type (Brushed vs Brushless)
- Motor power (this is a combination of voltage and amps)
- Motor gears or belts (helps transfer power from the motor to the wheels)
- Wheel diameter
- Wheel traction (How well does it grip the floor?)
- Battery type (voltage, LiPo/MiMh, amp output)
Things you can’t control
- Material used for the arena floor
- Driving skill and impact from other bots
- Other things you never thought of! (Trust me, this is a big and very real category; even for the pro’s.)
Example: The below graph gives you an example of how your motor RPM speed selection will impact your robot speed in the arena. To keep things simple, the gear ratio is 1:1 and the wheel diameter is 60 mm (2.4 inches). Only the motor RPM rating is changing.
In this example, you can see that at 100 rpm, your bot will take about 7.8 seconds to travel 8 ft. This is very, very slow and will make it very difficult to catch your competition. However, at 1,000 rpm, your bot will take about 0.8 seconds to travel 8 ft. That is an excellent speed for competition.
What rpm motors should I use? As with most aspects of the sport, this will come down to personal preference. Based on the data below and feedback from veteran builders, you should shoot for something that gets your across an 8 ft arena in 1-2 seconds or less. (According to the graph, this would be a range of 400 rpm minimum to 1,000 rpm; or higher.)
Key Lesson: Notice the “curve” of the graph above? This change is known as “exponential – A graph of such a rate would appear not as a straight line, but as a curve that continually becomes steeper or shallower”. In practical terms, this means the amount of actual speed gained (time to travel 8 ft) gets smaller and smaller as your increase your motor rpm.
Helpful Links and Additional Resources
Understanding DC (Direct Current) Motor Characteristics: A good fundamental walk through of how DC motors work and how to properly measure Torque, Speed, and Power.
Brushed or Brushless Motors? – Great article explaining the main differences between brushed and brushless motors.
Brushless Motor Shaft Replacement – Detailed and clear instructional video on how to safely remove the shaft on a brushless motor and replace it with a new one.