Brushless motors have almost completely replaced brushed motors. Their superior power and efficiency make them the obvious choice for powering your RC equipment. Here’s what you need to know to use them, and some helpful info on how they work.
Brushless Motor Benefits
Before going into how brushless motors work, here’s why they’re useful:
- More Efficient – Brushless motors are much more efficient than conventional brushed motors. This efficiency has been measured to be between 85% to 95% better than brushed motors.
- Less electrical energy is wasted as heat,and more is used to do useful work.
- Reduced Noise – Brushless motors have fewer mechanical parts than brushed motors, so they emit less sound.
- Longer Lifetime – Fewer moving parts are in mechanical contact than in brushed motors, reducing wear.
- Reduced EM Interference – Brushless motors emit less energy as electromagnetic (EM) waves than brushed motors do. This contributes to their efficiency, and helps reduce radio interference.
- Torque, Voltage, And RPM Linearly Related – This means that the amount of torque or RPM produced by the motor divided by the voltage put in is a constant, making it easy to predict how much power you’re going to get.
How Brushless Motors Work
On a fundamental level, an electric motor’s only job is to convert electrical energy (like that provided by a battery) into mechanical energy, like the turning of a propeller or rotor blade. There are two basic facts that allow electric motors to work:
- Electric and Magnetic Fields are Related - That is, every moving charge produces a magnetic field, and magnetic fields can produce electric charge.
- Magnets Interact – Magnets will align when placed near to each other. All electric motors basically consist of two magnets. One of them is permanent, the other is a coil of wire that, when charged, becomes a magnet.
The motor is designed such that the magnetic fields produced by each of the magnets are always out of alignment, causing the motor axil to rotate. This is similar to what happens when you hold a permanent magnet to a compass – the compass swings position so that it lines up with the magnets field.
With the brushed motor design, the magnetic fields are kept out of alignment by turning on the different coils of wire that surround the motor axil in succession. Metal brushes make mechanical contact with the rotating axil and the contacts with each metal coil. As the axil rotates, the brushes contact different coils. The end result is that current flows through different coils at different times, constantly changing the magnetic field and rotating the motor shaft.
It’s here that we see the main problem with the brushed design: the contact between the motor coils and the brushes causes friction, which increases with speed. The metal coils wear out over time, and are prone to sparking. They can also ionize surrounding air, creating ozone. So how can we get around these issues? The answer lies in the brushless motor design. Instead of using mechanical brushes to turn on the various wire coils, an ESC (electronic speed controller) is used instead. The ESC switches the motor coils on or off rapidly, and is synchronized to the motor axil position.
Always look for an ESC with a capacity (measured in amps) greater than that of the motor you’re pairing it with.
Some Common Terms Explained
There are a number of special terms associated with brushless motors. Here are explanations for some of the most common:
- RPM – This is a measure of angular speed, or how fast something is rotating. A motor’s RPM is simply how fast it can rotate.
- KV Rating - Remember how we said that the relationship between voltage, torque, and RPM was linear for a brushless motor? It turns out that the number of RPM provided by each volt is the same, called the KV number. The KV number’s useful because it let’s you figure out how many volts you need to achieve a certain RPM, or vice versa. For an example, a 980 KV motor powered by an 11.1 volt battery would spin at 980 x 11.1 = 10878 RPM with no load. The KV rating always assumes no load on the motor, so the actual RPM that your achieve will be less than the one you calculate.
- Continuous / Burst Current – Continuous current measures how much current a motor can handle continuously, for an extended period of time. Burst current measures how much current a motor can handle for a short amount of time, about a few seconds.
- Current Rating – This is the maximum current that a given motor can handle, measured in amps.
- Inrunner / Outrunner – These are the two major brushless motor designs. An inrunner brushless motor has stationary coils, and a rotating permanent magnet inside the coils on the motor shaft. An outrunner brushless motor is the opposite, it has a rotating permanent magnet, placed outside the stationary coils on the motor shaft . Outrunner motors have lower KV ratings, so they run at a lower speed with more torque. This could allow you to direct drive larger props without a gearbox. RC cars and boats tend to require inrunner brushless motors, rather than outrunners.
- Torque - Torque is a measure of angular force, or how much “push” a rotating shaft has. Watt – This is a measure of power, or how fast energy is used.
- Volt – This measures electric potential, or how much “push” the electrons from a battery have. A greater voltage means that more energy is being applied to a given amount of charge.
Choosing a Brushless Motor
Most airplane manufacturers will recommend certain brushless motors for different models. However, if this is not specified, a good starting point would be to check what other people are using locally,or search the web. We frequently visit RCGroups, RC Universe, and WattFlyer to see what the RC communities are using. If you have a brushed motor that you are replacing, choose a brushless motor that is the same physical size, and uses about the same wattage. To determine the wattage, multiply the current your old motor draws by the voltage it’s run at.