What Are Brushless Motors?

Last updated July 20th, 2020
A motor up close

Brushless motors are one of many electrical staples that are found in higher-level drones.

But what are they? How do they work? And why are they so important to these devices that have taken over our skies?

What Are Brushless Motors?

Brushless motors are motors that are powered by direct current through a switching power supply or inverter which transforms the direct current input into alternating current (AC) in order to power the workings of the motor.

The inverter powers each phase of the motor through a closed loop controller.

The controller supplies pulses of current to the motor windings that control the torque and the speed of the brushless motor.

Brushless motors were invented much later than brushed motors (invented in the 19th century), and they operate differently.

The main advantages that brushless motors have over brushed motors include a higher power-to-weight ratio, higher speed generation, electronic control, and lower maintenance costs.

These advantages have made brushless motors the device of choice for many electronic applications like computer accessories like disk drives and printers, hand held power tools, automobiles, and also aircrafts like drones.

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Why Brushless Motors Are Used in Drones

Brushless motors are the device of choice for the overwhelming majority of consumer drones available today.

The reason for this is that brushless motors provide a better thrust-to-weight ratio than brushed motors. And for aircrafts that makes all the difference.

In order to fly and be aerodynamic enough to navigate the skies, an aircraft has to be as light as possible while being able to generate the power it needs to fly against gravity towards the direction that is required.

In drones, the propellers are connected to the motors which are responsible for making them spin. The propellers themselves spin in order to provide the necessary thrust or force needed to take your drone into the skies.

There are a bucket load of other technical factors that come into play to provide the flying experience that one expects from drones. But it all starts from the motor and its interactions with the propellers.

Within the brushless motors are several electromagnets (coils) that are linked in particular pairs.

The motor controller — popularly called the electronic speed controller — controls the activity of the motor by activating and deactivating specific sections of the electromagnets found in the brushless motor.

This repeated activation and deactivation is done at very specific intervals to cause the rotor to spin under the influence of the magnetic force generated.

Because these electromagnets are connected to three main sections, brushless motors always come with three wires sticking out of them.

Components of Brushless Motors

There are two main components or parts that make up a brushless motor. These are the stator and the rotor.

The stator is the stationary part of the motor. It does not rotate and contains the electromagnets.

The rotor is the part that rotates and contains magnets placed in a radial pattern.

There are other parts of the motor that are found around these two structures, like coils, shafts, magnets, and so on. But the rotor and stator are the components of the brushless motor.

In-runner and Out-runner Motors

Brushless motors come in two categories: in-runner and out-runner brushless motors.

Multirotor drones and fixed wing aircrafts run only on out-runner motors; but they are not the only category that exists. In-runner motors are also a thing and are used elsewhere.

The difference between in-runners and out-runners is that for out-runners the rotor lies on the outer area of the motor. In-runners, on the other hand, have their rotating parts on the inside while the outer parts remain stationary.

Because they spin faster, you will normally see in-runner motors used in remote controlled cars.

However, out-runner brushless motors produce more torque or twisting power, which means that they are ideal for driving larger propellers. This makes them better for aircrafts and multirotor drones.

Dimension of a Brushless Motor

The dimension or size of a brushless motor depends on the size of its stator.

When shopping for a motor for your drone, you may come across devices that are sized in four digits like 2207 or 2306.

What these figures mean is that the stator is 22mm in width and 07mm in height or 23mm in width and 06mm in height respectively.

The first two digits indicate the diameter or width of the stator — which is usually approximated to be the diameter of the motor.

The second pair of numbers denotes the height of the stator, which is also taken as the height of the entire motor.

Why is it important to know the dimension of a brushless motor?

The size of the motor is a good indicator of the size of the drone it is meant for.

Larger quadcopter drones, for example, will use motors of the size 2212 which can be considered large. Smaller drones like racing mini-quadcopters tend to use brushless motors that are sized 1806 or 2204.

Performance Factors for Brushless Motors

There are few factors that determine how well a brushless motor does its job.

kv: The Velocity Constant

Kv, or the velocity constant, denotes the speed at which the brushless motor rotates for each volt input it receives.

To illustrate, imagine a motor that is designated 2300kv that is connected with a 3S drone battery.

This motor will spin at a rate of (2600 x 12.60) rpm — 32,760 revolutions per minute. This rpm is recorded when the motor is not connected to propellers.

When propellers are connected, the rpm reduces slowly due to the effect of air resistance.

Higher kv values usually mean lower resistance, lower efficiencies, and higher current draw.

Lower kv values mean higher resistance, lower current draws, and better efficiency.

But don’t let the phrases “better efficiency” and “lower efficiency” fool you.

When choosing a motor for your drone, the ideal rpm for you depends on the type of drone you have or are looking for.

For example, if you have eyes for a high performance, acrobatic-type drone, you will do well to go for a motor with a high kv value.

This makes sense, because drones like these are the smaller varieties that use smaller motors and, thus, smaller propellers.

Smaller propellers produce less thrust, so the motors have to work harder to produce the necessary revolutions per minute to lift the drone.

This need for higher rpm means that these motors use up more power and thus are less efficient. However, they get the job done.

On the other hand, if the drone you desire is one which needs to use the least amount of power as possible, in order to last longer in the air and be more stable, you would want to go for motors with lower rpm.

For example, if you desire or own a drone you intend to use for aerial photography, then this would be a larger drone that is capable of carrying a camera payload.

They require larger propellers for greater thrust during rotation. They usually require fewer revs per minute in order to fly.

Torque

Torque is simply defined as the spinning force that rotates the propellers. The torque of a brushless motor is affected by factors like:

  • Stator size: The bigger the stator is, the greater the torque produced.
  • Quality of materials like the quality of the copper windings in the rotor and the quality of the magnets.
  • Construction constraints like the presence of air gaps between the rotor and the stator

The performance of your drone relies heavily on the torque produced by the motor underneath. The torque also affects how responsive a drone is to commands from the pilot. The higher the torque, the more responsive the drone will be.

This relation between responsiveness and torque is important for some pilots like drone racing pilots.

For example, torque will influence how quickly a drone responds to a change in direction in mid-flight around a tight racecourse.

Also, there is the fact that if your drone motor produces less torque and comes with heavier props, it won’t spin the propeller as efficiently and this will affect thrust. Current draw will also be very high.

On the other hand, motors with high torques have a down side in that they feel more snappy and lead to more control errors or oscillations. This is not hard to imagine, given that drones with high torque motors can change their rpm much faster and may over respond to input, making them harder to control.

Efficiency

To get the efficiency of a brushless motor, you divide the thrust force produced by the motor while at 100% by the power produced by the motor.

The unit of measurement for efficiency is grams per watt (g/w).

The higher this ratio is, the more efficient the motor is, basically. So you want to pay attention to this is aspect of the motor when making a choice.

Keep in mind that you will not be flying your UAV at full speed all the time, so it is important to consider the efficiency of the brushless motor at various speed ranges from between 0% and 100%.

Some motors may be more efficient at one end of the speed spectrum than at the other. Some engines shine when speeds are low, while others come into their own when the devices is flown at high speed.

You want to, therefore, choose a motor that produces the highest efficiency for your style of flying.

Current Draw

The current draw of a brushless motor is important in determining the size of the electronic speed controllers required for that motor.

For example, a motor of size 1104 draws a maximum of 10A at 100% throttle, while some motors of size 2306 draw a maximum of 40A at 100% throttle.

Electronic speed controllers have to be selected in such a way that they are rated 20% more than the current drawn by the motor at 100% throttle.

So, if a motor draws 30A max at 100% throttle, it would require an electronic speed controller rated at 36A constant current.

Temperature

Temperature or heat is bad for motors over time. When exposed to heat for a long time, the magnets in the rotors tend to lose their strength progressively.

Eventually they become demagnetized due to the constant heat and this in turn results in a shortened lifespan of the motor.

There are a few reasons why a brushless motor might overheat, including 0ver propping and constant speeding over long periods of time.

For drone racers, flying at high speeds is expected. But if you are not a racer and you find your drone overheating, it is likely that your motor is over propped.

Motor manufacturers and drone manufacturers make up for this potential problem by installing cooling fins to help direct air into the motor to cool it and prolong its lifespan.

Brushed vs. Brushless Motors

When it comes to brushed motors and brushless motors in drones, there are a few differences in the way they are constructed and the advantages each possess.

Differences in Construction

A brushed motor is made with a rotating stator which acts as a two-poled electromagnet. The shell does not move, thus brushed motors can easily be mounted without the help of screws.

On the other hand, brushless motors use a permanent magnet and generate power by way of an inverter. There is no need for brushes, hence the name “brushless”. Because the motor bell moves, brushless motors require screws in order to be mounted.

Advantages of Brushed Motors

  • They are not expensive to manufacture
  • Can be made in the small sizes
  • Useful in extreme environments lacking in electronics
  • Replaceable brushes make for prolonged lifespan
  • Two wire control is simple
  • They do not require a speed controller since speed is fixed

Advantages of Brushless Motors

  • No brushes make for lower maintenance costs and longer durability
  • Better speed and higher torque due to the lack of brushes and the friction they produce as speeds increase
  • Wider speed range
  • Mode of construction make them better at dissipating heat than their brushed motors
  • More cost effective for high speed and high power operations

The advantages of brushed motors make them ideal for power tools, fan motors, computer peripherals, and small-sized motorized toys.

On the other hand, the advantages of brushless motors make them ideal for radio-controlled aircrafts like drones, electric cars, and also industrial machinery — stuff that require high rotation speed and power.

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