Brushless Electric Motor

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Outrunner brushless motor

Brushless electric motors are the most common type of electric motor used in present-day UAVs. While brushed motors use brushes to mechanically switch the phase of the windings in order to keep the motor running, brushless motors require the use of an Electronic Speed Controller (ESC) board to do this task. However, they provide various advantages over traditional brushed motors including better thrust-to-weight ratios, higher efficiencies, increased reliability, higher torque per weight, and lower noise. More recently, brushless motors are also being used for camera gimbals to provide inexpensive and effective stabilization of camera footage.


Disassembled outrunner brushless motor showing magnets and windings.

Brushless motors can be classified into outrunner motors and inrunner motors. In the outrunner motor configuration, the windings are fixed in the center while the magnets spin around it. The inrunner motor provides the inverse configuration, with the spinning magnets in the center and the fixed windings surrounding it. Outrunner configurations provide higher torque at lower speeds, but are typically of larger diameter and more dificcult to mount to the airframe because of the rotating outer frame. Inrunners typically provide higher rotation speeds but lower torques. For these reasons, outrunner motors are typically used for airplanes and multirotors while inrunner motors are usually used in helicopters and ducted fans.

Almost all outrunner brushless motors are designed to be used as direct drive, meaning that the propeller is directly attached to the motor shaft without using a gearbox. Inrunner motors sometimes use gearboxes, and they are often integrated into the motor design.


Brushless motor performance characteristics

The typical behavior for a brushless motor at a given operating voltage is shown in the diagram to the right. If no load is placed on the motor it will run at its maximum speed (called the no-load speed), where the input current is a very small value (called the no-load current). As a torque load is applied, the speed decreases and the input current and power increase. If the torque load reaches a certain level (called the stall torque) the motor would slow down to a halt, increasing the input power and current to its maximum value. The motor delivers its maximum power when operating at half of its maximum speed, and operates at maximum efficiency when running at a speed that is close to its maximum speed.

Increasing the operating voltage (for example, by using a battery with more cells) has the effect of increasing the output power and torque, as well as the input current and power. However, brushless motors also have a maximum current limit. If this current is exceeded, overheating may occur leading to complete failure of the motor. It is therefore important to ensure that this value is not exceeded in operating conditions.

Brushless motor perfromance is typically represented using four numbers: voltage constant K_{v} expressed in RPM per Volts, no-load current I_{0} in Amperes and its corresponding voltage V_{{I0}} in Volts (if this voltage is not provided, assume 8 Volts), and internal resistance R_{m} in Ohms. The performance of the motor at any voltage can be calculated using these numbers. The input current to the motor I in Amps and the output torque \tau in Newton-meters at any motor speed \omega in revolutions per minute can be calculated using the following equations:

\tau =\tau _{s}-{\frac  {2\pi }{60}}(\nu +{\frac  {1}{R_{m}(K_{v}{\frac  {2\pi }{60}})^{2}}})\omega

I={\frac  {V}{R_{m}}}-{\frac  {1}{K_{v}R_{m}}}\omega

Where \nu is a motor friction constant, \tau _{s} is the stall torque, which can be calculated by:

\tau _{s}={\frac  {60V}{2\pi K_{v}R_{m}}}

\nu ={\frac  {60I_{0}}{2\pi K_{v}(I_{0}R_{m}-V_{{I0}})}}

The input power P_{{in}}, the output power P_{{out}} and the motor efficiency \eta can then be calculated simply as follows:


P_{{out}}=\tau \omega {\frac  {2\pi }{60}}

\eta ={\frac  {P_{{out}}}{P_{{in}}}}

The maximum efficiency for a brushless motor is usually around 75%, and happens at around 90% of the maximum speed of the motor. From these equations it can be observed that increasing the K_{v} of the motor has the effect of decreasing the torque the motor can produce. Since the no-load speed, or the maximum motor speed, is approximately given by K_{v}V, increasing K_{v} also has the effect of increasing the maximum speed of the motor.


Brushless motors are selected so that they operate at maximum efficiency at a given operating condition. For example, a brushless motor for a quadcopter UAV should be selected so it operates at maximum efficiency in hover conditions. Due to the complexity of the propeller-motor dynamics, there is no simple way of achieving this. Thus propeller-motor selection often relies on experience, or following seller recommendations. Brushless motor and propeller performance prediction tools can also be used to find good propeller-motor combinations by trial and error.

A general rule of thumb is that higher diameter and higher pitched propellers will require lower K_{V} motors because they provide more torque at lower speeds, while low diameter and low pitch propellers will need higher K_{V} motors. Other factors to take into account are the weight and size of the motor. Lower K_{V} motors typically have larger diameters, and higher powered motors are heavier. In addition, the motor should be selected so its maximum current rating is never exceeded during operation.


Brushless motors are easy to setup and use. They have three cables which have to be connected to the electronic speed controller in any order. If the motor spins in the wrong direction, any two of the cables can be switched to reverse the direction of the motor.

A propeller can be attached to a brushless motor in various different ways. If a motor has its main rotating shaft extending out from the motor, a propeller can be attached to this shaft using a 'propeller adapter' device, which must be bought separately. Another way of connecting a propeller to a motor is by using a 'prop saver', which is a component that attaches the propeller to the main shaft of the motor using rubber bands. If the propeller hits something, the propeller detaches from the motor quickly and the chance of being damaged is reduced. Outrunner motors usually provide an alternate way of connecting the propeller, which consist of attaching another shaft directly to the outer rotating frame of the outrunner motor using screws. This shaft has threads and is of larger diameter than the main shaft, and therefore the propeller can be directly secured to it using an included nut known as a prop nut. This attachable shaft is sometimes included with the motor, but often times it must be bought separately (search for the motor's accessories).

It is sometimes desirable to balance an outrunner brushless motor in order to minimize vibrations during flight. This is especially important when the UAV carries cameras, as vibrations can cause bad video quality. There are various ways of balancing a brushless motor, including using a propeller balancer and a smartphone with vibration measuring software.

External Resources

More Information on Brushless Motors

Brushless Motor Databases

Brushless Motor Analysis Tools