Understanding Brushless Motors & Their Evaluation Techniques

Posted by jack wyatt on May 23rd, 2023

In the course of assembling a drone, the selection of an apt motor emerges as one of the primary stages. To extract the peak performance, it becomes indispensable to assess an array of motors and cherry-pick the most productive one for your blueprint. The market is inundated with many motors, including multiple electric motors specifically crafted for drone assembly.

Electric Motors and Their Varieties

Electric motors are divided into four groups: Universal, Alternating current (AC), Brushed Direct current (DC), and Brushless Direct current (DC) motors. We discuss only two types here: Universal and AC motors.

Universal motors are arguably the least preferred for drone usage owing to their low efficiency, substandard speed regulation, and optimal performance at exceptionally high RPM, which is generally unsuitable for drones that demand precise speed control for steady flight. Their primary application is in industrial tools and household appliances such as drills and vacuum cleaners.

The Alternating current (AC) motor operates by inducing its rotor to spin using an alternating current. Hence, it is commonly used when connected to a wall outlet. An AC motor would necessitate a transformer to operate on batteries.

How Do Brushless Motors Operate?

Before delving into the mechanics of the brushless system, let's familiarise ourselves with its components.

Elements of a Brushless Motor:

Electromagnets and Permanent Magnets:

A conductive wire coiled around a metallic base won't behave like a magnet, but when a current is passed through the wire, it prompts it to act as a magnet. This phenomenon is commonly known as an electromagnet. If a negative current flows through the same wire, the magnet now exhibits the opposite effect; it attracts another magnet instead of repelling it.

Rotor and Stator:

The parts visible on the inner circle are the electromagnets, while we have the permanent magnets on the outer circle. The motor's activation requires the delivery of an electric current to one of the electromagnets' coils. This action initiates the rotor's spin as the permanent magnet experiences repulsion from the like-electromagnet and strives to align with an opposite permanent magnet on the stator.

The ESC's Role:

An ESC or Electronic Speed Controller governs the electric motor by sending signals interpreted as speed changes or rotation reversals. It employs a direct current paired with a switch system to attain an alternate three-phase current. This output current can be altered by changing the switches' opening and closing rates in the circuit.

Inrunner or Outrunner - What's the Difference?

In an inrunner motor, the rotor is positioned inside the stator. These motors are typically characterised by high RPM and lower torque, making them ideal for applications that require higher speed but less force, such as drone propellers.

On the other hand, an outrunner motor has a rotor on the outside and a stator on the inside. This setup allows for higher torque at lower RPMs, making outrunner motors suitable for applications requiring more force, like larger, heavier drones or drones designed for lifting heavy payloads.

Assessing Performance and Efficiency

After comprehending the basic functioning of brushless motors, let’s assess their performance and efficiency.

Power: Power is the rate at which work is done or energy is transferred. In a brushless motor, power can be calculated by multiplying the torque (force) the motor produces by its speed (RPM).

Efficiency: Efficiency measures how much of the electrical power input to the motor is converted into mechanical power (useful work). It is calculated by dividing the output power by the input power and is usually expressed as a percentage. Higher efficiency means the motor can convert more electrical energy into useful work, resulting in less energy wasted as heat.

Motor selection

In electing an apt brushless engine, it's important to meticulously assess your stipulations and identify a congruous motor that fulfills your unmanned aircraft's demands. Principal factors to ponder encompass the drone's mass, the envisaged application of the drone (such as speed competition, aerial cinematography, and the like), and the dimensions and variety of propellers to be employed.

A more weightless motor might be consummate for a drone purposed for racing, wherein velocity and nimbleness are quintessential. Conversely, an engine boasting a higher torque capability might be imperative for a drone employed in photography, which may be entrusted with hauling hefty photographic apparatus. The dimensions and category of propellers slated for utilization also have a bearing on engine choice, as these elements will bear the burden sustained by the motor and its operational revolutions per minute.

Conclusion

To sum it up, understanding the mechanics, efficiency, and performance of brushless motors is fundamental to drone assembly. With this knowledge, you can decide the best motor for your drone, ensuring optimal performance and efficiency.