Decoding Brushless DC Motors

Another lesson on Mechatronics; the subject is quadcopters, and the component is Brushless DC Motor. While the first lesson on the topic explained how DC motors work, this lesson aims to understand the terminology commonly used to describe the characteristics and efficiency of the motors. Please note that both brushed and brushless DC motors can be used in quadcopters, but we will focus on brushless DC motors as they are more efficient and the preferred choice for almost all high-end quadcopters.

Understanding the terminology is the first step towards comprehending and improving the performance of a quadcopter or any multicopter containing brushless DC motors.

Almost all Brushless DC motors have stickers on them that provide some basic but important information. At a minimum, this information includes the width and height of the stator, as well as the KV rating. These parameters play an important role in the performance. You can see four numbers on the stickers on the first line. The first two numbers represent the width of the stator, given in mm. In this particular case, the width of the stator is 22mm. The remaining two numbers, 12 in this case, describe the height of the stator, also in mm. This means the height of the stator is 12mm. The third parameter after the slash is 13T, which describes the number of copper wire windings on each arm of the stator.

"KV" rating refers to the motor's velocity constant and it represents the RPM. RPM stands for Revolutions Per Minute. In simple terms, KV describes the number of revolutions per minute that a motor turns when one volt is applied with no load attached. This is the motor's velocity constant and should not be confused with "kilovoltage," which is abbreviated with a lowercase "k" and uppercase "V."

A motor with 1000 KV will rotate 1000 times in a minute if 1 volt is applied, provided there is no load attached. If 2 volts are applied, the same motor will rotate 2000 times in a minute with no load. A low KV motor has more winds of thinner wire on the armature — it can carry more volts at fewer amps, can produce higher torque, and can swing a bigger propeller. A high KV motor, on the other hand, has fewer winds of thicker wire that carry more amps at fewer volts and spin a smaller prop at high RPM.

Here is another picture of a different motor. Notice that the width and height of the stator are exactly the same as the previous motor, but the KV value is 2200. This motor will spin 2200 times per minute per volt.

As you may have already guessed, it is typically measured in units of revolutions per minute per volt, abbreviated as RPM/V. The KV of a BLDC motor is determined by the number of windings in the stator and the number of magnets in the rotor.

It's important to note that the choice of KV rating depends on various factors, including the drone's size, weight, propeller size, and intended use.

Low KV motors are generally used in larger drones or applications where high levels of torque are required. Here are some situations when you might consider using a low KV motor for a drone:

1. Heavy-lift applications: Drones designed to carry heavy payloads, such as professional cinematography drones with high-quality cameras or agricultural drones carrying specialized sensors, require a lot of torque to lift and maneuver effectively. Low KV motors provide the necessary torque for such tasks.

3. Stability and smoothness: Low KV motors with large propellers tend to provide smoother and more stable flights. This is desirable in applications where steady and controlled movements are crucial, like aerial surveying or mapping.

4. Reduced noise: Low KV motors typically produce less noise compared to high KV motors when spinning at the same RPM. This can be advantageous in scenarios where noise reduction is essential, such as wildlife monitoring or aerial surveillance.

High KV motors are typically used in smaller drones, especially those designed for racing, freestyle, or acrobatic flying. Here are some situations when you might consider using a high KV motor for a drone:

1. Agility and responsiveness: High KV motors spin at higher RPMs, resulting in fast response times and rapid acceleration. This makes them well-suited for racing drones, where quick changes in direction and agility are essential to navigate through tight courses and obstacles.

2. Aerobatics and freestyle maneuvers: Freestyle drones require the ability to perform complex tricks, flips, and rolls. High KV motors allow for quick motor response, enabling the drone to execute these maneuvers effectively.

3. Smaller drone sizes: In smaller drones, propellers are usually smaller too. High KV motors are necessary to achieve sufficient thrust with these smaller propellers, which helps keep the overall size and weight of the drone down.

4. Higher speed: If you want your drone to achieve high speeds, high KV motors are a suitable choice. They allow the drone to reach faster top speeds compared to low KV motors.

5. Racing competitions: In drone racing competitions, pilots often prefer high KV motors because of their responsiveness and ability to provide quick acceleration, allowing them to gain a competitive edge on the race track.

It's essential to understand that the choice of KV rating should be balanced with other factors, such as the drone's frame size, propeller size, and battery capacity. High KV motors may draw more current from the battery, leading to increased power consumption and potentially shorter flight times compared to low KV motors.

Motor KV ratings below 1000 KV are considered low KV for most multirotor drones.

Motor KV ratings between 1000 KV and 1800 KV can be considered as medium KV motors. These motors are versatile and can be used in a broader range of drone sizes and applications, striking a balance between torque and speed.

Motor KV ratings above 1800 KV are considered high KV motors. As mentioned earlier, these motors are commonly used in smaller racing drones, freestyle drones, and other applications where agility, responsiveness, and high RPMs are prioritized over torque.

Another important term, in the context of Brushless DC motors used, is "Max Efficiency Current," which refers to the level of current at which the motor operates with maximum efficiency. Efficiency in this context means the motor's ability to convert electrical power into mechanical power with minimal losses.

When a BLDC motor operates at its maximum efficiency current, it tends to draw the least amount of electrical current for a given mechanical output. This is the optimal operating point where the motor performs most effectively, resulting in the best balance between power consumption and mechanical output. Operating a motor close to its maximum efficiency point can lead to longer flight times and better overall performance for the quadcopter.

Manufacturers usually provide performance data and efficiency curves for their motors, indicating the optimal current range for efficient operation. Consulting these specifications and conducting tests will help determine the most efficient operating point for a specific quadcopter setup.