How to choose drone motors? KV and torque

Summary

This video explains how to select the right motor for a 7-inch drone, emphasizing two key factors: motor torque and KV. The video highlights the importance of motor size, particularly stator volume, in determining torque and performance, with larger motors offering faster propeller response. It also discusses KV, the number of revolutions per minute (RPM) per volt, and how it varies with propeller size and battery voltage. The ideal motor size for 7-inch drones is around 2207-2580, with the right torque and KV ensuring smooth, efficient flight performance.

Takeaways

• 😀 The correct motor for a 7-inch drone should balance both KV and motor size for optimal performance.
• 😀 Torque is a critical factor in motor selection, as it determines the motor's ability to change the propeller's speed and ensures smooth flight.
• 😀 A larger motor with more torque helps to reduce bobbing and oscillations, particularly on the pitch axis of the drone.
• 😀 The size of the stator directly impacts the torque; the bigger the stator's volume, the more torque the motor can produce.
• 😀 For 7-inch drones, you need a motor with a stator volume of at least 900 mm³ for proper performance.
• 😀 The most common mistake is choosing motors with too low a stator volume for a 7-inch drone, leading to performance issues like bobbing.
• 😀 Torque is calculated as force times the radius, and the stator's volume can be used to estimate the torque provided by the motor.
• 😀 Larger motors, like 2407 and 2508, provide more torque and are better suited for 7-inch drones than smaller motors like 2205 or 2206.
• 😀 KV is a measure of rotations per volt, and the required KV for your drone depends on its propeller size and the number of cells in the battery.
• 😀 For a 7-inch drone on 4S, a KV of around 1,600 is ideal for smooth operation, with adjustments needed based on the voltage and size of the motor.

Q & A

Why is choosing the correct motor important for a 7-inch drone?

Choosing the correct motor for a 7-inch drone is crucial because many people tend to focus only on the KV factor, overlooking the importance of motor size. A larger motor with more torque is needed to properly handle the larger propellers of a 7-inch drone, ensuring smooth flight and avoiding issues like bobbing during flight.

What is torque and why is it important when selecting a motor?

Torque is the force applied at a distance from the center of rotation, which helps the motor overcome the moment of inertia of the propeller. The higher the torque, the faster the motor can change the propeller's rotation speed, providing quicker responses and more stable flight. Low torque can result in unstable flight, particularly in larger drones.

How does the size of a motor affect its performance?

A bigger motor, with a larger stator diameter and height, can generate more torque. This increased torque allows the motor to overcome the propeller's inertia more quickly, leading to a faster and smoother response in flight. Larger motors are more suited to handling larger propellers like those used in 7-inch drones.

What happens if the torque of a motor is too low for the propeller size?

If the motor's torque is too low for the propeller, the drone may experience instability, such as bobbing or oscillating during flight. This occurs because the motor cannot respond quickly enough to changes in rotation speed, leading to inconsistent control and a less smooth flight experience.

What role does the stator volume play in motor selection?

The stator volume is key to determining the motor's torque capacity. A larger stator allows for more copper windings, which increases the motor's magnetic flux, thereby enhancing its torque. A motor with a larger stator volume can more effectively accelerate and decelerate the propeller, providing better control.

How can you calculate the stator volume of a motor?

To calculate the stator volume of a motor, you can use the dimensions provided in the motor's size code. For example, a motor labeled 2205 means the stator has a diameter of 22mm and a height of 5mm. The stator volume can be computed by multiplying the radius squared (half the diameter) by the height and then multiplying by π.

What is the minimum stator volume required for a 7-inch drone motor?

For a 7-inch drone, the minimum stator volume should be around 900 mm³. This ensures that the motor has enough torque to handle the larger propellers. Motors with smaller stator volumes, such as 2205 or 2306, are generally too small for efficient 7-inch drone performance.

Can increasing voltage solve issues with low torque in a motor?

Increasing voltage can slightly improve the torque and performance of the motor, but it does not fully resolve the need for a larger motor size. Even with higher voltage, the motor still needs to be sufficiently large to provide the required torque for handling the larger propellers, especially in a 7-inch drone.

What KV rating is ideal for a 7-inch drone motor?

For a 7-inch drone, the ideal KV rating is typically around 1,600 KV when using 6S batteries. This provides the right balance of power and efficiency to handle larger propellers without overloading the motor or compromising flight stability.

How do propeller size and number of cells in a battery affect motor KV selection?

The size of the propeller and the number of cells in the battery (such as 4S or 6S) influence the motor's KV rating. Larger propellers require motors with lower KV, as they rotate slower but generate more thrust. Similarly, higher cell counts in the battery require lower KV ratings to prevent the motor from spinning too fast, ensuring stable flight.