FPV Drone Motors: How to Choose the Right Size, Shape, and KV

Unlock the secrets of mini quad FPV Drone motors! We’ll explore key factors to consider when selecting the optimal motor for your desired performance, whether you’re a racing enthusiast, freestyle fanatic, or aerial photographer.

How Motors Generate Torque

Brushless motors generate torque through the interaction of permanent magnets on the rotor and electromagnets on the stator. Stronger magnetic fields, larger contact areas, and larger motor diameters all contribute to higher torque output.

More descriptive:

The magic behind brushless motor torque lies in the interplay between the rotor’s permanent magnets and the stator’s electromagnets. The strength of the magnetic field (flux density), the surface area where these fields interact, and the motor’s diameter all play crucial roles. A stronger magnetic field, a larger contact area, and a wider diameter all translate to a greater force acting on the motor shaft, resulting in increased torque.


More technical:

Torque generation in brushless motors is governed by the following key factors:

  • Magnetic flux density: A stronger magnetic field between the rotor and stator translates to a greater force.
  • Stator volume: This directly affects the contact area between the magnetic fields, influencing the resulting force.
  • Motor radius: The distance from the center of the shaft to the outer edge of the motor plays a role in amplifying the force into torque.

These options offer different levels of detail and formality while conveying the same core information about the factors affecting torque in brushless motors. Choose the one that best suits the overall tone and audience of your blog post.

Stator Volume is King for Torqu

This section effectively explains the relationship between stator volume and torque in brushless motors. Here are some minor suggestions for improvement:

  1. Clarify the area calculation: Briefly mention that “2π” is the constant representing the circumference of a circle. This can help readers unfamiliar with the formula understand its context.
  2. Emphasize trade-offs: While larger stator volume generally leads to higher torque, it’s worth mentioning that it can also increase weight and size, which might not be desirable in all applications.

Here’s a revised version incorporating these suggestions:

Now, let’s delve deeper into stator volume.

The area of the stator interacting with the rotor is roughly equivalent to the cylindrical surface area around the sides, excluding the end caps. This area can be calculated using the formula:

Area = 2πrh


  • r is the motor radius
  • h is the motor height

Substituting this into the torque equation, we get:

Torque (Magnetic Flux Density)² x (Stator Volume) / (Gap Permeability)

For a fixed magnetic flux density, the torque generated by a brushless motor is directly proportional to its stator volume!

This implies that a taller motor with the same radius will have a larger volume and consequently, generate higher torque. Therefore, maximizing stator volume is crucial for maximizing torque in brushless motors.

However, it’s important to remember that increasing stator volume also leads to a larger and heavier motor. This trade-off needs to be carefully considered when selecting a motor for specific applications where weight and size constraints might be crucial.

How Motor Shape Affects Torque and Responsiveness

Does wider always mean stronger? Not when it comes to pancake motors.

While it might seem intuitive that a wider motor would generate more torque, the key factor is actually the total volume of the stator, not its specific dimensions. As long as two motors have the same volume, their torque output will be comparable, regardless of their width or height.

In fact, narrower motors can often offer advantages in terms of both torque and responsiveness. This is because they have a lower moment of inertia, which is a measure of an object’s resistance to changes in its rotational state. The moment of inertia is influenced by the mass and distribution of mass within the object, and for motors, it’s heavily influenced by the radius squared (r^2).

Therefore, a narrower motor with the same volume as a wider one will have a lower moment of inertia. This translates to better responsiveness, making it easier for the motor to quickly accelerate and decelerate, which is crucial for agile flight maneuvers in freestyle applications.

However, there’s a trade-off to consider: cooling. While narrower motors excel in responsiveness, their taller design can sometimes lead to heating issues, especially in demanding freestyle setups. To mitigate this, maintaining a width-to-height ratio above 3:1 is generally recommended.

By understanding the interplay between stator volume, moment of inertia, and cooling, you can make informed decisions when selecting a motor that best suits your specific needs and flying style.

My rule of thumb:

  • Freestyle: Optimal width-to-height ratio around 3:1
  • Race/Cinewhoops: Up to 4:1 for better cooling under load
  • Avoid going over 6:1, as responsiveness suffers greatly.

Prioritize torque through stator volume, not width. But don’t go too narrow to compromise cooling.

How to Choose the Right Size Motor for Your Build

Here are a few ways to rephrase the sentence “Don’t just go by diameter – consider the whole prop design” while maintaining the same meaning:

  1. More concise:
  • Look beyond diameter – the entire prop design matters.
  1. More informative:
  • Prop diameter is just one piece of the puzzle. Evaluate the entire prop design for optimal performance.
  1. More specific:
  • Don’t be fooled by size alone! Consider factors like pitch, weight, and blade profile when selecting a prop that complements your chosen motor.

Understanding Motor KV and How it Affects Performance

KV is a measure of a motor’s RPM per volt applied.

Higher KV generally equals higher RPMs, all else being equal.

But KV also affects:

  • Torque vs RPM relationship– High KV maintains torque better at higher RPMs
  • Current draw and efficiency– High KV requires more current for a given torque, hurting efficiency

Let’s look at both of these in more detail:

Torque vs. RPM Relationship

Higher KV motors maintain their torque output better as RPM increases. As seen in this chart:

So, high KV motors have an advantage in peak power and speed. But…


Current Draw and Efficiency

High KV motors require significantly more current to produce the same torque as low KV ones.

The increased current causes more electrical losses due to:

  • Battery internal resistance
  • ESC resistance
  • Motor winding resistance

Therefore, high KV motors have lower efficiency (thrust per watt) than low KV ones.

This also limits the max torque – at very high current draws, battery voltage sag reduces power to the motors.

So, KV affects efficiency and maximum torque due to current draw issues.

Choosing KV – Two Methods

How do we choose the optimal KV for a build? Here are two approaches:

For freestyle/technical tracks

  • Look at static thrust data for your prop
  • Find current draw at max throttle
  • Choose KV where your battery can supply the needed current

For top speed/open tracks:

  • Test sustained full throttle current draw with different KVs
  • Increase KV until diminishing returns on current draw due to voltage sag
  • Make sure your battery can supply the needed current

Also consider:

  • Lower KV is better for larger props and heavy quads
  • Higher KV is better for lighter, high-speed quads

Match KV to your build and props for the power system to work optimally together.


Achieving good throttle feel and resolution

The final piece of the motor puzzle is tuning for a good throttle feel and resolution.

Symptoms of poor resolution:

  • Difficult to maintain a steady hover
  • Hard to make precise small throttle adjustment

If you’ve optimized components (right props, KV, etc.), your system may simply have more power than you’re used to.

In that case, use throttle curves in Betaflight to constrain the range and improve resolution:

  • Throttle limits– Reduce max throttle percent
  • Expo– Increase resolution around the center stick

Think of it like PID tuning vs. rates. PIDs make the quad match your input, and rates determine the input curve.

For throttle response:

  • Motor, prop, and KV choices affect the power
  • Throttle curves affect input sensitivit

So get the right hardware, then tune the curves for your preferred feel

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