The Ultimate Guide to Heavy Lift Drone Motors - JOUAV

What to Consider When Choosing the Best Heavy Lift Drone Motors?

Choosing the right motors for a heavy lift drone is a critical decision that directly impacts the drone's performance, stability, and efficiency. Here are key considerations to keep in mind:

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Drone Weight & Frame

To begin building a drone, start by calculating its weight, factoring in components such as the frame, flight controller, ESCs, motors, propellers, battery, camera, and antenna.

Add a 10-20% buffer to account for potential inaccuracies or future modifications.

Once you have the estimated drone weight, determine the frame size.

Ideally, the frame should accommodate a maximum propeller size equal to one-third of its dimensions.

This proportionality optimizes aerodynamic performance, striking a balance between lift and stability for efficient flight.

Drone frame. Source from unsplash.com

Thrust to Weight Ratio

Once you have determined the estimated weight of your drone and selected an appropriate frame size, the next crucial step in the design process is to establish the thrust requirements.

A fundamental guideline to follow is that the combined maximum thrust generated by all the motors should be at least double the weight of the drone.

For instance, if your drone weighs 1 kilogram, the collective thrust from all the motors should be a minimum of 2 kilograms. In the case of a quadcopter, this translates to each motor producing a maximum thrust of at least 500 grams.

This threshold ensures that the drone has the lifting capacity required for takeoff.

Ideally, for standard drones, a thrust-to-weight ratio of 3:1 or 4:1 is recommended. This ratio ensures that the drone not only lifts off effectively but also possesses the maneuverability needed for smooth and controlled flight.

Additionally, it allows the drone to accommodate extra payloads without compromising its overall performance.

PH-20 with MG-130E gimbal camera for aerial surveillance

Motor Size

A drone motor, whether brushed or brushless, consists of a stator with metal coils and a motor bell housing permanent magnets.

The stator's width and height, denoted by XXYY, determine the motor size. Larger motors offer more torque and thrust but are less responsive and heavier.

The metal coils, enameled for insulation, form the stator and generate a temporary magnetic field when an electric current flows through them.

The motor bell, attached to the inner side of the motor, protects the permanent magnets and coils. The motor shaft transfers torque from the motor to the propellers when the changing magnetic fields cause rotation.

Choosing the right motor size is critical. Larger motors provide more thrust but sacrifice responsiveness and add weight. For multi-copters, determining the required thrust from each motor based on a desired thrust-to-weight ratio is essential.

This ensures optimal performance by listing motors meeting the thrust requirement and selecting the smallest ones that fulfill these specifications, balancing power, responsiveness, and weight for efficient drone functionality.

Wider Motors

When selecting BLDC motors for drones, their dimensions—specifically, stator width and height—play a crucial role in performance.

Wider stator motors have greater inertia, making them less responsive to speed changes but offering effective cooling due to increased surface area. Additionally, their design allows for larger bearings, enhancing durability, efficiency, and stability.

Narrow stator motors are more responsive but may face challenges in cooling due to their compact design.

The choice between wide and narrow stators hinges on the drone's purpose. For drones lifting payloads, where responsiveness is less critical, wider motors are preferred.

Payload drones require careful piloting, making the sacrifice in responsiveness acceptable for the benefits of cooling efficiency and motor robustness.

An electric motor with all its copper windings. Source from unsplash.com

KV Rating

The next critical step is to consider the relationship between KV ratings and propeller selection for optimal drone performance.

Higher KV ratings indicate more revolutions per minute (RPM) when one volt is applied to an unloaded motor.

Motors with higher KV ratings typically have shorter windings and lower internal resistance, but they are prone to early heating.

This heating issue is more evident in taller motors with higher KV ratings due to their greater rotational speeds and thrust generation.

The conventional strategy involves pairing motors with higher KV ratings with lighter propellers and motors with lower KV ratings with heavier propellers.

This approach ensures a balance between motor characteristics and propeller load.

When a high KV rating motor is combined with a heavy propeller, it attempts to rotate the propeller at maximum speed, requiring more torque and drawing increased current.

This situation could potentially damage the Electronic Speed Controller (ESC) or MOSFETs.

Conversely, a low KV rating motor paired with a lighter propeller may struggle to produce sufficient thrust.

For those opting for wider motors to enhance maneuverability at slower speeds, a low KV rating motor with heavier propellers is recommended.

Conversely, for drones focused on rapid racing without carrying a payload, choosing a taller motor with a high KV rating and lighter propellers is more suitable.

It's crucial to note that the KV rating is a manufacturer-provided estimate, and actual motor RPM may vary due to factors like air resistance.

Whether selecting a low KV rating motor with a heavier propeller or a high KV rating motor with a lighter propeller, the key is to achieve the desired thrust-to-weight ratio.

Motor Torque

The torque a motor produces relies on factors like stator volume, magnet types, coil quality, and construction details (e.g., pole count, insulation gap).

Greater stator volume generally means a heavier motor, but if two motors share the same stator volume, the lighter one is preferred.

Motor torque affects responsiveness to pilot input.

Excessive torque can lead to jerky drone movements, causing difficulties in control and potential damage to the ESC unit due to voltage or current surges.

Choosing a lighter motor strikes a balance between power and control, mitigating these issues.

For optimal performance, selecting motors tailored to specific needs is crucial.

In scenarios where slow, steady flight with a payload is required, opting for motors with lower torque and RPM is recommended.

This ensures a precise and controlled flight experience while safeguarding electronic components.

KV vs. Torque Constant

The torque constant of a drone motor dictates the current required to generate torque.

Although not theoretically linked, practical observations reveal a trend: higher KV rating motors generally have higher torque constants, while lower KV rating motors have lower torque constants.

In practice, this means high KV rating motors draw more current to achieve a given torque, impacting energy efficiency.

High KV rating motors are less power-efficient than their low KV rating counterparts due to increased current consumption.

Optimal power efficiency requires choosing a KV rating that balances performance and efficiency, preventing excessive torque constant that hampers overall effectiveness.

Utilizing a motor with an excessively high torque constant poses risks, including damage to the Electronic Speed Controller (ESC) and motor heating issues.

Long-term consequences include reduced battery lifespan and increased wear on wires, motors, and MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).

Current Voltage & Efficiency

Selecting a suitable Brushless DC (BLDC) motor for a drone hinges on careful evaluation of voltage and current ratings.

The relationship between motor voltage and current draw is crucial—higher motor voltage typically results in increased current consumption from the battery during operation.

To determine the maximum current drawn by the motor, calculate this value when the motor operates at its highest voltage, generating maximum thrust.

This calculation is pivotal for selecting an Electronic Speed Controller (ESC) with an appropriate current rating.

When choosing an ESC, ensure that its current rating surpasses the maximum current drawn by the motor.

While the continuous current rating of the ESC is important, it does not necessarily need to exceed the maximum motor current.

However, it's imperative that the burst current rating is greater than the maximum motor current to ensure reliable and safe operation.

Ideally, opting for an ESC with a continuous current rating higher than the maximum motor current is advantageous.

This surplus capacity provides an additional safety margin, contributing to the longevity and reliability of the drone's propulsion system.

It ensures the ESC can handle unexpected spikes in current demand, preventing overheating and potential damage.

N & P in Motor

Drone motors are labeled with N & P ratings, such as 12N15P, indicating the number of poles in the motor stator and the permanent magnets.

Fewer poles, as seen in 12N15P, result in higher torque, while more poles contribute to smoother operation due to a uniform magnetic field.

As drone motors are three-phase, pole numbers are always multiples of 3. For 22XX and 23XX BLDC motors, the common configuration is 12N15P.

It's important to note that the number of poles and magnets doesn't directly impact motor performance but is essential for configuring flight controllers, like enabling RPM filters.

Understanding these ratings ensures optimal performance and responsiveness in drone systems.

Mounting Pattern

Drone motors, specifically the 22XX, 23XX, and 24XX series, feature versatile mounting patterns of 16x16mm or 16x19mm.

To ensure compatibility with various frames, a drone frame must support both of these patterns.

For the attachment of these motors, M3 screws are the standard choice.

The key consideration here is the length of these screws, which should exceed the thickness of the drone arm by 2mm.

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For example, if the drone arm is 5mm thick, the recommended length for the M3 screws is 7mm.

This precision in screw length is crucial for a secure and stable connection between the motors and the frame.

Following these guidelines ensures a reliable and robust assembly, contributing to the overall performance and structural integrity of the drone.

Motor Winding

The choice of motor winding significantly influences motor performance.

Thick wires handle higher currents but reduce the electromagnetic field, impacting torque.

Thin wires excel in creating strong electromagnetic fields and torque but struggle with high current draw due to increased internal resistance.

To navigate this balance, manufacturers often opt for thick copper wires with more windings.

This combination maintains current resilience while enhancing the stator's electromagnetic field, resulting in increased torque.

Motor windings come in two options: single-stranded and multi-stranded.

Single-stranded uses thick wires for larger current draw, suitable for high-voltage battery packs.

Multi-stranded, with three thinner strands, produces powerful electromagnetic fields and torque but faces a risk of damage from high current draw, leading to a lower KV rating.

Motor Bearing

The motor's bearing size directly influences its durability and operational smoothness.

Larger bearings enhance durability by distributing loads and dissipating heat effectively, making them suitable for heavy-duty applications.

On the other hand, smaller bearings contribute to stability and smooth operation, ideal for precision machinery.

The inner diameter of the bearing determines the motor shaft size, emphasizing the interconnected nature of motor components.

Some manufacturers promote motors with ceramic bearings for their smooth performance, though they may be more prone to breakage compared to steel bearings.

Motor Movements

Drone motors rotate in opposite directions for stability during flight. If all motors spun the same way, the drone would struggle to lift off and maintain control.

To achieve balance, motors mounted diagonally across from each other rotate in opposite directions—one clockwise, the other counter-clockwise.

This configuration counters torque, ensuring a stable and controlled flight, a crucial design principle adopted in multirotor drones for optimal performance.

Motor Connections

Drone motors, categorized as brushed DC or brushless DC, dictate the rotation direction — clockwise or counterclockwise.

Brushed motors have two wires, while brushless ones have three, all connecting to the Electronic Speed Controller (ESC), which, in turn, links to the flight controller.

Brushless DC motor with ESC. Source from tytorobotics.com

Swapping any two wires connected to the ESC reverses motor rotation.

This simple adjustment is instrumental in tailoring the drone's behavior to specific flight conditions or preferences.

Additionally, the flight controller, responsible for stability and orientation, can be programmed to further modify motor behavior, providing a centralized and efficient means of control.

FAQ

What Are the Different Motors on a Drone?

Drones typically have multiple motors that serve various purposes. These motors are responsible for generating thrust, controlling the drone's movement, and ensuring stability during flight.

The main types of motors on a drone include those for lift, propulsion, and control. The lift motors provide the upward force needed for takeoff and hovering, while the propulsion motors drive the drone forward.

Control motors adjust the orientation and stability of the drone by varying the speeds of individual rotors.

Each motor plays a specific role in achieving controlled and stable flight, allowing the drone to respond to pilot commands or automated flight systems.

What Types of Electric Motors are Used in Drones?

Drones typically employ either brushless or brushed motors.

Brushless motors are commonly found in larger, high-performance models like racing and cinematography drones, offering efficiency, longevity, and a favorable power-to-weight ratio.

In contrast, brushed motors are simpler and more cost-effective, often used in smaller drones, toy-grade models, and micro drones.

What Is a Good KV for a Drone Motor?

As a general guideline, drone motors can have KV ratings ranging from around 500 to KV or even higher. Lower KV motors are often used for larger drones carrying heavier payloads, while higher KV motors are suitable for smaller, lighter drones.

How Much RPM Motor Is Required for a Drone?

The required RPM (Revolutions Per Minute) for a drone motor can vary based on several factors, including the size and weight of the drone, the type of propellers used, the desired flight characteristics, and the overall design of the drone.

For small toy drones, the motor RPM might be in the range of 10,000 to 15,000 RPM, while larger and more professional drones may have motors with RPM values exceeding 20,000 RPM.

What is a High Efficiency Brushless Motor?

A high-efficiency brushless motor is designed to minimize energy loss and heat generation while providing maximum power output. These motors often use quality materials and advanced engineering for optimal performance.

What is the Power Output of a Brushless Motor?

The power range for BLDC motors can be approximately 30 to watts. However, it's important to note that there can be exceptions, and some motors may fall outside this range.

What is the Maximum Speed of a Brushless Motor?

While some high-performance brushless motors can indeed achieve speeds of 100,000 rpm or more, it's essential to note that the practical limits may be influenced by factors such as heat generation, mechanical stress on the components, and the capabilities of the materials used.

Motors designed for specific applications, like those used in drones or certain industrial machines, may be optimized for higher speeds.

How Fast is a W Brushless Motor?

A W brushless motor can have a rated speed of rpm/min and a maximum speed of rpm/min.

What are the Electric Motors in a Quadcopter for?

The electric motors in a quadcopter convert electrical energy into mechanical energy. This energy is used to spin the propellers, which generate the thrust necessary for lift and propulsion.

What is the Power Output of an Electric Motor that Lifts?

The power output of an electric motor that lifts depends on the weight and height it's lifting, and the time it takes to do so. The power output can be calculated using the following formula:

For example, the power output of an electric motor that lifts a 2 kg block 15 meters in 6 seconds is 49 watts.

Power =Work/Time

The mechanical power of an electric motor is defined as the speed times the torque. Mechanical power is usually measured in kilowatts (kW) or horsepower (hp). One watt is equal to one joule per second or one Newton-Meter per second.

Hi, dear colleges.

My name is Robin and I am doing my PhD on UAVs in France. I build my drones with raspberry Pi and pixhawks.

In terms of control a drone in the perspective of dynamics, the control inputs should be thrust force and torques, or angular velocity velocity of each motor (as there is a constant linear matrix between those two).

I found there are some discussion on this like
Thrust to throttle in /mavros/setpoint_raw/attitude
Control multicopter via thrust F and moments M

We are still trying to find a solution. If you are interested in this, let us work on that together.

@Jaeyoung-Lim Yes, this is what I am trying to do by sending commands directly to the /actuator_control through mavros.

There is not any mixer that for this purpose, this is why we discussed in the Thrust to throttle in /mavros/setpoint_raw/attitude .

I think I just need a simple mixer, identity transform, for this.

@Jaeyoung-Lim, but we can not control the body torque, right? In the /mavros/setpoint_attitude, we can not control the body torque, but only the pose of the drone.

If you mean the /actuator_control, I am not sure what the attitude control set to that by the default mixer are body torques. As I read some of the attitude control.cpp, I do not think the output of the attitude control is the body torque.

This is why I calculate the angular velocity by myself and want to send it to the drone directly.

You can control the torque by /actuator_control topic. /mavros/setpoint_attitude and /actuator_control are two different topics. If you use the actuator_control you are not using the attitude controller

You just need to set the control group accordingly : http://docs.ros.org/api/mavros_msgs/html/msg/ActuatorControl.html

I am not up to date on the current status of mavros on this, but I have used this a few years ago and I could hover with it at least in SITL. As you have communication delay, it won’t fly as nicely as modifying the controller inside the flight controller.

Yes, I agree with this as the fight control group 0. As the mixing/actuator says

0: roll (-1…1)
1: pitch (-1…1)
2: yaw (-1…1)
3: throttle (0…1 normal range, -1…1 for variable pitch / thrust reversers)

However I am not sure they are roll, pitch and yaw torques. (I am reading the atitude_control.cpp to get some ideas)

Let us say, If they are normalised torques, is there a way to map the real torque into this normalised way?

@robin-ecn So you simply need to scale it to a reasonable amount. Why not just try it in SITL?

I suspect that you are using ordinary ESCs which don’t have feedback. This means that you don’t know how much thrust the motors are going to generate exactly anyway. You can have a feel for example thrust, if the hover thrust is 0.5 your max thrust will be approximately double the weight of the vehicle.

Perhaps someone that is more familiar with the PX4 control scheme can correct me, but if you are running your own controller, you can calculate the forces and torques through PID or whatever other controller you may be using and then treat the mapping to the body as a different problem. This is what I do and I have had good success with this. You will have to define the mapping based on the geometry of your vehicle and the thrust curve of your particular motor/propeller combination.

I typically assume a linear thrust curve and just interpolate between 0 and my maximum measured static thrust. I’m guessing since this is your PhD work that you will want to be a little more precise with this. If I remember correctly, the thrust profile is approximately quadratic and you may or may not need to deal with dynamic thrust issues based on airspeed.

The goal of this is to take the forces and torques calculated in your controller and then map them to values between 0 and 1 (or -1 and 1 if you have reversible ESCs) for output to the actuators. Then you can just pass the outputs directly to your motors.

@robin-ecn Sorry for the delayed response. Yes that it what I did, I know that in reality, the relationship between force/torque and the ESC value is not linear (I think it is quadratic by the lift/drag equation), but a linear approximation worked pretty well. I use this on a real vehicle in practice and saw acceptable response. I’m assuming you will have some kind of PID or other controller? If so, the controller will take care of correcting for any errors from the linearization. If you want to be precise, you could measure the motor output as a function of input signal and use the data to create your output mapping.

I also used UAVCAN for all of my outputs and modified the driver to bypass the mixers, so you might have to do something a little different if you want to use the PWM outputs. I prefer having direct control over my output values, so I don’t bother with mixers or other intermediate steps.

So to go a little bit further, you probably want to find the forces to achieve each element separately. This gives you 4 forces for each motor that you need to sum up using the principle of superposition.

F = F_thrust + F_pitch + F_roll + F_yaw

F_thrust is the force required to keep the vehicle in the air
F_pitch is from the torque required to pitch or correct pitch errors
F_roll is from the torque required to correct for roll errors
F_yaw is from the torque required to correct for yaw errors

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