Selection and Optimization of Motor KV Values for Multi-Blade and High-Load Ratio UAVs

Abstract

This paper proposes a method to enhance the load capacity and reduce the volume of an Unmanned Aerial Vehicle (UAV) by optimizing the motor’s KV value (a measure of RPM per volt) to match the specific aerodynamic characteristics of a multi-blade propeller system. Given the fixed maximum output power of the motor when its volume and weight are determined, a higher-power-density motor structure can maximize blade lift by optimizing its output torque and speed. The electromagnetic structure of the motor is initially designed based on the aerodynamic characteristics of a 29-inch, four-blade model. Different motor Voltage-Turns (KV) value schemes are obtained by altering the number of winding turns and the wire diameter. Subsequently, the torque–speed output performance of the motor is calculated under constant power conditions. This data, in conjunction with the blade load, allow for a comparison of the motor’s output lift at different KV values. The optimal KV value for the motor is determined to be 116, resulting in a rated lift of 11.57 kg. This paper correctly analyzes the motor and blade as an integrated system, which is crucial for meaningful optimization in aerospace application.

1. Introduction

With the continuous development of electric motor and flight control technology, UAVs have been widely used in many fields [1,2,3]. For some new application scenarios such as disaster rescue, panoramic photography, power inspection, and environmental protection testing [4,5,6,7], there is a higher demand for the appearance and operational performance of UAVs, leading to research on miniaturization, high load-to-weight ratio, and long endurance. Figure 1. shows the prototype of high-load-ratio UAVs.

In the power system of UAVs, the blades are the main components that generate lift. There are many studies on the improvement of blade lift under the volume limitation of UAVs in recent papers. The maximum thrust–torque ratio was calculated considering the blade length, blade chord width, blade mass density, and blade twist angle [8]. The study [9] shows that use of a variable pitch propeller can increase the maximal takeoff weight of the aircraft and improve power efficiency in hover, especially if load varies for different missions. And the driving motors need to meet the load requirements of blade speed and torque, so the high power density is an important evaluation index for UAV motors. Currently, there are two main types of motors suitable for UAV power systems: outer rotor DC brushless motors and axial flux motors [10,11,12,13]. The electromagnetic structures of the two types of motors are shown in Figure 2.

In the paper by Carev V et al. [14], three BLDC motors stacked axially were used, which can increase the torque of the power system without increasing the outer rotor diameter of the motor, thereby loading more blades and improving lift. Liben M et al. designed a multi-blade integrated fan by increasing the blade diameter and number of blades, reducing the required motor speed, and designing an external drive for an axial flux motor with higher torque density [15]. This power system has better aerodynamic efficiency, can cool itself by air cooling, and can also reduce the energy consumption of the battery. The measured specific torque density of the air-cooled design is 6.25 N·m/kg—total mass, 11.1 N·m/kg—active mass only. In the paper by Hwang SW et al. [16], a dual-stator permanent magnet synchronous motor with separately controlled dual 3-phase winding was proposed to demonstrate impacts on the power density maximization, torque harmonic reduction, and efficiency improvement. Multiple slot poles, dual rotors, a slotless structure, and high winding factors can all improve the power density, torque density, and weight reduction in motors [17,18,19,20]. The current research only focuses on enhancing the lift of the blade or improving the power density of the driving motors separately. However, the blade as a load directly affects the output performance of the motor, and both must be designed and optimized as a whole system to achieve the optimal lift of the drone. Instead of treating the motor and propeller as independent components, this paper correctly analyzes them as an integrated system, which is crucial for meaningful optimization in aerospace applications.

This paper presents a methodological approach to enhancing the lift capacity of UAVs by optimizing the motor’s KV value to match the specific aerodynamic characteristics of a multi-blade propeller system. The core of the research is to find the ideal balance between motor torque and speed to maximize lift for a given power constraint.

The energy that UAVs can use is limited (carrying limited battery capacity), so the essence of optimizing the power system is how to allocate limited energy to achieve higher power efficiency (lift to power ratio). Specifically, it is the motor that maximizes the blade lift with the optimal output torque and speed. When applied to UAVs, the improved method is to adjust the KV value of the motor to better adapt to the load blade [21,22]. Low-KV motors tend to operate at lower speeds and generate greater torque, making them ideal for large UAVs. High-KV-value motors operate at higher speeds, making them an ideal choice for low torque, small, and fast rotating propellers [23,24,25]. Therefore, it is best to use high-KV motors for lighter and faster moving drones (such as walkthrough drones), while low-KV motors are better for heavier and slower-moving, high-load drones.

This paper is organized as follows. In Section 2, determine the blade scheme based on the high lift demand of high-load-ratio UAVs. In Section 3, determine the outer diameter limit of the motor based on the blade installation method, and optimize the number of poles and stator cores. In Section 4, the corresponding relationship between the winding and KV value is given, and the motor under no-load is analyzed by the finite element method. In Section 5, the aerodynamic characteristics of the blades are analyzed, and the optimal KV scheme with the highest lift is obtained by comparing the output performance of different motors. In Section 6, the experiment verified the correctness of the simulation results and the effectiveness of the optimization method. The final power system rated lift can reach 11.57 kg. Conclusions are given in Section 7.

2. Power System Composition

The power system of the UAV consists of a battery, an electronic speed controller (ESC), a throttle, a motor, and blades, as shown in Figure 3. In order to achieve a high load capacity and small volume of UAVs, the main design methods are as follows.

(1)

After determining the size and number of blades, increase the motor speed to enhance the overall lift.

(2)

Keep the blade speed constant, increase the number of blades, and at the same time increase the motor torque to meet the load demand, thereby improving lift.

Both methods require the motor to have a higher output power. Therefore, this paper selects an outer-rotor DC brushless motor as the driving motor, which has the characteristics of high power density and torque power, and is very suitable for high-load ratio UAVs.

Figure 3. UAV power system.

Figure 3. UAV power system.

According to the lift curve of the different number of blades in Figure 4, the number of blades increases from right to left. When the speed increases, the lift and blade efficiency enhance significantly. However, when the motor operates at high speed, high-frequency iron and copper losses occur, leading to severe motor heating and reduced efficiency [26]. Moreover, due to the compact structure, UAVs often use air cooling for heat dissipation. Excessive motor load temperature can cause the UAV to be unable to operate for a long time. Therefore, this paper adopts the method of increasing the number of blades and motor torque to enhance the lift of the power system.

3. Motor Design

3.1. Outer Rotor

Due to the direct connection between the blades and the motor shaft, the blades are tightly attached to the motor casing, as shown in Figure 5. In order to not affect the aerodynamic characteristics of the blade, it is first necessary to ensure that the maximum outer diameter of the rotor is less than the length of the blade root (not generate lift).

In the simulation test by ANSYS CFD 2022 R1, the flow field around the blade under the defined boundary conditions is shown in Figure 6a. There is a relatively stable air flow space below the blade root, where the air flow is very small. Combined with the model structure of the blade, the diameter of the blade root is 69 mm. As the angle of attack from the blade root to the blade tip changes from high to low, the air flow rate below is relatively low. The space below has a diameter of 146 mm and a height of 104 mm. There are other components (motor) within this limited space range, which will not have a significant impact on the lift of the blade.

Then, under the same number of slots, the number of poles is proportional to the torque, and the more poles there are, the greater the torque. Therefore, 30 poles of permanent magnets (PMs) are used to meet the high torque required for the blade. A larger number of poles can also make the magnetic circuit of the motor more reasonable, with smaller magnetic leakage and higher utilization of iron core material. Figure 7 shows the distribution of magnetic field lines of the motor with different poles simulated by ANSYS Maxwell. The larger the embrace of the PMs, the smaller the torque ripple. However, a higher embrace requires more PMs, which increase the weight of the motor. Taking into account the output performance and weight of the motor, a PMs embrace of 0.7 is selected. The performance comparison of different embrace is shown in

Table 1. The performance comparison of different embrace.

	Embrace = 0.7	Embrace = 1
Torque	4.71 N·m	4.85 N·m
Ripple	20.29	19.98
weight	73.1 g	104.4 g

. Simultaneously, considering rotor assembly and weight reduction, the rotor PMs require no back iron, and the PMs are directly fixed to the rotor housing.

3.2. Inner Stator

We selected the 36-slot structure based on the slot–pole matching of the motor. On the one hand, the distribution of the magnetic electromotive force generated by multi-slot windings is closer to sine, resulting in smaller torque output ripple. On the other hand, there is a better winding factor of 0.933 with 36-slot/30-pole. At the same time, a fractional slot can also reduce cogging torque. Due to the multi-pole structure of the motor, according to Equations (1) and (2), the magnetic field strength at the stator yoke is very low, so the material utilization rate in this part is poor. The magnetic field is mainly generated by the stator teeth, and the inner diameter of the stator can be further reduced, thereby decreasing the weight of the motor core. The distribution diagram of the stator magnetic field simulated by ANSYS Maxwell is shown in Figure 8.

$${\tau }_s={\displaystyle \frac{\pi \left(D_s-h_s\right)}{2p}}$$

(1)

$${\hat{B}}_s={\displaystyle \frac{{\hat{\Phi }}_m}{2k_{Fe}l{h}_s}}$$

(2)

where $D_s$ is stator diameter, $2p$ is poles, $h_s$ is yoke height, ${\tau }_s$ is yoke pole length, $k_{Fe}$ is iron core factor, $l$ is iron core length, ${\hat{\Phi }}_m$ is air magnetic flux density amplitude, and ${\hat{B}}_s$ is yoke magnetic flus density amplitude.

4. Motor No-Load Speed and KV Value

Firstly, a finite element model of the motor is established based on its main dimensions and rated parameters. Then, the rotational inertia of the rotor is obtained, and the damping is obtained by estimating the wind friction loss [27]. The main parameters of the motor are shown in

Table 2. Main dimensions and parameters of the motor.

Parameters	Values	Units
Rated power	1.7	kW
Rated speed	3000	rpm
Operating voltage	55	V
Slot/pole	36/30	-
Rotor outer diameter	114	mm
Stator inner diameter	88.5	mm
Stack length	20	mm
Magnet thickness	2	mm
Embrace	0.7
Moment of inertia	1.61 × 109	kg·mm2
Damping	4.28 × 104	N·m·s/rad
Active mass	~0.6	kg

.

For the UAV motor, the relationship between the peak voltage and speed of the motor under no-load conditions is mainly described by the KV value. When the motor rotates, the generated back electromotive force is proportional to the rotor speed. The KV value can be defined by the following formula:

$${\hat{B}}_s={\displaystyle \frac{{\hat{\Phi }}_m}{2k_{Fe}l{h}_s}}$$

(3)

where ${\omega }_{rpm}$ is the no-load speed.

The KV value provides an estimate of how much speed the motor will produce for each applied voltage. The rating helps to compare motors of the same physical size but with different performance characteristics due to their internal working principles. Generally speaking, as the number of windings in the coil increases, the KV value will decrease. Low-KV motors have more thin wire windings, which can carry higher voltages at lower currents. And high-KV motors have fewer windings, but a thicker wire diameter, which can carry higher currents with less voltage. The number of motor poles is inversely proportional to the KV value, mainly following two rules:

(1)

Large motors with more poles will require high torque and operate at low speeds; therefore, the motors will have lower KV values.

(2)

Small motors with fewer poles will require high speed operation and generate relatively low torque; therefore, the motor will have high KV values.

In order to achieve optimal performance of the motor under physical size limitation, this paper parameterizes the winding and solves for the no-load speed of the motor under different turns and wire diameters, as shown in Figure 9. Combined with Equation (3), it is converted into the corresponding KV value, as shown in

Table 3. Different motor windings, no-load speeds and KV values.

No.	Conductors Per Slot	Wire Diameter (mm)	No-Load Speed (rpm)	KV Value
a	10	1.085	7905	143.7
b	12	1.024	7182	130.5
c	14	0.912	6620	120.4
d	16	0.861	6457	117.4
e	18	0.813	6413	116.6
f	20	0.767	6320	114.9

.

In Figure 9, the blue area indicates that the motor load gradually decreases, and as the speed increases, it eventually reaches the maximum no-load speed. Due to the induction electromotive force being higher than the winding line voltage, the motor speed will not increase further. Therefore, the maximum no-load speed of the motor is 7905 rpm (10 turns of winding), and the maximum speed gradually decreases with the increase in turns.

5. Load Analysis

5.1. Blade Load Characteristics

In order to design a motor that matches the load and achieves better output performance, it is necessary to study the blade load characteristics. This paper uses the FLUXER PRO 29 × 8.7″ type of MAD components as the initial model of the blade, the structure diagram of the single blade is shown in Figure 10, and the basic parameters are shown in

Table 4. Basic parameters of propeller blades.

Parameters	Values	Units
Blade size	736.6 × 221	mm
Weight	92	g
Optimal speed range	1400~3600	rpm
Common lift	5.5~11	kg
Blade profile	NAC6309	-

. Considering that the blade efficiency effect will decrease with the increase in blade number, a moderate number of four blades was selected as the research object.

We established a model with four blades based on the profile, and calculated the lift and required torque of the multi-blade model at different speeds using ANSYS 3D computational fluid dynamics (CFD) [28]. Due to the high-speed rotation of the rotor, the fluid flow changes from laminar to turbulent. Compared with laminar flow, turbulent flow is more complex and the fluid produces irregular motion. The complexity of its motion makes it impossible for computers to directly simulate it, so turbulence models are needed to simulate it. This paper uses the realizable k-ε model for analysis. The k-ε model is a closure model for the Reynolds-averaged Navier–Stokes equation (RANS), which simulates the turbulent characteristics in complex flows by solving the transport equations of turbulent kinetic energy (k) and turbulent dissipation rate (ε). It is widely used in engineering fluid calculations. The boundary definition and grid partitioning are shown in Figure 11, and the inlet velocity is 0 m/s. The number of grids was 65,114.

As shown in Figure 12, when the blade speed is constant, increasing the number of blades can significantly improve the lift of the power system. At the same time, for the torque required to reach the same speed, the multi-blade model’s value is about twice that of the single-blade model.

5.2. Selection and Optimization of KV Value

The speed and torque performance curve of when the motor maintains a constant power output is shown in Figure 13. The pervious section has provided different KV schemes and lift cures of the multi-blade model. This section matches the speed and torque of the blade with the motor to obtain the maximum lift.

According to Figure 9, high-KV-value motors have a larger range of constant torque and are suitable for loads with large speed changes, while low-KV-value motors have a lager constant power range (green area), which can better meet the operational requirements of four blades when combined with the weak magnetic acceleration control of ESC. Therefore, by combining the lift curve of the four blades and comparing (a), (c), and (e) in the output performance of motors, the optimal matching of blade load in the constant power range is obtained, as shown in

Table 5. Comparison of lift with different KV values.

NO.	KV Value	Speed/rpm	Torque/N·m	Power/kW	Lift/g
a	143.7	2915	4.18	1.27	10,622
c	120.4	3064	4.63	1.48	11,801
e	116.6	3212	5.06	1.70	12,834

. When the KV value is set to 116.6, the speed is 3212 rpm, the torque is 5.06 N·m, and the lift is 12.83 kg.

6. Testing and Results

A prototype motor was constructed for the example four-blade UAV application. The stator was winded with a number of 18 conductors of 0.8 mm copper wire. The phase resistance was 27.5 mΩ. The total mass of the motor was 0.89 kg. ESC adopts the model AMPX of MAD: peak output current 200 A, maximum driving frequency 2166 Hz, PWM pulse width 200–2000 μS, and the control method was FOC. The battery capacity was 22 AH14S, the full charge voltage of a single battery was 4.35 V, and the discharge rate was 10 C. The battery power can provide a maximum voltage 60.9 V and a maximum current 220 A. The load blade was 29 inches, with a four-blade structure. The complete testing frame is shown in Figure 14.

The test data of the 0–100% throttle system under 55 V battery are shown in

Table 6. The test data of 0–100% throttle.

Throttle/%	Speed/rpm	Voltage/V	Current/A	Torque/N·m	Lift/g	Input Power/W	Output Power/W	Motor Efficiency/%	Blade Efficiency/g·W−1	System Efficiency/g·W−1
5	0	55.17	0	0	0	0	0	0	0	0
10	506	55.07	0.37	0.101	206	20.4	5.4	26.5	38.2	10.1
15	774	55.01	0.86	0.244	472	47.3	19.8	41.9	23.8	10
20	1035	54.94	1.62	0.449	941	89.0	48.7	54.7	19.3	10.6
25	1273	54.84	2.52	0.652	1401	138.2	86.9	62.9	16.1	10.1
30	1508	54.72	3.90	0.946	2125	213.4	149.4	70.0	14.2	10
35	1730	54.57	5.58	1.255	2884	304.5	227.4	74.7	12.7	9.5
40	1940	54.42	7.53	1.572	3635	409.8	319.4	77.9	11.4	8.9
45	2161	54.18	10.28	1.980	4620	557.0	448.1	80.4	10.3	8.3
50	2424	53.90	13.70	2.399	5597	738.4	609.0	82.5	9.2	7.6
55	2665	53.58	17.71	2.861	6649	948.9	798.4	84.1	8.3	7
60	2884	53.25	22.67	3.375	7984	1207.2	1019.3	84.4	7.8	6.6
65	3088	52.87	27.58	3.836	9037	1458.2	1240.5	85.1	7.3	6.2
70	3286	52.44	34.20	4.443	10,449	1793.4	1528.9	85.3	6.8	5.8
75	3462	51.94	39.90	4.902	11,569	2072.4	1777.2	85.8	6.5	5.6
80	3643	51.32	47.52	5.493	12,956	2438.7	2095.5	85.9	6.2	5.3
85	3832	50.72	54.76	5.924	13,848	2777.4	2377.2	85.6	5.8	5
90	3982	49.97	64.16	6.587	15,347	3206.1	2746.7	85.7	5.6	4.8
95	4132	49.15	74.98	7.273	17,128	3685.3	3147.0	85.4	5.4	4.6
100	4256	48.20	84.16	7.716	18,124	4056.5	3438.9	84.8	5.3	4.5

. The efficiencies were calculated by Equations (4), (5), and (6), respectively.

$${\eta }_b={\displaystyle \frac{L_b}{P_{out}}},$$

(4)

$${\eta }_s={\displaystyle \frac{L_b}{P_{in}}},$$

(5)

$${\eta }_m={\displaystyle \frac{P_{out}}{P_{in}}}\ast 100\%,$$

(6)

where ${\eta }_b$, ${\eta }_s$, and ${\eta }_m$ are the blade efficiency, system efficiency, and motor efficiency, respectively. $L_b$ is the blade lift, $P_{out}$ is the motor output power, and $P_{in}$ is the motor input power.

As is shown in Figure 15a, when the rotation speed is below 3600 rpm, the simulated and experimental lift values and the upward trend in lift are consistent. When the rotational speed exceeds 3600 rpm, the experimental lift is lower than the simulated value because the blade vibrated violently during high-speed rotation, causing lift loss. The motor torque also maintains the same trend in Figure 15b, when the experimental blade lift decreased, and the requirement of motor torque was also smaller than the simulation value. The maximum torque density of the motor is 8.64 N·m/kg—total mass, 12.86 N·m/kg—active mass only. The performance of this power system is superior to that mentioned in reference [15].

Usually, the throttle range for the normal operation of UAVs is 50–80%. According to Figure 16 and Figure 17, the lift continues to rise with the increase in the throttle, and when the blade operates within the rated speed range of 1400–3600 rpm, the load torque demand increases rapidly, requiring greater input power, resulting in a significant increase in current. And the motor efficiency remains at around 85%, indicating that the KV value motor selected in this paper can meet the requirements of multi-blade loads.

Figure 18 shows the variation in the blade and system efficiency with throttle, and both efficiencies decrease with increasing throttle. When the throttle is 75%, the lift is 11,569 g, and if the KV value of the motor increases, it is necessary to increase the throttle to achieve the same lift; additionally, at this time, the efficiency of the blade and the entire power system will decrease.

7. Conclusions

In this paper, an outer-rotor motor for UAVs was designed and optimized to enhance lift. The effectiveness of this method was demonstrated through finite element analysis and experimental verification. The research conclusions are as follows:

(1)

The method of optimizing the KV value of the motor to match the aerodynamic characteristics of the blade can maximize the lift of the power system and achieve better system efficiency.

(2)

The experimental results show that the multi-blade power system can enhance lift under the volume limitation of UAVs, with a load of four 29-inch blades providing 11 kg of rated lift, and that the motor efficiency can reach 85%.

(3)

When testing the high-speed operation of the blade, blade vibration can cause a decrease in lift. Subsequent research will consider the impact of blade shape variation on lift in CFD simulations to address the limitations of high-speed performance deviations.

The optimized KV value motor with a multi-blade structure can improve the lift of the power system and is suitable for high-load-ratio UAVs.