Introduction to Power System

Motor

A motor is a device that converts electrical energy into mechanical energy. There are various types of motors with different principles and structures suitable for different applications. The brushless motor used in UAVs is a high-efficiency, low-noise motor that achieves rotor commutation without carbon brushes. It is widely used in drones, electric vehicles, industrial equipment, and other fields.

Brushless motor structure:

Front cover, middle shell, rear cover: Main structural components that form the motor's overall structure. However, the shell of outer-rotor brushless motors also serves as the magnetic circuit path for the magnets, so it must be made of magnetically conductive material. The shell of inner-rotor motors is purely structural with no material restrictions. Inner-rotor motors have an additional rotor core that serves as the magnetic circuit path.

Magnets: Installed on the rotor, they are crucial components of brushless motors. Most performance parameters including power, speed, and torque are related to the magnets.

Silicon steel sheets: Essential components for slotted brushless motors (most brushless motors are slotted). They mainly function to reduce magnetic resistance and participate in magnetic circuit operation.

Shaft: The direct load-bearing component of the motor rotor. The shaft hardness must meet the requirements of high-speed rotation.

Bearings: Ensure smooth motor operation. Bearings can be divided into sliding bearings and rolling bearings (including deep groove ball bearings, needle bearings, angular contact bearings, etc.). Most brushless motors use deep groove ball bearings.

Brushless motor working principle:

It consists of stator and rotor. The stator windings are typically three-phase, while the rotor contains permanent magnets.

The working process has two main stages: sensor detection and electronic control.

In the sensor detection stage, built-in sensors (e.g., Hall effect sensors) detect rotor position by sensing changes in the magnetic field, feeding this information back to the electronic controller.

In the electronic control stage, the controller switches current through stator windings in specific sequences based on rotor position information, generating a rotating magnetic field. This interacts with the rotor's permanent magnets to induce rotation. The controller continuously adjusts current to maintain stable speed and operation.

Simply put, brushless motors work by changing the frequency and waveform of alternating current in stator windings to create a rotating magnetic field around the geometric axis, which drives the permanent magnets on the rotor. Performance relates to factors like magnet quantity, flux intensity, and input voltage.

Inner vs. outer rotor characteristics:

Outer rotor advantages: Large rotational inertia, smooth operation, high torque, secure magnet fixation.

Outer rotor disadvantages: Stator heat cannot dissipate, internal environment is unsealed allowing external contaminants.

Inner rotor advantages: Direct contact between windings and shell for heat dissipation, sealed interior preventing contamination.

Inner rotor disadvantages: Lower torque compared to outer rotor motors, more complex magnet fixation.

Brushless motor parameters:

KV Value: Indicates RPM increase per 1V voltage rise, a constant for brushless motors.

Surface-mounted vs. interior permanent magnet (IPM): Surface-mounted (SPM) motors have rotors on the surface and stators at the center. IPM motors have rotors at the center like traditional motors.

Dimensions: Motor model numbers like 2312 indicate stator outer diameter (first two digits) and height (last two digits). Larger motors have higher power and weight.

Slot and pole numbers:

1. Slots (N): Number of stator cores. Since brushless motors are three-phase, N is multiples of 3.
2. Poles (P): Number of stator magnets (always even as N/S poles pair).
3. Characteristics: Smaller N/P means higher RPM.
4. Same N: Larger P increases torque.
5. Larger N/P reduces cogging and vibration but complicates dynamic balance.

Power and efficiency: Output power = speed × torque. At same power, torque and speed are inversely related.

Electronic Speed Controller (ESC)

The UAV ESC is an electronic device that controls brushless motor speed, serving as a crucial component.

ESC main functions: Receive signals from flight controllers or remote controls, adjust current magnitude/direction to control motor speed/rotation. Converts signals to appropriate voltage/current output.

ESC working principle:

1. Input signal: Typically PWM signals from remote control indicating power level and speed control. (Pulse Width Modulation (PWM) remains a fundamental ESC protocol. PWM converts throttle input into timed power pulses. The duty cycle (on/off time ratio) determines power output and rotor speed.)

2. Signal processing: ESC decodes PWM signals and generates appropriate current output using microcontrollers.

3. Motor driving: Using power transistors/MOSFETs to switch current rapidly for precise speed/direction control.

4. Feedback control: Advanced ESCs use sensors to monitor speed and adjust output accordingly.

ESCs employ precise PWM control and often integrate protection features like overload, low voltage, and temperature protection.

Motor-battery-ESC selection logic: First determine motor → select ESC → finally choose battery. Never select battery/ESC blindly without considering motor specifications.

Propeller

Blade specifications:

1. Size: Typically 4 digits - first two indicate diameter (inches), last two pitch. E.g., 1045: 10" diameter, 4.5" pitch (1"=2.54cm).

2. Pitch: Forward distance at 70% diameter per revolution. Larger pitch increases lift but requires more torque.

3. Blade quantity vs efficiency: Fewer blades → higher efficiency (per blade). More blades increase lift but add weight/drag. Quadcopters generally have longer flight times than hexacopters with same specs.

4. Propeller efficiency: ep=T/P (lift (g) per mechanical power (W))

5. Total lift efficiency: eT=T/pm (lift (g) per motor electrical power (W))

Lift calculation: By Newton's laws: T=(mv)=v+m=v

Hover power: P=Tvi

Energy dissipation via airflow integration: P=(mv2)=

Combined: T==ρA（2）=2ρA

Battery

I. Introduction: Lithium Polymer (Li-Po) batteries power UAVs with high energy density and power output. A 3S battery contains three 3.7V cells in series (11.1V total), including battery cells and protection circuits.

II. Parameters:

1. S/P

S means series, and P means parallel.

Most UAVs require a voltage higher than 7.2V, so lithium-polymer battery packs commonly connect multiple cells in series. Each cell has a nominal voltage of 3.7V. A battery marked 2S therefore contains two lithium cells connected in series and provides 7.4V (3.7V × 2 = 7.4V); 3S means that three cells are connected in series and provide 11.1V (3.7V × 3 = 11.1V).

Another notation is 3S2P: three cells are connected in series in each group, and two such groups are connected in parallel. When two 3S/5200mAh packs are connected in parallel, their capacities are added while the voltage remains unchanged, resulting in a 3S2P/10400mAh battery pack.

2. Voltage: LiPo cell nominal voltage 3.7V. 3S=11.1V.

3. Capacity

The capacity unit is mAh:

• m = milli, or one thousandth;
• A = ampere, the unit of electric current;
• h = hour.

A 5200mAh marking indicates how much electrical energy the battery can store. It is equivalent to 5.2Ah, meaning that the battery can discharge at 5.2A for one hour.

4. Discharge Rate

The C rating of a lithium battery is its discharge rate; C is short for capacity. A 30C rating means that the battery can discharge at 30 times its rated capacity. For the 3S battery used in this example:

• At 5200mA, it can continue discharging for one hour.
• At ten times that rate, or 52000mA, it can continue discharging for 6 minutes (60 / 10).
• At a maximum rate of 30 times, or 156000mA, it can continue discharging for 2 minutes (60 / 30).

III. Characteristics:

• High voltage output.When batteries are connected in series, a 3S lithium battery pack delivers a relatively higher total voltage, enabling greater voltage output.
• High energy density. Lithium batteries, as a high-energy-density power source, achieve greater energy storage capacity through a 3S configuration (three battery cells connected in series), thereby extending the operational duration of the device.
• Enhanced safety through voltage balancing.Compared to a single lithium battery cell, the 3S lithium battery configuration demonstrates superior characteristics in voltage distribution and current balancing, delivering more stable and secure power supply.
• Low memory effect, self-discharge, and weight

IV. Precautions:

1. Voltage Range The nominal voltage of lithium batteries is 3.7V (volts). The charging cutoff voltage is 4.2V, and the absolute minimum discharge cutoff voltage is 2.8V. Voltages below 2.8V constitute over-discharge and are nearly irreparable, while voltages above 4.2V constitute overcharge and pose a fire hazard.

2. Charging Method LiPo batteries employ a constant voltage, variable current charging method. As voltage requirements are extremely precise, it is imperative to use a dedicated lithium battery balance charger. The quality of the charger affects charging accuracy, which in turn impacts battery lifespan.

3. Charging Current The optimal charging current for LiPo batteries is 0.7C. For example, a 5200mAh (5.2Ah) battery should be charged at 3.6A-3.7A (5.2Ah*0.7C=3.64A).

While most lithium polymer batteries currently support 3-5C fast charging, this reduces battery lifespan due to the chemical reaction characteristics of LiPo batteries. When time permits, charging at 0.7C is recommended.

4. Charging Temperature Cell temperature during charging must remain within 0-45°C.

V. Storage Requirements:

1. The optimal storage voltage is approximately 3.85V per cell. Fully charged LiPo batteries experience active internal chemical reactions. Prolonged storage at full charge accelerates aging and reduces discharge capacity. The ideal storage voltage is approximately 3.85V per cell. If the battery won't be used within 3 days after use, charge each cell to 3.85-3.90V for storage. Similarly, if a fully charged battery goes unused for any reason, discharge it to 3.85-3.90V within 3 days for storage.

2. For long-term storage (exceeding 3 months):

• Avoid direct sunlight and humid environments
• Keep away from high temperatures
• Store in dry, cool locations
• Use fireproof bags or explosion-proof containers
• Recommended ambient temperature: 10-25°C
• Storage area should be dry and free of corrosive gases

(Note: This translation strictly adheres to the original technical specifications, maintaining all critical parameters, safety warnings, and operational guidelines without modification or omission. All units and values are preserved exactly as in the source text.)