A Genuine Guide to High-Quality Brushless DC Motors

A Genuine Guide to High-Quality Brushless DC Motors

When it comes to the brushless DC motor (BLDC), it is a widely known and utilized device. Functioning as a direct current motor, it replaces the conventional commutator with electronic commutation, thus achieving a brushless operation. Compared to ordinary DC motors, the BLDC offers remarkable advantages such as heightened efficiency, reduced noise, extended lifespan, and minimal maintenance requirements. The essence of this text aims to elucidate the criteria for determining the superiority among BLDC motors with identical specifications, and help you identify the most suitable option. Through this article, we shall elucidate the characteristics of an exceptional brushless DC motor by exploring multiple facets, including its static structure, rotational performance, and electrical parameter analysis.

1. Static structure

1.1 Appearance craftsmanship

A premium brushless DC motor should possess a visually appealing appearance, free from noticeable scratches, dents, or oil stains. The casing is often constructed using durable aluminum alloy, ensuring stability with no signs of looseness or cracks. Upon inspecting the motor's interior, the magnetic components responsible for generating the magnetic field should be made of high-performance rare-earth permanent magnet materials, such as neodymium iron boron, which provides a significantly strong magnetic force. The internal iron core should consist of neatly wound high-purity copper wire. Furthermore, the motor's wiring should be securely soldered without any traces of soldering iron marks or burn marks. The solder joints should be smooth and tidy, without any excess solder.


Figure: Neatly wound core

1.2 Axial and radial rocking

One can gently shake the rotor of an external rotor motor or the output shaft of an internal rotor motor. In the case of a high-quality brushless DC motor, there should be no noticeable movement in the axial or radial direction, as such movement could indicate issues with bearings, bent shafts, rotor imbalance, or design and assembly errors. These factors, if present, can potentially compromise the motor's performance, safety, and noise levels when in use.

2. Rotational performance

2.1 No-load short-circuit rotation

Brushless DC motors are powered by three-phase power supply, where each phase has a phase difference. Therefore, if one tries to short-circuit any two of the three power lines and gently rotates the motor by hand, they will feel a specific position where rotational resistance occurs. In the case of a high-quality brushless DC motor, the copper wire windings should have the same number of turns and be arranged neatly. As a result, regardless of which two lines are short-circuited, the perceived rotational resistance felt by hand should be similar.

2.2 Rotating noise

Due to high-quality bearings, well-matched structural components, and efficient lubrication, a high-quality brushless DC motor tends to operate with significantly reduced noise levels during rotation.

2.3 Thermal performance

A high-quality brushless DC motor, benefiting from high-purity copper wire and excellent winding techniques, exhibits lower internal resistance with the same number of windings. This characteristic results in reduced heat generation during operation and enhanced motor efficiency. Moreover, some motors nowadays incorporate ventilation holes on their outer casing to improve heat dissipation. However, caution must be exercised during usage to prevent foreign objects from entering through these openings.

3. Electrical parameter analysis

3.1 Electrical efficiency

Electrical efficiency refers to the ratio of output power to input power, and higher efficiency signifies greater energy savings under similar performance conditions. Generally, motors with greater torque tend to have lower efficiency at the same input power level. In the case of model aircraft motors, a more commonly used parameter is power-to-thrust ratio (g/W), which represents the ratio of generated thrust in conjunction with the propeller blades to the input power.


Figure：Force efficiency data table of a certain motor under different throttles

3.2 The resistance of each of the three phases

Previously, it was mentioned that high-purity copper wire and excellent winding techniques result in lower internal resistance for the same number of windings. If we have a multimeter at hand, we can measure the resistance between the three phases. High-quality brushless DC motors typically exhibit relatively small differences in resistance among the three phases.

4. Other specifications

4.1 Power supply line specifications

Premium brushless DC motors should be equipped with power supply wires that have clear markings indicating their gauge. The wire gauge should be matched to the motor's performance parameters, including the maximum continuous current and maximum peak current that the wire can handle.


Figure：American Wire Gauge (AWG) Cable/Conductor Sizes and Properties

4.2 Reducer type

Some brushless DC motors are equipped with gear reducers, or even have built-in gear reducers, to achieve the required torque. Different types of reducers can result in varying load performance at the output shaft. Furthermore, the noise characteristics and transmission efficiency also differ among different types of reducers. Here is a brief overview of the advantages and disadvantages of several common reducers:

4.2.1 Gear Reducers:

4.2.2 Planetary Reducers:

4.2.3 Cycloidal (Harmonic) Reducers:

4.2.4 Harmonic Reducers:

Indeed, the methods I shared for evaluating motor quality based on hardware and rotational performance are mainly used to compare multiple motors and determine which one is better. They do not represent an absolute judgment of whether a motor is good or bad. In reality, brushless motors have mature manufacturing processes, and by selecting reliable well-known brands, one can obtain high-quality motors with comprehensive after-sales service. Additionally, opting for popular models from reputable brands also means there will be numerous reviews and shared experiences on social media platforms, which can greatly assist in the practical use, troubleshooting, and optimization processes.