With the advancement of modern power electronics and materials science, brushless DC motors (BLDC) have become the core power components in the fields of industrial automation, consumer electronics, and new energy, thanks to their advantages such as high efficiency, long service life, and precise control. Among them, the outrunner structure, with its unique mechanical layout and electromagnetic characteristics, demonstrates significant advantages in specific application scenarios. Moreover, the pole number design, as a crucial parameter of the motor’s magnetic circuit system, directly affects the motor’s torque output, speed range, and operating efficiency.
The Relationship between Torque and Speed
Torque Enhancement
The outrunner design has a relatively large moment of inertia. Since the rotor is located on the outside, the diameters of the magnet steel and the air gap are larger, enabling it to provide higher torque output. The six-pole design further increases the number of magnetic poles, enhancing the uniformity of the magnetic field distribution. As a result, it can provide smoother torque at low speeds, making it suitable for applications that require high starting torque, such as drones and power tools.
Speed Limitation
An increase in the number of poles leads to an increase in the electrical frequency of the motor. Under the same power supply frequency, the maximum speed of a six-pole motor is lower than that of a motor with fewer poles (such as a four-pole motor). Therefore, six-pole outrunner motors are generally more suitable for medium-low speed and high torque scenarios.
Efficiency and Losses
Iron Loss and Eddy Current Loss
The six-pole design may result in a higher alternating frequency of the magnetic circuit, increasing the eddy current loss and hysteresis loss in the stator core. However, due to the larger heat dissipation area of the outrunner structure (the rotor is directly exposed), this loss may be partially offset, improving the heat dissipation efficiency.
Copper Loss Optimization
The stator windings of an outrunner motor are usually more compact, and the winding ends are shorter, which reduces the copper loss. The six-pole design may require a more complex winding arrangement, but the impact on efficiency can be balanced by optimizing the coil design.
Mechanical Characteristics
Moment of Inertia
The moment of inertia of the outrunner design is significantly higher than that of an inrunner motor. The six-pole structure further increases the mass distribution radius of the rotor, which may lead to a slower dynamic response (longer acceleration/deceleration time). However, this characteristic also makes it more suitable for scenarios that require stable operation, such as fans and hub motors.
Vibration and Noise
The magnetic field harmonics of the six-pole design are fewer than those of the four-pole design, and the torque ripple is lower, thus reducing vibration and noise. Combined with the symmetry of the outrunner structure, the overall operation is quieter.
Heat Dissipation and Temperature Rise
Heat Dissipation Advantage
The rotor of the outrunner structure is directly exposed to the external environment and can dissipate heat directly through the housing, making it suitable for long-term high-load operation. However, since the stator is located inside, additional heat dissipation designs (such as thermal conductive adhesive and internal air ducts) may be required to prevent heat accumulation.
Magnet Steel Stability
The magnet steel of the outrunner is directly attached to the inner side of the rotor housing. There is a risk of demagnetization of the magnet steel in a high-temperature environment, so it is necessary to select magnetic materials with a high temperature resistance grade, such as N52SH.
Adaptability to Application Scenarios
Low-speed and High-torque Scenarios
Typical applications include hub motors of electric bicycles, propeller drives of drones, fans, industrial servo systems, etc. These scenarios require high torque and stable operation, and the six-pole outrunner design can well meet these needs.
Space Limitation
The outrunner structure is usually flatter (shorter in the axial direction), making it suitable for scenarios where the installation space is limited but there is sufficient radial space, such as hub motors or disc motors.
Control Complexity
Drive Frequency
The electrical frequency of a six-pole motor is relatively high, requiring the controller to support a higher switching frequency, which places higher demands on the performance of MOSFETs or IGBTs.
Sensor Requirements
The Hall sensors of the outrunner structure are usually installed inside the stator and may require more precise positioning to avoid magnetic field interference.
Manufacturing and Cost
Process Difficulty
Pasting the magnet steel and debugging the dynamic balance of the outrunner are relatively complex. The six-pole design requires higher magnetic pole positioning accuracy, which may increase the manufacturing cost.
Material Cost
The outrunner requires a larger volume of magnet steel and more robust housing materials (such as aluminum alloy), which may result in a higher cost compared to the inrunner design.
The core advantages of a six-pole outrunner brushless motor lie in its low-speed and high-torque performance, low noise, and good heat dissipation, but it sacrifices part of its speed and dynamic response capabilities. Whether this design is suitable needs to be considered in combination with specific application scenarios:
Recommended Scenarios: Drones, hub motors of electric vehicles, fans, household appliances (such as air conditioning fans).
Not Recommended Scenarios: Ultra-high-speed motors (such as high-speed electric spindles), servo systems that require fast dynamic response.
With the continuous progress of technology, six-pole outrunner brushless motors are expected to demonstrate their unique advantages in more fields. Through improved design, innovative heat dissipation solutions, and enhanced controller performance, these motors are expected to achieve significant breakthroughs in efficiency, performance, and reliability, injecting new impetus into technological development and driving various industries towards a more efficient and environmentally friendly direction.