What is a hub motor ? As the name suggests, it's a motor integrated into the wheel hub, directly driving the wheel. Hub motor technology, also known as in-wheel motor technology, is characterized by integrating the power, transmission, and braking systems into the wheel hub, thus greatly simplifying the mechanical components of electric vehicles. Hub motor technology is not new; electric vehicles equipped with hub motors on the front wheels were manufactured as early as 1900. In the 1970s, this technology was applied in fields such as mining vehicles. For hub motors used in passenger cars, Japanese manufacturers started researching and developing this technology earlier and currently hold a leading position. International automotive giants, including GM and Toyota, have also ventured into this technology.
In-wheel motor structure
A hub motor power system typically consists of a motor, a reduction gear, a brake, and a cooling system. Hub motor power systems are mainly divided into two structural types based on the rotor type: internal rotor and external rotor. The figure shows a simplified structural diagram of these two types of hub motors. Typically, the external rotor type uses a low-speed external rotor motor with a maximum speed of around 1000-1500 r/min. It has no reduction gear, and the external rotor is fixed to or integrated with the wheel rim, with the wheel rotating at the same speed as the motor. The internal rotor type uses a high-speed internal rotor motor and is equipped with a reduction gear with a fixed transmission ratio. To achieve higher power density, the motor speed is typically as high as 10000 r/min. The reduction gear structure usually uses a planetary gear reduction device with a transmission ratio of around 10:1, with the wheel rotating at around 1000 r/min.
Internal rotor type
High-speed internal rotor hub motors offer high power-to-weight ratio, are lightweight, compact, efficient, quiet, and low-cost. However, they require a reduction gear, reducing efficiency and increasing unsprung mass. The maximum speed is also limited by coil losses, friction losses, and the capacity of the transmission mechanism. Low-speed external rotor motors, on the other hand, are simple in structure, have small axial dimensions, high power-to-weight ratio, can control torque over a wide speed range, and have a fast response. The external rotor is directly connected to the wheel, eliminating the need for a reduction gear, resulting in high efficiency. However, to achieve higher torque, the engine size and weight must be increased, leading to higher costs, lower efficiency during acceleration, and higher noise. The figure shows two types of hub motors. Both structures are currently used in electric vehicles, but with the advent of compact planetary gear transmissions, high-speed internal rotor drive systems are more competitive in terms of power density than low-speed external rotor systems.
External rotor type
Because the electric braking capacity of in-wheel motor power systems is relatively small and cannot meet the braking performance requirements of the entire vehicle, an additional mechanical braking system is usually required. The brakes in in-wheel motor systems can be drum or disc brakes, depending on the structure. Due to the electric braking capacity of the motor, the design capacity of the brake can often be appropriately reduced. Most in-wheel motor systems use air cooling, while some use water cooling or oil cooling to dissipate heat from the motor, brakes, and other heat-generating components, but these methods are more complex.
Classification of motor magnetic field types
Hub motor systems are classified into two types based on their magnetic field: radial magnetic field and axial magnetic field. A comparison is as follows:
(1) The structure of the axial flux motor is more conducive to heat dissipation, and its stator does not need to be cored;
(2) The forces between the stator and rotor of the radial flux motor are relatively balanced, and the magnetic path is obtained by stacking silicon steel sheets, which is simpler and more mature.
Hub motors are categorized into permanent magnet, induction, and switched reluctance types. Their characteristics are as follows:
(1) Induction (asynchronous) motors are simple in structure, robust and durable, low in cost, reliable in operation, have small torque ripple, low noise, do not require position sensors, and have high speed limits; the disadvantages are that the drive circuit is complex and the cost is high, and the efficiency and power density of asynchronous motors are relatively low compared to permanent magnet motors.
(2) Brushless permanent magnet synchronous motors can adopt cylindrical radial magnetic field structure or disc axial magnetic field structure, which have high power density and efficiency as well as a wide speed range. They have a very broad development prospect and have been applied in a variety of electric vehicles at home and abroad.
(3) Switched reluctance motors have the advantages of simple structure, low manufacturing cost and good speed/torque characteristics, and are suitable for electric vehicle drive; the disadvantage is that the design and control are very difficult and precise, and the operating noise is high.
