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Internal Rotor Motor Drive Vs. External Rotor Motor Drive

Hub motors could be driven by internal rotor motors or external rotor motors. Installed within the wheel, the greatest advantage of the hub motor is that it combines the power system, transmission system, and braking system into one unit. Moreover, electric vehicles using this motor have simpler chassis structures and high transmission efficiency. Depending on whether a reduction mechanism is installed; hub motors can be split into two categories: reduction drive and direct drive.

Internal Rotor Motor Drive

Reduction drive, also known as internal rotor motor drive, employs a high-speed internal rotor motor with a maximum speed of 15,000 rpm. As this type of motor operates at relatively high speeds, it needs a fixed-ratio reducer between the drive motor and wheels. Because these rotors have a smaller diameter, the power density is very high and the moment of inertia is low. This makes the dynamics of these motors particularly wonderful, as the speed and direction can be changed in milliseconds.


Internal Rotor Motor Drive Vs. External Rotor Motor Drive

Image Source: Portescap

The reducer controls the motor’s output speed and increases torque to meet the wheels’ output needs, propelling the vehicle forward. The output torque becomes more substantial after the reducer improves torque power, enhancing the vehicle’s hill-climbing capability. The car can maintain a large torque for an extended period during low-speed operations.

However, hub motors driven by internal rotor motors have some potential problems, such as lubrication issues, rapid gear wear and tear which is difficult to repair, motor durability issues, heat dissipation challenges, and noise reduction.

External Rotor Motor Drive

Direct drive, or external rotor motor drive, uses a low-speed external rotor motor, generally controlled at around 1,500 rpm. The motor’s outer rotor directly aligns or connects with the automobile wheel rim, eliminating the need for a reducer. Consequently, the motor’s rotation speed signifies the wheel’s rotation speed. The direct drive system does not incorporate a reducer and offers benefits like a simpler structure, relatively small axial size, and high transmission efficiency.

The driving method for the hub motor used in the pure electric vehicle is a direct drive or external rotor motor drive, wherein the motor’s stator, rotor, and inverter are combined into a single unit comprised of eight sub-motors that share a common rotor, with algorithms in place to realize the independent and cooperative control of each sub-motor.

outer rotor motor vs. internal rotor motor

Image Source: Portescap

This distributed structure reduces the power demand on each sub-motor, thus enabling the use of smaller, cost-effective electronics, and allowing for a highly compact integration of the entire motor. Simultaneously, if any of the sub-motors were to malfunction, the remaining motors could continue to operate normally, preventing an immediate vehicle breakdown.

However, during the car’s start-up, climbing, and other special working conditions, it requires a high current input. This makes it difficult to ensure the car’s battery and permanent magnets will not be damaged.

Currently, hub motors are successful in electric vehicles. With ongoing in-depth research and development, hub motors will become widely used in more industries.

Motor Solution Provider: Tengye

At Tengye, we’re thrilled to be early participants in your new projects. Our passion for magnets extends from material selection and design optimization to providing magnetic solutions. We hope to bring our knowledge and experience to the forefront by actively engaging in your upcoming projects. Fill out the form and work with experts to choose suitable magnet rotor assemblies for motors.


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Jonah Jin

Jonah Jin
Managing Director

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