Common BLDC Motor Structures

Although all brushless DC motors operate on the same basic principle, their mechanical structures can vary significantly depending on the application. The original article introduces several common motor configurations to help readers understand why different BLDC motors look and perform differently.

(Illustration: Comparison of several common BLDC motor structures, including inner-rotor, outer-rotor, slotless, and flat (pancake) motors.)

Inner-Rotor and Outer-Rotor Designs

As introduced earlier, BLDC motors are generally divided into inner-rotor (inrunner) and outer-rotor (outrunner) designs.

However, the differences extend beyond the location of the rotor—they also affect cooling, inertia, torque characteristics, and application suitability.

Inner-Rotor Motors

In an inner-rotor motor:

  • The rotor rotates inside the stator.
  • The stator is fixed to the motor housing.
  • Heat generated by the windings is transferred efficiently to the housing.
  • The rotor has relatively low rotational inertia, allowing rapid acceleration and deceleration.

These characteristics make inner-rotor motors ideal for applications requiring high speed and precise dynamic control.

Typical examples include:

  • Servo motors
  • Industrial automation equipment
  • Medical devices
  • High-speed electric tools

(Illustration: Cross-sectional view of an inner-rotor BLDC motor with arrows indicating the heat dissipation path from the stator to the housing.)

Outer-Rotor Motors

In an outer-rotor motor:

  • The stator is located at the center.
  • The rotor surrounds the stator and rotates externally.
  • Permanent magnets are attached to the inside of the rotating outer shell.

Because the rotor has a larger diameter, the motor naturally produces higher torque.

Advantages include:

  • Higher starting torque
  • Lower operating speed
  • Reduced need for gear reduction
  • Compact overall length

Outer-rotor motors are widely used in:

  • Cooling fans
  • UAVs and drones
  • Gimbal motors
  • Small electric vehicles
  • Direct-drive systems

(Illustration: Cross-sectional view of an outer-rotor BLDC motor showing the rotating outer shell and centrally mounted stator.)

Slotted and Slotless Stators

Another important structural difference is the design of the stator.

Slotted Stator

Most commercial BLDC motors use a slotted stator, where copper windings are placed inside slots in the laminated steel core.

Advantages include:

  • High torque output
  • Efficient magnetic coupling
  • Mature manufacturing process
  • Lower production cost

However, the interaction between the rotor magnets and stator teeth can create cogging torque, which may cause slight torque ripple at very low speeds.

(Illustration: Close-up of a slotted stator with copper windings installed inside the stator slots.)

Slotless Stator

In a slotless motor, the windings are not placed inside teeth. Instead, they are arranged in a smooth cylindrical structure.

Compared with slotted motors, slotless motors provide:

  • Extremely smooth rotation
  • Very low cogging torque
  • Lower acoustic noise
  • Excellent precision at low speeds

Their disadvantages include:

  • Lower torque density
  • More complex manufacturing
  • Higher production cost

For these reasons, slotless motors are commonly used in high-precision applications such as medical instruments, aerospace equipment, and precision motion-control systems.

(Illustration: Comparison of slotted and slotless stator structures, highlighting the absence of stator teeth in the slotless design.)

Flat (Pancake) Motors

Some applications require a motor with a very short axial length while maintaining a relatively large diameter.

In these cases, engineers often use flat motors, also known as pancake motors.

Compared with conventional cylindrical motors, pancake motors offer:

  • Compact axial dimensions
  • Large rotor diameter
  • High torque within a limited installation space

They are commonly found in:

  • Robotic joints
  • Medical imaging equipment
  • Precision automation systems
  • Compact direct-drive mechanisms

(Illustration: Comparison of a conventional cylindrical BLDC motor and a flat pancake-style BLDC motor.)

Direct Drive and Gear Drive

Depending on the application, a BLDC motor may be used either directly or together with a gearbox.

Direct Drive

In a direct-drive system:

  • The motor shaft connects directly to the load.
  • No mechanical reduction is used.
  • The system has fewer moving parts and higher transmission efficiency.

Direct-drive systems are commonly used where smooth motion, low maintenance, and high precision are required.

(Illustration: BLDC motor directly connected to a driven load without a gearbox.)

Gear Drive

When higher output torque or lower output speed is required, a gearbox can be attached to the motor.

Common gearbox types include:

  • Spur gearboxes
  • Planetary gearboxes
  • Worm gearboxes

Adding a gearbox allows:

  • Higher output torque
  • Lower output speed
  • Improved load capability

However, gearboxes also introduce:

  • Mechanical losses
  • Additional noise
  • Increased maintenance requirements

(Illustration: BLDC motor connected to a planetary gearbox driving an output shaft.)

Cooling Methods

As motor power increases, effective cooling becomes increasingly important.

Common cooling methods include:

Natural Air Cooling

Heat is dissipated through the motor housing without additional cooling devices.

Suitable for:

  • Small motors
  • Intermittent-duty applications

Forced Air Cooling

A cooling fan increases airflow around the motor housing, improving heat dissipation.

Suitable for:

  • Medium-power motors
  • Continuous-duty operation

Liquid Cooling

High-power motors may use water or other coolant circulating through cooling channels.

Liquid cooling provides:

  • Superior thermal performance
  • Higher continuous power output
  • Improved temperature stability

It is commonly used in electric vehicles and industrial servo systems.

(Illustration: Comparison of natural air cooling, fan-assisted cooling, and liquid cooling for BLDC motors.)

Key Takeaways

  • BLDC motors are available in a variety of structural configurations optimized for different applications.
  • Inner-rotor motors excel at high-speed, high-dynamic applications, while outer-rotor motors provide higher torque at lower speeds.
  • Slotted stators offer higher torque density, whereas slotless stators provide smoother rotation with minimal cogging torque.
  • Flat (pancake) motors are designed for applications with limited installation height.
  • BLDC motors can be used in direct-drive systems or combined with gearboxes when additional torque reduction is required.
  • Proper cooling is essential for maintaining efficiency, reliability, and service life, especially in high-power applications.

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