Conventional brushless DC motors are constructed with a permanent magnet rotor located inside a wound stator. But one type of DC motor is designed with the rotor on the outside and the stator and housed inside the rotor. Permanent magnets are mounted on the inner diameter of the rotor housing (sometimes referred to as the ‘bell” or “cup”), and the rotor rotates around the internal stator with windings. This design is often referred to as an external rotor motor, but can also be called an outer rotor motor, an outrunner motor, or a cup motor.

The external rotor design provides several performance advantages. First, to house the stator, the rotor of an external rotor motor is by necessity larger than the rotor of a conventional DC motor. And a larger rotor means higher inertia, which helps to dampen torque ripple (a common problem in conventional DC motors) and provide smooth, stable operation, even at low speeds.

Another advantage of external rotor motors is that they can typically produce higher torque than comparably sized internal rotor designs. Recall that torque is a product of the magnetic force times the radius of the air gap (length of magnetic flux). For a given motor diameter, external rotor motors have a larger air gap area than inner rotor designs, and the larger air gap allows a higher force to build. They also have a larger air gap radius, which increases the “lever arm” for torque production. The larger diameter (and, therefore, circumference) of the rotor in external rotor designs also means the rotor can accommodate more poles, which further increases magnetic flux.

External rotor motors are axially shorter than inner rotor motors with similar performance characteristics. This compact size and high torque production make them ideal for directly driving the propellers of remote-controlled model airplanes and drones. In high-precision applications, such as optical drives, their smooth, consistent speed is a benefit over other motor types. And in applications with varying loads, such as industrial power tools, pumps, fans, and blowers, the high inertia of external rotor motors can help to “push through” load fluctuations and provide steady output torque.

Fan and blower applications are one of the more common uses for external rotor motors thanks to a specific design benefit: The external rotor can serve as the hub of the fan or blower impeller. This provides a compact package and allows the impeller to act as a large, rotating heat sink and assist with motor cooling.

But integrating the rotor into the impeller also increases the motor’s mechanical time constant — the amount of time required for the motor to reach 63.2 percent of its final speed for a given voltage — an important parameter for ensuring the motor doesn’t overheat.

τ_m = mechanical time constant of the motor
R = winding resistance
J = rotor inertia
K_e = back EMF constant
K_t = torque constant

As shown in the equation, the motor’s mechanical time constant depends in part on the rotor inertia. When the rotor is integrated into the impeller, the inertia of the rotor and impeller are considered together. This higher inertia results in a higher mechanical time constant, and therefore, a longer time for the motor to reach its required speed.