Outer rotor motors are widely used in unmanned ground vehicle driving system because of their compact size and large torque to volume ratio. However, these motors tend to suffer severe thermal condition due to geometry constraints.
This is caused by back electromotive force (EMF) that forms when the windings are energized. The direction of back EMF polarity opposes the main voltage that the windings receive.

High Inertia
Most motors have a fixed part called the stator that provides a magnetic field. The rotor has magnets that are attracted and repelled by the stator magnetic field to make it spin. The rotor can be rotated by applying DC voltage.
In order to control the rotational velocity of a motor system, it is important to know the total system inertia. A precise estimate of the inertia is obtained by conducting acceleration and deceleration tests.
Typical inertia estimates are given in the motor datasheets. However, a direct relationship between the motor inertia and the load's inertia is difficult to achieve because of a variety of small, hard to quantify effects such as lubricant viscosity and nut pitch vs. efficiency.
In general, if the load's inertia significantly exceeds the motor's inertia, the motor will struggle to reach the load's required position or speed and draw excessive current. This can lead to overheating of the motor and other electrical components and may increase overall system costs.

Low Noise
For brushless motors, a major source of noise is generated when the brushes and commutator switch between being open and closed. This instantaneous switching generates significant electrical noise that can get coupled into sensitive circuitry.
To reduce this, the motor can be designed with a more gradual change in airflow direction or spacing between stationary parts to reduce the noise. Another solution is to add extra heat dissipation features to the rotor.
Noise can also be reduced by removing sharp edges and burrs from all structural components touching the airstream or by increasing the distance between them. This can be done with a laser vibrometer or by using a more robust housing. Noise measurements from a small outer rotation motor showed that spatially-averaged measurement pressure was highest at frequencies of 84 Hz, 252 Hz and 508 Hz. The higher response at 1360 Hz was possibly caused by internal resonance, although further investigation is required. It is difficult to identify internal acoustic resonances using run-up tests and impulse tests alone.

Flexibility
Using a genetic algorithm, the stator outer diameter and motor axial length were optimized. This resulted in optimal rated speed, efficiency, cogging torque, and specific electric loading. The rotor yoke and air gap flux density were also optimized.
Unlike an inner rotor design, an outer rotation motor allows for flexible mounting. This is especially important for unmanned ground vehicle (UGV) driving systems where the motor must be positioned at different positions in the wheel.