Use of BLDC Motors for Low-Speed Rotation Mechanisms*

The brushless DC motors (BLDC) are increasingly used in various applications, particularly to drive mechanisms with low rotation speed, such as wheel motors.

However, to achieve a high torque at these low speeds, the integration of a gearbox is often essential.

This article presents the role of gearboxes, their main types, and the basic calculations necessary to optimise the use of BLDC motors in this context.

1. At low speed and high torque: the importance of the gearbox

Gearboxes play an essential role in converting the motor's rotation speed into available output torque.

They allow multiplying the transmitted torque to the wheel or output shaft, which is crucial in applications requiring significant force at low speed (mobile robots, direct drives, precision drives, etc.).

Calculation of the reduction ratio

The reduction ratio R is defined as follows:

\(R = \frac{n_{motor}}{n_{output}}\)

where:

\(n_{output}\) : motor rotation speed (rpm)
\(n_{motor}\) : output rotation speed of the gearbox (rpm)

The output torque is then given by the relation:

\(T_{output} = T_{motor} \times R \times \eta_{trans} \)

with:

\(T_{motor}\) : nominal motor torque
\(R\) : reduction ratio
\(\eta_{trans}\) : overall efficiency of the transmission system

This formula shows that the output torque increases proportionally to the reduction ratio, considering mechanical losses (friction, play, deformations, etc.).

2. Specific case of the wheel motor

In a wheel motor application, the gearbox does not only adapt the speed:

it also supports radial and axial loads generated by the vehicle's weight and dynamic forces.

Thus:

the sizing of the bearings must be carefully done,
the mechanical rigidity of the gearbox must ensure good shock resistance,
and sealing is often a major reliability criterion (outdoor environment, dust, humidity, etc.).

3. Types of gearboxes suitable for BLDC motors

Type of gearbox	Advantages	Disadvantages	Typical applications
Worm gear	High reduction ratio, compactness	Low efficiency, possible overheating	Motorised doors, actuators
Planetary	Excellent compactness, good efficiency, high robustness	Higher cost, precise assembly required	Wheel motors, robots, AGV
Spur gear	Simplicity, good efficiency	Noise, bulkiness	Machine tools, fixed drives
Belt drive	Positioning flexibility, shock absorption	Possible slippage, regular maintenance	Conveyors, flexible drives

4. Safety and maintenance in case of power failure

For systems where the position must be maintained at a standstill, it is recommended to use a fail-safe brake.

This type of brake, often spring-operated, releases only under tension (via an electromagnet).

Advantages:

Increased safety in case of power failure
Mechanical holding of the system on a slope or in a fixed position
Protection against unintentional movements

This device is particularly useful for mobile applications, lifts, or any system requiring a safe stop.

5. Conclusion

The use of BLDC motors for low-speed rotation mechanisms is an efficient and reliable solution.

The addition of a well-sized gearbox allows to:

maximise the available torque,
optimise the overall efficiency,
and ensure the safety and durability of the system.

By selecting the appropriate type of gearbox and performing the appropriate calculations, it is possible to design efficient drives that precisely meet the requirements of each application.

*: The technical information presented in this article is provided for guidance only. It does not replace the official manuals of the manufacturers. Before any installation, handling or use, please consult the product documentation and follow the safety instructions. Torque.works cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.