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Understanding the Torque/Speed Curve of BLDC Motors (Simplified Case)*

The BLDC (Brushless DC) motors are valued for their efficiency, instantaneously available torque, and their ability to adapt to demanding applications such as agriculture, robotics, or electric vehicles. An essential concept for their sizing and optimal use is the torque/speed curve, which describes how the torque delivered by the motor changes with the rotational speed.

This article presents a simplified model of this curve and its practical implications.

1. The Quasi-Constant Torque Zone (Current Limited)

In most BLDC motors, for low to moderate speeds, the delivered torque remains almost constant.

  • Principle: The motor is current limited, and the torque is directly proportional to this current.

  • Simplified expression:

\(C≈C_{nom}\)

  • Typical range: from stop to transition speed.

  • Characteristic: The motor provides its nominal torque, ideal for starting, carrying heavy loads, or inclines.

Caution: The current must not exceed the motor's maximum limit, to avoid overheating or triggering protections.

2. The Quasi-Constant Power Zone

Beyond the transition speed, the torque begins to decrease as the speed increases.

  • Principle: The available voltage to the motor becomes the limit. As the back electromotive force (EMF) increases with speed, the motor can no longer maintain the nominal torque.

  • Simplified expression:

\(P≈C⋅ω≈constante⟹C∝ \frac {1}{ω}\)


  • Typical range: speeds above the transition speed.

  • Characteristic: The motor operates at quasi-constant power, which is typical during fast movements or light loads at high speed.

3. Practical Application for Sizing

The torque/speed curve guides the choice of reduction ratios and control strategies.

  1. Choose an appropriate reduction ratio:

    • Allows staying in the quasi-constant torque zone during critical phases (starting, climbing inclines).

    • Avoids overcurrents and optimises efficiency.

  2. Avoid the low torque zone for heavy loads:

    • At high speed, the torque decreases. An excessive load may block the motor or saturate the current.

  3. Optimise power usage:

    • Understanding the torque → constant power transition allows for properly sizing the motor and controller, ensuring safe and durable operation.

4. Simplified Example

Imagine a 48V BLDC motor with:

  • Nominal torque: 10 Nm

  • Transition speed: 3000 rpm

  • 0 → 3000 rpm: quasi-constant torque ≈ 10 Nm

  • >3000 rpm: torque decreases with speed, quasi-constant power

For an agricultural machine, it is recommended to choose a reduction ratio that keeps the motor below 3000 rpm at maximum load, thus ensuring sufficient torque to start or climb a slope without overcurrent.

Conclusion

The torque/speed curve of a BLDC motor, even in a simplified model, is an essential tool for:

  • Properly sizing the motor and the gearbox.

  • Ensuring sufficient torque at start or on inclines.

  • Optimising efficiency and protecting the motor against overcurrents.

Understanding this curve allows for getting the best out of 48V BLDC motors in all applications where torque and performance are critical.


*: 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.