What Is Continuous Torque, & Why Should You Care About It?

Continuous torque refers to the maximum torque that a motor can continuously deliver without overheating or sustaining damage. It is a crucial parameter in motor design and application, ensuring that the motor operates efficiently and reliably under constant load conditions. 

Different duty cycles are defined for motors (S1 to S10). The key duty cycle for continuous torque is S1, which is defined as the continuous duty cycle at constant load for a given operating temperature post the achievement of thermal equilibrium at that point. The typical duration used for S1 definition is 60 minutes.

Axial Flux Motor Continuous Torque

Continuous torque’s value is particularly important for axial flux motors because of their unique design and applications. Here are a few key reasons why original equipment manufacturers (OEMs) should focus on these parameters:  

• Efficiency and performance: Axial flux motors are known for their high efficiency and power density. Maintaining axial flux motor continuous torque ensures that the motor operates efficiently under constant load conditions, which is crucial for applications requiring sustained performance.
• Thermal management: Continuous torque ratings help manage the thermal load on the motor. Axial flux motors, with their compact design, can generate significant heat. Ensuring that the motor operates within its continuous torque limits prevents overheating and potential damage.
• Application suitability: Many applications of axial flux motors such as electric vehicles (EV) and industrial machinery require consistent and reliable torque output. Continuous torque ratings help ensure that the motor can handle these demands without performance degradation.
• Design optimization: The design of axial flux motors—including the arrangement of magnets and coils—is optimized to deliver high torque in a compact form. Continuous torque ratings help in designing motors that can sustain high torque over long periods, maximizing their utility in various applications.

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Importance in Axial Flux Motors 

Continuous torque is dictated by the continuous current rating, which is limited by thermal constraints (cooling, resistance losses, and winding temperature). Voltage does not directly affect torque but determines the ability to supply current at higher speeds.

Higher voltage allows the motor to operate at higher speeds while maintaining torque by overcoming back electromotive force, which is the voltage generated by a source, such as a battery or an electric motor, that causes current to flow in a circuit. It is measured in volts and represents the energy per unit charge supplied to move electrons through the circuit. Operating voltage is a critical parameter for motors for several reasons:

• Performance and efficiency: The operating voltage directly affects the performance and efficiency of the motor. Motors are designed to operate optimally at a specific voltage range. Deviating from this range can lead to reduced efficiency and performance.
• Torque and speed characteristics: The voltage applied to a motor influences its torque and speed characteristics. For example, in direct-current motors, increasing the voltage typically increases the speed and torque. In alternating-current motors, the voltage affects the magnetic field strength and, consequently, the motor’s torque.
• Thermal management: Operating a motor at the correct voltage can help manage its thermal load. Overvoltage can cause excessive heat generation, leading to insulation breakdown and the potential for motor failure. Undervoltage can cause the motor to draw more current to maintain performance, which also leads to overheating.
• Longevity and reliability: Consistently operating a motor at its rated voltage helps ensure its longevity and reliability. Motors subjected to incorrect voltages may experience premature wear and tear, thus reducing their operational lifespan.
• Safety: Operating a motor at the correct voltage is essential for safety. Overvoltage can lead to electrical hazards, and undervoltage can cause the motor to stall or operate unpredictably, thus posing mechanical risks.

The Difference Between Continuous Torque & Continuous Power Density

Continuous torque and continuous power density are related but distinct concepts in motor performance.Continuous torque is the maximum torque a motor can deliver continuously without overheating or sustaining damage. It ensures that the motor can handle a constant load efficiently and reliably.

Continuous power density refers to the amount of power a motor can continuously deliver per unit volume or weight. It is a measure of how efficiently a motor can convert electrical energy into mechanical power while maintaining thermal stability.

In examining the relationship between the two, consider power calculation: Power (P) is the product of torque (T) and angular velocity (ω): 

P = T ⋅ ω 

This equation shows that power is directly proportional to torque and speed. Therefore, continuous torque contributes to continuous power density, especially at higher speeds.

With thermal management, continuous torque and continuous power density are influenced by the motor’s ability to dissipate heat. Efficient thermal management allows a motor to sustain higher torque and power densities without leading to overheating.

Motors designed for high continuous power density often have optimized cooling systems and materials to handle higher continuous torque. This ensures that they can deliver high performance in a compact form factor.

In summary, while continuous torque focuses on the motor’s ability to deliver sustained rotational force, continuous power density emphasizes the motor’s efficiency in delivering power relative to its size or weight. Both are crucial for OEM applications requiring high performance and reliability.

Applications

Continuous torque is crucial in many applications where sustained performance and reliability are essential in ensuring the efficiency and safety of various systems and applications. One application is EVs, for which continuous torque is vital to provide consistent acceleration and maintain speed, especially during long drives. It ensures that the vehicle can handle varying loads and driving conditions without being subjected to overheating.

In manufacturing and automation with industrial machinery, continuous torque is necessary for machines such as conveyor belts, robotic arms, and CNC machines. These machines often operate for extended periods and require reliable torque to maintain precision and efficiency.

Wind turbines rely on continuous torque to convert wind energy into electrical power efficiently. The ability to sustain torque under varying wind conditions is crucial for maximizing energy output and ensuring the longevity of the turbine.

In aerospace applications, it is important for actuators and control systems that manage the movement of aircraft components. Reliable torque helps to ensure the safety and performance of these critical systems.

Continuous torque is also essential for marine engines and propulsion systems to provide steady thrust and maneuverability over long distances. This is particularly important for commercial ships and submarines.

In medical equipment such as MRI machines and surgical robots, continuous torque helps ensure precise and reliable operation. This is crucial for patient safety and the effectiveness of medical procedures.