Understanding Torque-Speed Curves of BLDC Motors

What is a BLDC Motor?

A BLDC motor is an electric motor that operates without brushes and uses electronic commutation instead of mechanical commutation. In a traditional brushed motor, brushes physically contact the commutator, and create friction and wear. A BLDC motor eliminates this issue by using a permanent magnet on the rotor and an electromagnet on the stator. This structure provides a long lifespan and increases the efficiency of the BLDC motor. 

Construction of the BLDC motor includes two main components: stator and rotor. Stator is the stationary part of the BLDC motor and contains a series of windings. Rotor is the rotating part of the BLDC motor and consists of permanent magnets. 

Watch the YouTube Video by "The Engineering Mindset" to know about the workings of BLDC Motors. 

The working of the BLDC motor depends on the current supplied by the controller to the stator’s windings. A controller supplies the current to energise the stator winding and creates a rotating magnetic field. The rotor interacts with the rotating magnetic field and causes rotation. By adjusting the timing and power supplied to the windings of the stator, the controller modulates the speed and torque of the BLDC motor with high precision to achieve smooth operation. 

BLDC motors come into two main configurations: Inrunner BLDC motor where the rotor is inside the stator and outrunner BLDC motor where the rotor is outside the stator. They are widely used in applications requiring precise control, like robotics, electric vehicles, and drones, due to their high efficiency, fast response, and low heat generation. 

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What is the Torque and Speed Curve of BLDC Motor

The torque and speed curve of the BLDC motor represents the relationship between the motor’s output torque and speed. This curve is essential to understand how the BLDC motor performs under varying load conditions and speed. The torque and speed curve of the BLDC motor can be divided into three regions:

• Constant Torque Region

At low to moderate speed, the BLDC motor operates at constant torque. In this region, the BLDC motor can maintain a steady output torque as the speed increases. It is the region where the BLDC motor performs most efficiently. 

• Constant Power Region

After reaching a certain speed, the BLDC motor enters the constant power region. After this point, the torque starts to decrease as the speed increases but power remains constant. 

• Field Weakening Region

At a very high speed, the BLDC motor enters the field weakening region. At this point, the torque drops sharply as the speed increases and the BLDC motor is no longer able to produce enough torque to overcome load. 

The curve is crucial for understanding the brushless DC motors motor efficiency, load handling, and optimal operating speeds. It helps in selecting the right motor for specific applications, ensuring the motor can deliver the necessary torque at the required speed while avoiding overloads or inefficient operation.

Factors Affecting the Torque-Speed Curve of BLDC Motor

The torque-speed curve of a BLDC (Brushless DC) motor is influenced by several factors that impact its performance. Key factors include:

• Supply Voltage

The supply voltage significantly impacts the BLDC motor’s speed and torque. The higher the voltage, the higher the brushless DC motor’s ability to achieve high speed and maintain torque. As the voltage increases the torque at higher speeds remains constant. Conversely, a decrease in supply voltage also decreases both speed and torque.

• Winding Resistance

The electrical resistance and inductance of the BLDC motor play an important role in determining the torque and speed relationship. Higher winding resistance leads to high power losses and reduces the torque. The inductance impacts how quickly the current can build and be supplied to the motor’s winding which influences the BLDC motor’s response at varying speeds.

• Using Back-EMF

Back-EMF is the voltage generated by the rotating motor which opposes the input voltage. As the BLDC motor’s speed increases, the back EMF also increases and opposes the applied voltage to the windings and limits the speed. The design of the motor, including the number of poles and magnetic properties, affects the back EMF characteristics.

• External Load

The external load connected to the BLDC motor directly impacts the torque and speed. A high load requires more torque to maintain a specific speed, which curves downwards. If the load exceeds the motor’s torque capacity at a given speed, the motor may slow down or stall.

• Controller Performance

The performance of controllers affects the torque and speed of the BLDC motor. Advanced controllers optimise the BLDC motor’s torque by adjusting the current and voltage in real-time. Controllers with poor performance can lead to a less efficient torque-speed curve, especially at higher speeds.

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Conclusion

Understanding the torque-speed characteristics of BLDC motors is essential for engineers and designers aiming to match BLDC motor’s performance with application requirements. By analysing torque-speed curves, one can identify the most suitable BLDC motor and optimise system efficiency, reliability, and longevity.