Brushless motors operate using different current control methods depending on the drive system. To regulate the motor's rotational speed and torque, it is essential to manage the amount of current flowing through the motor windings. One widely used technique for controlling motor current is Pulse Width Modulation (PWM).
PWM is an electrical control method that adjusts the voltage applied to the motor windings by rapidly switching a power transistor on and off. This creates a pulse-shaped voltage waveform, where the average voltage can be controlled by varying the duty cycle—essentially the ratio of the time the switch is on compared to the total cycle time.
Figure 3.12 illustrates a basic PWM circuit for a DC motor, while Figure 3.13 shows how the voltage and current waveforms change as the pulse width modulates. The key idea is that by adjusting the duty cycle, the average voltage delivered to the motor increases or decreases, which in turn controls the motor current.
However, due to the inductance of the motor windings, the current does not immediately follow the voltage changes. Instead, it lags behind when the voltage is increased and gradually decreases when the voltage is removed.
In PWM control, the duty cycle is determined by comparing a reference voltage with a triangular waveform. When the reference voltage is higher than the triangular signal, the switching element turns off, and when it's lower, the element turns on. By increasing the reference voltage, the duty cycle increases, leading to a higher average voltage and more current. Conversely, lowering the reference voltage reduces the duty cycle and the current.
For brushless motors using a square wave drive system, PWM is employed to control the switching elements that energize the motor windings, allowing for precise control over speed and torque.
In a sine wave drive system, the reference voltage is shaped into a sinusoidal waveform, enabling a smooth and continuous current flow. This approach ensures that the current waveform closely matches the desired output, resulting in smoother motor operation and reduced torque ripple.
To achieve accurate sine wave driving, the motor driver must precisely detect the rotor position, often using Hall-effect sensors or encoders. Oriental Motor employs both methods to meet varying requirements for speed accuracy.
If you're interested in learning more about brushless motor systems, check out our previous post on brushless motor drive systems and torque, or explore our next post on the technology used in brushless motors. You can also discover more about Oriental Motor’s AC and DC input brushless motor systems, or take a look at our overview of brushless motor systems.
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