Brushless motors rely on various methods to control current flow, depending on the drive system being used. In addition to managing motor speed, it's essential to regulate the amount of current flowing through the motor windings to control torque. One widely used technique for this purpose is PWM (Pulse Width Modulation).
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, allowing precise control over the average voltage supplied to the motor. By varying the pulse width—also known as the duty cycle—the average voltage can be adjusted, which directly affects the motor current.
The concept is illustrated in Figure 3.12, where a basic PWM circuit for a DC motor is shown. In Figure 3.13, you can see how the voltage and current waveforms change with different pulse widths. When the pulse width increases, the duty cycle rises, resulting in a higher average voltage and more current flowing through the motor. Conversely, reducing the pulse width lowers the duty cycle, decreasing the average voltage and motor current.
Due to the inductance of the motor windings, the current does not immediately follow the voltage changes. Instead, it lags behind during the ON phase and gradually decreases when the voltage is turned off. This behavior is important to consider when designing or analyzing PWM systems for brushless motors.
In PWM control, the duty ratio is determined by comparing a reference voltage with a triangular wave signal. If the triangular wave voltage is lower than the reference, the switching element turns ON; if it's higher, it turns OFF. Adjusting the reference voltage allows for fine-tuning of the motor current. Increasing the reference voltage raises the duty ratio, leading to higher average voltage and increased current. Lowering the reference voltage has the opposite effect.
For brushless motors using a square wave drive system, PWM is applied to the switching elements that energize the motor windings. This enables control over both motor speed and torque. However, in sine wave drive systems, the reference voltage is shaped into a sinusoidal waveform, allowing for smoother current flow and improved performance. In such cases, the current waveform closely follows the reference signal, resulting in a more efficient and controlled operation.
To achieve sine wave driving, it's crucial for the motor driver to accurately detect the rotor position. This is typically done using Hall-effect sensors or encoders, enabling what's known as a "closed-loop" control system. Oriental Motor employs both sensor types to meet varying requirements for speed accuracy and performance.
If you're interested in learning more about brushless motor technologies, check out our previous post on Brushless Motor Drive Systems and Motor Torque. For more insights into the technology behind these motors, visit our next post on the technology used with brushless motors.
Explore more about Oriental Motor’s AC and DC input brushless motor systems, or take a closer look at our brushless motor systems overview. Don’t forget to subscribe to our blog for monthly updates and new content!
forklift, electric forklift, diesel forklift, warehouse forklift, pallet forklift, industrial forklift
SHANDONG KEN STONE HEAVY MACHINERY CO.,LTD , https://www.chinaexcavator.com