Technical Manual Series: Brushless Motor Drive Systems

Brushless motors and their drive systems share a deep connection, and numerous approaches have been developed to enhance the control and performance of these motors. In this article, we’ll explore two primary drive systems: square wave drive and sine wave drive.

3.1.1 Square Wave Drive System (120° Conduction Mode)

Brushless motors come in a variety of drive configurations, and one commonly used method is the 120° conduction mode, which employs a square wave drive. As illustrated in Fig. 3.1, this technique switches the excitation states based on the combinations of hall effect IC outputs. While there may be speed fluctuations at lower speeds, the system offers high output power with a relatively simple circuit design. Consequently, it’s widely adopted for controlling brushless motors in office equipment and power devices.

3.1.2 Sine Wave Drive System

The sine wave drive system precisely controls the applied voltage to create a sinusoidal motor current, as depicted in Fig. 3.2. This drive system minimizes torque ripple and ensures smooth rotation. Additionally, it reduces the noise produced by the motor.

However, performing sine wave driving requires accurate detection of the rotor magnetic pole positions. Historically, a high-resolution encoder was necessary for this task, but it increased both the overall motor length and its cost.

Today, instead of relying on encoders:

• Software processes the hall effect IC signals.
• Magnetic pole positions are detected using motor inductance and back EMF.

As a result, various methods can now be used to determine the magnetic pole positions and achieve sine wave driving.

3.1.3 Current Waveforms for Each Type of Drive System

Figure 3.3 shows the actual current waveforms. The square wave drive exhibits some distortion compared to the applied voltage waveform, but its shape is closer to a sine wave than a true square wave. The sine wave drive, on the other hand, closely matches the applied voltage waveform.

 

In upcoming posts, we’ll delve deeper into how different drive systems impact motor performance.

 

 

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