The Effect of PWM Frequency on Permanent Magnet Synchronous Motor Noise
Many applications utilizing permanent magnet synchronous miniature motors demand driving the motors either at more than one load point or through specific load cycles. Running the motor at usable load points requires a variable, controllable power source; this can be achieved through continuous linear regulation power supplies or through Pulse Width Modulation (PWM). PWM is a technique used to vary the effective voltage applied to a permanent magnet synchronous motor by switching the supply voltage on and off at a fast rate. The frequency of these voltage pulses can vary from a few hundred Hertz to several kilohertz, depending on the application requirements. However, the PWM frequency has a direct impact on the acoustic noise generated by the motor.
How Does PWM Frequency Impact Permanent Magnet Synchronous Motor Noise?
The effect of PWM frequency on permanent magnet synchronous motor noise can be explained by understanding the underlying mechanism of noise generation in motors. Permanent magnet miniature synchronous motors produce noise due to the interaction between the magnetic field and the mechanical structure of the motor. The commutation or switching of current in the motor’s windings, which is controlled by the PWM signal, can also contribute to this noise.
At lower PWM frequencies, the switching of current in the motor’s windings is slower, leading to a more pronounced acoustic noise. This is due to the magnetic field of the motor suffering from more instability due to the slower switching, thereby creating more vibrations in the motor's mechanical structure. At higher PWM frequencies, however, the switching of current is faster, leading to less audible noise. In this case, the magnetic field suffers from less instability and creates fewer vibrations in the motor's mechanical structure.
PWM frequency also affects the magnitude of mechanical noise produced by permanent magnet miniature synchronous motors. The higher the frequency, the lower the amplitude of the noise. The reason behind this relationship is the motor's inertia and response to the PWM signals. When the frequency is low, the motor tends to respond slowly and irregularly to the signals, leading to a jerky and noisy operation. However, when the frequency is high, the motor responds quickly and smoothly to the signals, resulting in a quieter and more stable operation.
In addition to reducing the mechanical noise, increasing the PWM frequency also has other benefits, including:
- Higher Efficiency. A high-frequency PWM signal reduces the motor's power losses, improving its overall efficiency (see Graph 1).
- Smoother Operation. A higher PWM frequency results in smoother movement and less vibration, making the motor ideal for precision applications.
- Reduced Temperature Rise. A high-frequency PWM signal reduces the motor's temperature rise, reducing the risk of thermal damage.
Graph 1: PWM frequency v/s Ripple Current
PWM frequency has a significant impact on the noise generated by permanent magnet synchronous motors. A higher frequency results in smoother and quieter motor operation, with additional benefits like higher efficiency, smoother operation, and reduced temperature rise. With this in mind, it’s recommended to use a higher PWM frequency for applications where noise reduction is a crucial factor.
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