The device responsible for frequency conversion and speed control is known as a frequency converter. A typical frequency converter consists of several key components, including a rectifier, a filter, a drive circuit, a protection circuit, and a controller (such as an MCU or DSP). The process begins with the input AC power—either single-phase or three-phase—which is first converted into DC voltage through a rectifier and then smoothed out by a capacitor. This DC voltage is then fed into an inverter, where it is converted back into AC using the on/off switching of power components. The resulting output is a rectangular pulse waveform, whose shape can be controlled to achieve desired performance.
By adjusting the width of these pulses, the voltage amplitude can be regulated, while varying the modulation period controls the output frequency. This allows for simultaneous control of both voltage and frequency at the inverter output, enabling U/f coordinated control, which is essential for efficient variable frequency speed regulation. One of the main advantages of PWM (Pulse Width Modulation) is its ability to suppress or eliminate low-order harmonics, allowing the motor to operate under a near-sine wave voltage. This results in reduced torque pulsation and a broader speed control range.
However, the motor speed achieved through PWM control has an upper limit, typically not exceeding 7000 rpm for compressors. In contrast, PAM (Pulse Amplitude Modulation) allows for a speed increase of approximately 1.5 times, significantly improving acceleration and deceleration performance. Additionally, PAM offers better current waveform shaping during voltage adjustments, leading to higher efficiency compared to PWM. It also excels in anti-interference capabilities, effectively suppressing higher harmonics and reducing electrical pollution on the power grid.
After implementing this frequency conversion speed control technology, significant improvements are observed: the stator current of the motor decreases by 64%, the power frequency drops by 30%, and the glue pressure is reduced by 57%. According to motor theory, the speed of an asynchronous motor can be expressed as:
n = 60·f·(1 - s)/p
Where:
- n is the motor speed,
- f is the stator frequency (grid frequency),
- p is the number of pole pairs,
- s is the slip ratio.
As shown in the equation, as long as the slip ratio remains relatively small, the motor speed n is directly proportional to the stator frequency f. This means that by continuously adjusting the supply frequency, the motor can achieve smooth and continuous speed variation over a wide range. For example, a motor rated at 3000 rpm can be operated between 5 Hz and 50 Hz via an inverter, allowing it to run at any speed between 300 and 3000 rpm. Starting from the grid, the motor runs smoothly with high torque and energy savings.
The 50 Hz, 380 V mains power is first converted into DC through a rectification and filtering process. Then, it is transformed into AC with adjustable frequency and amplitude through the inverter stage. In the main circuit of the inverter, the electric energy undergoes an AC-DC-AC conversion, which is why this type of inverter is referred to as an AC-DC-AC inverter.
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