Energy-saving principle of frequency converter speed control operation - News - Global IC Trade Starts Here Free Join
A frequency converter is a device used to achieve variable speed control by converting the frequency of the power supply. It typically consists of several key components, including a rectifier, a filter, a drive circuit, a protection system, 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 through a rectifier and then smoothed out using a capacitor. This DC voltage is then fed into an inverter, where it is converted back into AC with adjustable frequency and voltage.
The inverter generates a rectangular pulse waveform by controlling the on/off state of its power components. By adjusting the width of these pulses, the output voltage can be regulated, while changing the modulation period controls the output frequency. This allows for simultaneous control of both voltage and frequency, enabling efficient and smooth speed regulation of the motor. This method, known as U/f control, ensures stable operation across a wide range of speeds.
One of the main advantages of Pulse Width Modulation (PWM) is its ability to reduce or eliminate low-order harmonics, allowing the motor to operate under a nearly sinusoidal voltage waveform. This results in reduced torque pulsation and a broader speed control range. However, PWM-controlled motors have a limitation in terms of maximum speed, typically not exceeding 7000 RPM for compressors.
In contrast, Pulse Amplitude Modulation (PAM) allows the compressor speed to increase by approximately 1.5 times, significantly improving acceleration and deceleration performance. Additionally, PAM helps shape the current waveform during voltage adjustment, resulting in higher efficiency compared to PWM. It also offers better noise suppression, reducing harmonic distortion and minimizing interference with the power grid.
After implementing frequency conversion speed control technology, significant improvements are observed. For example, the stator current of the motor can be reduced by 64%, the power frequency by 30%, and the pressure of the glue by 57%. According to motor theory, the speed of an asynchronous motor can be expressed as:
n = 60·f·(1 - s)/p
Where: n = Motor speed (in rpm), f = Stator frequency (grid frequency), s = Slip ratio, p = Number of pole pairs.
As shown in the equation, as long as the slip ratio remains small, the motor speed n is approximately proportional to f. This means that by continuously varying the power supply frequency, the motor can achieve smooth and continuous speed control over a wide range. For instance, a motor rated at 3000 rpm can run at any speed between 300 and 3000 rpm if the inverter is set to start at 5 Hz and operate up to 50 Hz. This allows for soft starting, high torque, and energy savings compared to direct power supply.
Finally, the 50Hz, 380V mains electricity is first converted into DC through a rectification and filtering process. Then, it is transformed into AC with adjustable frequency and amplitude via the inverter stage. Since the main circuit involves AC-DC-AC conversion, this type of inverter is referred to as an AC-DC-AC inverter.
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