The primary circuit in a frequency converter is responsible for supplying a regulated voltage and modulated frequency power source to the asynchronous motor. This circuit can generally be categorized into two main types: voltage-source frequency converters, which transform direct current (DC) from a voltage source into alternating current (AC), with the DC circuit typically featuring a capacitor for filtering; and current-source frequency converters, which convert DC from a current source into AC, with the DC loop employing inductors for filtering. The primary circuit consists of three key components: a rectifier that converts utility frequency power into DC power, a smoothing circuit that absorbs voltage ripples from the inverter, and an inverter that transforms DC back into AC power.
(1) Rectifier: Modern frequency converters predominantly use diode-based rectifiers to convert AC power into DC. However, transistor-based converters can also be employed, allowing for bidirectional power flow and enabling regenerative operations.
(2) Smoothing Circuit: After rectification, the DC voltage contains ripple voltages at six times the power supply frequency, along with ripple currents generated by the inverter. To mitigate these fluctuations, inductors and capacitors are used to absorb the ripple. For smaller systems, if sufficient margins exist between the power supply and the main circuit, the inductor may be omitted, and a simpler smoothing circuit can suffice.
(3) Inverter: Unlike the rectifier, the inverter converts DC power into AC power at the desired frequency. By precisely controlling the switching of three transistors, it generates a 3-phase AC output. Voltage-type PWM inverters provide a good example of how this process works.
The control circuit, on the other hand, provides control signals to the main circuit of the asynchronous motor, adjusting both voltage and frequency. It comprises several subsystems: an arithmetic circuit for calculating frequency and voltage, a voltage and current detection circuit for monitoring the main circuit, and a speed detection circuit for the motor. These are integrated with a driver circuit that amplifies control signals and a protection circuit for safeguarding the inverter and motor.
(1) Calculation Circuit: External commands such as speed and torque are compared with detected current and voltage signals to determine the output voltage and frequency of the inverter.
(2) Detection Circuit: This circuit is electrically isolated from the main circuit to measure voltage and current safely.
(3) Driver Circuit: This circuit controls the activation and deactivation of the main circuit devices while maintaining isolation from the control circuit.
(4) Speed Detection Circuit: Signals from the motor's speed detector (such as tachometers or pulse encoders) are processed and used to adjust motor speed according to command inputs.
(5) Protection Circuit: Monitors voltage, current, and other parameters of the main circuit. If an anomaly like overload or overvoltage occurs, it prevents damage to the inverter and motor by halting operation or suppressing voltage/current levels.
Frequency converters play a crucial role in modern industrial applications, offering precise control over motor performance while enhancing energy efficiency and reducing wear on mechanical components. Their design ensures robustness and adaptability across various operating conditions, making them indispensable tools in automation and manufacturing environments.
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