Frequency converter working principle - Solutions - Huaqiang Electronic Network

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GW MB10F 0.8A 1000V rectifier bridge

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Probe high frequency needle JTK-50L-Q has 5 needles in the high frequency needle

Photocoupler

A frequency converter is an electronic device that transforms a standard power supply (either 50 Hz or 60 Hz) into an AC power source with variable frequencies, enabling precise control of motor speed. The main components include the control circuit, which manages the operation of the system, the rectifier circuit that converts AC to DC, and the DC smoothing circuit that stabilizes the rectified voltage. Finally, the inverter circuit reconverts the DC power back into AC with the desired frequency. For advanced systems like vector-controlled inverters, which require complex calculations, a CPU may be used for torque management along with additional circuits. This understanding is crucial when diagnosing and repairing inverter issues.

Frequency converters can be classified in various ways. Based on the main circuit operation, they are either voltage-source inverters or current-source inverters. In terms of switching methods, they can be categorized as PAM, PWM, or high-frequency PWM inverters. From a functional perspective, they fall into V/f control, slip frequency control, and vector control types. In repair scenarios, inverters are often divided by purpose—such as general-purpose, high-performance, high-frequency, single-phase, or three-phase models.

Non-intelligent control methods in AC drives include V/f coordination, slip frequency control, vector control, and direct torque control. V/f control adjusts the motor's speed while maintaining constant magnetic flux, making it suitable for most general-purpose inverters. However, it operates in an open-loop mode, limiting its performance at low speeds, where torque compensation is necessary. Slip frequency control directly manages torque by adjusting the output frequency based on the motor’s actual speed. Vector control separates the stator current into excitation and torque components using coordinate transformations, allowing more precise motor control. Direct torque control simplifies the process by using space vector concepts, eliminating complex transformations and improving response time and accuracy.

The main circuit of an inverter handles the power conversion for an AC motor. It typically consists of three parts: a rectifier that converts AC to DC, a DC smoothing circuit that filters out voltage ripples, and an inverter that converts DC back to AC. Voltage-type inverters use capacitors for DC filtering, while current-type inverters rely on inductors. These components work together to ensure stable and efficient power delivery.

The rectifier stage usually employs diode-based converters to convert AC from the grid into DC. Some systems also use transistor-based converters, allowing for regenerative operation due to bidirectional power flow. The smoothing circuit helps reduce voltage fluctuations caused by both the rectifier and inverter stages. Small devices may omit the inductor and use a simpler smoothing configuration if the system design allows for it.

The inverter stage is responsible for converting DC power into AC with the required frequency. By controlling the switching of three transistors at specific intervals, a three-phase AC output is generated. The timing and waveform of these switches are critical for achieving the desired performance, especially in PWM inverters.

The control circuit manages the operation of the main circuit by providing signals for voltage and frequency adjustment. It includes an arithmetic unit, voltage and current detection circuits, a motor speed detection system, a drive circuit to amplify control signals, and a protection circuit to safeguard the inverter and motor from faults like overvoltage or overload.

The operation circuit compares external commands with feedback signals to determine the inverter’s output. The voltage and current detection circuit isolates the main circuit to monitor its status. The drive circuit ensures safe switching of the main components, while the speed detection circuit uses sensors like tachogenerators or pulse generators to track motor speed. Lastly, the protection circuit detects abnormalities and takes action to prevent damage to the system.