Variable frequency power supplies are specially designed and manufactured for import and export equipment, and they correspond to the power supply systems of Europe and the United States. They can be used as power supplies for imported 60Hz electrical appliances and production lines, and can also be used as tests for production lines of exported electrical products.
What are the characteristics of three-phase variable frequency power supply rectification?
The input of the three-phase power supply system is a three-phase three-wire 380V/50Hz AC network, and the output is a three-phase four-wire 0-500V, 60Hz AC, which can be divided into two parts: the main power conversion circuit and the control circuit. In order to improve the adaptability of the three-phase output to unbalanced loads, the main circuit and control circuit of the three-phase power supply are designed according to three sets of independent single-phase power supplies.
The main circuit adopts an AC-DC structure, including a rectifier, a DC filter, an inverter, an AC filter and a transformer. Among them, the AC-DC part is a bridge rectifier, which is slowly started by an AC contactor and an electrolytic capacitor, and filtered to obtain a stable current.
The rectification has the following characteristics:
1. When the rectifier part is started, an AC contactor is used to provide a "soft" start and reduce the impact on the power grid.
The DC-AC inverter part adopts a single-phase full-bridge structure. The three-phase system has three sets of identical single-phase inverters, which share the DC bus that constitutes the core of the power supply. The inverter uses IGBT as a switching element. Using a higher IGBT switching frequency, the inverter is controlled by sinusoidal pulse width modulation (SPWM), and the stable DC power is converted into pulse width modulated AC output. The basic frequency of the AC power is the required output power.
The pulse width modulation wave output from the inverter is filtered by the LC output filter circuit, and the AC sine wave current is output. The outputs of the three single-phase inverter circuits differ from each other by 120 electrical degrees. They are electrically independent of each other on the primary side of the transformer and are connected in a star configuration on the secondary side of the transformer to output the required three-phase AC power.
In order to improve electromagnetic compatibility, noise filters are connected to the input and output terminals of the power supply.
The control circuit consists of intelligent central monitoring, single-phase voltage and waveform control, IGBT inverter control, output detection, fault detection and protection, operation display interface, control power supply and other parts to complete the control of output frequency, voltage and waveform.
Power supply system control, system fault diagnosis and protection, operation and status and other functions. Among them, the voltage and waveform control of the single-phase output uses three independent single-phase controls, so each phase of the three-phase power supply can be used independently as a single-phase power supply and is suitable for any single-phase power supply. End load. This improves the load adaptability of the power supply unit.
2. IGBT drive current and protection circuit
The control and protection circuit of IGBT is designed for a single-phase inverter bridge. The control and protection circuit consists of a printed circuit board equipped with a single-phase inverter bridge and a cooler to form a single-phase inverter unit module.
The front stage of the drive circuit is a PWM signal processing circuit. After the single-channel PWM signal sent by the control circuit is shaped and inverted by the voltage comparator, two mutually different 180° signals are used as control signals for the upper bridge arm and the lower bridge arm.
The signal passes through the dead zone circuit, and its rising edge is delayed by 3-4μs to ensure that the dead zone of the upper bridge arm and the lower bridge arm is not less than 3μs, and then they are sent to the control circuit.
This power supply overcurrent protection uses a dual overcurrent protection scheme that combines online protection with centralized protection of the tube and bridge arm. The online protection with tube is completed by the internal protection circuit M57962. The central protection circuit uses an HL current sensor with extremely fast response speed to detect the intermediate circuit current. If the circuit exceeds the set threshold, the protection circuit blocks the control signal of all IGBTs of the reverse bridge.
The surge protector uses a DC bus in parallel with a non-inductive capacitor to absorb voltage spikes during the switching process. For the voltage spikes generated by large currents during overcurrent protection, shortening the DC line to reduce distributed inductance, appropriately reducing the protection threshold, and increasing absorption capacity can solve the problem. In addition, the driver board is equipped with two protection devices to prevent overheating of the power supply unit and undervoltage protection of the intermediate circuit.
3. Control circuit
This power supply uses three phases, independent of the output control and central monitoring system. It consists of three sets of single-phase control circuits and one set of central monitoring circuits. The single-phase control circuit completes frequency, voltage and waveform control. The central monitoring circuit completes the settings of output voltage and frequency, each functional unit of the power supply system, control panel, and I/O logic control, error detection and display.
The voltage is set to analog quantity, and the frequency is set to a ten-bit address selection signal. The setting signal, control and protection logic signal, and control power supply form the signal bus of the system. Three sets of single-phase control circuits, centralized monitoring circuits and power supply circuits are integrated into one.
（1）Three-phase control circuit
The target of waveform control is single-phase voltage output, and a two-loop control scheme with an inner current loop is used. In the voltage waveform system composed of two control loops, the current loop is the inner loop, and the controlled object of this loop is the current Ic of the filter capacitor.
The voltage waveform control loop is outside the current loop, and this loop affects the instantaneous value of the output voltage. Control is performed so that the output voltage and filter capacitor current are detected and shaped by the detection circuit, and then sent directly to the waveform loop compared with the standard sine wave and waveform loop, generating PWM control pulses after dual loop adjustment.
The standard sine wave generation uses a typical lookup table method for addressing the lookup table. The standard sine data is stored in the EPROM. The EPROM is controlled according to the output frequency sequence, and the sine digital output of the EPROM is converted into an analog signal by a D/A converter.
The analog quantity has a positive polarity and is symmetrically shifted downward by the operational amplifier circuit. After the capacitor is blocked, the sine standard signal is output.
The voltage control is performed by a Billy setting with closed-loop control. The AC output voltage signal sent by the detection circuit becomes an adjustable DC feedback variable after amplitude adjustment, absolute value exchange and active filter circuit. Compared with the feedback signal, the deviation is sent to the proportional controller, and after being amplified by the controller, it is sent to the standard sine wave amplitude, so that the average value of the output voltage is kept constant and the output is stable.
Frequency control is controlled by the standard sine wave setting.
The storage capacity of one cycle of the standard sine data is 1024 bytes, the frequency of the standard sine wave corresponds to the nominal output frequency at 60 Hz, and the frequency of the EPROM address selection signal should be 409.6kHz. Use crystal oscillator frequency division to obtain signals so that the output frequency is accurate and stable, and the performance is well guaranteed.
The dedicated frequency modulation circuit can be set to a frequency range of 45-60Hz. In the three sets of single-phase control circuits, the standard sinusoidal data stored in the EPROM is 120 electrical degrees different from the i phase.
（2） Central monitoring circuit
This circuit is based on a 16-bit 80C196 microcontroller. It uses the 8-channel A/D conversion interface in the CPU to complete analog quantity detection, uses the external CPU and PIO interrupts to complete error detection and operation logic, and the control panel to indicate control.
Overvoltage protection of input and output and overload protection of output are implemented in the software.
The detection circuit consists of three parts: output voltage detection, output current detection, and filter capacitor current detection. In order to improve the control speed of the bushing and ensure the quality of power supply, the sensing element connected to the bushing uses an HL magnetic balance sensor, and all detection signals are electrically isolated from the main control circuit.
