The developed frequency converter adopts the variable frequency speed regulation integrated chip SA868 produced by British MITEL Company as the control core, and the synchronous PWM waveform of variable frequency and variable voltage required for output speed regulation constitutes the most convenient frequency conversion of driving three-phase induction motor. Speed ​​system, when the input voltage is too high, the three-phase inverter output current is too large, the inverter will immediately turn off the PWM waveform output, which can protect the power stage semiconductor device. The governor input is 50Hz, 220V AC available anywhere, so it is especially suitable for low power and ultra-low power speed regulation.
The block diagram and working principle of the inverter can be seen from Figure 1. The input unidirectional 220VAC is rectified and filtered to obtain smooth high voltage DC, which is used as the primary voltage of the auxiliary power supply and the input voltage of the three-phase inverter. The overvoltage and overcurrent detection unit will periodically check the voltage across the rectifying and filtering capacitor and the current of the DC link, and send the signal to the dedicated integrated control chip to provide corresponding security protection. The special integrated chip is the control core of the inverter. It adopts the British MITEL company SA868. The film has strong functions, few external components, simple connection and convenient use. According to the speed regulation command input by SET1~SET4, six PWM PWM waveforms are outputted, and the six outputs are isolated, and the three-phase inverter is driven by the power amplifier. The three-phase inverter is composed of a six-cell IGBT module with a freewheeling diode. composition. Thereby, the connection is greatly reduced, the volume is reduced, the reliability is improved, the installation is convenient, and the debugging is easy. The three-phase inverter output can directly drive three-phase low-power motors.
The principle circuit is designed and debugged according to the above minimum system block diagram. The specific circuit of each unit is briefly described as follows:
The rectifier filtering and overvoltage and overcurrent detection circuit are shown in Figure 2.
In Figure 2, RT1 is a thermistor used to limit and reduce the inrush current at the moment of start-up. The high voltage direct current filtered and filtered by D1~D4 and C1 is directly sent to the DC input terminal of the 6-unit IGBT module. The C3 capacitor is as close as possible to the IGBT module to filter out various high frequency interferences in the DC bus. The overvoltage detection signal, the overcurrent detection signal and the analog ground of the rectification filter constitute a detection unit output, and the two sets of outputs are sent to the control unit.
The dedicated integrated control chip dedicated variable frequency speed control integrated chip SA868 is a three-phase pulse width modulator.
The pin is introduced as follows:
: Reset terminal, active low, when the input is low, the following functions are completed:
â— All PWM outputs are set to zero;
â— All internal counters are set to zero;
â— The instantaneous frequency is set to zero, and the direction selection position is 1;
DIR: Direction setting terminal. When the bit input is high and the internal signal is allowed, it will rotate in the positive direction; this bit input is at this time and rotates in the opposite direction.
SET1~SET4: Speed ​​setting terminal, the new target frequency can be set by changing these four digit values;
VMONITOR: inverter input voltage monitoring terminal;
IMONITOR: Inverter input current monitoring terminal;
Raccel: acceleration rate setting terminal, external resistor and capacitor;
Rdecel: Deceleration rate setting terminal, external resistor and capacitor;
XTAL1: clock input;
XTAL2: clock output;
UPPER OUTPUT: high-end output of the inverter bridge arm;
LOWER OUTPUT: low-end output of the inverter bridge arm;
SET TRIR: Output closes the setting terminal. When the input is high, after 3~4 main clock frequency cycle delays, it is still high, then the PWM output signal is disabled, and the mechanism state can be cleared.
: The off state indication terminal is active low, and the low level indicates that the PWM signal is turned off, and the terminal can directly drive the LED;
The chip features a waveform generator designed for civilian products. The operating temperature is 0°C~70°C. The driver is built-in or directly drives the optocoupler to achieve isolation. The carrier frequency can be selected up to 24KHz for noise-free operation. The modulation frequency range is wide. :0~4000Hz;
All parameters defined by the user are programmed by the manufacturer in the ROM; bipolar modulation; minimum pulse width, dead time selection; acceleration time, deceleration time selection by external resistor; three waveforms available for user selection; SA868 Available in 24-pin, it comes in dual in-line and flat small form factor packages.
Chip connection method In the ultra-small inverter, the specific connection of SA868 is shown in Figure 3.
In Figure 3, the overcurrent detection signal is sent to the current monitoring terminal of SA868 via the non-inverting amplifier composed of operational amplifier 358. If an overcurrent occurs, SA868 will temporarily disable the PWM signal output, and the voltage detection signal is sent to the voltage monitoring terminal. R7, R8, C5, and C6 determine the frequencies of the acceleration and deceleration oscillators, respectively. By changing the R and C values, you can set the desired acceleration and deceleration frequency. The clock of this chip is provided by a 20MHz crystal oscillator. To ensure the normal operation of the crystal, a 10pF~68pF capacitor should be connected between the two pins of the crystal to the digital ground. The operational amplifier 358B and its resistor and voltage regulator form an undervoltage detection circuit. When the operating voltage is too low, the op amp 358B outputs a high level, which sets the SET TRIP terminal to 1, instantaneously turns off the PWM output, and protects the power stage from damage. . Speed ​​settings and protection alarms are input and output through the P1 socket.
The isolated drive circuit of the SA868 six-output is identical. The following is an example of one of them, as shown in Figure 4. The 20MHz signal output from the XTAL2 terminal of the SA868 is divided by two 74HC74 to obtain a 5MHz square wave output. The output is connected to the 74F126, so that the push-pull switching circuit composed of BC337 and BC227 operates at a frequency of 5MHz. The transformer T2 also operates in a 5MHz high frequency mode. The power supply voltage applied to the transformer T2 is the PWM signal RPHT. Therefore, the control signal is transmitted to the secondary side of T2 through high frequency chopping, and the secondary side is full-wave rectified by D6~D9. It is restored to the original control signal and added to the G and E of the module. The drive circuit realizes isolation and the volume is very small.
Conclusion The ultra-small inverter has strong function, simple peripheral circuit, high flexibility, easy to use, and easy to form an ultra-small variable frequency speed control system. When the inverter power module is sufficiently cooled, its output can reach 1KVA. â–
references:
1. Hou Zhenyi, etc., pulse width modulator SA868 of AC inverter circuit [J]. Foreign electronic components, 1998, NO.9
2. Ding Daohong, Power Electronics Technology [M]. Beijing: Aviation Industry Press, 1999. Revised Edition
3.Mitel company online information,
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