When the inverter supplies power to the three-phase motor, the eight basic voltage vectors of the six switching vectors V1 to V6 of the inverter and the two zero switching vectors V0 and V7 are combined into one space voltage vector, which can approximately simulate a constant amplitude rotation vector. At this time, the air gap flux of the motor is approximately circular.

Great savings in CPU overhead. The waveform generator simplifies the control software and external hardware that generate the synchronous pulse width modulation waveform, making the control of the AC frequency conversion device simple. In order to ensure the speed stability of the output control circuit, speed feedback can be used. The optical code disc outputs two pulse signals with a mutual difference of 90, and the capture pins EPA0 and EPA1 are accessed. The T1 counter can calculate the motor speed.

If the speed accuracy is not high, open loop operation is possible. As the system is equipped with voltage feedback digital regulator, so the speed control accuracy can reach 0.05%. To ensure the reliable operation of the system, it also has overvoltage, undervoltage, overcurrent, overload, stall, overheating and other protection functions. In addition to a piece of 8-bit microcontroller, complete the keyboard scan and display. The drive circuit is the same for the same type of IGBT, so it will not be repeated here. Control circuit diagram.

Waveform Generator Interrupt Service Program Flow System Software Design The AC frequency conversion speed control device has many functions and the application software is more complex. The system software consists of main program, voltage space vector PWM calculation, SPWM waveform generation program, timer interrupt service program and fault interrupt service program.

The main program completes the initial system check and initialization of each register; the timing interrupt service program completes the task of communication with the keyboard and display, and the fault detection task; the waveform generator completes acceleration/deceleration, voltage space vector PWM calculation, and waveform generator loading tasks.

The voltage space vector PWM calculation and waveform generation block diagram are given here.

When a three-phase sinusoidal voltage is applied to a three-phase motor stator winding, the air-gap flux is rotated at a constant angular velocity, and the trajectory is circular. When the inverter is used to supply the three-phase motor, the eight switching vector vectors V1 to V6 and the two zero switching vectors V0 and V7 of the inverter synthesize a single space voltage vector, which can approximately simulate a constant-amplitude rotation vector. At this time, the air gap flux of the motor is approximately circular.

In the figure: T1=TS2msinP3-H; T2=TS2msinH; T0=T7=TS2-T1-T2/236 where m-modulation coefficient; T1, T2- non-zero vector; T0, T7-zero vector.

According to the required frequency in the software, by the V/f control principle, after the output voltage Vout (ie m) is determined, T1, T2, T0 and T7 can be calculated, and the SPWM wave can be output using the 87C196MH waveform generator. The reference voltage Vout is in the space voltage vector PWM switching mode within the illustrated sector.

Concluding remarks The all-digital 280kW variable frequency speed control device is now officially put into industrial applications, one of which is used in a ring furnace blower in a factory. The operating results show that the economic benefits are considerable, the mechanical life is high, the heating quality of the blank is improved, the control accuracy is high, the reliability is good, and the anti-interference ability is strong.

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