Analysis of high frequency chopping cascade speed regulation system

3.3 Asynchronous Motor Model

Figure 3 shows the internal model structure of the asynchronous motor simulation subsystem, which is encapsulated as a Motor subsystem module with four input terminals and seven output terminals (see Figure 2).

The parameters of the asynchronous motor module (Asynchronous Machine) are set as wound-rotor motors, and the parameters are converted to the rotor side. The three-phase transformer (linear transformer) connected in series on the stator side is an inverter transformer.

3.4 Simulation of the startup process

Figure 4 shows the internal model structure of the startup link simulation subsystem, which is encapsulated into a Start subsystem module with 9 input terminals and 3 output terminals (see Figure 2), completing the task of smooth starting of the motor.

When the motor starts, 1KM is closed, 2KM and 3KM are opened, and a three-phase frequency-sensitive resistor PF is connected in series to the motor rotor circuit to limit the starting current. When the motor speed increases to the set allowable value, the device automatically closes 2KM, cuts off the frequency-sensitive resistor, and the motor rotor circuit is short-circuited through 1KM to enter the full-speed working state. After stable operation, 1KM is disconnected, 2KM and 3KM are closed, and the cascade speed control system is connected to enter the speed regulation operation state, and the duty cycle is adjusted to change the speed.

3.5 Simulation of Cascade Speed ​​Control System

Figure 5 shows the internal model structure of the simulation subsystem of the chopper-type cascade speed control system. After encapsulation, it is a Speed ​​Control subsystem module with 7 input terminals and 3 output terminals (see Figure 2). The motor speed control task is completed by adjusting the duty cycle of input 7.

The three core units of the speed control system in Figure 5 are: ① Universal Bridge is a universal bridge module, which is used to simulate a three-phase full-wave rectifier unit to convert the three-phase AC of the rotor circuit into DC so as to apply series DC potential control to the rotor circuit. ② IGBT is a chopper unit, which works in a constant frequency width modulation mode. The external high-frequency pulse signal is used as the gate signal of the IGBT, and its duty cycle and frequency are determined by the pulse signal. ③ Thyristor bridge is a 6-pulse thyristor bridge module, which is used to simulate a three-phase full-bridge active inverter, and inverts the slip power after chopping control into a three-phase industrial frequency AC and sends it to the internal feedback winding to achieve energy saving.

4 Simulation Examples

A simulation test was conducted on the cascade speed control system for large-capacity square torque loads such as fans and pumps. The model shown in Figure 2 was established, and the relevant parameters were set as follows: the power supply was 6KV, 50HZ three-phase AC power supply; the asynchronous motor was rated power 2240KW, pole pair number 2, and moment of inertia 140 kg.m2. The simulation time was 0 to 10 seconds, and the speed control system was put into use at t=3.5 seconds, with a duty cycle of 100%. After t=5 seconds, the duty cycle was reduced by 10% every 1 second, and the simulation test was carried out.

Figure 6 shows the simulation curves of the speed, A-phase rotor current, rectifier voltage and DC current during the motor startup and speed regulation process. It can be seen that during the startup process, the motor smoothly increases from zero speed to full speed, and the rotor current is effectively suppressed. The speed regulation process is smooth and fast, and as the duty cycle decreases, the equivalent additional DC electromotive force Ub increases, the rotor current I2 decreases, and the speed decreases, which is consistent with the theoretical analysis in Section 2.2.

5 Conclusion

This paper makes a detailed analysis of the AC and DC circuits of the high-frequency chopping cascade speed regulation system, and based on the electrical principle structure diagram, establishes a simulation model for the cascade speed regulation system using the SimPowerSystem toolbox and packaging technology in the MATLAB/Simulink environment.

Judging from the results of the simulation examples, the model realistically reproduces the dynamic processes of the actual system such as startup and speed regulation operation, which shows that the simulation method is effective and has practical engineering value.

The author's innovation: The simulation model of the cascade speed control system established in the MATLAB/Simulink environment using the SimPowerSystem toolbox and packaging technology conforms to the composition structure of the actual engineering design, and the simulation effect is realistic, providing a theoretical basis and verification means for the engineering design of the motor speed control system.