Direct torque motor control technology based on NPC three-level SVPWM

This paper introduces a direct torque control technology based on NPC three-level SVPWM. It uses the Matlab platform to build a simulation model. It verifies the ideal waveform of the torque, magnetic chain, and current of the NPC-SVPWM direct torque control system under high speed and low speed with no load and load. This technique improves the waveform of the stator current and stator magnetic chain and solves the problem of the switching frequency not being fixed. Through this technique, it not only retains the robustness of the direct torque technique to the parameter change, but also effectively reduces the torque pulsation.


Introduction
As a commonly used type of inverter, the two-level inverter not only has a simple structure but also has small noise and heat loss, but its voltage output is fixed, and the waveform is not smooth.The harmonics are generated, and the switching loss is large.The three-level is used to achieve different outputs of three voltages by increasing the intermediate level, and its output is more stable and accurate.The multi-step output voltage generated by a 3-level inverter is a better approximation of a sine wave, and the action of a 3-level inverter is half that of a conventional 2-level inverter.However, the increased number of switches creates complexity in its control algorithms [1].The direct torque control structure has the characteristics of simple control, rapid dynamic response, and good robustness.The method of spatial vector pulse width modulation proposed in [2] solves the problem of large torque ripple caused by transmission direct torque control technology.This modulation method is mostly used in two-level inverters, and the current harmonic content is higher due to the small number of voltage levels.Therefore, this paper adopts the NPC-SVPWM-DTC method to improve power quality.

SVPWM-DTC
SVPWM-DTC technology is based on the stator magnetic field orientation combined with the PI regulator to suppress the disturbance, overcome the dynamic deviation between the given and actual torques and flux, and fix the IGBT switching frequency ƒm while reducing the flux and current ripples.Compared with the traditional DTC control technology, SVPWM-DTC has a similar mechanism and has made certain changes to the traditional structure of DTC.The improvements of SVPWM-DTC are embodied in stator flux vector angle, flux, and torque PI regulator, and space vector modulation [3].This is shown in Figure 1.The switching logic voltage selector is replaced with a space voltage vector modulation module, and the flux-torque hysteresis comparator is replaced with a PI regulator.

Time register.
Voltage vector action time:T a is the u ref duration of action, and T b is the duration of action.

Swith sequence selection.
The switching time of each phase bridge arm T m0 , T m1 , and T m2 are shown in Equations ( 7)(8)(9): 2.2.4 PWM carrier modulation.During the sampling time T s , the drive signal for the gate switch is generated by the triangular wave model.This is shown in Figure 3.

NPC-SVPWM-DTC
The Neutral Point Clamped inverter is shown in Figure 4.In terms of structure, it can be divided into Itype, T-type, and improved I-type.It can output a variety of level states.Compared with the two-level inverter, it has a lower output voltage total harmonic rate and larger capacity [5][6].
According to the magnitude of the amplitude, the spatial voltage vectors are divided into four types: large vectors, medium vectors, small vectors, and zero vectors [7][8][9] [10].This is shown in Figure 5. IOP Publishing doi:10.1088/1742-6596/2761/1/0120134 2.4.1 Two-level and three-level inverter switching conversion tables.Switching state transition relationship: 12-sector number is selected according to the ideal voltage vector and a 6-sector PWM signal is generated to jointly change the three-level inverter (P, O, N) signal.This is shown in Table 1.

Simulation results of NPC-SVPWM-DTC based on Matlab simulation
The no-load and load experiments were carried out with the NPC three-level as the core, and the good dynamic and static performance of NPC-SVPWM-DTC was verified by the analysis of the results.This is shown in Figures 7.

Analysis of simulation results
According to the simulation model described above, the switching frequency is set to 5 kHz, and the stator flux is = 0.3 Wb.The initial speed is set to be 2 rad/s.After 1 second, the given speed is 1000 rad/s.After 2 seconds, the given speed is 500 rad/s.The initial load is 5 N•m, and the torque load is increased to 50 after 2 s, and the dead time is set to 10 .
The results of the analysis is shown in Figures 8, 9

Conclusion
According to the FFT analysis, the current waveform of NPC-SVPWM-DTC is smooth, the sinusoidal degree is good, the harmonic content is small, and the midpoint potential is basically zero.It is not affected by the speed compared with SVPWM-DTC; the performance of NPC-SVPWM-DTC not only does not lose static stability but also has a fast torque response and small torque ripple.

Table 1 .
Switch selection table.