The conventional switching-table-based direct torque- controlled (DTC) ac machine drive is usually afflicted by large torque ripple, as well as steady-state error of torque. The existing methods, which optimize the duty ratio of the active vector, are usually complicated and parameter dependent. Based on the analysis of instantaneous variation rates of stator flux and torque of each converter output voltage vector, a simple and effective method considering the effect of machine angular velocity is proposed to obtain the duty ratio. The experimental results carried on a dSPACE platform with a laboratory prototype of the permanent-magnet machine verify that the proposed duty-based DTC method can achieve excellent transient response, less torque ripple, and less steady-state error, without resorting to the complicated control method over a wide range of operating regions.
1. Direct torque control (DTC)
2. Duty ratio
3. Permanent-magnet synchronous machines (PMSMs)
4. Steady-state error
5. Torque ripple
Fig. 1. Control diagram of DTC of PMSM.
EXPECTED SIMULATION RESULTS:
Fig. 2. Comparison of steady-state performance of various DTC methods (rated condition: 400 r/min, 5 N • m). (a) Conventional DTC method. (b) M1. (c) M2. (d) Proposed DTC method.
Fig. 3. Dynamic performances of torque response with inner torque loop control only and without outer speed loop. Reference torque from 2 to −2 N • m. (a) Conventional DTC method. (b) M1. (c) M2. (d) Proposed DTC method.
Fig. 4. Dynamic and steady-state performances when reference speed changes from 200 to −200 r/min. (a) Conventional DTC. (b) Proposed DTC.
Fig. 5. Steady-state performance of the proposed DTC method with different control parameters: Ka = 0.7, Kb = 0.0005 (rated condition: 400 r/min,5 N • m).
This paper has proposed, analyzed, and experimentally verified a simple and effective method for determining the appropriate duty ratio in DTC three-phase PMSM drives to reduce the torque ripple and the steady-state error of torque, accounting for the influence of machine angular velocity. A simple estimated method is proposed to obtain the range of the key control parameters. Compared to the existing duty-based DTC methods, the proposed method can achieve the decent performance of torque and flux at the lower price of increased average communication frequency.
The proposed duty ratio determination has the following features.
1) Simple structure: Compared to conventional DTC, just a very simple duty ratio regulator is added.
2) Parameter independent: Unlike the previous duty-based methods, where many parameters such as stator inductance and PM flux are required, in the proposed DTC method, only the torque error and speed are needed to compute the duty ratio, which makes it robust to parameter variation.
3) Outstanding steady-state performance over a wide range of operating regions, even when speed is reversed.
4) Similar excellent transient response to the conventional DTC.
Although the analysis and experiments in this paper are based on the DTC of three-phase PMSM drives, the proposed duty ratio determination can be also extended for general use and applied to the other machines of switching-table-based direct torque and power control methods, which may exhibit the same problem of ripple and/or steady-state error.
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