ABSTRACT:
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.
KEYWORDS:
1.
Direct torque
control (DTC)
2.
Duty ratio
3.
Permanent-magnet
synchronous machines (PMSMs)
4.
Steady-state error
5.
Torque ripple
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
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).
CONCLUSION:
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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