Abstract
Multiphase motors have multiple control planes, and harmonics are decoupled in different planes. Multiphase motors can improve magnetic field distribution, power density and core utilization by injecting certain harmonic currents into the harmonic planes. In the harmonic plane control process, due to the switching frequency of the inverter being limited, the ratio of the switching frequency to the current frequency (the carrier ratio) of the harmonic plane is low, the digital control delay increases, and the inverter output current contains more harmonics, which makes it difficult for the proportional-integral (PI) current controller to effectively control the d-axis and q-axis currents of the harmonic plane and thus unable to track the given values stably. Moreover, the PI current controller is relatively dependent on the motor parameters. For these reasons, a model predictive current control method with predictive error compensation is proposed. Taking a nine-phase induction motor as an example, the control voltage is calculated by the cost function and corrected by the current predictive error, which realizes the current control method at a low carrier ratio. Additionally, the robustness of the control method is analyzed after the parameters of the multiphase motor have large errors. The experimental results show that the proposed method can control the current of the harmonic plane at low carrier ratio, accurately track the harmonic current commands and attain strong robustness for the motor parameters.
Highlights
Due to its own characteristics, multiphase motors (n ≥ 5) mostly use a concentrated full-pitch winding structure
Since the existing normal model predictive control (MPC) method could not achieve accurate calculation of the optimal control voltage at a low carrier ratio, we proposed a solution to this problem by using current prediction errors to correct the optimal control voltage
When the third harmonic plane of the nine-phase induction motor was at a low carrier ratio of 16.2, the maximum static error of the third harmonic current tracking of the traditional PI controller without and with a load was 2.17 A and 0.62 A, respectively
Summary
Due to its own characteristics, multiphase motors (n ≥ 5) mostly use a concentrated full-pitch winding structure. When the carrier ratio of the harmonic plane is relatively low, the rotor position of the multiphase motor changes greatly in one control period, so the stator voltages of the harmonic plane change greatly in one control period [23], which makes it difficult for the CCS-MPC to accurately calculate the optimal control voltage, and the current control performance of the harmonic plane decreases. A novel general MPC method is proposed to solve the problem of deteriorated current control performance when the harmonic planes are at a low carrier ratio. Considering that it is difficult for the existing normal MPC methods to accurately calculate the optimal control voltage at a low carrier ratio, a scheme for correcting the control voltage using current prediction error is proposed. The phase currents are decoupled to d1 − q1 and d3 − q3 by Equation (1), and MPC models need to be established in d1 − q1 and d3 − q3, respectively
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