IEEE Transactions on Transportation Electrification, 2020
Operations at low sampling-to-fundamental frequency (S2F) ratios of induction machines are requir... more Operations at low sampling-to-fundamental frequency (S2F) ratios of induction machines are required for high-power or high-speed applications. The traditional indirect field-oriented control is open-loop control on torque, which is sensitive to machine parameters and degrades at low S2F ratios. Deadbeat-direct torque and flux control (DB-DTFC) is closed-loop control on torque but also degrades at low S2F ratios with inaccurate discrete-time approximations. This paper presents a methodology of accurate discrete-time modeling for the flux and current observer and the deadbeat controller used in DB-DTFC. The current, flux and torque estimation is improved with the proposed observer model at very low S2F ratios. The torque control is enhanced and insensitive to machine parameters under high-speed low-S2F-ratio conditions. The increased computational time of the proposed model is insignificant for real-time implementations. Index Terms-Torque control, observer, discrete-time modeling, low switching frequency, high speed.
Flux weakening techniques have been developed for field oriented control (FOC) drives to achieve ... more Flux weakening techniques have been developed for field oriented control (FOC) drives to achieve the maximum torque capability above the base speed. Deadbeat-direct torque and flux control (DB-DTFC) has the advantages of precise torque and flux control and reduced parameter sensitivity compared to FOC, thus it has the benefit of more consistently achieving the maximum torque capability in the flux weakening region more accurately. This paper introduces a flux weakening strategy for DB-DTFC. Considering voltage and current limits, a simple solution to the optimal trajectory for the stator flux magnitude is derived to achieve the maximum feasible torque over the entire operating range. Results show that the speed control is extended to a significantly higher range beyond the base speed. Torque control is less sensitive to machine parameters and has better dynamics in the flux weakening region compared with the traditional FOC drives. Self-sensing is also investigated in the flux weakening region. Challenges of self-sensing are addressed to significantly extend the high-speed operating range.
Indirect field oriented control (IFOC) has become a widely adopted solution for AC motor drives. ... more Indirect field oriented control (IFOC) has become a widely adopted solution for AC motor drives. Standard IFOC controls torque and rotor flux linkage via q- and d- axis current. Alternatively, deadbeat-direct torque and flux control (DB-DTFC) has emerged as a promising motor control strategy for the future, which manipulates Volt-sec. vector directly. Air-gap torque and stator flux linkage are decoupled and independently controlled over each switching period. Stator flux linkage is used as a separated degree-of-freedom to manipulate losses dynamically without compromising torque dynamics and torque ripple. In voltage-limited operations, direct selection of Volt-sec. allows DB-DTFC to fully utilize the dc bus voltage and produce fast torque. A single control law is used over a wide speed range. This paper aims to provide a comparative overview of the two motor controls regarding their sensitivity to parameters, current- and voltage-limited operation, loss manipulation, and torque ripple during signal injection. Based on the comparison, the ultimate objective is to demonstrate the opportunities and remaining challenges in DB-DTFC.
This paper introduces an enhanced speed self-sensing method that tracks the stator flux linkage i... more This paper introduces an enhanced speed self-sensing method that tracks the stator flux linkage in induction machines at very low speeds. Volt-second sensing technology is used to mitigate the inverter nonlinearity, which is the major source of flux estimation error in the very-low-speed range. By integrating volt-second sensing in the Gopinath style closed-loop flux observer, the rotor speed information with more precision and lower noise is extracted from the estimated stator flux. Experimental results show that the proposed self-sensing method extends the low speed operating range, reduces the speed and torque ripple, and increases the disturbance rejection capability compared with the traditional self-sensing method based on the voltage integration method and the back-EMF tracking method.
As a result of dead-time, device on-state voltage drop, dc bus voltage measurement error, etc., v... more As a result of dead-time, device on-state voltage drop, dc bus voltage measurement error, etc., volt-second errors degrade precise control of torque and flux linkage, particularly at low speeds. This is true for deadbeat-direct torque and flux control (DB-DTFC), which directly manipulates the volt-second vector sourced by inverters as well as for indirect field oriented control (IFOC) drives. This paper introduces a real-time sensing scheme to measure the motor terminal volt-second vectors for each switching period with negligible phase lag. Based on the volt-second sensing, a model reference adaptive system (MRAS)-based approach is developed to decouple the volt-second errors from inverter nonlinearity and dc bus voltage fluctuation and measurement error. By delivering an accurate volt-second vector for each switching period, torque and flux control accuracy, self-sensing performance and parameter estimation accuracy are significantly enhanced.
IEEE Energy Conversion and Congress Exposition (ECCE), 2018
High power applications with low switching frequencies as well as high speed applications with hi... more High power applications with low switching frequencies as well as high speed applications with high fundamental frequencies operate at a low switching-to-fundamental (S2F) frequency ratio. With a low S2F frequency ratio, it is challenging to achieve high torque control accuracy and dynamic performance. This paper identifies control issues that yield performance degradation at a low S2F frequency ratios, for traditional field oriented control (FOC) and for deadbeat-direct torque and flux control (DB-DTFC) induction machine drives. It is documented that by eliminating synchronous reference frame current regulation, DB-DTFC demonstrates advantages to achieve a more desirable torque control performance in low-S2F-ratio applications. Three progressively more accurate DB-DTFC torque models are compared in terms of torque control accuracy and computation time to FOC and standard high switching frequency DB-DTFC. A general guideline is proposed to select the most suitable model for DB-DTFC drives at an arbitrary S2F ratio for high power or high speed applications.
IEEE Transactions on Transportation Electrification, 2020
Operations at low sampling-to-fundamental frequency (S2F) ratios of induction machines are requir... more Operations at low sampling-to-fundamental frequency (S2F) ratios of induction machines are required for high-power or high-speed applications. The traditional indirect field-oriented control is open-loop control on torque, which is sensitive to machine parameters and degrades at low S2F ratios. Deadbeat-direct torque and flux control (DB-DTFC) is closed-loop control on torque but also degrades at low S2F ratios with inaccurate discrete-time approximations. This paper presents a methodology of accurate discrete-time modeling for the flux and current observer and the deadbeat controller used in DB-DTFC. The current, flux and torque estimation is improved with the proposed observer model at very low S2F ratios. The torque control is enhanced and insensitive to machine parameters under high-speed low-S2F-ratio conditions. The increased computational time of the proposed model is insignificant for real-time implementations. Index Terms-Torque control, observer, discrete-time modeling, low switching frequency, high speed.
Flux weakening techniques have been developed for field oriented control (FOC) drives to achieve ... more Flux weakening techniques have been developed for field oriented control (FOC) drives to achieve the maximum torque capability above the base speed. Deadbeat-direct torque and flux control (DB-DTFC) has the advantages of precise torque and flux control and reduced parameter sensitivity compared to FOC, thus it has the benefit of more consistently achieving the maximum torque capability in the flux weakening region more accurately. This paper introduces a flux weakening strategy for DB-DTFC. Considering voltage and current limits, a simple solution to the optimal trajectory for the stator flux magnitude is derived to achieve the maximum feasible torque over the entire operating range. Results show that the speed control is extended to a significantly higher range beyond the base speed. Torque control is less sensitive to machine parameters and has better dynamics in the flux weakening region compared with the traditional FOC drives. Self-sensing is also investigated in the flux weakening region. Challenges of self-sensing are addressed to significantly extend the high-speed operating range.
Indirect field oriented control (IFOC) has become a widely adopted solution for AC motor drives. ... more Indirect field oriented control (IFOC) has become a widely adopted solution for AC motor drives. Standard IFOC controls torque and rotor flux linkage via q- and d- axis current. Alternatively, deadbeat-direct torque and flux control (DB-DTFC) has emerged as a promising motor control strategy for the future, which manipulates Volt-sec. vector directly. Air-gap torque and stator flux linkage are decoupled and independently controlled over each switching period. Stator flux linkage is used as a separated degree-of-freedom to manipulate losses dynamically without compromising torque dynamics and torque ripple. In voltage-limited operations, direct selection of Volt-sec. allows DB-DTFC to fully utilize the dc bus voltage and produce fast torque. A single control law is used over a wide speed range. This paper aims to provide a comparative overview of the two motor controls regarding their sensitivity to parameters, current- and voltage-limited operation, loss manipulation, and torque ripple during signal injection. Based on the comparison, the ultimate objective is to demonstrate the opportunities and remaining challenges in DB-DTFC.
This paper introduces an enhanced speed self-sensing method that tracks the stator flux linkage i... more This paper introduces an enhanced speed self-sensing method that tracks the stator flux linkage in induction machines at very low speeds. Volt-second sensing technology is used to mitigate the inverter nonlinearity, which is the major source of flux estimation error in the very-low-speed range. By integrating volt-second sensing in the Gopinath style closed-loop flux observer, the rotor speed information with more precision and lower noise is extracted from the estimated stator flux. Experimental results show that the proposed self-sensing method extends the low speed operating range, reduces the speed and torque ripple, and increases the disturbance rejection capability compared with the traditional self-sensing method based on the voltage integration method and the back-EMF tracking method.
As a result of dead-time, device on-state voltage drop, dc bus voltage measurement error, etc., v... more As a result of dead-time, device on-state voltage drop, dc bus voltage measurement error, etc., volt-second errors degrade precise control of torque and flux linkage, particularly at low speeds. This is true for deadbeat-direct torque and flux control (DB-DTFC), which directly manipulates the volt-second vector sourced by inverters as well as for indirect field oriented control (IFOC) drives. This paper introduces a real-time sensing scheme to measure the motor terminal volt-second vectors for each switching period with negligible phase lag. Based on the volt-second sensing, a model reference adaptive system (MRAS)-based approach is developed to decouple the volt-second errors from inverter nonlinearity and dc bus voltage fluctuation and measurement error. By delivering an accurate volt-second vector for each switching period, torque and flux control accuracy, self-sensing performance and parameter estimation accuracy are significantly enhanced.
IEEE Energy Conversion and Congress Exposition (ECCE), 2018
High power applications with low switching frequencies as well as high speed applications with hi... more High power applications with low switching frequencies as well as high speed applications with high fundamental frequencies operate at a low switching-to-fundamental (S2F) frequency ratio. With a low S2F frequency ratio, it is challenging to achieve high torque control accuracy and dynamic performance. This paper identifies control issues that yield performance degradation at a low S2F frequency ratios, for traditional field oriented control (FOC) and for deadbeat-direct torque and flux control (DB-DTFC) induction machine drives. It is documented that by eliminating synchronous reference frame current regulation, DB-DTFC demonstrates advantages to achieve a more desirable torque control performance in low-S2F-ratio applications. Three progressively more accurate DB-DTFC torque models are compared in terms of torque control accuracy and computation time to FOC and standard high switching frequency DB-DTFC. A general guideline is proposed to select the most suitable model for DB-DTFC drives at an arbitrary S2F ratio for high power or high speed applications.
Uploads
Papers by Yang Xu