Finally, the conclusions are summarized in Section 6. The comparison of control strategies based on current, voltage, VA rating, and power factor requirement as a function of torque is described in Section 5. In Section 4, simulation results are presented to verify the unique feature and capability of the control strategies introduced in the paper emphasizing their merits. Section 3 presents the detailed derivation and implementation of five control strategies for PMSM drive. In Section 2, the dynamic model and decoupled control of PMSM are explained shortly. This paper is organized in the following manner: Section 1 begins by providing a brief introduction about PMSMs and a study of different existing control strategies. A detail analysis and comparison of these control strategies have been made so as to choose the control strategy that optimizes the operation of a particular speed control system. Similarly, a maximum efficiency control reduces the net loss in the motor and is appropriate for applications where saving the energy is important. For example, the constant torque angle control forces the electromagnetic torque to be proportional to the stator current magnitude but results in low power factor, optimum torque per ampere current control strategy provides maximum electromagnetic torque for a given stator current, a unity power factor control strategy optimizes the volt ampere (VA) requirement of the system, and a constant mutual air gap flux linkages control limits the flux linkage of the air gap equal to rotor permanent magnet flux linkage which helps to avoid the saturation of core. Each of these control strategies has its own merits and limitations. With the help of phasor diagrams, this paper analyses the characteristics of both surface and interior mounted permanent magnet motors. The comprehensive analysis of control strategies for the speeds lower than base speeds is made and compared in this paper. For the speeds lower than base speed, the control strategies for PMSM are constant torque angle control, optimum torque per ampere control, unity power factor control, constant mutual air gap flux linkages control, and maximum efficiency control, while, for the speeds higher than base speed, control strategies are six-step voltage and constant back emf. Therefore, in this paper, different control strategies such as constant torque angle control, optimum torque per ampere control, unity power factor control, constant mutual air gap flux linkages control, and angle control of air gap flux and current phasor are considered in detail for the variable speed motor drive. But decoupled control of torque and flux is not only the performance requirement for PMSM drive.
Also control of PMSM is comparatively simpler than that of induction motor and high performance of PMSM can be achieved by means of vector control as it provides decoupled control of torque and flux. Recently, PMSM drive has emerged as a top competitor amongst AC drives for industrial servo drives, hybrid electric vehicles, and other applications due to features like high speed, low power waste, large starting torque, high power factor, and high efficiency. Hence, it can be a good control strategy to consider. Based on the comparative study, it can be concluded that CMFLC is superior to CTAC, ACAGF, OTPAC, and UPFC. In addition, performances of these control strategies are compared, which is a key to select optimum strategy depending on requirements. Based on the simulation results, a conclusion is drawn that OTPAC is superior in normalized torque per unit normalized stator current ( ) ratio whereas UPFC yields very low ratio. The performance characteristics for each strategy under steady state are modelled and simulated in MATLAB environment. This paper therefore presents some important control strategies for PMSM along with merits and limitations which provide a wide variety of control choices in many applications. Additional control methods such as constant torque angle control (CTAC), optimum torque per ampere control (OTPAC), unity power factor control (UPFC), constant mutual flux linkages control (CMFLC), and angle control of air gap flux and current phasor (ACAGF) can also be implemented. Vector control is accepted widely due to its decoupling effect but it is not the only performance requirement. To make PMSM a high performance drive, effective control system is required. Permanent Magnet Synchronous Motor (PMSM) has been considered as the best choice for numerous applications.
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