Permanent magnet synchronous motors have many advantages as mentioned in the previous sections. These features permit this machine to be operated not only in the constant torque region but also in the constant power region up to a high speed by flux-weakening. However, considering the drawbacks listed in above
literature survey for PMSM drives, it is understood that further improvements in the design and control techniques of PMSMs for constant power operation are possible. Therefore, the objectives of this research are:
• Analysis of constant power speed range for IPMSM Drives;
• Design optimization of a prototype interior PMSM;
• Control of the prototype interior PMSM in wide speed operation.
1.6.1 Analysis of Constant Power Speed Range for IPMSM Drive
An accurate mathematical modelling of PMSMs provides a foundation for the de- sign and control of PMSMs. The accuracy of the model directly affects the outcome of the design and control. A two-phase PMSM equivalent circuit model in the ro- tor d-q reference frame has been widely adopted by Pillay [2], Sebastian [82] and Rahman [83] due to its relatively simple composition and reasonable accuracy. The d-q model, also known as the analytical model, can be derived directly from d- and q-axis magnetic circuits or can be obtained from the three phase model of a PMSM by using Park’s transformation [84].
One objective of this research is to determine the relationship between steady state equivalent circuit parameters of interior PMSMs and their power capability over a wide speed range. In all cases it will be assumed that the motor is supplied from a variable frequency inverter with current and rotor position feedback so that
both current magnitude and phase angle of current vector can be independently controlled. The system performance will be investigated primarily by the shape of the motor power capability curve which is a plot of the maximum power attain- able versus motor speed subject to current and voltage magnitude constraints. A simple equivalent circuit model, which takes account of saturation effect in q-axis inductance, will be used in order to determine the best choice of motor parameters necessary for maximum power capability over a wide speed range.
1.6.2 Design Optimization of Interior PMSM
While the design process for conventional exterior PMSMs is well developed in Slemon and Panigrahi’s works [19, 21], it is hard to borrow these experiences to the design of interior PMSMs because of their complicated rotor structures and complex influence of magnetic saturation. In the modelling and computing pro- cess, FEM can be used to accurately compute the air gap flux density and ro- tor magnetic saliency ratio that are nonlinear functions of the machine flux level.
However, applying FEM in the every stage of design and optimization process is time-consuming, and sometimes not practical.
In this work, the design procedure combines the FEM and RSM for the mag- netic analysis, and genetic algorithms (GA) are used as a search method in the optimization procedure. The effects of a wide range of geometric variables on the performance of PMSM are investigated and the optimum design is selected for further studies. Using the proposed design methodology, a 400 W prototype in-
terior PMSM with the optimum constant power speed range has been designed and manufactured. Various steady state tests are conducted to verify the design optimization method. Test results are compared with those from the numerical analysis. The close agreement validates the proposed RSM model for the accurate estimation of motor parameters. The tests also provided an understanding of the performance of the prototype PMSM and laid a foundation for the control of the PMSM drive system.
1.6.3 Control of IPMSM in Wide Speed Operation
For a PMSM drive the speed is determined by the frequency of its power supply and the torque is determined by both the magnitude of the stator current and the torque angle. The PMSMs can be controlled at constant torque below the base speed and at constant power above the base speed. However, without proper control strategy, the PMSM drives cannot exploit its potential power capability even with a well designed motor.
In this research, a conventional current vector control strategy is imple- mented. The performances on constant torque and constant power operations are investigated, and their advantages and disadvantages are examined. A novel mod- ified DTC scheme based on stator flux control is also proposed. The experimental results confirm that the new control strategy not only improves the torque capa- bility in the flux-weakening range, but also extends the speed range at constant power operation.