Interior permanent-magnet (IPM) synchronous motors possess special features for adjustable-speed operation which distinguish them from other classes of ac machines. They are robust high powerdensity machines capable of operating at high motor and inverter efficiencies over wide speed ranges, including considerable ranges of constant-power operation. The magnet cost is minimized by the low magnet weight requirements of the IPM design. The impact of the buried-magnet configuration on the motor's electromagnetic characteristics is discussed. The rotor magnetic circuit saliency preferentially increases the quadrature-axis inductance and introduces a reluctance torque term into the IPM motor's torque equation. The electrical excitation requirements for the IPM synchronous motor are also discussed. The control of the sinusoidal phase currents in magnitude and phase angle with respect to the rotor orientation provides a means for achieving smooth responsive torque control. A basic feedforward algorithm for executing this type of current vector torque control is discussed, including the implications of current regulator saturation at high speeds. The key results are illustrated using a combination of simulation and prototype IPM drive measurements.

Interior Permanent-Magnet Synchronous Motors for Adjustable-Speed Drives

The interior permanent magnet (IPM) synchronous motor is compatible with extended-speed-range constant-power operation by means of flux-weakening control. Flux weakening uses stator current components to counter the fixed-amplitude magnetic airgap flux generated by the rotor magnets, performing a role similar to field weakening in a separately excited dc motor. The nature of current regulator saturation caused by the finite inverter dc source voltage is described, marked by premature torque and power degradation at high speeds in the absence of flux-weakening control. This is followed by presentation of a new flux-weakening control algorithm developed as a modification of an established feedforward IPM torque control algorithm described previously in the literature. Attractive features of this new algorithm include smooth drive transitions into and out of the flux-weakening mode, fast response, as well as automatic adjustment to changes in the dc source voltage. Simulation and empirical test results from a 3-hp laboratory IPM motor drive are used to confirm the constant-power operating envelope achieved using the new flux-weakening control algorithm.

Flux-Weakening Regime Operation of an Interior Permanent-Magnet Synchronous Motor Drive

It is demonstrated that the rotor design of synchronous reluctance machines can be optimized in terms of a key geometric parameter, i.e., the ratio of the rotor insulation width to the rotor iron width so as to obtain maximum torque production. The equation which gives the maximum motor power factor in terms of the saliency ratio has been derived and it is shown that the power factor of 0.8 is a realizable value with the optimal rotor design. An experimental motor has been fabricated and the results of measurements of the motor parameters prove the validity of the rotor design optimization. >

Rotor design optimization of synchronous reluctance machine

The author presents several linear and nonlinear models suitable for transient and steady-state analyses of induction machine drives The models are presented in equivalent circuit form to preserve the identity of nonlinear parameters The circuits, designated as Gamma or inverse Gamma form, are simpler than the conventional T-form circuit Their parameters are readily determined from terminal measurements The major effects of magnetic nonlinearity are included in the models in a way that is more accurate than that usually obtained with the conventional T-form circuits Modeling of time harmonics is also discussed >

Modelling of inductance machines for electric drives

Induction motors are modeled by nonlinear higher-order dynamic systems of considerable complexity. The dynamic analysis based on the complex notation exhibits a formal correspondence to the description using matrices of axes-oriented components; yet differences exist. The complex notation appears superior in that it allows the distinguishing between the system eigenfrequencies and the angular velocity of a reference frame which serves as the observation platform. The approach leads to the definition of single complex eigenvalues that do not have conjugate values associated with them. The use of complex state variables further permits the visualization of AC machine dynamics by complex signal flow graphs. These simple structures assist to form an understanding of the internal dynamic processes of a machine and their interactions with external controls. >

The representation of AC machine dynamics by complex signal flow graphs

The analysis presented in a previous paper is extended to include a consideration of synchronizing and damping torque during continuous and transient oscillations, the description of an equivalent circuit which has been found useful in calculating the decrement of the short circuit current of interconnected machines, and the derivation of simple approximate formulas for the roots of the equation determining the wave shape of the current on three-phase short circuit.

R. H. Park

Papers

Two-reaction theory of synchronous machines — Part II