Torque maximization for permanent magnet synchronous motors

Abstract

This paper discusses the problem of maximizing the torque of permanent magnet synchronous motors in the presence of voltage and current constraints. Formulas are given that are suitable to operation with voltage and current source inverters and to real-time computation. Generally, the available torque is limited by current constraints at low speeds and by voltage constraints at high speeds, and there is often an intermediate range of speeds where both the voltage and the current constraints are active. The paper specifies how optimal operation can be achieved in all three ranges, and at what speeds the optimal operation transitions from one mode to another. An example is discussed to illustrate the contributions of the paper and experimental results demonstrate the usefulness of the formulas in a high-performance robotic application.

Torque Maximisation Of The Pmac Motor For High Performance, Low Inertia Operation

“…Electrical torque developed by the motor is proportional to the q-axis current T e = k t i q where k t is the motor torque constant. This transformation also applies to current and flux linkage quantities, and the three phase quantities may be obtained from the d and q axis variables by application of an inversion of the matrix in equation (1). The q-axis inductance is equivalent to the armature inductance, and the d-axis inductance is equivalent to the field inductance.…”

“…Torque maximisation for smooth rotor (surface mounted magnets) permanent magnet machines has been addressed [1] using steady-state (i.e. constant rotor velocity) system description with the aim of yielding smaller motors for given applications, or faster operation for a given size of motor.…”

Automatic flux-weakening control of permanent magnet synchronous motors using a reduced-order controller

“…In this study, the hexagon is replaced by a circle as the boundary of all saturated voltage vectors for convenience. Under such a condition, the relation between the actual output voltage and the saturated voltage can be described by the following equation: (15) where the magnitude of the saturated voltage is a constant value and can be expressed as .…”

“…Flux-weakening control of PMSM's employs a negative -axis current (demagnetizing current) to suppress the back emf. References [12]- [15] proposed several flux-weakening controls of PMSM's. In these methods, a -axis current command is determined to achieve maximum torque output or constant power output.…”

“…Furthermore, implementing an overmodulation strategy allows the controller to automatically generate a demagnetizing current to expand the speed range when input voltage saturation occurs. This flux weakening control is also optimal in the sense of minimum power dissipation, and differs from the maximum output torque design or the constant power design in the general flux-weakening control methods [12]- [15]. Stability analyzes based on the Lyapunov's linearization method indicate that, to stabilize the closed-loop system, the control gains in the reduced controller must be both upper and lower bounded.…”