In order to produce the maximum torque, which main component is proportional to q-axis component of the armature current, it is convenient to control the inverter-fed PMSM by keeping the direct, d-axis, current component to be id as long as the inverter output voltage doesn’t reach its limit.
At that point, the motor reaches its maximum speed, so-called rated speed (called al so base speed when talking about flux-weakening). Beyond that limit, the motor torque decreases rapidly toward its minimum value, which depends on a load torque profile. To expand the speed above the rated value, the motor torque is necessary to be reduced. A common method in the control of synchronous motors is to reduce the magnetizing current, which produces the magnetizing flux. This method is known as field-weakening. With PM synchronous motors it is not possible, but, instead, the air gap flux is weakened by producing anegative d-axis current component, id.
Because nothing has happened to the excitation magnetic field and the air gap flux is still reduced, so is the motor torque, this control method is called flux-weakening. As a basis for this analysis, the PMSM current and voltage d-q vector diagrams from the previous section Fig are used. During flux-weakening, because the demagnetizing (negative) id current increases, a phase current vector is rotates toward the negative d-semi-axis.
The rotation of the phase voltage vector is determined by a chosen flux weakening strategy, but at the end of flux-weakening it always rotates toward the positive q- semi axis because of iq current, i.e vd voltage magnitude decrease.
Hence, the voltage-to-current phase shift decreases to zero and increases in negative direction either to the inverter phase shift limit (usually 300), or a load torque dictated steady-state (zero acceleration), or to the zero motor torque condition (no load or generative load). A big concern of flux-weakening control is a danger of permanent demagnetization of magnets. However, large materials such as Samarium-Cobalt, allows significant id current which can extend the motor rated speed up to two times. Three commonly used flux- weakening control strategies are:
1) constant-voltage-constant-power (CVCP) control;
2) constant-current-constant-power (CCCP) control; and
3) optimum-current-vector (OCV or CCCV-constant-current-constant-voltage) control.
