The key to maintaining motor output power stability during field-weakening control lies in dynamically adjusting magnetic field strength and current distribution to balance speed increase with torque reduction. When the motor speed exceeds the base frequency (rated frequency) and enters the field-weakening region, the back EMF increases with speed, causing the terminal voltage to approach the inverter's output limit. Without adjusting the control strategy, voltage saturation can lead to current runaway, further compromising power stability. The frequency converter injects a negative direct-axis current (Id < 0) to partially offset the permanent magnet's magnetic field, reducing the air gap flux and maintaining the terminal voltage within the limit at higher motor speeds, thus providing the foundation for power stability.
During field-weakening control, the frequency converter must precisely coordinate the distribution of direct-axis current (Id) and quadrature-axis current (Iq). The direct-axis current regulates magnetic field strength, while the quadrature-axis current determines torque output. During the field-weakening phase, the frequency converter weakens the magnetic field by increasing the negative direct-axis current while dynamically adjusting the quadrature-axis current to compensate for torque loss. For example, when the speed increases to 1.5 times the base frequency, the frequency converter may adjust the direct-axis current from zero to -20% of the rated value while simultaneously increasing the quadrature-axis current to 110% of the rated value to ensure that the product of torque and speed (i.e., power) remains stable. This dynamic optimization of the current vector relies on the frequency converter's built-in vector control algorithm, which decouples the magnetic field and torque components to achieve independent regulation.
Maintaining power stability also relies on the frequency converter's strict control of voltage limits. In the field weakening region, the motor terminal voltage is limited by the inverter DC bus voltage, forming a voltage limit ellipse. The frequency converter monitors the terminal voltage in real time and automatically triggers field weakening control when the voltage approaches the limit. It adjusts the current vector angle to ensure that the voltage vector remains within the limit ellipse. For example, when the terminal voltage reaches 90% of the DC bus voltage, the frequency converter immediately increases the negative direct-axis current and reduces the voltage vector amplitude to avoid power interruption caused by overvoltage protection. This closed-loop control mechanism ensures power continuity in the motor's high-speed range.
The torque compensation strategy is a key component in maintaining power stability. During the field weakening phase, motor torque decreases inversely as speed increases. Without compensation, power will decrease due to torque decay. Frequency converters achieve torque compensation in two ways: first, increasing the quadrature-axis current amplitude to maintain torque by increasing current density during field weakening; second, optimizing the current phase angle to align the current vector more closely with the optimal torque direction. For example, in high-speed, light-load scenarios, the frequency converter may prioritize current phase optimization to reduce copper loss; whereas, in heavy-load startup scenarios, it may increase the quadrature-axis current to ensure instantaneous torque requirements.
The frequency converter's parameter adaptation further enhances power stability. When motor parameters (such as resistance and inductance) vary with temperature, the frequency converter dynamically adjusts control parameters using an online identification algorithm. For example, when motor temperature increases, causing stator resistance to increase, the frequency converter automatically adjusts the resistance value in the voltage equation to avoid field regulation loss caused by parameter errors. This adaptive mechanism ensures the effectiveness of the field weakening control strategy under various operating conditions and prevents power fluctuations caused by parameter drift.
In practical applications, the frequency converter also needs to optimize field weakening control based on load characteristics. For loads such as fans and pumps, whose torque is proportional to the square of the speed, power demand naturally decreases during the field weakening phase, simplifying the control strategy. However, for constant-power loads such as machine tool spindles, the frequency converter requires more precise current regulation to maintain power output. For example, during high-speed cutting on a CNC milling machine, the frequency converter uses feedforward compensation to proactively adjust the current vector to offset power transients caused by sudden load changes.
From a control architecture perspective, the frequency converter employs a layered design to achieve power stability. The bottom layer is the current loop, which uses a PI regulator to rapidly track the direct and quadrature axis currents. The middle layer is the velocity loop, which dynamically adjusts the torque reference in conjunction with a load observer. The top layer is the power monitoring layer, which calculates input and output power in real time and feeds it back to the control algorithm. This layered structure ensures comprehensive power stability, from transient response to steady-state accuracy, enabling the motor to operate at high speeds during field weakening while precisely matching power output to load demand.
