Constant torque operation is the core control mode for speed-regulating motors in the low-speed to base-speed range. Its core objective is to maintain a constant output torque to meet the demands of high-load conditions such as starting, climbing, and heavy-load feeding. It is widely used in machine tool feed axes, conveyor belts, and lifting equipment. Its core control logic involves precisely adjusting the stator current and voltage frequency ratio to ensure stable air-gap magnetic flux, thereby achieving constant and controllable torque output while balancing operational stability and load adaptability.
The core principle of constant torque control is to follow the basic logic of “voltage-frequency proportional control (V/F control)” while combining it with vector control to optimize accuracy. Below the base speed, the motor flux is positively correlated with the stator voltage and frequency. Strict synchronization of voltage and frequency regulation is required to ensure a constant U/f ratio, thereby maintaining stable air gap flux and preventing torque fluctuations caused by flux saturation or insufficiency. For different types of motors, such as permanent magnet synchronous motors and asynchronous motors, targeted optimization of control strategies is necessary. Asynchronous motors require stator voltage drop compensation to improve low-speed torque accuracy, while permanent magnet synchronous motors achieve constant torque by regulating the q-axis torque current component.
Precise current control is a crucial aspect of constant torque operation. This requires real-time acquisition of the stator three-phase current via current sensors, which is then transformed into current components in the dq coordinate system. The q-axis current directly corresponds to the torque output and must be stabilized at a set value through closed-loop control. The d-axis current maintains constant excitation to ensure stable magnetic flux. Simultaneously, current limits are set to prevent inrush currents during startup or sudden load changes, thus preventing motor overload and damage to power devices, and ensuring smooth, fluctuation-free torque output.
Operating condition adaptation and disturbance compensation must be implemented simultaneously to improve control reliability. Real-time monitoring of motor speed, load changes, and temperature parameters is crucial. When load changes abruptly, the voltage and current ratios should be adjusted quickly to compensate for the impact of load disturbances on torque. For parameter drift caused by temperature increases, a dynamic calibration mechanism should be implemented to correct deviations in stator resistance, inductance, and other parameters. Furthermore, the control logic during startup should be optimized, employing soft start or graded voltage boosting to avoid instantaneous torque surges during startup, balancing startup smoothness with load-bearing capacity, and ensuring stable motor operation across the entire low-speed, heavy-load range.
Post time: Mar-09-2026