1. Low Speed Inevitably Reduces Torque Output
When reducing the speed of a brushless DC motor by lowering the PWM duty cycle, the electromagnetic torque it delivers decreases significantly. The system naturally enters a low-torque operating regime. The lower the speed, the weaker the motor’s ability to overcome external loads.

2. Heavy Loads Can Cause Stalling
If the pump is operating under a heavy load—for instance, due to high discharge pressure, high vacuum or positive pressure output, or increased pipeline resistance—the torque available from the motor may become insufficient to keep the pump rotating. As a result, the pump head can come to a forced stop, leading to a locked-rotor condition.

3. The Driver Triggers a Latched Protection Mode
Motor drivers continuously monitor the system’s status through current sensing, back-EMF detection, speed feedback, and other diagnostic methods. Once a stall is confirmed, the driver immediately activates a hardware-level protection mechanism and latches this fault state. Most drivers employ a non-self-recovering latching scheme. This means that even if you subsequently increase the PWM command to request higher speed, the protection state will not clear automatically, and the motor will remain unresponsive.

4. Only a Power Cycle Can Clear the Latched Fault
To exit the stall-induced latched state, a complete power cycle—fully disconnecting and then reapplying power—is required. This operation clears the driver’s fault memory and reinitializes the control logic. Therefore, simply adjusting the PWM signal is not sufficient to restore motor operation; a power-cycle reset is the only reliable way to restart the system.