There are three possible reasons:
1. Closed circuit in the motor windings
This occurs when the A+ and A− or B+ and B− leads of the motor are connected, forming a closed circuit. Under the influence of the permanent magnet, the motor generates a small amount of braking torque. This is normal behavior. In fact, this method is commonly used to identify which two wires belong to the same phase.
2. Contaminants inside the motor
In this case, the resistance feels uneven and random. The contaminants that cause this issue may be small metal shavings from the machining process, or tiny metal particles from the environment that are attracted to the rotor magnet during assembly. Because these particles appear randomly, they may not be detected during factory inspection. In some cases, minor resistance caused by small contaminants may diminish as the motor runs. However, if the resistance persists or the motor exhibits noticeable sticking or locking at specific positions, the motor is defective and should be replaced.
3. Rotor or stator misalignment due to external forces
When the motor is subjected to a significant external force—such as drops, impacts, or shocks—the shaft, rotor, or stator may become misaligned, causing the rotor and stator to come into contact. This situation should be avoided whenever possible. Such damage may also occur during shipping. If you find obvious damage to the external packaging or the product itself, please contact our customer support team.