When a stepper motor is not powered on, its shaft may become difficult to rotate (effectively locked) if the two wires of the same coil phase (winding) are short-circuited.
How a Stepper Motor Works
A stepper motor consists of a stator with multiple coil windings and a rotor containing a permanent magnet. The rotor is essentially a strong magnet with distinct north (N) and south (S) poles, while the stator houses several sets of coil windings.
When current flows through a specific set of stator coils in a given direction, it generates a magnetic field within the stator. The permanent magnet rotor then aligns itself according to the principle of attraction between opposite poles (N and S), following the principle of minimum reluctance. By sequentially changing the magnitude and direction of the current in the stator coils, the stator's magnetic field rotates at a specific speed and direction, causing the rotor to follow the rotating magnetic field.
Why the Shaft Locks When the Wires Are Short-Circuited
The permanent magnet rotor generates magnetic flux. When the motor is not powered and the rotor is manually turned, the rotating magnetic field of the rotor induces an electromotive force (back-EMF) in the stator coils.
If the two wires of the same coil phase are short-circuited, this creates a closed loop. The back-EMF drives a large induced current through the short-circuited winding. This current generates a magnetic field that opposes the rotor's motion — a phenomenon known as the dynamic braking effect. As a result, the motor shaft becomes very difficult to turn, effectively locking it.
Troubleshooting
If you experience unexpected shaft locking on an unpowered stepper motor, follow these steps:
- Use a multimeter to check whether any coil phase is short-circuited. Measure the resistance between the two wires of each phase.
- Compare the reading against the expected resistance for your motor model. A significantly lower-than-expected resistance may indicate a short circuit.
- Inspect the wiring and connectors for damage or accidental contact between wires.