Holding Torque vs. Running Torque of Stepper Motors

Holding torque, also known as static torque, refers to the maximum torque a stepper motor can generate when it is energized but not rotating (in a stationary state).

Here's an intuitive way to understand it: When you supply the rated current to the stepper motor's windings (and lock it), if you try to turn the motor shaft with a wrench, you need to apply a certain force to overcome the magnetic resistance and make it rotate. The minimum torque required to start rotating the motor shaft is the holding torque.

Key Characteristics and Points

1. Static Metric: It is measured at zero speed and represents the motor's "locking" or "self-locking" capability when stationary.
2. Maximum Torque Point: Holding torque is the theoretical maximum torque the stepper motor can provide.
3. Proportional to Current: The magnitude of the holding torque is directly related to the current flowing through the windings. The higher the current applied (without exceeding the maximum allowable current for the motor and drive), the greater the holding torque.
4. Not the Running Torque: This is crucial. Once the motor starts rotating, the actual torque it can output (called the "running torque") decreases significantly as the speed increases and is less than the holding torque. This is mainly due to factors such as the inductance of the motor windings and the back EMF generated during rotation.

Engineering Significance of Holding Torque

• Core Basis for Selection: Holding torque is one of the primary parameters for selecting and comparing different models of stepper motors. It directly reflects the strength of the motor. For example, when needing to move a heavy load in a linear motion, you first need to calculate the required torque and select a motor whose holding torque is greater than this value.
• Maintaining Position: In applications where the motor needs to firmly hold its position when stationary (e.g., a 3D printer nozzle during a print pause, a robotic arm after reaching a specified point), a high holding torque ensures the position does not shift due to external forces.

As shown in the diagram, the stepper motor model 23HS30-2804S has a static holding torque of 2 Nm. However, during actual operation, its dynamic torque decreases as the speed increases. This is reflected in the speed-torque curve, where its maximum running torque values are all lower than the static holding torque value.

I. Inductive Characteristics of the Windings

The stator windings of a stepper motor are substantial inductive components. The core characteristic of an inductor is to "resist changes in current." Under static conditions, the windings carry a stable DC current, which is constant, so the inductor presents no opposition.

However, when the motor needs to rotate, the drive sends high-frequency pulsed voltage signals to the windings to switch the magnetic field rapidly. At this point, the presence of the inductance causes a critical issue: the current builds up much more slowly than the voltage changes.

• At low speeds: The pulse frequency is low, allowing relatively sufficient time for the current to rise to the rated value. Therefore, the torque is close to the holding torque.
• At medium to high speeds: The pulse frequency increases, and the duration of each pulse becomes extremely short. The current doesn't have enough time to reach its peak before the next pulse may arrive. This causes the average current in the windings to decrease significantly.

Conclusion: Torque is proportional to current. Because inductance prevents the current from building up rapidly, the average current during rotation is less than the static current. Consequently, the actual output torque is necessarily less than the holding torque.

II. Back EMF (Electromotive Force)

When a stepper motor rotates, it acts not only as a motor but also as a generator. The rotating permanent magnet rotor cuts the magnetic field lines of the stator windings, generating, according to the law of electromagnetic induction, an induced electromotive force (EMF) that opposes the driving voltage – this is the Back EMF.

• Effect: The Back EMF counteracts part of the voltage supplied by the drive. According to Ohm's law, the effective voltage across the windings is reduced, making it harder to push current into them.
• Relationship with Speed: The magnitude of the Back EMF is proportional to the rotational speed. The higher the speed, the greater the Back EMF, the stronger its opposing effect on the current, and the more severe the torque drop.

Important Relationship: Back EMF is not an independent cause but an inevitable consequence of electromagnetic induction once the motor rotates. A motor with high inductance will typically also generate higher Back EMF. Thus, inductance is the "cause," and Back EMF is the "effect"; they work together to cause the torque to drop as speed increases.

III. Other Losses

During high-speed operation, the magnetic field in the motor's core changes at a high frequency, generating eddy current losses and hysteresis losses. This energy is dissipated as heat and cannot be converted into mechanical work, further reducing the motor's output efficiency and available torque.

Therefore, when selecting a stepper motor, you must never simply use the holding torque to evaluate its ability to drive a load during operation. You must refer to the Torque-Speed Curve provided on our website. Ensure that the motor can still provide torque greater than the load resistance at the maximum speed required by your application, and leave sufficient margin (typically 30%-50%) to prevent lost steps.

Updated on: 30/09/2025