How to Understand Static Torque and Dynamic Torque in Stepper Motors?

When selecting a stepper motor, torque is one of the most important performance parameters to consider. In motor datasheets, two key specifications often attract attention: Static Torque (Holding Torque) and Dynamic Torque.

Both parameters describe the motor's torque capability, but they represent different operating conditions.

If you only focus on holding torque while ignoring dynamic torque, you may encounter problems such as missed steps, vibration, or insufficient driving force during operation, even if the motor can hold its position perfectly when stationary.

So, what is the difference between static torque and dynamic torque? And which parameter should you focus on when selecting a stepper motor?

1. What Is Static Torque in a Stepper Motor?

Static torque generally refers to the torque a stepper motor can resist while remaining stationary.

One of the most commonly used specifications related to static torque is holding torque.

When a stepper motor stops rotating, the driver continues supplying current to the motor windings, creating a stable magnetic field in the stator. The rotor interacts with this magnetic field and maintains its position.

If an external force attempts to rotate the motor shaft, the motor generates an opposing electromagnetic torque to resist the movement.

The higher the holding torque, the greater the motor's ability to resist external rotational loads when energized and stationary.

Static torque has several key characteristics:

• The motor remains stationary at 0 RPM.

• The driver maintains current in the windings to generate holding torque.

• It is primarily used to resist external rotational loads.

• It depends on winding current, magnetic circuit design, and motor construction.

Typical applications:

• Maintaining position in CNC equipment when machining stops.

• Holding the position of automated positioning platforms.

• Maintaining stationary positions in robotic arm joints.

• Preventing unwanted rotation in transmission mechanisms.

It is important to note that holding torque does not mean the motor can lock its shaft under all conditions. If the external load torque exceeds the motor's holding torque, the rotor may move away from its original position.

2. What Is Dynamic Torque in a Stepper Motor?

Dynamic torque refers to the output torque a stepper motor can deliver while rotating.

Unlike static torque, dynamic torque describes the motor's ability to drive a load during operation.

When a stepper motor continuously receives pulse signals, the driver changes the winding currents in a specific sequence. This creates a rotating magnetic field that drives the rotor to rotate continuously.

During this process, the motor must overcome the load resistance while providing sufficient torque to maintain speed and accelerate the mechanical system.

Dynamic torque has several important characteristics:

• The motor operates in a rotating state.

• Output torque is closely related to operating speed.

• Available torque generally decreases as rotational speed increases.

• Performance is affected by supply voltage, drive current, winding inductance, and load conditions.

Typical applications:

• Automated conveyor systems.

• 3D printers.

• CNC machines.

• Packaging and sorting equipment.

• Precision positioning and motion control systems.

3. What Are the Differences Between Static Torque and Dynamic Torque?

The most fundamental difference is the operating condition of the motor.

One important point to remember:

A motor's static holding torque does not directly represent its output capability at high operating speeds.

For example, a stepper motor with a holding torque of 0.5 N·m does not necessarily deliver 0.5 N·m at 1,000 RPM.

Its actual output torque at a specific speed must be evaluated using the torque-speed curve.

4. Why Does Dynamic Torque Decrease as Stepper Motor Speed Increases?

This is one of the most important considerations when selecting a stepper motor.

The main reason is that as motor speed increases, the winding current becomes more difficult to establish within the limited time available.

Three major factors contribute to this phenomenon.

4.1 The Effect of Winding Inductance

Stepper motor windings have a certain amount of inductance, which resists rapid changes in current.

When the driver changes the winding current, inductance limits how quickly the current can rise.

At higher speeds, there is less time for the current to reach its target value. As a result, the motor may produce less electromagnetic torque.

4.2 The Effect of Back EMF

When the motor rotates, it generates back electromotive force (Back EMF) in its windings.

Back EMF generally increases with rotational speed.

This voltage opposes the applied voltage and makes it more difficult to establish the required winding current, limiting the available torque at higher speeds.

4.3 The Effect of Drive Voltage

Increasing the supply voltage, within the limits of the driver and motor system, can help the winding current rise more quickly and improve high-speed performance.

However, the drive voltage must comply with the driver's specifications and be used with appropriate current limiting.

A higher voltage does not automatically mean better performance in every application.

As illustrated by a typical torque-speed curve, a stepper motor generally produces higher torque at low speeds, while its available torque gradually decreases as speed increases.

Therefore, when selecting a stepper motor, do not rely solely on holding torque. Always check whether the motor can provide sufficient dynamic torque at the required operating speed.

5. How to Select a Stepper Motor Based on Static and Dynamic Torque

Different applications have different torque requirements. Motor selection should always consider the actual operating conditions.

Case 1: Equipment Requires Strong Position Holding

Examples include vertical lifting platforms and positioning mechanisms.

These applications require the motor to resist external loads when stationary. Therefore, holding torque should be a key selection parameter.

For vertical loads, additional protection against falling or back-driving is necessary during power loss. Energized holding torque alone cannot provide this protection.

Case 2: Equipment Requires Continuous High-Speed Operation

Examples include high-speed conveyors and automated machining equipment.

For these applications, focus on the motor's dynamic torque at the target operating speed rather than simply comparing holding torque ratings.

Case 3: Equipment Requires Frequent Starting and Acceleration

Examples include automated sorting systems, robotic arms, and reciprocating motion mechanisms.

In addition to dynamic torque, consider the torque required to accelerate the load's inertia and ensure sufficient torque margin throughout the operating speed range.

Testing under actual load conditions is recommended to verify stable operation and prevent missed steps.

6. Why Is the Torque-Speed Curve Essential for Motor Selection?

The torque-speed curve illustrates how a stepper motor's output torque changes at different rotational speeds. It is one of the most important references for evaluating dynamic motor performance.

When reviewing a torque-speed curve, pay attention to the following factors:

1. Target operating speed: Confirm the rotational speed required by the application.

2. Available output torque: Check how much torque the motor can deliver at that speed.

3. Drive conditions: Verify the supply voltage, current settings, and drive mode used to generate the curve.

4. Torque margin: Ensure sufficient torque reserve to accommodate load variations and acceleration requirements.

It is also important to understand that the dynamic torque of the same stepper motor may vary under different driver configurations, supply voltages, and current settings.

Therefore, always check the test conditions when comparing torque-speed curves from different manufacturers.

Conclusion

Static torque and dynamic torque are two essential parameters for evaluating stepper motor performance.

• Static Torque: Indicates the motor's ability to hold its position and resist external rotational loads when energized and stationary.

• Dynamic Torque: Indicates the motor's ability to drive a load while rotating.

For applications requiring position holding, holding torque deserves particular attention. For equipment involving continuous operation, high-speed movement, or frequent acceleration, dynamic torque and the torque-speed curve are more important.

Choosing the right stepper motor is not about selecting the motor with the highest holding torque. It is about selecting a motor that can deliver sufficient torque under the actual operating speed and load conditions.