Stepper Motor Torque Explained: Holding Torque vs Running Torque

Introduction

Stepper Motor Torque is the key parameter that determines a motor’s load capacity and positioning accuracy. Among all torque-related indicators, Holding Torque and Running Torque are the two most easily confused, and they are also the top concerns for engineers and purchasers. In daily work, many people mix up these two torques—mistakenly using Holding Torque as Running Torque when selecting stepper motors. This mistake often leads to issues like the motor failing to drive loads, step loss, and positioning errors, which disrupt normal equipment operation. This article breaks down the definitions, differences, and influencing factors of Holding Torque and Running Torque, combined with practical application scenarios such as 3D printers and CNC machine tools. It will help you select the right stepper motor, avoid common mistakes, and master the key knowledge of Stepper Motor Torque to solve practical problems in selection and use.

Basic Understanding of Stepper Motor Torque

What is Stepper Motor Torque

Stepper Motor Torque refers to the rotational force output by a stepper motor, which enables the motor to overcome load resistance and achieve precise rotation. It is usually measured in Newton-meters (Nm) or Ounce-inches (oz-in), and is a primary reference when choosing a stepper motor for any application. This torque directly affects how much load the motor can drive, its ability to maintain stable positioning, and the smoothness of its operation—all critical for equipment like 3D printers and CNC machine tools to perform reliably.

Common Types of Stepper Motor Torque

Stepper motors have several key torque types, with Holding Torque and Running Torque being the most important for practical use. Holding Torque applies when the motor is energized but stationary, while Running Torque applies when the motor is rotating at a steady speed. Other related types include pull-out torque, pull-in torque, and positioning torque—understanding these helps avoid confusion during selection. Note that Running Torque is also referred to as Dynamic Torque in some technical documents.

Detailed Explanation of Holding Torque

What is Holding Torque of Stepper Motor

Holding Torque is the maximum force a stepper motor can exert to resist external loads and keep its rotor stationary when energized. It acts like a "locking force" that keeps the motor shaft in place even when force is applied. The magnitude of Holding Torque is directly related to the current in the motor’s stator windings, and it can be calculated using the formula t = i × k (where t = Holding Torque, i = winding current, and k = torque constant). For example, when a 3D printer pauses mid-print, the Holding Torque is what keeps the nozzle fixed in position.

Factors Affecting Holding Torque

- Winding Current: Higher current (within the motor’s rated limit) increases Holding Torque; excessive current can cause overheating, so it’s important to stay within the manufacturer’s specifications.
- Motor Size & Design: Larger motors typically generate stronger magnetic fields, resulting in higher Holding Torque. The number of poles and winding configuration also play a role in torque output.
- Motor Type: Bipolar stepper motors generally have higher Holding Torque than unipolar motors, as they use dual windings to create a stronger magnetic force.

Common Applications of Holding Torque

Holding Torque is essential in scenarios where the motor needs to stay stationary for extended periods while maintaining position. Examples include CNC machine tool spindle positioning, robot arm stagnation, 3D printer pause states, and station locking in automated production lines. Insufficient Holding Torque can lead to position drift or load slippage, which impacts equipment performance.

Detailed Explanation of Running Torque

Stepper Motor Running Torque Definition

Running Torque (also called Dynamic Torque) is the continuous force a stepper motor outputs when its rotor rotates at a steady speed. It is always lower than Holding Torque and decreases as the motor’s speed increases. This is due to winding inductance and back electromotive force, which limit current flow at higher speeds. The formula for Running Torque is t = 5252 × p / n (where t = Running Torque, p = motor power, and n = speed).

Factors Affecting Running Torque

- Speed: Running Torque drops significantly at high speeds. The speed-torque curve (provided in most motor datasheets) clearly shows this relationship—higher speeds lead to greater torque attenuation.
- Power Supply: Higher voltage helps reduce torque loss at high speeds by overcoming back electromotive force. Current must match the motor’s rated value; too low current causes insufficient torque, while too high current leads to overheating.
- Load Inertia: Heavier loads or higher inertia require more Running Torque to start and maintain rotation, increasing the risk of step loss if the torque is insufficient. A small torque margin (10-20%) is recommended to avoid overload.
- Microstepping & Step Angle: Microstepping improves operational smoothness but slightly reduces effective torque. Smaller step angles generally provide higher torque per unit step.

Common Applications of Running Torque

Running Torque is critical for applications where the motor must drive loads continuously. Examples include 3D printer nozzle movement, CNC machine tool feed motion, automated conveyor belts, and robot joint rotation. Insufficient Running Torque can cause the motor to stall, lose steps, or operate unevenly—all of which affect production efficiency.

Holding Torque vs Running Torque – Key Differences

Side-by-Side Comparison

How to Correctly Match Torques for Selection

When selecting a stepper motor, both Holding Torque and Running Torque must be considered—one cannot replace the other. For static positioning tasks, prioritize Holding Torque; for continuous rotation tasks, focus on Running Torque at the required operating speed. Always reserve a 10-20% torque margin to prevent overload. For example, when selecting a motor for a 3D printer, the Holding Torque must be sufficient to keep the nozzle stationary during pauses, while the Running Torque must handle the nozzle’s movement speed without step loss. Refer to the motor’s datasheet and speed-torque curve to ensure a proper match.

Practical Stepper Motor Torque Selection Tips & Common Mistakes

Three-Step Torque Selection Process

1. Clarify Equipment Requirements: Determine whether the motor needs to maintain static positioning, continuous rotation, or both, and note the required operating speed range.
2. Calculate Load Needs: Based on the load weight and transmission structure, calculate the minimum required Holding Torque and Running Torque, and add a 10-20% margin to avoid overload.
3. Match Motor Specifications: Use the motor’s datasheet to check Holding Torque and Running Torque at the required speed. Refer to the speed-torque curve to ensure the motor performs reliably at high speeds without excessive torque loss.

Common Selection Mistakes to Avoid

- Focusing only on Holding Torque: This leads to insufficient torque for rotation, causing step loss or motor stalling.
- Ignoring speed-torque relationship: Assuming Running Torque remains constant at all speeds can result in inadequate performance at high speeds.
- Overlooking current limits: Increasing current beyond the motor’s rating to boost torque causes overheating and damage.
- Confusing Holding Torque with positioning torque: Positioning torque is the static force when the motor is de-energized, which is much lower than Holding Torque.

Summary & Practical Guidance

Holding Torque and Running Torque are both critical for stepper motor performance—Holding Torque keeps the motor stationary when energized, while Running Torque drives continuous rotation. Understanding their differences and how to match them to your application is key to selecting the right motor and avoiding common issues like step loss and positioning errors.
If you’re unsure about selection, refer to the motor’s datasheet for torque specifications and speed-torque curves. For specific applications like 3D printers or CNC machines, calculate the required torque based on your load and operating speed. You can also adjust voltage or current (within rated limits) to boost torque if needed.
By mastering these concepts, you can ensure your stepper motor operates reliably, improving equipment performance and reducing downtime.