Tech Guide: Understanding Stepper Motor Holding Torque vs. Detent Torque

When selecting a stepper motor for your application, “torque” is the most critical specification. However, stepper motor torque is defined in two very different ways: Holding Torque and Detent Torque.

Understanding the difference between these two is essential for choosing the right motor for your needs.

1. Holding Torque

Holding torque is the most important specification of a stepper motor. When someone refers to a stepper motor’s “torque,” they are almost always referring to the holding torque.

What is Holding Torque?

Definition: Holding torque is the maximum amount of torque that can be applied to an energized motor (with windings powered) while the rotor is stationary, before it is forced to rotate one full step.

In simple terms, this is the “locking” strength of the motor when it is powered on and told to stand still.

Why is it Important?

• The Core Advantage: This ability to “lock” in place is one of the primary advantages of a stepper motor over other motor types. It allows the motor to hold a load steady against a force without needing a mechanical brake.
• Selection Benchmark: The question “how much force does my application need to hold in place?” is answered directly by the motor’s holding torque rating.
• Torque Characteristic: Holding torque is typically the maximum torque the motor can produce. Once the motor begins to spin, the available dynamic torque (running torque) will decrease as speed increases.

Key Point: The torque value listed on a stepper motor’s datasheet (e.g., 1 N.m) almost always refers to its holding torque.

2. Detent Torque

Detent torque is a secondary characteristic, but it still has an important effect on the motor’s performance.

What is Detent Torque?

Definition: Detent torque is the amount of torque produced by a motor when its windings are not energized (power is off).

This is the “cogging” or “pulsing” resistance you feel when you turn the shaft of an unpowered stepper motor by hand. It is caused by the magnetic interaction between the rotor’s permanent magnets and the steel teeth of the stator.

What Does it Do?

Detent torque can be both beneficial and a drawback:

• The Benefit (A Natural Brake):
  • When the motor stops, the detent torque helps bring it to a halt more quickly by resisting the rotor’s inertia.
  • In the event of an accidental power loss, it provides a small amount of resistance to help keep a load from moving freely.
• The Drawback (An Internal Resistance):
  • To start and run, the motor must first expend energy to overcome its own detent torque.
  • This means the motor’s usable running torque is slightly less than its ideal value, as a portion of its power is always fighting this internal resistance.

Summary Comparison