Why Do Stepper Motors with the Same Size Have Different Torque?

When selecting a stepper motor, many customers notice an interesting phenomenon: two stepper motors with the same frame size and mounting dimensions can have significantly different torque outputs.

For example, why can two NEMA 17 stepper motors have holding torques of 0.4 N·m and 0.6 N·m respectively, even though their external dimensions are identical?

The answer is that motor dimensions alone do not determine torque. Internal electromagnetic design, material selection, winding configuration, and manufacturing processes all play important roles.

Let's explore the six key factors that explain these differences.

The stator and rotor are the core components responsible for generating electromagnetic torque.

Even when two motors have identical external dimensions, their internal electromagnetic structures may differ in several ways:

• Stator tooth design: Tooth shape, width, pitch, and slot geometry affect magnetic field distribution and flux utilization.

• Rotor structure: Rotor tooth geometry, tooth count, and magnetic pole design influence electromagnetic performance.

• Air gap: The gap between the stator and rotor affects magnetic reluctance and overall magnetic circuit efficiency.

An optimized electromagnetic design improves magnetic flux utilization, allowing a motor to generate higher torque within the same frame size.

Hybrid stepper motors typically use permanent magnets in their rotor assemblies. The performance of these magnets directly affects the motor's magnetic field strength.

Different permanent magnet materials have different characteristics, including:

• Residual flux density

• Coercivity

• Maximum energy product

Under otherwise similar design conditions, higher-performance permanent magnets can help increase magnetic flux density and improve torque output.

Magnet dimensions, magnetization methods, and assembly accuracy also influence the final motor performance.

The winding configuration is another important factor affecting stepper motor torque.

Even with identical frame dimensions, two motors may have different winding designs.

• Wire diameter: Affects winding resistance and current-carrying capacity.

• Number of turns: Determines magnetomotive force and electrical characteristics.

• Slot fill factor: Determines how efficiently copper wire is utilized inside the stator slots.

• Rated phase current: Influences the magnetic field strength and temperature rise.

Within the normal operating range, increasing phase current can increase holding torque, provided the magnetic circuit does not reach significant saturation.

However, higher current is not always better. Excessive current can cause overheating, damage winding insulation, and shorten motor service life.

The stator core is generally made from stacked electrical steel laminations. Its material properties and effective stack length also influence motor torque.

Key factors include:

• Silicon steel material: Different materials have different magnetic permeability and core loss characteristics.

• Core stack length: Under otherwise similar conditions, a longer effective stator core can generally provide higher electromagnetic torque.

• Core manufacturing precision: Punching quality, lamination alignment, and stacking accuracy affect magnetic circuit performance.

Therefore, two motors with the same frame size may still have different effective core lengths and torque capabilities.

Even with similar designs and materials, manufacturing quality can create differences in motor performance.

Important factors include:

• Concentricity between the stator and rotor

• Consistency of the air gap

• Winding arrangement and insulation quality

• Rotor magnetization consistency

• Dimensional accuracy during assembly

These factors influence magnetic field distribution, operating stability, and production consistency.

High-quality manufacturing processes help motors achieve their designed torque while maintaining consistent performance across production batches.

It is important to distinguish between a motor's rated torque specifications and its actual operating torque.

Several driver-related factors can affect actual torque output:

• Driver current settings

• Supply voltage

• Microstepping configuration

• Operating speed

At high speeds, winding inductance limits how quickly current can build up. If the supply voltage is insufficient, the actual phase current may not reach the target value, resulting in reduced torque.

Therefore, when comparing two stepper motors, their driving conditions and testing methods should also be consistent.

When comparing two stepper motors with identical frame dimensions, the following parameters should be considered.

One important point: A motor with higher holding torque does not necessarily deliver higher torque at every operating speed.

For high-speed applications, it is especially important to compare torque-speed curves under identical driving conditions rather than relying solely on holding torque.

Stepper motors with identical external dimensions can have significantly different torque outputs because of differences in electromagnetic design, permanent magnet performance, winding configuration, core materials, and manufacturing processes.

For stepper motor manufacturers, the real challenge is not simply making motors smaller. It is achieving higher torque, better operating stability, and greater reliability within limited installation space.

When selecting a stepper motor, don't focus on frame size alone. Pay attention to its internal design and actual performance.