Stepper vs servo torque: what should you compare?
Stepper and servo motors differ in how much torque they can deliver at a given speed, and for how long. A stepper motor can be an effective choice for holding a position and moving at low speed. A typical industrial servo motor can offer a wider useful speed range and short bursts of peak torque. Neither motor type wins every torque comparison.
Holding torque is not running torque
A stepper's holding torque is measured at standstill with its windings energised at the specified current. It does not describe the torque available throughout the speed range. For motion, consult the torque–speed curve of the actual motor and drive combination. Pull-out torque is the limiting torque at a given speed before the motor loses synchronism; leave operating margin below that boundary.
Servo specifications separate continuous torque, rated torque and peak torque. Rated torque applies at the rated speed under stated thermal conditions. Continuous torque at standstill may be listed separately. Peak torque is a temporary capability, constrained by the motor, drive and permitted overload duration. Match the torque rating and measurement unit before comparing catalogue values.
How their torque behaviour differs
| Operating condition | Stepper motor | Typical industrial servo |
|---|---|---|
| Holding a stationary load | Check energised holding torque. | Check continuous torque at zero speed and thermal limits. |
| Increasing speed | Available torque generally falls substantially; the drive affects the curve. | Can maintain torque over a wider speed range, with limits at higher speeds. |
| Acceleration and deceleration | Check running torque and the commanded ramp. | May use the permitted peak torque for short periods. |
| Exceeding capacity | An open-loop system can miss steps or stall. | May reach a torque limit, develop following error or issue an overload alarm. |
| Main sizing information | Motor–drive curve at the required speed, with torque margin. | Continuous and peak envelopes, plus the thermal demand of the motion cycle. |
Why does stepper torque drop as speed rises?
Torque depends on winding current. As the motor runs faster, there is less time to establish the required current before the next phase change. Winding inductance and back EMF can prevent the drive from reaching its current target. The result is less available torque at higher speeds. This is why stepper torque performance depends on the drive, supply voltage, current setting and winding connection as well as the motor.
Increasing the supply voltage may improve performance within the motor and drive's permitted operating conditions, but a catalogue curve cannot simply be assumed valid with a different supply or drive. Treat stepper motor and drive selection as one task. Raising the current without checking the ratings creates a thermal problem rather than a sound sizing method.
Could a 1.3 N·m servo outperform a 3 N·m stepper?
Yes, at a particular operating point. Consider an illustrative example: a stepper has 3 N·m holding torque, but its motor–drive curve allows only 0.6 N·m at 1000 rpm. A servo can provide 1.3 N·m continuously at that speed. A shaft load requiring 1 N·m continuously exceeds the stepper's running capability. The servo passes that continuous-torque check, although acceleration and thermal checks are still required.
These are hypothetical values, not specifications for a particular product. They show why a larger standstill rating may be less useful at the required speed. Sizing a motor for torque starts with the operating point and its torque–speed relationship, rather than the largest number in a catalogue.
Acceleration needs additional torque
A motor that moves a load comfortably at constant speed may struggle with an aggressive acceleration command. A simplified shaft-side calculation is Trequired = Tload + Jtotal × α. Load torque includes opposing friction; J is the total inertia referred to the motor shaft, and α is angular acceleration. Shortening the acceleration time increases the acceleration torque requirement. Include the mechanical transmission's effects in the calculation.
A servo's permitted peak torque can help during acceleration, but there is no universal peak-to-rated torque ratio. The motor and drive combination sets the available envelope. Correct speed and torque control, speed-loop adjustment and auto-tuning help the axis respond predictably; they cannot compensate for unlimited load or inadequate mechanical capacity.
Peak torque can pass while thermal sizing fails
Repeated movements also require an RMS torque check. Divide the cycle into segments and calculate TRMS = √[Σ(Ti² × ti) / Σti]. This considers both torque and time. Positive and negative torque do not cancel in the calculation because each torque value is squared.
Suppose a repeating one-second cycle requires 3 N·m for 0.2 seconds and 1 N·m for 0.8 seconds. Its RMS torque is √(3² × 0.2 + 1² × 0.8) ≈ 1.61 N·m. A servo with 1.3 N·m continuous capability under the relevant conditions may be thermally undersized even if its peak rating accommodates 3 N·m. Cooling, speed-dependent limits and the allowed peak duration must also be checked.
Does feedback make a stepper equivalent to a servo?
An encoder-equipped stepper with a suitable closed-loop drive can monitor position error, correct it within the system's capabilities or report a fault. Feedback does not remove the motor's torque–speed limits. In a stepper–servo comparison, distinguish the control method from the motor's physical capability.
Microstepping is not a torque booster. It provides smaller command increments and smoother motion. Nor is it generally correct to say that 1/16 microstepping makes the motor's total torque exactly 1/16 of its original value. Incremental positioning torque and overall torque capacity are different quantities. Actual performance depends on current regulation and the motor–drive pairing.
Sources: Oriental Motor — Stepper torque characteristics · Oriental Motor — Comparing steppers and servos · Kollmorgen — RMS torque calculation · Oriental Motor — Microstepping and drive performance