Stepper Motor Torque Calculator
Calculate stepper motor holding and pull-out torque from specs and operating conditions. Enter values for instant results with step-by-step formulas.
Formula
Pull-out Torque = Holding Torque x (I_speed / I_rated) | I_speed = (Vs - BackEMF) / Z
Where I_speed is the achievable current at operating speed limited by winding impedance Z = sqrt(R^2 + (2*pi*f*L)^2), Vs is supply voltage, BackEMF is the speed-dependent counter-electromotive force, R is phase resistance, L is phase inductance, and f is step frequency.
Worked Examples
Example 1: Torque at Operating Speed
Problem: A NEMA 23 stepper motor has 0.5 Nm holding torque, 2A rated current, 1.5 ohm resistance, 3 mH inductance, 1.8 degree step angle. Calculate pull-out torque at 300 RPM with 24V supply.
Solution: Steps/rev = 360/1.8 = 200\nStep frequency = 300 x 200 / 60 = 1000 Hz\nTime constant = L/R = 0.003/1.5 = 2.0 ms\nBack-EMF constant Kb = 0.5/(2 x 1.414) = 0.177\nBack-EMF at 300 RPM = 0.177 x 31.42 = 5.56V\nAvailable voltage = 24 - 5.56 = 18.44V\nImpedance = sqrt(1.5^2 + (2pi x 1000 x 0.003)^2) = sqrt(2.25 + 355.3) = 18.9 ohm\nMax current = 18.44/18.9 = 0.976A\nCurrent ratio = 0.976/2.0 = 48.8%\nPull-out torque = 0.5 x 0.488 = 0.244 Nm
Result: Pull-out Torque: 0.244 Nm | Current at Speed: 0.976A | Step Freq: 1000 Hz
Example 2: Microstepping Resolution
Problem: Calculate the positioning resolution for a 1.8-degree stepper motor with 16x microstepping.
Solution: Full step angle = 1.8 degrees\nMicrostep angle = 1.8 / 16 = 0.1125 degrees\nMicrosteps per revolution = 200 x 16 = 3200\nLinear resolution with 5mm lead screw:\nResolution = 5 mm / 3200 = 0.00156 mm = 1.56 micrometers\nNote: Practical accuracy limited to about 3-5% of full step\nRealistic resolution = ~0.05 to 0.09 degrees
Result: Microstep Angle: 0.1125 deg | 3200 microsteps/rev | 1.56 um linear resolution
Frequently Asked Questions
What is stepper motor holding torque and how is it measured?
Holding torque is the maximum torque a stepper motor can produce when the windings are energized at rated current but the motor shaft is stationary. It represents the peak force the motor can resist before the rotor slips from its detent position. Holding torque is measured by applying a gradually increasing torque to the motor shaft using a torque wrench or dynamometer while the motor is energized in a fixed position. The torque value at which the shaft slips is the holding torque. This specification is the most commonly quoted torque rating for stepper motors and serves as the starting point for calculating performance at various operating speeds and conditions.
How does stepper motor torque change with speed and why does it decrease?
Stepper motor torque decreases as speed increases due to several electrical and magnetic effects. At low speeds, the motor windings have enough time during each step to reach the full rated current, producing maximum torque. As speed increases, the step pulse frequency increases, giving the current less time to reach its full value in each winding due to the inductance of the coils. The current follows an exponential rise limited by the L/R time constant of the winding. Additionally, back-EMF generated by the rotating motor opposes the supply voltage, further limiting current flow. At very high speeds, the available torque may drop to only 10 to 20 percent of the holding torque, eventually reaching zero at the maximum speed.
What is the difference between pull-in torque and pull-out torque?
Pull-in torque is the maximum torque at which a stepper motor can start and stop without losing steps at a given pulse rate. It is always lower than pull-out torque at the same speed. Pull-out torque is the maximum torque the motor can deliver while running at a given speed without stalling. Once a motor is already spinning, it can handle higher loads than it can start with because the rotor has momentum and the magnetic coupling is already established. The region between pull-in and pull-out torque curves is called the slew range, where the motor can operate but cannot start or stop without acceleration and deceleration ramp profiles. Understanding both curves is essential for proper motion control system design.
How does supply voltage affect stepper motor performance?
Higher supply voltage dramatically improves stepper motor performance at speed by overcoming the inductive reactance of the motor windings more quickly. When a step pulse occurs, the current must rise from zero to the rated value through the winding inductance. With higher voltage, the current rises faster according to the relationship di/dt = V/L. A motor rated at 3V at 2A will perform much better when driven at 24V or 48V with a current-limiting chopper driver. The higher voltage pushes current through the inductance faster, maintaining closer to rated current at higher step rates. Modern chopper drives use voltages 10 to 20 times the motor rated voltage while precisely regulating the current to the rated level.
What is microstepping and how does it affect torque and resolution?
Microstepping divides each full step into smaller increments by controlling the current ratio between two motor phases. Instead of switching current fully between phases, a microstepping driver varies the current sinusoidally, creating intermediate positions. Common microstep divisions include 2, 4, 8, 16, 32, 64, and 256 microsteps per full step. A 1.8-degree motor with 256 microstepping has a theoretical resolution of 0.007 degrees per microstep. However, microstepping reduces the available torque at each microstep position. At the first microstep from a full step position, the torque is approximately the sine of the microstep angle times the holding torque. Practically, positional accuracy beyond 8 to 16 microsteps is limited by mechanical factors.
How do you select the right stepper motor size for an application?
Selecting the right stepper motor requires matching the motor torque-speed characteristics to the application requirements with appropriate safety margins. First, calculate the total load torque including friction, gravity, acceleration, and any process forces. Then apply a safety factor of at least 50 percent, meaning the motor should deliver at least 1.5 times the required torque at the operating speed. Check the torque-speed curve to verify adequate torque throughout the entire speed range, not just at the target speed. Consider the inertia ratio between the motor rotor and the load, ideally keeping it below 10 to 1 for good dynamic response. Finally, verify that the motor temperature rise stays within acceptable limits at the required duty cycle.