When testing stepper motors, pull-in and pull-out torque are two critical parameters that define the motor’s performance. These characteristics determine whether your motor will start, stop, and run reliably under varying loads and speeds. Understanding and accurately measuring these torques is essential for applications ranging from precision robotics to industrial automation.
What Is Pull-In Torque?
Pull-in torque is the maximum load torque at which a stepper motor can start and accelerate to synchronous speed from rest without losing steps.
- At low speeds, pull-in torque is high because the motor has enough time to align with the commanded steps.
- As speed increases, pull-in torque decreases due to limited time for the rotor to catch up with each step.
Accurately determining pull-in torque helps engineers avoid startup issues, especially in systems that require frequent starts and stops.
What Is Pull-Out Torque?
Pull-out torque is the maximum load torque the motor can sustain without losing synchronization while running at a given speed. It defines the upper limit of the motor’s operating range during steady-state motion.
- Pull-out torque usually peaks at moderate speeds and decreases at both very low and very high speeds.
- Exceeding this torque causes step loss, resulting in position errors or complete stall.
For motion-control systems, ensuring the application load remains below the pull-out torque curve is essential for stability.
Why Does It Matter?
- System Reliability: If a motor fails to start under load (pull-in torque too low), the entire system could malfunction.
- Precision: Loss of steps during operation (exceeding pull-out torque) can cause positional errors in robotic systems.
- Efficiency and Safety: Accurate torque data prevents overloading, reduces wear, and improves motor lifespan.
How to Measure Pull-In and Pull-Out Torque
Stepper Motor Testing Systems, such as Sugawara’s SMC-2 / SMT-2, use precise load-control Prony braking combined with intelligent software for automated torque testing.
Prony braking is a simple method for applying a controlled load to a rotating motor during torque testing. It works by wrapping a brake belt or thread around a pulley attached to the motor shaft and tightening it to create friction. The difference in tension on the two ends of the brake belt generates a measurable braking force. By multiplying this force difference by the pulley radius, the torque on the motor can be calculated. This method is widely used because it is effective for measuring pull-in and pull-out torque of stepper motors.
Key Features for Accurate Measurement:
- Automated Test Sequences: The tester gradually adjusts load while monitoring step loss, plotting frequency vs. torque curves for clear visualization.
- High Sensitivity Sensors: Capable of detecting micro torques (<0.1 mN·m) for micro stepper motors.
- Minimal Inertia Effect: Using low-inertia brakes ensures measurement accuracy, especially at low torque values.
Conclusion
For engineers and designers, understanding pull-in and pull-out torque is fundamental to optimizing stepper motor applications. Using advanced testing systems like Sugawara’s Stepper Motor Torque Tester ensures accurate, repeatable measurements critical for both R&D and quality control.