Torque-Speed Curve Generation in Stepper Motors

Stepper motors feature both static and dynamic torques. Static torque is the torque that a motor can produce when it is not rotating; in stepper motors, holding torque and detent torque are referred to as static torques. Conversely, dynamic torque is the torque that a motor can produce in rotating conditions. In stepper motors, pull-in and pull-out torques are referred to as dynamic torque, with the motor’s torque-speed curves consisting of the pull-out torque curve and the pull-in torque curve. These dynamic torque curves are always defined for one motor and for one specific driver; the driver selected has a huge influence on the output torque & power. Now let’s explore the steps for generating dynamic torques.

Pull-Out Torque Explained

The pull-out torque is the maximum dynamic torque the motor can deliver at a given speed without losing synchronism, meaning that the stepper motor is executing steps as per supplied pulses without losing steps. For open loop applications where no feedback is provided to the motor, the torque in the application must be at least 30% lower than pull-out torque. For closed loop applications where the motor receives continuous feedback from the system to adjust its speed/current, the customer can decide on the factor of safety on the pullout torque. In these applications, you can start with a load at lower PPS (pulse per second) and then accelerate to reach the required higher PPS.

Methodology for Measurement – Pull-Out Torque

Figure 1: Torque vs. Speed Characteristics for Stepper Motors

1. Make sure there is no load on the motor.
2. Reach the given speed by increasing the frequency of the pulses.
3. Load the motor shaft.
4. Note the torque for which we lose the synchronism. Use the tachometer and confirm the speed with input pulses /speed.
5. Repeat above steps for another speed until we reach the limit.
6. Then plot the curve. 

Pull-In Torque Explained

Pull-in torque is a combination of the maximum torque and speed at which motor the can perform frequent start and stops. Like the pull-out torque, a sufficient factor of safety can be considered as per application requirements.

Methodology for Measurement – Pull-In Torque

1. A load is applied to the motor.
2. We try to start motion at different speeds by generating a constant pulse frequency (no ramping).
3. Note the highest speed we get with this load.
4. Repeat the above steps for different torques.
5. Plot the pull-in curve.

Test Setup and Instruments Required to Measure Pull-In and Pull-Out Torque

To measure the pull-out and pull-in torque of a stepper motor, you’ll need the following instruments: a hysteresis brake, a motor driver, an oscilloscope, a tachometer, mounting fixtures, couplings, and a power supply. Figure 2 illustrates how to set up the testing environment.

Figure 2: Pull-Out and Pull-In Torque Measurement Test Setup

To measure the motor’s pull-out torque, follow the below steps:

1. Fix the motor and hysteresis brake in the fixtures and couple them using the couplings. Ensure the shaft of the brake and motor are perfectly aligned.
2. Set the required supply PPS and current in the driver settings (Chopper or L/R drive).
3. Keep the brake’s power supply off and set the current to 0.01A (as low as possible) and start the motor.
4. After starting the motor, the motor accelerates and attains its defined speed. Until now the brake power supply is off, and it is freely rotating with the motor.
5. Re-confirm current and PPS in each phase of the motor using the oscilloscope.
6. Start the brake power supply and slowly increase the supply current to the brake. As we increase the current, the hysteresis brake generates braking torque. Increase the current until the motor stalls.
7. Record the current supplied to braking at which the motor stalled.
8. Repeat the above steps for different PPS.
9. After recording the current supplied to the brake at which the motor stalled (or lost synchronism), check the torque generated by the hysteresis brake at the corresponding supply current using the torque watch gauge.

Would you like more details on the in’s and out’s of measuring and testing dynamic torque in stepper motors? Reach out to us directly – our engineers are always available to assist!