# Speed and Pulse Frequency of Stepper Motor

The speed of the stepper motor is proportional to the pulse frequency, that is, the higher the pulse frequency, the higher the speed of the stepper motor. However, although the pulse frequency is increased, the speed is increased, but the torque is lost. The reason why the torque decreases with the increase of the pulse frequency: The two reasons for the stepper motor to lose step are:

1. The control pulse frequency is high. At this time, the acceleration of the rotor is less than the speed of the rotating magnetic field of the stepping motor stator. After the stepper motor power supply is designed, the charging time constant of the stator coil is basically fixed, assuming that the time constant is 0.02S (0.02S is charged to 63% of the maximum value), if the pulse period accepted by the stepper motor is greater than 0.04S (The duty cycle is 50% and the frequency is less than 25HZ), the stator coil can obtain enough energy to generate enough torque to drive the rotor. If the pulse frequency is too high, such as 50HZ (duty ratio is 50%, and the pulse period is greater than 0.02S), the charging time obtained by the stator coil is only 0.01S, which is half of the charging time, and the torque generated is greatly reduced, resulting in the rotor Can’t keep up with the speed of the stator’s rotating magnetic field, each step is behind the equilibrium position it should reach, and it gets further and further away from the equilibrium position. The result of accumulation is loss of step.
Of course, the frequency of 50HZ is too small. This example is just for the convenience of illustration, and a random solution is given:
• Reduce the pulse frequency, don’t think it is troublesome, most of the debugging stepper motor is the process of adjusting the pulse frequency.
• If you don’t want to reduce the frequency and cause the speed to be too low, then increase the power supply current of the stepper motor.
• Reduce the load of the motor.
2. The control pulse frequency is low, at this time the speed of the rotor is higher than the speed of the rotating magnetic field of the stepping motor stator. Taking the above 0.02S charging time constant as an example, the pulse frequency is low, the stator coil is fully charged, and the torque generated is large. At this time, if the load of the motor is light, the rotor will exceed the equilibrium position that should be reached, and the stator magnetic field will increase. To pull the rotor back to the equilibrium position, when it returns to the equilibrium position, it will overtake the equilibrium position and fall behind the equilibrium position. It is precisely at this time that the next pulse arrives, so the rotor has to start a new round of steps at the place behind the equilibrium position. This cycle also causes each step to fall behind the equilibrium position that should be reached, and the distance from the equilibrium position is getting farther and farther. The result of accumulation is loss of step.
Solution:
• Increase the pulse frequency.
• If you don’t want to be too high-speed, then reduce the power supply current of the stepper motor.
• Both of the above cannot be adjusted, and the motor with small torque can be replaced. The torque-frequency characteristic diagram, or the relationship table between torque and speed is generally given in the manual of the servo motor. It can be seen from the torque frequency characteristics of most brands of stepping motors that the output torque of the stepping motor is normal when the speed is less than 600 rpm. When it exceeds 1000 rpm, the torque drops sharply (of course, some motors have normal torque output at 1200 rpm). Therefore, it is an ideal choice to set the maximum speed of the stepper motor as 600 rpm.

Of course, this 600 rpm is not a general data, and you have to consult the manufacturer for the torque-frequency characteristics of the stepping motor. The definition of 600 rpm is just to tell you that when choosing a motor or designing the speed in the early stage, you should take into account the small speed of the stepper motor!