Stepping motors are multi-polar (generally 50 pairs of poles) synchronous motors. Each time the current polarity of the stator is changed, the rotor can turn by 1.8 degrees. Therefore, it can be used as an electrical pulse signal to convert into angular displacement or linear displacement. mechanism. In fact, the operation of a stepping motor is mainly driven by the torque generated by the current flowing through the stator to drive the rotor. The control of the motor current is completed by a stepper motor driver. It converts the pulse signal sent by the controller into the angular displacement of the motor. Therefore, the stepper driver can be used as a power amplifier. Its main task is to modulate the motor operation required by PWM. Of current. The current controlled by the stepper driver of the stepper motor is mainly affected by the stator resistance, inductance of the motor itself, and the back electromotive force generated during the step motion. The inductance and current of the stepper motor are relatively small. The biggest influence on the current is the back electromotive force generated by the rotor in the stator. Therefore, the influence of back-EMF on motor torque is mainly analyzed. The fundamental electromotive force generated by the motor is a sine wave when running at a constant speed. Its expression is:
E = pφmω, sin (θT) where p is the number of pole pairs of the stepper motor, p = 50; φm is the motor back-EMF induction constant, which is related to the material used for the motor rotor, ωT is the angular velocity of the rotor, and θT is the rotor position. The number of pole pairs of a stepper motor is much larger than that of a normal synchronous or AC motor. Therefore, the same speed will generate several times more back electromotive force. In addition, the larger the rotor, the larger the back electromotive force constant, and the resulting The electromotive force will also increase in multiples. The task of the stepping driver is to modulate the current required by the stator according to the load. In fact, it is necessary to meet the following physical equations:
U = IR + L dI / dt + E, the voltage U needs to be generated by PWM modulation. In the PWM of each cycle T, the on-time ton needs to be calculated so that U = ton / t Ubus (0≤ton≤T) . Because the resistance R and inductance L of the motor are relatively small, the back electromotive force is the main factor affecting the stepper driver. It can be known from experiments that the average 86 stepper motor is about 300 revolutions, and the maximum back electromotive force generated will exceed 48V. The back electromotive force generated at about 450 rpm is about 48V. After the motor back-EMF exceeds a given bus voltage, the drive PWM modulation enters a "saturated" state and cannot provide energy to the motor. Instead, it absorbs energy, so the torque of the motor will gradually decay; the faster the speed, the easier it is to lose steps.
Therefore, the choice of stepper driver voltage is related to the type of motor being driven. The higher the stepper driver voltage, the larger the PWM modulation range, which can compensate the back electromotive force generated by the motor at high speed, which is more conducive to the high speed operation of the motor. High-torque motors need to be equipped with high-voltage stepper motor drivers to achieve their high-speed performance. For example, 86-stepper motors will not perform as well as 57-motors at 24V, mainly because 86-motors tend to generate relatively high back-EMFs at high-speed conditions. As a result, the PWM modulation of the stepping driver quickly enters the saturation region, and the current decays sharply, causing the torque to decay rapidly. Under high voltage conditions, the torque of the 86 motor will be exerted, producing a torque greater than 57. This is a key issue to pay attention to when selecting a stepper driver and a stepper motor.