Phase number and its characteristics of stepper motor
The two-phase and three-phase stepping motor is taken as an example to illustrate the relationship between the phase number and characteristics of the stepping motor. A comprehensive overview of the number of phases and characteristics is:
According to the equation θs=180°/PNr, the step angle is 180/PNr, so the larger the phase number P, the higher the angular resolution. Increasing the resolution can improve positioning control accuracy, improve low-speed out-of-step, make multi-phase control possible, and improve damping (improve braking performance, reduce overshoot and braking time at stop). Details are in the Drive Technology section.
As shown in the figure below, the torque fluctuations of the two-phase and three-phase stepping motors are shown. The more the number of phases, the more the relative error between the intersection torque value Tg and the maximum static torque Th of the commutating two-phase winding dynamic torque curve small. Tg is the lower limit of the load torque of the motor, and (Th-Tg)/Th is the relative error of the torque ripple. The more the number of phases, the smaller the value is, which is more advantageous for reducing the vibration. That is, the more the number of phases, the smaller the amplitude of the torque ripple generated by the motor, and the higher the frequency, the smaller the vibration generated.
The advantage of a multi-phase stepper motor is its high speed response. The stepping motor is a synchronous motor, and the winding current frequency is proportional to the rotor speed. If the motor runs at a high speed, the winding current angular frequency ω increases, so that the reactance ωL generated by the winding inductance L increases, thereby reducing the current and causing the torque to drop.
When the stepper motor is driven by thousands of pps, the motor winding impedance Z will greatly increase the reactance ωL compared with the DC resistance. When the motor runs at high speed, if the voltage V is constant, the motor phase current is V/ωL. The mechanical angular velocity ωm is ω=Nrωm, and the current is inversely proportional to Nr for a motor of the same mechanical angular velocity.
According to the formula θs=180°/PNr, when the two phases Nr=50, the step angle is 1.8°; when the five-phase Nr=20, the step angle is 1.8°. When the two stepping motors rotate at the same speed at a high speed, the current of the five-phase winding is 2.5 times that of the two phases. Since the current is small, the torque is small, so the torque of the five phases is larger than that of the two phases.