Basic characteristics of stepper motors: static, dynamic, transient torque characteristics
The basic characteristics of a stepper motor include motor static characteristics, continuous motion characteristics (dynamic characteristics), motor starting characteristics, and motor braking characteristics (transient characteristics). The following are introduced separately:
Static torque characteristic
When the coil of the stepping motor is through direct current, the relationship between the electromagnetic torque of the loaded rotor (the recovered electromagnetic torque generated by balancing the load torque is called static torque or static torque) and the rotor power angle is called angle-still. Torque characteristics, this is the static characteristics of the motor.
Since the rotor is a permanent magnet and the resulting air gap magnetic density is sinusoidal, the theoretical static torque curve is a sine wave. This angle-stationary torque characteristic is an important indicator of the ability of the stepping motor to generate electromagnetic torque. The larger the maximum torque, the better, and the closer the torque waveform is to the sinusoid, the better. In fact, there is cogging torque under the magnetic pole, which causes the combined torque to be distorted. For example, the cogging torque of the two-phase motor is 4 times harmonic of the static torque angle period, and is added to the sinusoidal static torque.
Dynamic torque characteristic
Dynamic torque characteristics include drive pulse frequency-torque characteristics and drive pulse frequency-inertia characteristics.
Pulse frequency-torque characteristic
The pulse frequency-torque characteristic is an important feature of the stepper motor. As shown in the figure below, the vertical axis is the dynamic torque (dynamictorque), the horizontal axis is the response pulse frequency, and the response pulse frequency is expressed in pps (pulsepersecond) as the unit, that is, the number of pulses per second.
The dynamic torque generation of the stepping motor includes two factors: pull-out-torque and pull-in-torque. The former is called out-of-step or lost torque, the latter is called starting or pulling torque. The pull-in torque range is from zero to the maximum self-start pulse frequency or the maximum self-start frequency range. The area enclosed by the curve is called the self-starting area. The motor synchronously starts the forward and reverse start operation, and is the running zone between the pull-in and the out-of-step zone. The motor can run synchronously with the corresponding load in this zone. The out-of-range load torque will not run continuously, and the step-out phenomenon will occur. . The stepping motor is open-loop drive control, and there is a margin between the load torque and the electromagnetic torque, and the value should be 50%~80%.
The out-of-step torque is equal to the pull-in torque at 0 pps. As the control pulse frequency increases, the load carrying capacity decreases. At the beginning of the operation, the control pulse frequency should be increased slowly to take advantage of the large torque at low speed to provide the acceleration torque required by the motor at low speeds and reduce the acceleration time. The smaller the inductor design of the stator coil of the stepper motor, the larger the maximum response pulse frequency, so that the slow acceleration drive can be changed to the fast acceleration drive operation.
Pulse frequency-inertia characteristic
Stepper motors must have sufficient starting acceleration when starting with inertia loads. Therefore, if the inertia of the load increases, the starting pulse frequency decreases. For this reason, both of them should be considered comprehensively when selecting a stepping motor.
The vertical axis of the figure below shows the maximum self-starting frequency, the horizontal axis is the load inertia, and the curve shows the relationship between the load inertia and the maximum self-starting pulse frequency. Here, a PM type claw pole stepping motor (two phases, step angle 7.5 °) is taken as an example. Under the load PL, the relationship between the maximum self-start pulse frequency PL and the load inertia Jc is as follows:
In the formula, JR stepper motor rotor inertia, Ps is the maximum self-starting frequency of no-load.
