A stepper motor is a type of induction motor. Its working principle is to use electronic circuits to convert direct current into multi-phase timing control current that is supplied in a time-sharing manner. Only when this current is used to power the stepper motor can it work normally. The driver is a multi-phase timing controller that supplies power to the stepper motor in a time-sharing manner.
Although stepper motors are widely used, they cannot be used like ordinary DC or AC motors under normal conditions. They require a control system composed of dual-ring pulse signals and power drive circuits to function. Therefore, using stepper motors effectively is not easy; it involves expertise in mechanics, electrical engineering, electronics, and computers. As an actuator, the stepper motor is a key product in mechatronics and is widely used in various automated control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing daily, and they are used in various sectors of the national economy.
Understanding the torque-frequency characteristics of stepper motors
What is the power of a stepper motor ? How is it calculated? P = F × S / t = F × v = F × r × ω = F × r × 2πf
The following graph shows the torque-frequency characteristic curve of a 4Nm stepper motor (86 motor, 95mm). Looking at this graph, you might be alarmed by its steep downward trend. Don't worry, we'll calculate it point by point.
0. First, look at section A, which is a relatively horizontal straight line. The torque is 4 Nm, and the output power is proportional to the speed. The maximum speed in section A is 60 r/min = 1 Hz.
Therefore, the maximum output power of region A is P0 = 4 (Nm) × 2 × 3.14 × 1 (Hz) = 25.12 (W).
1. The torque at point 1 is 3.9 Nm, and the corresponding speed is 80 r/min = 1.33 Hz. Therefore, the output power P1 = 32.57 (W).
2. The torque at point 2 is 3.3 Nm, and the corresponding speed is 200 r/min = 3.33 Hz. Therefore, the output power P2 = 69.01 (W).
3. The torque at point 3 is 2.75 Nm, and the corresponding speed is 300 r/min = 5 Hz. Therefore, the output power P3 = 86.35 (W).
4. The torque at point 4 is 2.1 Nm, and the corresponding speed is 400 r/min = 6.67 Hz. Therefore, the output power P4 = 87.96 (W).
5. The torque at point 5 is 1.5 Nm, and the corresponding speed is 600 r/min = 10 Hz. Therefore, the output power P5 = 94.20 (W).
6. The torque at point 6 is 0.95 Nm, and the corresponding speed is 900 r/min = 15 Hz. Therefore, the output power P6 = 89.49 (W).
7. The torque at point 7 is 0.3 Nm, and the corresponding speed is 2000 r/min = 33.33 Hz. Therefore, the output power P7 = 62.79 (W).
It can be seen that the motor reaches its maximum output power of 94W at 600r/min. Converted to a three-phase AC motor, this is P×η=94W. Assuming η=0.75, the rated power P=125W. The power of this stepper motor decreases by 67% at 2000r/min.
Let's look at another example: an 86 motor with a diameter of 125mm.
0. First, look at area A. The torque is 5.8 Nm. The maximum speed of area A is 70 r/min = 1.17 Hz. Therefore, the maximum output power of area A is P0 = 5.8 (Nm) × 2 × 3.14 × 1.17 (Hz) = 42.62 (W).
1. The torque at point 1 is 5.3 Nm, and the corresponding speed is 400 r/min = 6.67 Hz. Therefore, the output power P1 = 222.00 (W).
2. The torque at point 2 is 4.8 Nm, and the corresponding speed is 600 r/min = 10 Hz. Therefore, the output power P2 = 301.44 (W).
3. The torque at point 3 is 4 Nm, and the corresponding speed is 800 r/min = 13.33 Hz. Therefore, the output power P3 = 334.85 (W).
4. The torque at point 4 is 3.5 Nm, and the corresponding speed is 900 r/min = 15 Hz. Therefore, the output power P4 = 329.7 (W).
5. The torque at point 5 is 3 Nm, and the corresponding speed is 1000 r/min = 16.67 Hz. Therefore, the output power P5 = 314.06 (W).
6. The torque at point 6 is 1.35 Nm, and the corresponding speed is 2000 r/min = 33.33 Hz. Therefore, the output power P6 = 282.57 (W).
It can be seen that the motor reaches its maximum output power of 335W at 800r/min. Converted to a three-phase AC motor, this is P×η=335W. Assuming η=0.75, the rated power P=447W. The stepper motor's power decreases by 84% at 2000r/min.
These are two torque-frequency characteristic curves I randomly selected, from products of the same manufacturer. Comparing these two examples, we can conclude that even stepper motors from the same manufacturer have different hard characteristics and are suitable for different applications. It's also important to note that the drive voltages in these two graphs are different.
How do we choose a stepper motor? The most important thing is to obtain its torque-frequency characteristic curve, preferably a set of curves under different drive voltages. For stepper motors, once they start rotating, with fixed driver parameters, regardless of speed, the difference in power consumption is small, but the difference in output power is large, i.e., a large difference in efficiency. If we frequently operate the motor under low-efficiency conditions, the power loss will generate heat. The lower the efficiency, the more heat is generated, which is detrimental to the motor's lifespan.
P = Ω·MΩ = 2π·n/60P = 2πnM/60
P represents power in watts, Ω represents angular velocity in radians per second, n represents rotational speed per minute, and M represents torque in Newton-meters.
