An electric motor is a machine that converts electrical energy into mechanical movement. In particular, a stepper motor converts electrical pulses into mechanical rotation using small, non-continuous motion. Fundamentally, a stepper motor consists of a gear shaped piece of iron surrounded by electromagnets (EM), as shown in the image below. The rotational motion of a motor is generated by turning on the electromagnets in sequence. When the first electromagnet is turned on, it attracts the teeth of the gear causing the teeth to align with this particular electromagnet. Subsequently, the second electromagnet is powered, causing another slight rotation, and so on. Using the correct order, continuously powering the electromagnets in quick succession generates a seemingly continuous rotation. The nature of stepper motors allows for them to be turned by precise angles, thus making them useful in operations where precision is key.
The following applications demonstrate the concept of a basic stepper motor. In the first activity, click the buttons to manually power the individual electromagnets. In the second activity, you can program a sequence of button presses rather than manually activating the stepper motor. The loop button allows for the sequence to be repeated indefinitely. Adjusting the delay slider will adjust the pause time in between button presses; the lower the dial, the shorter the pause.
Consider these questions:
1) What can you do to rotate the motor clockwise in a seemingly smooth fashion?
2) How can you achieve a very specific rotation of 36°?
Answers may vary. Possible Loop: EM 2, EM 3, EM 4, EM 1
The central rotor has 25 teeth and it takes four steps for the gear to shift forward by one tooth, as it aligns successively with magnets EM1, EM2, EM3 and EM4. Therefore, the gear will undergo a full rotation in 25 * 4 = 100 steps. As a result, each step rotates the gear 360°/100 = 3.6°. To get a 36° rotation, 36 / 3.6 = 10 successive steps are required.
More About Stepper Motors
There are various basic types of stepper motors:
A permanent magnet stepper motor has a central rotor, which is separated into poles and has no teeth. The electromagnets are powered in pairs and energized to opposing polarities, causing the rotor to align along the magnetic field. See the image on the right for a visual representation.
Operation of a basic permanent motor
A variable reluctance stepper motor has a rotor which is made of soft iron and has teeth like a gear. It is not a permanent magnet. When the electromagnet is energized, the rotor rotates in order to minimize the distance between the electromagnet and the teeth. When the teeth of the rotor align with one electromagnet, they are offset with the next. Increasing the number of teeth allows for smaller step angles. The stepper motor in the activity above is an example of a variable reluctance stepper motor.
A hybrid stepper motor contains characteristics from both a permanent magnet and a variable reluctance motor. It works using the same principle.
The phase of a stepper motor refers to the number of different conditions possible. So far, two-phased motors and four-phased motors have been discussed. More complex stepper motors can have many more phases. Increasing the number of phases increases the resolution of the motor, allowing for smaller steps.
Stepper motors are generally used when precision is required. Applications of these motors includes printers and scanners, conveyor belts and machine tools.
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