 Welcome to the Variable Reluctance stepper motor. The Variable Reluctance stepper motor has a rotor that turns through a number of degrees and then stops. It is used in applications where it moves a physical object from one location to another in a linear or rotary motion with high precision. A theoretical VR stepper motor is shown here. It consists of six pairs of stator poles with wire wound around them to form electromagnets. The rotor, which is in the shape of a disk with teeth and slots around the outer edge, is made of soft iron that is not magnetized. The stator poles in each pair are located directly across from each other and are energized at the same time because their coils are series connected. Because the coils of each pair are wound in the opposite direction, the poles will always have different polarities. As a reference, the rotor tooth with the black dot is aligned with the poles that are energized, A and A' Magnetic flux lines flow between the poles through the rotor. Notice that the other teeth of the rotor are not lined up with the other stators, which are de-energized. To step the motor to the next counterclockwise position, coils A and A' are de-energized, and coils B and B' are energized. The rotor teeth closest to the B coils will align themselves with the B poles. The rotor turns for the following reasons. It is made of soft iron with a lower reluctance value than air. Magnetic flux lines pass through a material with lower reluctance more easily than through a material with higher reluctance. The difference in reluctance between iron and air creates a force that causes the rotor to turn so that the flux lines can pass through the iron teeth rather than through the air slots between them. The next step movement is made by de-energizing the B and B' coils and energizing the C and C' coils. The step movement continues by repeatedly energizing the coil pairs in the sequence of A, B, and C. The number of degrees the rotor turns per step is called the step angle. The number of rotor teeth and the number of poles determines the step angle. The theoretical stepper motor has 12 stator poles that are equally spaced at 30 degrees apart. The rotor has 8 teeth that are equally spaced at 45 degrees apart. Each time a step occurs, the rotor turns 15 degrees, which is the difference of alignment between the stator and the rotor. A more realistic stepper motor is shown here. The rotor has 50 teeth and it turns the distance of one quarter-tooth on each step. Therefore, four steps are required to turn the rotor the width of one tooth with one space between the teeth. With 50 teeth and 50 spaces on the rotor, it takes 200 steps to complete one revolution. The step angle is computed by dividing 360 degrees by 200 steps. Coming up, we have a couple of quiz questions. Please pause the video if you need more time to answer. Question 1. It is blank for magnetic flux lines to pass through a material with low reluctance than to pass through material with high reluctance. The answer is A. Easier. Question 2. If the rotor teeth of a stepper motor are 30 degrees apart and the stator poles are 20 degrees apart, how many degrees would the rotor turn per step? The answer is 10 degrees. You have completed this learning object, Variable Reluctance Stepper Motor.