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Sunday, December 2, 2007

Variable-reluctance (VR) Stepper Motors

Stepper Motor

The variable-reluctance (VR) stepper motor differs from the PM stepper in that it has no permanent-magnet rotor and no residual torque to hold the rotor at one position when turned off. When the stator coils are energized, the rotor teeth will align with the energized stator poles. This type of motor operates on the principle of minimizing the reluctance along the path of the applied magnetic field. By alternating the windings that are energized in the stator, the stator field changes, and the rotor is moved to a new position.

The stator of a variable-reluctance stepper motor has a magnetic core constructed with a stack of steel laminations. The rotor is made of unmagnetized soft steel with teeth and slots. The relationship among step angle, rotor teeth, and stator teeth is expressed using the following equation:

Figure 1-5 shows a basic variable-reluctance stepper motor. In this circuit, the rotor is shown with fewer teeth than the stator. This ensures that only one set of stator and rotor teeth will align at any given instant. The stator coils are energized in groups referred to as phases. In Figure 1-5, the stator has six teeth and the rotor has four teeth. According to the equation, the rotor will turn 30° each time a pulse is applied. Figure 1-5 (a) shows the position of the rotor when phase A is energized. As long as phase A is energized, the rotor will be held stationary. When phase A is switched off and phase B is energized, the rotor will turn 30° until two poles of the rotor are aligned under the north and south poles established by phase B. The effect of turning off phase B and energizing phase C is shown in Figure 1-5(c). In this circuit, the rotor has again moved 30° and is now aligned under the north and south poles created by phase C. After the rot or has been displaced by 60° from its starting point, the step sequence has completed one cycle. Figure 1-5(d) shows the switching sequence to complete a full 360° of rotation for a variable-reluctance motor with six stator poles and four rotor poles. By repeating this pattern, the motor will rotate in a clockwise direction. The direction of the motor is changed by reversing the pattern of turning ON and OFF each phase.

Figure 1-5

The VR stepper motors mentioned up to this point are all single-stack motors. That is, all the phases are arranged in a single stack, or plane. The disadvantage of this design for a stepper motor is that the steps are generally quite large (above 15°). Multistack stepper motors can produce smaller step sizes because the motor is divided along its axial length into magnetically isolated sections, or stacks. Each of these sections is excited by a separate winding, or phase. In this type of motor, each stack corresponds to a phase, and the stator and rotor have the same tooth pitch.

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