Electrical Engineering ⇒ Topic : Principle of Operation of stepper motor
Principle of Operation
The stator of the motor has several poles (normally eight) whose polarities are changed by electronic switches. Both the stator and rotor have small teeth machined on them. On one shaft there may be multiple rotors
Figure (a) Stepper motor: Typical step angles are 15°, 7.5°, 2°, 0.72°.
When the stator and rotor teeth are aligned, the reluctance of the magnetic structure is minimum and self inductance of stator is maximum. As the rotor is turned in clockwise direction, the reluctance is maximum midway between two stators and reaches a minimum when the rotor and stator are again aligned. So the reluctance describes a sine wave and the inductance also describes a sine wave but with a phase change of 1800 with respect to reluctance.
The rotor and stator teeth are slightly misaligned. A pulse is applied to each pair of stator teeth successively with the result that the average north and south poles of the stator are rotated.The rotor's north pole aligns with the south pole of stator. The average stator field rotates with each step pulse and the rotor follows it in step. For four pulses, the rotor rotates through one pitch and this is known as four-step switching. A pitch is the angular displacement between one rotor tooth and the succeeding stator tooth. For 100 teeth on the rotor, it would make 400 steps at 4 steps per tooth for one complete revolution
The pulse rate cannot be too high as the rotor may fail to respond and drive the load from one position to another. So there is minimum start range. The slew range is one in which the load position follows the pulse rate without losing step.
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