As mentioned earlier, these are used to obtain small step size, typically ranging between
2 to 15°.
In a m stack motor, the motor is divided into a m number of magnetically isolated sections called stacks, along its axial length.
The m stacks of stator have a common frame while the rotors are mounted on a common shaft. The stators and rotors have the same number of poles (teeth).
The stator poles in m stacks are aligned while the rotor poles are shifted by (1/rn) of the pole pitch from one another.
All the stator windings in a stator stack are excited simultaneously hence each stator stack forms a phase.
So number of stator phases is equal to number of stator stacks. Generally three stack stepper motors are used.
The Fig. 7.10 shows the arrangement in three stack stepper motor along with shifting of the rotor poles by (1/3) of the pole pitch from one another.
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