The Principle Operation of Stepper Motor

           MICROSTEPPING

•     In the above discussion we have assumed that the windings are excited one at a time. if the two phases are excited simultaneously i.e. keeping AA’ excited, the BB’ is also excited with switch SWI and SW2 closed, then the stator magnetic axis shifts to a mid position rather than along BB’.

•     Hence rotor gets aligned along this moves through a half step i.e. 15°.

•     A logical extension of this technique is to control the currents in the phase windings so that several stable equilibrium positions are created.

•     Normally the step angle is reduced by  

  

•      This technique is called micro stepping.

• A further reduction in step angle can be achieved by increasing the number of poles of I the stator and rotor or by adopting different constructions such as,

1.     Using reduction gear mechanism
2.     Using multi stack arrangement

REDUCTION GEAR STEPPER MOTOR
   

• Fig. 7.9 shows a reduction gear stepper motor. The stator has 8 salient poles and four phases for use as exciting winding.

•     The rotor has 18 teeth and 18 slots uniformly distributed around. Each salient pole of the stator consists of two teeth, forming an interleaving slot of the same angular periphery as the rotor teeth or slots.

•     When the coil A-A’ is excited, the resulting electromechanical torque brings the rotor to the position as shown in the Fig. 7.9.

 


















                               Fig. 7.9 Reduction gear stepper motor

•     With this arrangement, the step angle reduces to 5°. By successive excitation of coils A- A’, B—B’, C—C’ and D—D’, the rotor makes 72 steps to complete one revolution.

•     The general relationship between step angle a , number of stator phases q and rotor poles or teeth N remains at an angle  as,

•     By choosing different combinations of number of rotor teeth and stator phases, any desired step angle can be achieved