Permanent  Magnet Stepper Motor

The stator of PMSM  consists  salient pole  with  concentrated winding the  rotor  of this  type of  stepper motor  is generally taken  as cylindrical pole  type  which  is made of permanent  magnets and hence this motor  is called  permanent magnet stepper motor.

                                       Figure A -2 pole PMSM.

When phase A is  excited with  the stator produced  poles  attract the  rotor  permanent  magnet  poles  so that  their magnetic axes coincide  N- pole  attract the rotor aligns with the phase a pole  on the left. The  rotor turns through 900 when  phase  A is  de energized and phase B is excited with current in. If  the above  currents made  negative  the direction of  rotation  will be opposite. The  sequence of exciting the  stator  phase  winding is A, B, A, B, A......for  clockwise rotor  movement.

PMSM has high  detent  torque  higher  torque  per ampere of stator current  higher  inertia and lower  torque  to inertia ration, slower acceleration and  response. The major  disadvantage of PMSM  is that they  cannot  be constructed with PM rotors  with large  number of poles and small  steps  are not  possible.