Permanent magnet step motors:

The principle of operation of permanent magnet motor illustrated below:

1) In this type of motor rotor is magnetized to produce a permanent magnet poles and stator contains two phase windings that should be excited by either polarity currents viz

2)

3) The magnetic poles produced by stator current cause rotor to move as shown below:

4) Due to force of attraction for the excitation

shown in above fig:

5) The step angle in the phses would be of 45 degrees from the movement point P

6) The direction of the rotation will be decided by the sequence in which motor will be excited:

1) Motor torque is generated due to the interaction between the magnetic fields produced by the stator and rotor.

2) The step angle of permamant magnet is large and it is difficult to manufacture motors with smaller step angles.

3) The inertia of this type of motor is very high as compare to the variable reluctance type.

4) This type of motor needs the bipolar excitation viz current which is flowing through the winding needs the reverse direction or polarity for an Ex A , B which we can't see in variable reluctance types of motor.

5) Variable reluctance motor require unipolar voltage drive alongwith the less number of power devices per phase but in case of permanent magnet the power devices will be more as compared to variable reluctance.

Advantages of permanent magnet motor:

1) Due to inbuilt permanent magnet these are having lower power requirements.

2) Due to permanent magnet these are having higher detent torque.

Limitations:

1) Due to use of permanent magnet the motor gives lower torque per unit volume

2) Step size is negatively large

3) Bipolar excitation is used