Stepper Motor Part 4
The hybrid stepper motor uses the principles of the permanent magnet and variable relucatnce stepper motors. In the hybrid motors, the rotor flux is produced by the permanent magnet and is directed by the rotor teeth to the appropriate parts of the airgap. The permanent magnet is placed in the middle of the rotor. It is magnetized in the axial direction. Each pole of the magnet is surrounded with soft-toothed laminations.
The construction of hybrid stepper motor is shown in the Fig. 10.
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| Fig. 1 Hybrid Stepper motor |
The main flux path is from the north pole of the magnet, into the end stack, across the airgap through the stator pole, axially along the stator, through the stator pole, across the airgap and back the magnet south pole via other end stack.
There are usually 8 poles on the stator. Each pole has between 2 to 6 teeth. There is two phase winding. The coils on poles 1, 3, 5 and 7 are connected in series to form phase A while the coils on poles 2, 4, 6 and 8 are connected in series to form phase B. The windings A and B are energized alternately.
When phase A carries positive current, stator poles 1 and 5 become south and 3 and 7 become north. The rotor teeth with north and south polarity align with the teeth of stator pole 1 and 5 and 3 and 7 respectively. When phase A is de-energized and phase B is excited, rotor will move by one quarter of tooth pitch.
The torque in a hybrid motor is produced by interaction of the rotor and the stator produced fluxes. The rotor field remains constant as it is produced by the permanent magnet. The motor torque Tm is proportional to the phase current.
Following are the main advantages of the hybrid stepper motor.
1. Very small step angles upto 1.8o
2. Higher torque per unit volume which is more than in case of variable reluctance motor.
3. Due to permanent magnet, the motor has some detent torque which is absent in variable reluctance motor.