The hybrid stepper motor uses the principles of the permanent magnet and variable reluctance stepper motors. In the hybrid stepper motor, the rotor flux is produced by the permanet magnet and is directed by the rotor teeth to the appropriate parts of the airgap.
The main flux path is from the north pole of the magnet, into the end stack, across the airgrap through the stator pole, axially along the stator, through the stator pole, across the air gap and back to the magnet south pole via the 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 energised 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 poles 1 and 5 and 3 and 7 respectively. When phase A is de energised and phase Bis exicited, are energised alternately.
The torque in hybrid motor is produced by the 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 T is proporatinal to the phase current.
Following are the main advantages of the stepper motor drive:
1. Very small step angles upto 1.8
2. Higher torque per unit volume which is more than in cae of variable reluctance motor
3. Due to permanet magnet, the motor has some detent torque which is absent in variable reluctance motor.
These are the various types of the stepper motors. After duscussing the various types and the operating principle, let us discuss the important parameters related to a stepper motor. The stepper motor characteristics are mainly the indication of its important parameters.
1. Holding Torque:
It is defined as the maximum static torque that can be qpplied to the shaft of an excited motor without causing a continuous rotaing.
2. Detent Torque:
It is defined as the maximum static torque that can be qpplied to the shaft of an unexcited motor without causing a continuous rotation.
Under this torque the rotor comes back to the normal rest position even if excitation ceases. Such positions of the rotor are referred as the detent positions.
3. Step Angle:
It is defined as the angular displacement of the rotor in response to each input pulse.
4. Critical Torque:
It is defined as the maximum load torque at which rotor does not move when an exciting winding is energised. This is also called pullout torque.
5. Limiting Torque:
It is defined for a given pulsing rate or stepping rate measured in pulses per second, as the maximum load torque at which motor follows the control pulses without missing any step. This is also called pull in torque.
6. Synchronous stepping rate:
It is defined as the maximum rate at which the motor can step wihout missing steps. The motor can start, stop or reverse at this rate.
7. Slewing rate:
It is deined as the maximum rate at which the motor can step unidirectionally. The slewing rate is much higher than the synchronous stepping rate. Motor will not be able to stop or reverse without missing steps at this rate.