Sunday, December 16, 2012

Permanent Magnet Stepper Motor

Stepper Motor Part 3
       The stator of this type is multipolar. As shown in the Fig. 7, the stator has four poles. Around the poles the exciting coils are wound. The number of slots per pole per phase is usually chosen as one in such multipolar machines.
Fig. 7 Four phase permanent magnet stepper motor

       The rotor may be salient or smooth cylindrical. But generally is is smooth cylindrical type as shown in the Fig. 7. It is made out of ferrite material which permanently magnetized. Due to this the motor is called permanent magnet stepper motor.
        The voltage pulses to the stator winding can be obtained by using a driving circuit. The basic driving circuit for four phase permanent magnet stepper motor is shown in the Fig. 8.
Fig. 8 Basic drive circuit for permanent magnet four phase stepper motor

1.1 Opeartion
       As soon as the voltage pulses are applied to various phases with the help of driving circuit, a rotor starts rotating through a step fir each input voltage pulse.
1. At first, switch SW1 is closed exciting the phase A. Due to its excitation we have N pole in phase A as shown in the Fig. 9(a). Due to the electromechanical torque developed, rotor rotates such that magnetic axis of permanent magnet rotor adjusts with the magnetic axis of the stator, as shown in the Fig. 9(a).
2. Next phase B is excited with switch SW2, disconnecting phase A. Due to this, rotor further adjusts its own magnetic axis with N pole of phase B. Hence it rotates through 90o further in clockwise direction as shown in the Fig. 9(b).
Fig. 9  Steps in four phase permanent magnet stepper motor

       Similarly when phase C and phase D are sequentially excited, the rotor tends to rotate through 90in clockwise direction, every time when phase is excited. When such sequence is repeated, it results into a step motion of a permanent magnet stepper motor.
       The stepper motors with permanent magnet rotors with large number of poles can not be manufactured in small size. Hence small steps are not possible. This is the biggest disadvantages of permanent magnet stepper motor. This is overcome by the use of variable reluctance type stepper motor.

Comparison Between Variable Reluctance and Permanent Magnet Stepper Motor

However, now a days a disk type of permanent magnet stepper motors are designed which have the low inertia and smaller step angles.

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