Variable Reluctance Stepper Motor - Construction & Principle of Operation


Construction of Variable Reluctance Stepper Motor  :


     It is the most basic type of stepper motor. The stator of this of motor consists of three phase wounded type. It has six salient poles with concentrated windings around each one of them. The rotor has 4 slots to produce the effects of 4 poles.

Variable Reluctance Stepper Motor

      The two stator windings which are in opposite direction are connected in series. These windings are then energised by a three different switches as shown in figure. 



Principle of Operation :


     The basic principle of working of reluctance motor is based on the various reluctance positions of rotor. When a voltage signal is given to any one of the phase of  stator and excited, it setups magnetic field whose axis lies along the poles. Now the rotor tries to rotates in such a direction that it has to achieve minimum reluctance. Such a rotation means a position where axis of magnetic field setup by a stator will be equal to the axis passing through any two poles of the rotor.

      Let us see the operation when the windings A, B and C are connected in sequence and energised one after the other, with the help of switches SW1, SW2 and SW3.


Case - 1

     When the supply is connected across the terminals of AA¹, with closing the switch SW1. There exists of magnetic poles between the windings AA¹. Now due to the force of attraction of this magnetic poles rotor tries to achieve minimum reluctance position. Therefore axis of the rotor 1 and 3 tries to get in align with axis of the poles AA¹.

Variable Reluctance Stepper Motor


Case - 2

     Now when the supply is connected across the windings BB¹ . There is a change of magnetic axis of stator poles axis from to BB¹. Now the rotor again tries to achieve minimum reluctance path, thereby moving the rotor.

Variable Reluctance Stepper Motor

     Here we can see that the rotor pole axis 2 and 4 are nearer to the windings BB¹. This causes a rotation of rotor so that axis of the rotor poles 2 and 4 get aligned with BB¹. Therefore moving the rotor to a step movement of angle 30 degrees as shown in below figure.


Case - 3

     Similarly when the windings CC¹ gets supplied with switch SW3 and AA¹ and BB¹ are dis-connected. The rotor pole axis tries to align there position with stator pole axis. Here we get an anther 30° of rotation by rotor. Totally we get an angle of 60° of step motion from phases AA¹ to CC¹.

Variable Reluctance Stepper Motor


     By successively exciting the three phases in the specific sequence, the motor takes 12 steps to complete one revolution. The direction of rotation of the motor depends upon the sequence of supply given the each stator phases. The torque developed by the motor, which acts on the rotor is proportional to the square of the phase current ( T α i).



Important Observations :


From the above discussion, the following observations can be made

1. When the sequence of supply to phases are A-B-C-A...., the movement of rotor is in anti-clockwise direction.

2. When the sequence of supply to phases are C-B-A-C..., the movement of rotor is in clockwise direction.

     Therefore step angle, is given by

αs  =  360 / n p

Where

       n = No. of stator phases and

       p = No. of rotor pole/rotor teeth



Micro Stepping :


     To get an micro step movement of the rotor. The supply to two phases AA¹ and BB¹ are given at a same time with closing the switches SW1 and SW2. Here the axis of the magnetic poles of stator lies between the two phases. Hence the rotor tries to align with its axis, i.e., half step 15°. 



Advantages :


1. High torque to inertia ratio.

2. High rates of acceleration.

3. Fast dynamic response.

4. Simple and low cost machine.

5. Rotor construction is robust due to absence of brushes.




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