VARIABLE RELUCTANCE MOTORS
o Self starting capability.
- It is the most basic type of stepper motor. This helps to explain the principle of operation of the stepper motors.
- The motor has a stator which is usually wound for three phases.
- The stator has six salient poles with concentrated exciting windings around each one of them. The stator construction is laminated and assembled in a single stack.
- The number of poles on the stator and rotor are different. This gives the motor ability,
o Self starting capability.
- The rotor is made out of slotted steel laminations. If the number of stator poles are N and the number of rotor poles are Nr then for a. three phase motor, the rotor poles in terms N q are given by,
- where q = Number of phases For example for N =6 and
- For our discussion, 4 pole rotor constructi6n is elected. So rotor has 4 salient poles without any exciting winding as shown in the Fig. 7.6.
- The coils wound around diametrically opposite poles are connected in series and the three phases are energized from a d.c. source with the help of switches.
- The basics driving circuit is shown in the Fig. 7.7.
- The operation is based on various reluctance positions of rotor with respect to t When any one phase of the stator is excited, it produces its magnetic field whose axis I along the poles, the phase around which is excited.
- Then rotor moves in such a direction as to achieve minimum reluctance position. Such a position means a position where axis magnetic field of stator matches with the axis passing through any two poles of the rotor.
- Let us see the operation when phases A, B and C are energized in sequence one after the other, with the help of switches SW1, SW2, and SW3.
- When the phase AA’is excited with the switch SW1 closed, then stator magnetic axis exists along the poles formed due to AA 1 e vertical
- Then rotor adjusts itself in a minimum reluctance position i.e. matching its own axis passing through the two poles exactly wit stator magnetic axis.
- This position is shown in the Fig. 7.8 (a)
- When the phase BB’ is excited with the switch SW closed and phase AA’ de energized with the switch SW open, then stator magnetic axis shifts along the pales formed due to BB’, shown dotted in the Fig. 7.8 (b).
- Then rotor tries to align in the minimum reluctance position and turns through 3Ø in anticlockwise direction.
- So axis passing through two diagonally opposite poles of rotor matches with the stator magnetic axis. This is the new minimum reluctance position.
- The point P shown on the rotor has rotated through 30° in anticlockwise direction as shown in the Fig. 7.8 (b)
- When the phase CC’ is excited with the switch SW closed and the phases AA’ and B are de energized, then the stator magnetic axis shifts along the poles formed due to CC’, shown dotted in the Fig. 7.8 (c).
- Then to achieve minimum reluctance position, rotor gets subjected to further anticlockwise torque. So it turns through further 30° in anticlockwise direction.
- Hence point P is now at 6G° from its starting position, in anticlockwise direction as shown in the Fig. 7.8 (c).
- By successively exciting the three phases in the specific sequence, the motor takes twelve steps to complete one resolution.
- Now if i is the current passing through the phase which is excited then the torque developed by the motor, which acts on the rotor is expressed as,
- where L is the inductance of the relevant phase at an angle 0.
- Since the torque is proportional to the square of the phase current (T x i ) , it is independent of the direction of i.
- The direction of rotation is totally decided from the sequence in which the phases are excited.