Question

Electric Drives: Reluctance Drives Question

Determine the phase excitation sequence, and draw the rotor positions, in a variable-relcutance step drive for a counter-clockwise rotation.

Please answer with details.

Thanks in advance :)

Answer 1

A four phase, 8/6 poles switch reluctances motor is shown in the following figure.

Here the stator has 8 poles and the rotor has 6 poles. When a stator phase (say phase 1-1’ in the figure) is excited, the reluctance torque make the rotor to move towards the minimum reluctance position. As you know Reluctance is directly proportional to the air gap, the minimum reluctance will be achieved when the rotor tooth is completely aligned with the stator tooth due to the minimal air gap between them. Or as in the figure if is zero, then the air gap is maximum and hence the reluctance is maximum.

                So when the rotor reaches the position of minimum reluctance (i.e. Take the tooth 1 of rotor as reference so here we are talking about when tooth 1 of rotor is having minimum air gap), excitation is shifted to the next phase. Thus shifting of the position of minimum reluctance is ahead of rotor. Hence with the help of rotor position sensors, position of minimum reluctance is continuously shifted by shifting the excitation from on phase to another and the reluctance torque makes the rotor to move continuously.

Speed of rotation depends on the average torque acting on the rotor, which in turns depend on the magnitude of the phase currents. As direction of reluctance torque is independent of the direction of phase currents, motor can be controlled by unidirectional phase currents. The direction of rotation can be changed by changing the phase sequence.

So for the rotor to rotate in counter clock wise direction, the rotor tooth 1 has to move towards the 4’ tooth of the stator and to achieve this, we have to excite phase 2 so that the position of minimum reluctance is ahead of rotor reference tooth. Then continue shifting the phase to phase-3 and then phase-4 to get counter-clockwise rotation.

Hence to get counter-clockwise rotation, the phase excitation sequence will be 1-2-3-4-1 (referring to the given diagram).