Question 18.4: Analysis of Variable-Reluctance Step Motor Determine the cur...
Analysis of Variable-Reluctance Step Motor
Determine the current excitation sequences required to achieve 45◦ steps in the VR step motor of Figure 18.11.
Known Quantities: Phase currents.
Find: Current excitation sequence for 45^{\circ} steps.
Assumptions: The motor currents at the start of the sequence are i_{1}>0 and i_{2}=0.
Analysis: The operation of the variable-reluctance (VR) step motor (with a salient-pole rotor) is simpler than that of the PM type, because the rotor is not magnetically polarized, and therefore it is not necessary to have bipolar currents to achieve the desired rotor motion. The stator of Figure 18.10 is excited by DC currents supplied by a single (unipolar) voltage supply. The switches shown in the figure could be controlled by a logic circuit similar to the ones described in Chapters 13 and 14 . Note that four separate coils are used.
Figure 18.12 depicts how the first three steps of the sequence could be achieved. These are summarized in Table 18.4.
Table 18.4 Current excitation sequence for VR step motor
\begin{array}{c|l|l|l|l}\hline \boldsymbol{S}_{\boldsymbol{A}} & {\boldsymbol{S}_{\boldsymbol{B}}} & {\boldsymbol{S}_{\boldsymbol{C}}} & {\boldsymbol{S}_{\boldsymbol{D}}} & \text{ Rotor position } \\\hline 1 & 0 & 0 & 0 & 0^{\circ} \\1 & 1 & 0 & 0 & 45^{\circ} \\0 & 1 & 0 & 0 & 90^{\circ} \\0 & 1 & 1 & 0 & 135^{\circ} \\0 & 0 & 1 & 0 & 180^{\circ} \\0 & 0 & 1 & 1 & 225^{\circ} \\0 & 0 & 0 & 1 & 270^{\circ} \\1 & 0 & 0 & 1 & 315^{\circ} \\1 & 0 & 0 & 0 & 360^{\circ} \\ \hline \end{array}
Comments: Note that the circuit required to drive this circuit is even simpler than the one required by the \mathrm{PM} step motor.
