A variable-reluctance stepper motor can be the single-stack type or the
multiple-stack type. The latter (shown below) is used to provide smaller
step sizes. Its rotor is segmented along its axis into magnetically isolated
sections called stacks, which
Image from commons.wikimedia.org
are excited by a separate
winding called a phase.
Although VRSMs with up to
seven stacks and phases
are used, three-phase
arrangements are more
often used.