Working Variable Reluctance Stepper Motor

Working Variable Reluctance Stepper Motor

The stepper motor works on the principle that the rotor aligns in a particular position with the teeth of the excitation pole in a magnetic circuit wherein minimum reluctance path exist. Whenever power is applied to the motor and by exciting a particular winding, it produces its magnetic field and develops its own magnetic poles.

Due to the residual magnetism in the rotor magnet poles, it will cause the rotor to move in such a position so as to achieve minimum reluctance position and hence one set of poles of rotor aligns with the energized set of poles of the stator. At this position, the axis of the stator magnetic field matches with the axis passing through any two magnetic poles of the rotor.

When the rotor aligns with stator poles, it has enough magnetic force to hold the shaft from moving to the next position, either in clockwise or counter clockwise direction.

Consider the schematic diagram of a 3-phase, 6 stator poles and 4 rotor teeth is shown in figure below. When the phase A-A’ is supplied with a DC supply by closing the switch -1, the winding become a magnet which results one tooth become North and other South. So the stator magnetic axis lies along these poles.

Due to the force of attraction, stator coil North Pole attracts nearest rotor tooth of opposite polarity, i.e., South  and South Pole attract nearest rotor tooth of opposite polarity, i.e., North. The rotor then adjusts to its minimum reluctance position where the rotor magnetic axis exactly matches with stator magnetic axis.

When the phase B-B’ is energized by closing switch -2 keeping phase A-A’ remain de-energized by opening switch-1, winding B-B’ will produce the magnetic flux and hence the stator magnetic axis shifts along the poles thus formed by it. Hence the rotor shifts to the least reluctance with magnetized stator teeth and rotates through an angle of 30 degrees in the clockwise direction.

When the switch-3 is energized after opening switch-2, the phase C-C’ is energized, the rotor teeth align with new position by moving through an additional angle of 30 degrees. By this way, the rotor moves clockwise or counterclockwise direction by successively exciting stator windings in a particular sequence. The step angle of this 3-phase 4-pole rotor teeth stepper motor is expressed as, 360/ (4 × 3) = 30 degrees (as step angle = 360 / Nr × q).

The step angle can be further reduced by increasing the number of poles on the stator and rotor, in such case motors are often wound with additional phase windings. This can also be achieved by a adopting different construction of stepper motors such as multi stack arrangement and reduction gear mechanism.