Single Stack Variable Reluctance Stepper Motor

A Single-stack Variable Reluctance (VR) stepper motor’s operation is based on the flux lines’ ability to occupy the low Reluctance path. Therefore, in a stepper motor with Variable Reluctance, the stator and rotor are aligned so that the Magnetic Reluctance is as low as possible.

Construction of Single Stack Variable Reluctance Stepper Motor

A salient pole stator makes up a Single-stack Variable Reluctance stepper motor. Over the stator poles are Concentrated Windings in the stator. The connection of the stator coils determines the number of stator phases. The Single-stack Variable Reluctance stepper motor’s rotor is an unwound, slotted structure made of Ferromagnetic material.

The stator and rotor of a Single-stack Variable Reluctance stepper motor are both Constructed of High-quality Magnetic materials with extremely high Permeability, Resulting in a very low excitation current requirement for the motor.

Operation of Single-stack Variable Reluctance Stepper Motor

A magnetic field is created in the motor when the stator phases are excited from a DC source in the right order with the aid of semiconductor switches. The ferromagnetic rotor is positioned where there is the least amount of resistance in the stator field’s path. As a result, the magnetic field of the stator’s stator aligns with the rotor’s axis.

Consider a single-stack variable reluctance stepper motor has 4-phases, 4/2-pole (i.e., 4-poles in the stator and 2-poles in the rotor). The 4-phases viz. A, B, C, D of the motor are connected to a DC source through the semiconductor switches SA, SB, SC and SD respectively. The stator phase windings of the motor are excited in the sequence A, B, C, D, A.

When the phase winding A is excited, the rotor aligns with the axis of the phase A. The rotor will remain stable in this position and cannot move until the phase A is de-energised.

Next, the phase B is energised and the phase A is de-energised. The rotor will move through 90° in the clockwise direction to align with the stator field which now lies along the axis of phase B.

The rotor now rotates 90 degrees in a clockwise direction as phase C is excited and phase B is de-energised. In this position, the rotor lines up with the stator’s magnetic field, which is now located along the phase C axis. As a result, the rotor rotates 90 degrees clockwise at each transition as the phases are excited in the order A, B, C, D, A. Through four steps, the rotor makes one revolution. By switching the windings in the opposite order, the direction of the rotor’s rotation can be changed. For example, for rotation in the anticlockwise direction, the phase excitation would go A, D, C, B, A.

It is evident from the discussion above. That the direction of the rotor’s rotation in a single-stack variable reluctance stepper motor depends only on the order. In which the phases are switched and is unrelated to the flow of current through the phases.

The magnitude of the step angle for any variable reluctance stepper motor is given by

Step angle,α=360°/(𝑚𝑠𝑁𝑟)

Where,

• m_s is the number of stator phases
• N_r is the number of rotor poles.

If N_s being the number stator poles,. Then the step angle of the variable reluctance stepper motor can also be expressed as

Step angle,α=(Ns−Nr)×360°/(NsNr)

The step angle of a variable reluctance stepper motor can be reduced from 90° to 45° by exciting the phases in the sequence A, A+B, B, B+C, C, C+D, D, D+A, A. Here (A+B) means the phases A and B are excited together. And hence the resultant stator magnetic field will be midway between the poles carrying the phase windings A and B, i.e.. The resultant field axis makes an angle of 45° with axis of the pole A in the clockwise direction.

As a result, when phase A is excited, the rotor lines up with its axis. The rotor rotates 45 degrees anticlockwise when the phases A and B are excited simultaneously. Micro-stepping is the term used to describe this method of gradually transitioning excitation from one phase to another with an intermediate step. So Smaller steps are realised using micro-stepping. More stator poles and rotor teeth are used to reduce step angle, which results in lower step angle values.