In this theory, construction and working of full step (single phase ON) and full step (two phase ON) and half step (single phase and two phase ON) in different step mode is given.

Variable Reluctance Stepper Motor

Construction of Variable Reluctance Stepper Motor

In the VR stepper motor, the stator consists of winding whereas the rotor has salient poles.
The position of rotor depends upon magnetic reluctance between stator and rotor therefore it is called as – Variable reluctance stepper motor.
The permanent reluctance stepper motor is designed for two phases whereas the variable reluctance stepper motor is designed for minimum three phases.
The variable reluctance stepper motor is designed for three phases, four phase or five phase. It may be single stack or multi stack.

Operation of Variable Reluctance Stepper Motor

Figure shows variable reluctance stepper motor in which six projecting poles are shown.
These poles are energized in proper sequence by DC supply.
There are four projecting poles on rotor, therefore it is called as 6/4 variable reluctance stepper motor.

Figure shows three winding XX’, YY’ and ZZ’ on the stator winding. These three windings are connected to DC supply through common terminal.
The switching of each phase is done through solid semiconductor switch. Each stator winding is connected such that its one pole becomes North pole whereas opposite pole becomes South pole.
These poles are shown by X – X’, Y – Y’ and Z – Z’.
When any one stator winding is energized, rotor teeth attracts towards energized stator poles.
When rotor teeth and stator pole both are on same direction, we can say that the rotor is in minimum reluctance position.

Step angle for three phase, four rotor poles

Step angle θ = 360⁰/ mN_r

Where m = Number of poles

N_r= Number of rotor poles

Therefore step angle θ = 360⁰/ 3 × 4 = 30⁰

When stator winding is energized, the rotor rotates in the forward or backward direction.

Stepping Modes: Full step ( Single Phase ON )

At any time, only one stator winding is energized therefore it is called as single step ON or full step ON operation.

Step 1

When switch S1 is closed, stator winding XX’ is energized and magnetic field produced in that direction resulting rotor teeth 1 and 3 attracted towards stator poles XX’ which is at minimum reluctance position.

Step 2

When switch S2 is closed, stator winding YY’ is energized and magnetic field produced in that direction resulting rotor teeth 2 and 4 attracted towards stator poles YY. The rotor rotates at 30 degree step angle.

Step 3

When switch S3 is closed, stator winding ZZ’ is energized and magnetic field produced in that direction resulting rotor rotates further 30 degree step angle. ( The rotor rotates at 60 degree from its initial position. )
Once again when switch S1 is closed, rotor rotates at 30 degree step angle. We can say that when switch S is closed, rotor rotates at 30 degree step angle. When switch S1 – S2 – S3 – S1 – S1 – S2 – S3 – S1 is closed, rotor rotates at 180 degree or say half cycle.

Step	Energization of Phase			Step Angle
Step	XX’	YY’	ZZ’	Step Angle
1	+	0	0	0⁰
2	0	+	0	30⁰
3	0	0	+	60⁰
4	+	0	0	90⁰
5	0	+	0	120⁰
6	0	0	+	150⁰
7	+	0	0	180⁰

Full Step ( Single Phase – ON ) Operation

Stepping Mode: Full step ( Two Phase ON )

When two stator windings are energized simultaneously, it is called as full step two phase ON operation of the stepper motor.

Step 1

When switch S1 and S2 are closed simultaneously, stator winding XX’ and YY’ are energized, resulting torque produced due to interaction of fluxes due to winding XX’ and winding YY’.
The torque on the rotor exerted due to energization of both winding XX’ and YY’.
The rotor will rotor at 15 degree from its initial position.

Step 2

When switch S2 and S3 are closed simultaneously, stator winding YY’ and ZZ’ are energized, resulting torque produced due to interaction of fluxes due to winding YY’ and winding ZZ’.
Now, the rotor will rotor further 30 degree.
The rotation of rotor is at 45 degree from its starting position.

Step 3

When switch S3 and S1 are closed simultaneously, stator winding ZZ’ and XX’ are energized, resulting torque produced due to interaction of fluxes due to winding ZZ’ and winding XX’.
Now, the rotor will rotor further 30 degree.
The rotation of rotor is at 75 degree from its starting position.

Step	Energization of Phase			Step Angle
Step	XX’	YY’	ZZ’	Step Angle
1	+	+	0	15⁰
2	0	+	+	45⁰
3	+	+	0	75⁰
4	+	+	0	105⁰
5	0	+	+	145⁰
6	+	+	0	175⁰

Full Step ( Two Phase – ON ) Operation

The holding torque in the two phase ON operation is higher than that of single phase ON operation in the stepper motor.

Half Step ( Single Phase ON & Two Phase ON )

The combined operation of single phase ON and two phase ON in the stepper motor is called as Half step operation.
The rotor of stepper motor rotates at 15 degree ( Half of full step ) therefore it is called as half step operation.
The stator winding is energized in the sequence of XX’ – XX’ & YY’ – YY’ – YY’ & ZZ’ – ZZ’ – ZZ’ & XX’ – XX’.

Step 1

When stator winding XX’ is energized, the stator poles XX’ and rotor teeth face to each other.

Step 2

When stator winding XX’ and YY’ are energized simultaneously, the rotor rotates at 15 degree in the forward or clockwise direction.
The half step operation is shown in the Figure.

The resolution in the half step operation is double to that of full step operation due to half step angle. The rotor rotates smoothly due to half step operation and effect of resonance is reduced.