Variable Reluctance Type Stepper Motor:

1) This motor works on the principle of the minimum reluctance path which means magnetic circuit gets completed by the use of the flux which takes minimum reluctance path, like an electric circuit where current flowing through it takes the minimumresistance path.

2) The minimum reluctance for the flux depends on the stator excitation whether motor align itself.

3) For example let's consider an simplified version of the single stack, three phase variable reluctance motor which is having 12 teeth n stator, and 4 teeth per phase which can be excited and an unexcited rotor with 8 teeth.

4) Each phase consist of a 4 stator teeth and winding on them are connected in series with each other as:

5) At a time only one phase is allowed in this excitation toallow a DC Current to flow through it.

6) The rotor which is used in this type of motor is made up of ferromagnetic material having very high permeability.

7) The direction of rotation is depends on the sequence in which they are switched.

Phase A Excitation:

When Phase A of the stator winding is excited the rotor will align itself such that four of the rotor teeth are in perfect alignment with four of the phase A stator teeth so as to provide a minimum reluctance path Refer figure in which 1,3,5 and 7 are aligned.

Phase B Excitation:

When phase B of the stator is excited the rotor will align itself in such a way that four rotor teeth (2, 4, 6 & 8) which are closest to the four phase B Stator teeth are in perfect alignment with four phase B stator. Four phase B stator teeth a minimum reluctance path for the flux; this will cause the rotor to rotate in anti-clock wise direction by an angle of 15 degrees

Phase C Excitation:

Now after Phase A and B if Phase C is excited as shown in fig then this type of phase the rotor will align itself like four rotor teeth which is near to the four phase C rotor teeth get perfectly aligned to them. And due to this rotor shaft starts rotating in clock wise direction at an angle and equal to the difference of stator and rotor tooth pitches i.e. 15 degrees. Hence at the end if we continue to excite the Patten in sequence like A,C, B, A,C,B,A……..and so on to rotate the rotor in Clockwise direction.

The rotor tooth pitch is equal to the(360/Nr) degrees where Nr is the number of rotor teeth.

Since all step angles are same for three of the excitations of three phase motors is given as:-

360/(3Nr)=15 degrees

In general for A 'X' phase motor with Nr tooth rotor is given as:-


The motor which we had discussed till now is an single stack variable reluctance motor and on same working principle there are many types of variable reluctance motors are there.

I n this type of motors it is important to note that it requires the unipolar drive viz current which is flowing thorough the winding is unidirectional as:-

Advantages of variable reluctance Motor:

1) Simple to construct.

2) Rotor construct is robust

3) Cost is low

4) Fast dynamic response

5) Brushes are not used.

6) High torque to inertia ratio