Technelec Ltd.

Specialists in brushless motors and generators

 Flux Switching Motors and Drives

The flux switching (FS) motor [1] is a unique combination of an inductor alternator [2,3,4], a two phase switched reluctance machine with fully pitched windings and the Laws relay limited motion actuator [5].  Published work on switched reluctance motors with fully pitched windings has concentrated on three phase machines [6].  The unique benefit of the inductor alternator and the proposed flux switching motor is that one of the two windings can carry dc current leaving only one winding requiring electronic control.  

            The flux switching motor is shown in Fig. 1(a and b) with eight stator teeth and 4 rotor teeth.  A field winding in 4 of the slots is fed with direct current at all times.  This establishes a 4 pole magnetic field.  The other 4 slots contain an armature winding also pitched over two teeth.  The direction of current in the armature winding determines which set of four stator poles carry flux and hence the position of the rotor.

Figure 1.    A flux switching motor with an 8 slot stator and 4 pole reluctance rotor.  Rotation is achieved by reversal in the direction of the armature current, A, (a) to (b).  The field current, F is continuous.

Each reversal of armature current (shown by the transition between Fig. 1(a) and Fig. 1(b) causes the stator flux to switch between two sets of alternate stator teeth; hence the name flux switching motor.  The flux does not rotate but oscillates clock-wise and anti-clockwise by 45° with each armature current reversal.  With appropriate control of the armature current, the reluctance rotor can rotate continuously and at a speed controlled by the armature current frequency.

            The armature winding requires an alternating current reversing in polarity in synchronism with the passing of each rotor tooth.  For automotive applications the cost of the power electronic controller must be as low as possible.  This is achieved by placing two armature coils in every slot so that the armature winding comprises a set of closely coupled (bifilar) coils [7].  The inverter can then be made from two ground referenced MOSFETs as shown in Fig. 2.  The cost of isolated or floating gate drives is eliminated.  The diode in parallel with each MOSFET plays an integral part in the operation of the circuit.

Figure 2.    The complete power electronic controller for the flux switching motor for an automotive application.

As the point for armature current reversal approaches (rotor teeth are approaching the aligned position for the excited set of stator teeth) the appropriate MOSFET is turned off by the micro-controller.  The stored energy associated with the armature inductance (not the total stored energy in the motor as some of that is associated with the field winding) causes the armature current to transfer to the closely coupled second armature winding, and a current flows in the MOSFET diode back to the battery via the diode in parallel with the field winding.  When the armature current reaches zero, the second MOSFET can carry the armature current in the forward direction.  As the second armature: winding is connected in the opposite polarity to the first the armature excitation is therefore reversed.    There will be some leakage energy not coupled from one bifilar winding to the other at the point of switching.  This energy is small and is absorbed in a very brief avalanche breakdown of the appropriate power mosfets immediately after turn off.  It is important to note that this avalanche energy is not the same as the energy dumped in a mosfet in some semiconductor relay applications where a diode is not present around the motor windings and all the motor’s magnetic energy is dumped in the mosfet.  

Since the field winding needs continuous dc current it can be connected to a dc source without the need for electronic control.  It is connected in series with the armature electronics in the circuit shown in Fig. 2.  The field winding, is thus connected in series with the incoming supply from the battery.  The field current is therefore automatically modulated with the magnitude of the armature current.  Furthermore, the field winding provides a significant amount of EMC filtering to the motor drive.