Sensorless commutation of printed circuit brushless direct current motors

The reliability and overall cost of brushless direct current (BLDC) motors can be significantly improved by implementation of an effective sensorless commutation control method. A recently proposed technique termed the `equal inductance method` offers commutation control and starting performance equal to that obtained when using Hall sensors. However, the method applies to nominally symmetrical machines with high enough phase winding time constants. This study is about sensorless commutation of a class of motors which is characterised by significant asymmetry and relatively low-phase winding time constants. The motors in question are axial flux machines with ironless stators that may be wound or made up of printed circuits. The basic reason for the asymmetry is that the three-stator phases are stacked with each one of them located at a different axial distance from the rotor. The low-phase time constant is due to the ironless nature of the stator. The focus is on the printed circuit version of the motor. A new sensorless commutation technique has been developed, which is effectively a generalisation of the originally proposed equal inductance method. The theoretical basis behind the technique and practical results demonstrating its effectiveness are presented.