Load commutation of the circular winding brushless DC machine(CWBLDC)

A circular winding brushless DC (CWBLDC) machine is essentially a trapezoidal-EMF multiphase permanent magnet machine with the associated commutation circuit. It can achieve the same level of torque ripple performance as sinusoidal-EMF permanent magnet synchronous machines (PMSMs), yet boast much higher torque density since the iron core is more fully utilized with trapezoidal EMFs. The commutation circuit is an integral part of this machine and should be properly controlled. This paper presents a load commutation method for CWBLDC machine, which relies on the EMF of its load (the motor) to reverse the phase current direction. Power devices of the commutation circuit can be turned off with no current flowing through, which can be considered as a form of soft switching. The load commutation process is analytically derived and critical conditions for successful commutations are highlighted. Field-circuit coupled analysis have been performed to verify the load commutation method. A 10 kW, 8-pole, 46-slot CWBLDC prototype has been designed, fabricated and tested to verify the analysis.