The present invention relates to the combination of a brushless DC motor and encoded drive circuit and more particularly to such a combination in which the arcuate length and numberof rotor magnet poles, the manner of connection of multiphase stator windings and the encoded sequence of energization of the windings cooperate to produce a more efficient and better operating motor.
Recently six-transistor, encoded integrated circuits have been made available and have been proposed for brushless DC motors. These circuits permit "full wave" excitation of each winding, i.e., current flow in forward and reverse directions. Previous three-transistor stator winding energization circuits permitted only "half wave" excitation, current flow in one direction only. THe six-transistor circuits have been proposed for four-pole, three-phase brushless DC motors. See for example, "Hexfets in Hybrid Packages Make Compact Motor Drives," P. Wood and D. Grant, Proceedings of PCI, October 1985. The art, however, has not made full advantage of these encoded, six-transistor drive circuits such that each stator winding is energized through a larger mechanical rotor angle to contribute to the torque produced and fewer slots are utilized in the stator lamination stack whereby more magnetic material is present to conduct flux, thus improving efficiency. This is particularly true in small brushless DC motors where the number of slots takes on greater significance in the smaller diameter stator stock.
With higher speed brushless DC motors it is, moreover, desirable to decrease the number of switching occurrences per rotor revolution because switching losses reduce efficiency and with higher speeds occur more frequently. A typical four-pole, three-phase brushless DC motor utilizes three Hall devices angularly spaced at 30.degree. or 60.degree. mechanical to produce 12 commutations for every 360.degree. mechanical.