Small copper loss unidirectional current type BLDC motor and the motor system

In an electric car, wide range speed driving is required. In order to increase the rotation speed of a bidirectional current type cylindrical BLDC motor, the applied voltage needs to be increased because of the increase of back-EMF. When that voltage gets to a maximum value, torque current, i_q stop increasing. In order to increase i_q, negative direct axis armature current, i_d, is generated. In other words, the weak field control is carried out. The motor´s efficiency lowers because of the copper loss increase, if i_d flows in the control circuit. From the above facts, the motor, in which high speed rotation is possible and the efficiency is not changed in the high speed rotation, is desired. Furthermore, the induction brake current, i_B flows in the armature winding during high speed rotation. The i_B lowers the rotation speed and motor´s efficiency The i_B can be prevented by the control circuit of the motor system. Another method is to change the parallel circuit number of the armature winding. The back-EMF constant can be lowered by increasing the parallel circuit number. The armature current is increased by lowering the back-EMF constant.