High-torque brushless DC motor for a valve actuator

The design of a high torque brushless DC motor to replace the induction motor and worm gear for valve actuators is described. Minimum motor cost, short axial length and low electronics rating are achieved by selection of open slots and coils wound on bobbins. First and second harmonic designs are compared, with either 2 or 4 magnet poles facing 3 teeth. As well as being cheaper from having fewer coils, the second harmonic design is found to be superior in having lower copper loss at peak torque. The peak tangential force density across the gap was 5.8 /spl times/ 10/sup 4/ N/m/sup 2/. Measurements are reported on 75 Nm and 150 Nm peak torque prototypes, which are intended to "run" at 180 rpm and at 20 Nm and 40 Nm respectively. Though 36 pole motors, their no load loss would be sufficiently low for higher speed operation, even to around 2000 rpm. Successful operation as an actuator was observed on a valve.