Design specifications for the brushless DC motor were established based on the principle that the motor should be easy to construct with readily available parts, and should provide qualitative performance similar to many commercially available DC motors, such as those used in small electric fans.
The motor was designed as a 3-phase, 4-pole brushless DC motor with 4 - N52 neodymium magnets on the rotor, and 3 wire wrapped solenoids connected to the stator. The brushless design was chosen because of the increased efficiency, reduced number of mechanical parts, and lower friction. The N52 magnets were chosen for their strength, price, and easy availability. Brushless motor control is discussed further in the ‘BLDC Motor Control’ section. Table 2 shows a comparison of brushless and brushed DC motors.
The solenoids are powered at 8-12 volts and controlled by an electrical switching circuit. 3 hall effect sensors will provide location information telling the circuit when to perform commutation.
The following equations were used to estimate the performance of the motor and therefore create the initial motor design.
These equations are messed up if you want to see them take a look at the pdf linked in the intro.
The force between two magnets separated by some distance can be roughly approximated by the following equation: F=BmAmBsAs/4g2
where B is the magnetic field density at the surface of the magnet, A is the area of the magnet, and g is the distance between the two magnets. Bs, the magnetic field of the solenoid is given by: B=NIl
where I is the current, N is the number of wraps, and l is the length of the solenoid. In the motor the max torque was estimated as: t=2Fr
where r is the radius, chosen to be 25 mm.
Combining these equations a linear expression relating the output torque to the input current can be obtained for a given solenoid geometry. f =2rBmAmAsN4g2lI
The desired torque constant was chosen to be 40 m-Nm/A based on the desired performance relative to other available motors .