Measurement of permanent magnet rotor magnetization characteristics of DC brushless motors

The cylindrical permanent-magnet rotors of DC brushless motors are magnetized in special patterns for the main-field region and the commutation-field region so as to provide starting torque reduce cogging, and minimize losses. The resultant magnetic-field patterns are difficult to measure accurately as a function of axial and circumferential position. A measurement system is described which was developed to make such measurements on a series of small DC brushless motors. The system utilizes a Hall probe for sensing the radial or tangential-B components of the field, a synchronizing device, an oscilloscope for display and digitizing data and a computer for processing and printing out the data. An example is shown of a typical readout of the radial-B field for a motor magnetized in the N-S-O-N-S-O pattern. The system is designed for checking on the field strength of the magnets, the required pattern, and the correlation between the main-field and commutation-field patterns.<>