Application Characteristics of Permanent Magnet Synchronous and Brushless dc Motors for Servo Drives

The permanent magnet synchronous motor (PMSM) and the brushless dc motor (BDCM) have many similarities; they both have permanent magnets on the rotor and require alternating stator currents to produce constant torque. The difference in these two machines is that the PMSM and the BDCM has sinusoidal and trapezoidal back emfs, respectively. This means these two machines have different operating characteristics and control requirements. For application considerations, these two motor drives have to be differentiated on the basis of known engineering criteria. Some of the criteria used to assess these two machines include power density, torque per unit current, speed range, feedback devices, inverter rating, cogging torque, ripple torque, and parameter sensitivity. Guidelines for the appropriate machine to be used for a given application are given based on the results of the criteria listed above.