Brushless permanent magnet motors with low inertia rotors are used in high-performance servo applications such as machine tools or pick and place applications where fast, precise movements are required, and motors with high inertia rotors (and high pole-numbers) suit low-speed applications such as gearless lift systems.
The performance characteristics of these drives are summarized below:
They have excellent dynamic performance at speeds down to standstill when position feedback is used.
For precision positioning the position feedback must define the absolute position uniquely within an electrical revolution of the motor. This can be provided with a position sensor or alternatively a sensorless scheme can be used. The performance of a sensorless scheme will be lower than when a position sensor is used.
Field weakening of permanent magnet motors is possible to extend their speed range, but this requires additional motor current, and so the motor becomes less efficient in the field weakening range. This form of control also increases the rotor losses and raises the temperature of the magnet material, thereby increasing the risk of demagnetization. Care is also needed in such applications to avoid overvoltage at the motor and drive terminals in the event of a loss of control: at high speeds, the open-circuit voltage will exceed the rated value.
Brushless permanent magnet motors exhibit an effect called cogging that results in torque ripple. It is caused by magnetic reluctance forces acting mainly in the teeth of the stator, and can be minimized by good motor design, but can still be a problem in sensitive applications.
Brushless permanent magnet motors can be very efficient as the rotor losses are very small.