The permanent magnet excited synchronous motor has no excitation losses.
It is free of wear out and therefore needs no maintenance.
The synchronous motor needs no speed sensor - all spindles run at exactly
the same speed.
The construction of a permanent magnet excited synchronous motor is cost efficient because the air gap might be wider compared to an induction motor. The rotor of smaller machines consists only of a simple ring magnet, sticked on the shaft [1]. The motor can be fed with a high voltage to reduce the diameter of the copper wires in the textile machine.
This kind of permanent magnet excited synchronous motor needs a low frequency to start up, because there is no damping cage on the rotor, which would allow a induction start. So each spindle needs its own frequency inverter. Another disadvantage is, that the permanent magnet excited synchronous motor will not operate at higher speeds because of instability caused by the small damping - a result of the high power efficiency [3]
The reactive current depends on the load, so torque and current are not proportional. To reach a high power efficiency reactive current must be controlled by the frequency inverter.