35 Electric Motors sensitive to under-voltage situations which can also result in falling out of synchronism. Renewed synchronisation takes place automatically when the load torque is lower than the synchronising torque. However, the rotor will stop if the motor is loaded with a torque that is greater than its induction stalling torque. Drawbacks of the concept are the infl uence of the magnets while starting the motor. Torque oscillations and torque peaks, paired with noise, arise during the start up. Furthermore the starting torque is lower compared to an asynchronous motor as the magnets create a negative torque component (1). Torque Tcage Tmagnet TLSPM S=0 Speed Fig.
1.27 Starting torque of LSMP is reduced compared to the pure squirrel cage torque LSPM motors are typically used in fans and pumps, available in the power range up to approximately some 10 kW, but can also be used in low inertia applications. 1.4.4 Reluctance Motors For creating a motor movement these types of motors utilise magnetic reluctance, which is also called magnetic resistance. Similar to electric circuits the magnetic fl ux follows the path of the lowest resistance. As in asynchronous motors, the magnetic fi eld is created by applying a suitable voltage to the stator windings. The rotor rotates towards the position with minimum magnetic reluctance.
If the rotor is now forced out of this position a torque is created in order to move it back to the position where the reluctance is minimised. The torque resulting from the magnetomotive force depends on the relationship between the inductances in the d-axis and q-axis, known as the saliency ratio. The saliency ratio results directly from the rotor lamination design. Cut-off s in the lamination are utilised to shape the equivalent air gap of the machine by controlling (1) Source – 2014 .J Sorgdrage, A.J Grobler and R-J Wang, Design procedure of a line-start permanent magnet synchronous machine.