37 Electric Motors Similar to a LSPM motor design, (see section 1.4.3 Line Start PM Motor (LSPM Motor)) the motor accelerates to near synchronous speed when connected to a three phase grid, if the produced torque is suffi cient for the load. When approaching the synchronous speed the rotor is pulled into synchronism and runs at synchronous speed despite the absence of rotor excitation. Fig. 1.30 Torque characteristic of a reluctance motor Under load, the salient rotor poles lag behind the stator rotating fi eld by the load angle. Again the behaviour is similar to LSPM when the load torque becomes too high.
The motor is pulled out of synchronism, continues to operate like an asynchronous motor and regains synchronisation automatically when the load torque is lower than the synchronising torque. The possibility to start direct on line (DOL) and run at synchronous speed make the motor interesting for several applications. Power range ends often at approximately 10 kW. The drawback is a reduced effi ciency, especially when operated by FC’s, as the rotor windings act as an additional damper. 1.4.6 Synchronous Reluctance Motor (SynRM) The design of a new generation of reluctance motors focuses on energy effi ciency.
This highly effi cient motor type is often meant when synchronous reluctance motors are addressed and should not be confused with reluctance motors which focus on high torque density or the possibility to start on mains. The key to the effi ciency is the new rotor design.