34 Electric Motors Modern EC/ECM utilise the same control principles as the PM motors. In building automation EC motors are often used as hubs in EC fans. This results in a very compact fan unit with a very effi cient motor. Unfortunately the placement of the motor in the middle of a centrifugal fan creates air turbulences which reduce the total fan effi ciency. In comparison to a direct-driven fan the diff erence at same motor effi ciency can be in the range of 3-6%. 1.4.3 Line Start PM Motor (LSPM motor) A line start PM motor is a hybrid of a squirrel-cage asynchronous motor and a PM motor where the magnets are placed internally to the rotor. Fig.
1.26 The position of magnets in the rotor infl uences the motor characteristics When connected to a three phase grid the motor develops a torque and accelerates like a standard asynchronous motor to near synchronous speed, if the motor torque is greater than the load torque throughout acceleration. When the rotor has roughly reached the speed of the rotating fi eld, a synchronising torque (reaction torque) is produced due to magnetic coupling between the rotating stator fi eld and the rotor poles, which pulls the rotor into synchronism. After synchronisation, the motor continues to run at synchronous speed.
As there is no speed diff erence between the magnetic fi eld and the rotor, no currents are induced in the cage. This results in a high effi ciency with a good power factor. When load changes take place the squirrel cage is still working as a damper. This is also the case when the motor is operated by a FC where the additional damper can reduce the effi ciency by approximately 5-10 %. If the motor is loaded with a torque that is greater than its synchronous stalling torque, it is pulled out of synchronism and continues to operate like an asynchronous motor at a load-dependent speed. Depending on the design, the motor is more or less