30 Electric Motors In order to magnetise the motor in the best way the controller needs to know the rotor angle at any point in time. In many applications sensorless strategies for determining the rotor angle are suffi cient. If the controller is not capable of sensorless control or in high dynamic servo applications, external position feedback devices are used. In the equivalent diagram the magnets are represented by a voltage source Up because turning the rotor will result in a voltage induced in the stator. This voltage is called back EMF, see section 1.4.1.1 Back EMF.
The absence of motor slip, rotor resistance and inductance indicates that no losses are created in the rotor which results in the very good effi ciency. U1 supply voltage I1 stator current U P Voltage generated by permanent magnets R1, X1 describe the coils in the stator using X1 = X1 h + X1σ R 1 X 1 I 1 U 1 U 1 U p U p ~ Fig. 1.23 Simplifi ed PM motor equivalent circuit diagram ln general PM motors can be divided into motors with rotors where the magnets are placed on the surface (SPM motor) or internally (IPM motor). The placement of the magnet results in diff erent shapes of the resulting magnetic fi eld and is described by the inductances L d and L q . Fig.
1.24 Magnet placement a) SPM and b) IPM