38 Electric Motors Fig. 1.31 Special rotor lamination design results in high effi ciency at low torque ripples The stator construction and the windings are similar to an asynchronous motor. By applying a suitable voltage to the distributed windings, a harmonic fi eld is created which creates low harmonic losses. Also the design of the rotor is optimised to reduce harmonic losses and operate with low torque ripples. As the motor cannot start directly on mains, a frequency converter is required to control the motor. For magnetising the cut-off s in the rotor lamination, higher apparent power is required than for an Asynchronous motor (see section 1.4.4 Reluctance Motors).
If the converter and the capacitors in the intermediate circuit are suitably sized they will deliver the additional apparent current. In this case the grid is not loaded with the higher apparent power and the low cos φ. For operating the motor, the FC needs to know the rotor angle. Depending on the angle, the converter will energise the diff erent windings. The determination of the rotor angle is often done sensorless without an additional device. In order to achieve an energy effi cient control, the converter must also take care of the L d and L q behaviour in operation. Inductance L d 1/2 D-axis inductance (L d ) L dSat 1/2 L q L dSat Q-axis inductance (L q ) I d /I q in % of nom.
motor current 100% Fig. 1.32 Example of L d /L q relationship to I d /I q saturation factor