33 Electric Motors T [Nm] Nominal power Nominal torque Field weakening n [min -1 ] Nominal speed Related to back EMF Critical speed Fig. 1.25 Operation in fi eld weakening area The greatest risk in fi eld weakening operation is switching off the motor control at too high speed, as the high back EMF can destroy the FC (see section 1.4.1.1 Back EMF). Another possibility for extending the speed range is to change the star confi guration of a motor to delta, if the motor provides this feature. Similar to asynchronous motors, a delta connection results in a higher voltage on the windings, because it is not reduced by the factor 1.73 or 3 as for a star confi guration.
1.4.2 Brushless DC (BLDC) or Electronically Commutated (EC) Motors EC (Electronically Commutated Motor) and BLDC (Brushless DC) are basically diff erent names for the same technology. In the original BLDC concept only two phases were energised with a trapezoidal voltage. Compared to a distribution over three phases this result in 1.22 time higher current. For determining the rotor position Hall sensors have been used. Drawbacks of the concept were worse torque ripples and iron losses. In practice there are many diff erent types of EC motors, such as small servo motors with power ratings of a few watts or motors in building automation systems up to approximately 10 kW.
In general BLDC/EC has a reputation for extremely high effi ciency. This is fully deserved, in particular for very small devices – the original application area for these motors – where they are distinctly better than universal or split-pole motors (effi ciency approximately 30%). Above a few hundred watts the effi ciency is comparable to standard PM motors. Nominal speed range Above nominal speed