Operating Torque Estimation of High-Speed Slotless Brushless DC Machine Considering Power Loss

Torque is very essential for performance evaluation of electric machines because it is directly connected with power loss for dynamic characteristics. To evaluate dynamic performance, torque with harmonic component according to time is introduced. Particularly, eddy current loss is significantly increased at high speed, so it is difficult to express the dynamic characteristic without reflection about losses. Therefore, in this paper, rotor eddy current loss wave is derived by fast Fourier transform which uses the phase current wave obtained from finite element analysis according to two conditions: one has rotor eddy current loss, the other does not. The core loss is deduced by a modified Steinmetz equation and additional losses are considered from an experiment. Those losses are worked as drag torque which is inverse force to performance of a machine. The operating torque estimated from the difference between drag torque and electric torque closely reflects the dynamic characteristics.