Design and Voltage Supply of High-Speed Induction - Aaltodoc

The different rotor topologies were modeled with their own parameterized element meshes. First
order elements were used in the optimization, since modeling with second order elements would
have been too time consuming. The calculation times are shown in Table 9.1 in Section 9.1. The
resulting designs were calculated afterwards with second order elements to see the possible
differences in the results.
This study concentrated only on the rotor design. Thus, the stator core and stator winding were kept
constant. No mechanical or manufacturing constraints were enforced. Friction loss was not taken
into account. Rotors not having a smooth surface generate more friction loss but they could be
coated with a cylindrical sleeve. No thermal modeling was used in this study.
The objective function was simply the total electromagnetic loss of the motor. The motors were
modeled at the same operational point yielding the same shaft power. Shaft power of 50.1 kW at
50100 1 /min was selected, as this was the nominal operation point for the tests performed and
reported in Chapters 5 and 6. Nominal magnetization was used in the calculations.
The supply of the motors was modeled as a voltage supply giving one ideal voltage pulse per half a
cycle. This corresponds to the output of a PAM voltage source inverter. Constant amplitude for the
pulse was assumed, i.e., a constant DC-link voltage in an inverter and no voltage drops in power
electronic circuits. The rise time of the pulse was considered zero but from the modeling point of
view it was determined by the time discretization of the time-stepping method in FEM.
The rotors were modeled without endrings in order to keep the parameterization of the rotors
simple. However, the stator was modeled together with the circuit equations for the end winding
impedance. This means that the relative importance of stator winding loss was increased with
respect to the rotor loss. The results will show that compared to a pure 2D calculation, the stator
winding loss is overestimated by a factor of 2.6 – 3.1 as seen in Table 9.3. This more than
compensates the effect of circulatory currents in a stator winding discussed in Chapter 8. If the end
rings were modeled, the corresponding factor would be smaller for the rotor loss because of the
smaller diameter relative to the 2D length of the motor.
9.3.1 Optimization results
The quarter cross sections of the initial and optimized motors are presented in Figs. 9.7. – 9.13. An
example of post-processed results of the FEM model for the coated motor are shown in Appendix
A. The results of the numerical optimization are presented in Table 9.3. The objective function
value, i.e., the total electromagnetic loss, is given together with stator current, power factor, air gap
flux and rotor slip. The different loss components are also given. The results of the optimized
115