Design and Voltage Supply of High-Speed Induction - Aaltodoc

8 CIRCULATORY CURRENTS IN A STATOR WINDING
The difference between the electromagnetic power loss measured and calculated in Section 7.3 was
disturbing. The difference seemed to be a function of the input current or load of the high-speed
induction motor. At the nominal operation point, the model used in the calculations underestimated
the loss by almost 20%. This raises questions about the usability of the model in designing and
optimization of the high-speed machines.
Circulatory currents in the filamentary stator windings were suspected as a possible reason for the
difference in the results. These currents can flow in parallel strands and paths of a filamentary
conductor winding. This would mean an increased loss in a stator winding. The loss component
would be a function of current in the same way as is the discrepancy in the results of comparison
mentioned above. Thus, the high-speed motor in question was disassembled for the measurement of
the possible circulatory current effect. Fig. 8.1 shows the stator prepared for the measurement.
The reason for the circulatory currents is the magnetic flux penetrating the windings inside the
stator slots. The flux causes eddy currents that change the current density distribution in the parallel
strands. The amplitude and the phase angle of the currents in parallel conductors are different from
each other.
In the FEM model used, the current density in the stator winding is assumed to be constant. The
eddy-current loss is modeled separately from the field solution by using Eq. A11. Thus, the
circulatory currents in the parallel strands and paths of the filamentary winding are not modeled. In
order to model the filamentary winding precisely, every individual strand should have its own
voltage equation (Eq. A3) solved together with the field solution. Doing this kind of model is
difficult. Every strand should be meshed in detail. As there can be thousands of strands in the stator
winding, the size of the model may become unfeasibly difficult to solve. Using macro elements as
described by Szücs (2001) could decrease this size of the system of equations.
Even if solving the modeling problem is possible, one problem remains. The positions of the strands
inside the slot of a manufactured machine are not known. Accurate and correct positioning of
strands of a filamentary winding is considered expensive and time-consuming, if not impossible.
This difference between modeling and reality will inherently generate a difference in calculated and
measured results for the stator winding loss.
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