Study of the electric loading aspects of the BDFM using a lumped parameter thermal model

Abstract

Details of a lumped parameter thermal model for studying thermal aspects of the frame size 180 nested loop rotor BDFM at the University of Cambridge are presented. Predictions of the model are verified against measured end winding and rotor bar temperatures that were measured with the machine excited from a DC source. The model is used to assess the thermal coupling between the stator windings and rotor heating. The thermal coupling between the stator windings is assessed by studying the difference of the steady state temperatures of the two stator end windings for different excitations. The rotor heating is assessed by studying the temperatures of regions of interest for different excitations.

Design considerations for the brushless doubly‐fed (induction) machine

“…It is recognised that the acceptable conductor current density will need to conform to limits determined by the size of the BDFM and its particular cooling arrangements and this will affect the overall electric loading of the machine. Work on the thermal modelling of the BDFM [21] shows that the two stator windings in the same stator slots are thermally closely coupled so an overall loss figure is appropriate for thermal calculations for small imbalances between the two windings. The resistance (R)o f a stator coil of N turns is given by…”

“…Results from the TSFEA simulations were used to compute the peak airgap flux densities of the 4-pole and 8-pole fields together with the peak flux densities in the teeth and core back of the rotor and stator. These are given together with peak flux density values computed using (20) and (21), but with the same airgap flux densities calculated using TSFEA. The peak core back flux densities of both the stator and rotor compare well with a maximum error of 6.7%, for the stator core back values of the full load operation.…”

“…It is recognised that the acceptable conductor current density will need to conform to limits determined by the size of the BDFM and its particular cooling arrangements and this will affect the overall electric loading of the machine. Work on the thermal modelling of the BDFM [21] shows that the two stator windings in the same stator slots are thermally closely coupled so an overall loss figure is appropriate for thermal calculations for small imbalances between the two windings. The resistance ( R ) of a stator coil of N turns is given bywhere ρ is the resistivity, A is the conductor cross‐sectional area, d w is the winding diameter, l is the effective stack length, α is the arc length of the end winding and c is the end winding length correction factor, slightly greater than 1.…”

“…However, the turns ratio for each loop will be different and so not only will loop currents differ, but there will also be a degree of circulating currents. The uneven current distribution has implications for the electric loading, but recent work [21] confirms that the rotor bars are thermally well coupled. The winding will exhibit an effective turns ratio, and this typically shows a small variation with the frequency of rotor currents.…”