Evaluation and modelling of cross saturation due to leakage flux in vector controlled induction machines

A detailed magnetic, reluctance based, computational model of an induction machine into which a traditional, rotor-flux-orientated, vector, control-scheme has been grafted, is used to examine the influence of saturation of both main and leakage flux paths upon vector controlled drive performance. Individual machine teeth, windings and conductors are incorporated, as is pseudo 3 dimensional modelling of skew. The cross saturation effect is therefore determined from the basic dimensional, and winding, design data of the machine. Effective compensation methods can then be derived for practical implementation. Direct, rotor flux orientation and magnitude control is used to ensure that inaccuracies in a machine parameter based vector control scheme are not reflected in the results for cross saturation. Results for a 15 kW, 4 pole induction motor with rotor skewed 1 stator slot pitch show that skew leakage flux is predominantly responsible for cross saturation. The skew leakage flux is shown to heavily saturate one end of the machine and to reduce the air gap flux density at the other. At 200% rated load current this equates to a 20% reduction in direct axis flux if i_d is kept constant. To compensate for this reduction a 40% increase in i_d would be required.