SINGLE PHASE INDUCTION MOTOR TRANSIENTS BASIC INFORMATION

Single-phase induction motors undergo transients during starting, load perturbation or voltage sags etc. When inverter fed, in variable speed drives, transients occur even for mechanical steady state during commutation mode.

To investigate the transients, for orthogonal stator windings, the cross field (or d-q) model in stator coordinates is traditionally used. [1] In the absence of magnetic saturation, the motor parameters are constant.

Skin effect may be considered through a fictitious double cage on the rotor. The presence of magnetic saturation may be included in the d-q model through saturation curves and flux linkages as variables.

Even for sinusoidal input voltage, the currents may not be sinusoidal. The d-q model is capable of handling it. The magnetisation curves may be obtained either through special flux decay standstill tests in the d-q (m.a) axes (one at a time) or from FEM-in d.c. with zero rotor currents.

The same d-q model can handle nonsinusoidal input voltages such as those produced by a static power converter or by power grid polluted with harmonics by other loads nearby.

To deal with nonorthogonal windings on stator, a simplified equivalence with a d-q (orthogonal) winding system is worked out. Alternatively a multiple reference system + - model is used [3]

While the d - q model uses stator coordinates, which means a.c. During steady state, the multiple reference model uses + - synchronous reference systems which imply d.c. steady state quantities.

Consequently, for the investigation of stability, the frequency approach is typical to the d-q model while small deviation linearization approach may be applied with the multiple reference + - model.

Finally, to consider the number of stator and rotor slots-that is space flux harmonics-the winding function approach is preferred. [4] This way the torque/speed deep around 33% of no load ideal speed, the effect of the relative numbers of stator and rotor slots, broken bars, rotor skewing may be considered.

Still saturation remains a problem as superposition is used. A complete theory of single phase IM, valid both for steady-state and transients, may be approached only by a coupled FEM-circuit model, yet to be developed in an elegant computation time competitive software.