NEW APPROACH FOR THE ANALYSIS OF A 3-PHASE INDUCTION-MOTOR OF DIFFERENT RATINGS CONNECTED TO A SINGLE-PHASE SUPPLY

The problem of operating a three-phase induction motor connected to a single-phase supply system using two convertors is discussed. Special attention is focused on the starting torque and minimum unbalance requirements for different motor power ratings. A new approach is suggested here, concerning the calculation of the starting and running sizes of the convertors, to enable the motor to start under full load conditions with minimum unbalance factors. These sizes are also modelled as functions of the motor power, with a wide applicable range. A method of determining the switching instant of the first convertor sizes is also introduced and modelled. Numerical application of the proposal has been carried out on different induction motors to investigate its validity. The results prove a reasonable minimum unbalance factor of 5.8 per cent during normal running conditions. They also demonstrate that a sufficient starting torque is at least equal to the full load figure. The models proved to be accurate, so that there would be no need for further analysis for any motor power in the range studied (0.37-15 kW). This approach has no complex or special hardware control system; it needs only a simple facility to implement the switching process from starting to running sizes for one of the two convertors. The other convertor will remain fixed at its optimum size, independent of the speed.