Adjustment of Machine Voltage in Drives Operating at Constant Power: Consequences for the Inverter

The reduction of the stator flux in an electrical machine moving at constant speed causes the reduction of the voltage at the machine terminals. In the field of electrical drives this phenomenon leads to the common convincement that the reduction of the reference stator flux, while leaving the speed unchanged, always avoids the saturation of the voltage source inverter connected to the machine, thereby preventing the potential loss of current control. Such a statement is correct only if any faithful model of the real interconnection network between the inverter and machine is characterized by ideal conductors directly connecting the inverter and machine terminals. Clearly, in the general case, the interconnection network is far from being faithfully modeled by ideal conductors. The present paper proves analytically that the nature of the interconnection network, together with a very common control strategy for the machine, can render the reduction of the reference stator flux below a certain value detrimental for the inverter, instead of beneficial. This is in contrast with the widespread convincement. The paper proves also that the interconnection network imposes an upper limit on the average power that can be exchanged between the inverter and the machine. The results are generalized for the case in which more inverters share the motor current equally and it is highlighted that both are simply two manifestations, just seen from different points of view, of the same intrinsic nature peculiar to the electrical circuit.