An intelligent constant-flux AC drive

A new constant flux control technique for pulse width modulated (PWM) AC drives using conventional cage induction machines, based on observations of zero sequence harmonics generated by nonlinearities in the magnetising inductance of the machine is discussed. It achieves flux constancy within 1-2% for frequencies above ≈0.5 Hz for both motoring and overhauling loads yet it requires no modifications to the machine. It is simple to implement with a microprocessor based controller and does not involve significant hardware costs or software overheads. Inverters based on this principle allow the drive´s torque/speed characteristics to be accurately determined and in a prototype inverter such working variables can be downloaded over an RS232-C link to give a real-time display, on a low-cost computer VDU screen, of the drive´s performance. The computer can also interact with the drive over the same link to give a programmable drive with data collection capability