Scalar control: an alternative AC drive philosophy

A new control philosophy for inverter driven induction motors causes the airgap flux to vary over a wide range in proportion to load-induced changes in the scalar magnitude of the machine current. A controller based on this principle outputs a constant slip frequency at high efficiency under all useful operating conditions. It is stable with good damping factors and good response times, yet it requires no external transducers. It allows for accurate torque estimates, but it can be implemented with low cost microprocessors that can be interfaced to computers for interactive plant control and data analysis. Measured results from a practical inverter are shown to be in close agreement with both the theoretical analysis presented and with computer simulations.