I-F starting method with smooth transition to EMF based motion-sensorless vector control of PM synchronous motor/generator

This paper proposes a novel hybrid motion- sensorless control system for permanent magnet synchronous motors (PMSM) using a new robust start-up method called I-f control, and a smooth transition to emf-based vector control. The I-f method is based on separate control of i_d, i_q currents with the reference currents i_d* = 0 and i_q* constant, and the reference frequency having ramp variation. This solution allows ultra low- speed sensorless control without initial rotor-position estimation, and without machine parameters identification. A first-order lag compensator is employed to ensure a smooth transition from I-f to emf sensorless vector control when the frequency reaches a certain level, and back. The PMSM rotor position and speed are extracted by using a PLL state-observer from the estimated rotor-flux, which is based on an equivalent integrator in close- loop with a PI speed-adaptive compensator to eliminate dc-offset and phase-delay. Digital simulations for PMSM start-up with full load torque are presented for different initial rotor-positions. The transitions from I-f to emf motion-sensorless vector control and back as well, at very low-speeds, are fully validated by experimental results. This method is suitable for both surface and interior PMSMs, but the paper refers directly only to surface PMSM. Other field of application might be in wind generators safe (faster) self-starting when they are connected to the grid, or moving the rotor a little for inspections/repairs, and for general industrial variable speed drives where slightly hesitant but full- load self-starting is allowable.