Uncontrolled Generator Operation of PM Synchronous Machine Drive With Current-Source Inverter Using Normally on Switches

This paper investigates the uncontrolled generator (UCG) operating characteristics of permanent magnet (PM) synchronous machine drives using current-source inverters (CSIs) with normally on switches. This special mode occurs when a PM machine is spinning at a high speed and gating signals are simultaneously removed from all of the inverter switches. This investigation shows that the UCG operating characteristics are quite different when the PM machine is combined with a CSI that uses normally on switches, compared to the use of a conventional voltage-source inverter (VSI). The presence of the stiff dc-link current prevents high transient currents from being developed in the machine phases that can expose the machine's rotor magnets to significant demagnetization risks. As a result, the UCG characteristics of the CSI-based PM machine drive with normally on switches are more benign than those of comparable VSI-based drives, representing an advantage of the CSI drive configuration during fault-mode operation. Experimental test results for UCG-mode operation with a 2.6-kW current-source PM drive are included that exhibit good agreement with the analytical predictions.