Many users of wound-rotor machines have replaced them with less expensive and more reliable squirrel-cage units, with inverter power supplies for speed control. One way of doing such a conversion is simply to short-circuit the collector rings. That has the advantages of eliminating the brushes and the external resistance. It may not be practical, however. Because the speed-torque characteristic of a wound-rotor motor can be varied by the external resistance in the rotor circuit, "locked-rotor torque" is not the concern that it is in a squirrel-cage machine. Conversion to a squirrel-cage design therefore normally means replacing the rotor. Such replacement usually includes the shaft as well, because there is no longer any need for the shaft-mounted collector ring assembly connecting the rotor winding to the external circuit. Rotor replacement also involves several mechanical issues. Converting a wound-rotor motor to squirrel-cage operation is frequently possible, but is evidently not a simple matter and calls for engineering supervision to make sure the result is suited to the proposed application.