By relieving the shaft of an electronically-controlled turbocharger (ECT) in a central region of where a rotor of an electric machine couples with the shaft eases assembly of the electric machine onto the shaft. On either side of the relieved section, the fit between the shaft and the rotor may be a slip fit or an interference fit. Alternatively, the rotor is relieved in a central section. In some embodiments, the shaft is welded to the rotor. In yet other embodiments, the outside of the shaft and the inside of the rotor are threaded with a nut or a pin to secure the shaft to the rotor or the rotor itself has threads to engage with threads on the shaft. Such arrangements ease assembly and allow adjustment of dynamic characteristics of the rotor system.