A single external power supply variable speed induction motor and a control method thereof are disclosed. The method includes inserting between two elements, at least one wave shim, which is a wave-shaped compressible body with a slot subdividing a portion of the body at the end into a pair of wave-shaped extending arms, and a tab attached to the body at a side opposite the wave-shaped extending arms. The switching device has a common source region and multiple drain regions, each drain region connected to an associated RSE from said plurality of RSEs. This method can effectively protect mobile hosts from denial-of-service attacks and is transparent to NATNAPT firewalls. In addition, it is possible to reduce impact applied to the ink and obtain good print quality, since the ink is ejected using the electrostatic field.