Process for switching between main supply and a frequency inverter and vice versa for an escalator drive

An electrical controller for controlling the drive of a line voltage connection (Netz) with an essentially constant line frequency (fNetz), a drive motor (26) and a conveyance demand signal generator (48) which signals the demand to switch the operating mode between the load and the idle mode of a switchable conveyor installation in the form of an escalator (10) or a moving sidewalk, comprising:

• a frequency converter (42) whose output frequency can be adjusted;
• a controllable switch-over installation (K1, K2) with a load operation mode in which the drive motor (26) is directly coupled to a line voltage connection (Netz), and an idle mode in which the drive motor (26) is coupled to the line voltage connection (Netz) via the frequency converter (42);
• a synchronizer (44) whereby the output frequency of the converter (42) can be adjusted, and the output voltages of the line connection (Netz) and the frequency converter (42) are comparable with respect to frequency and phase position;
• the frequency converter (42) can be set for an output frequency that has a predetermined frequency spacing (Δfup, Δfdown) from the line frequency (fNetz);
• and at the time (t1, t4) after a switch-over demand has been signalled, when the output frequency of the converter (42) has reached both the predetermined frequency spacing (Δfup, Δfdown) from the line frequency (fNetz), as well as a predetermined phase spacing between the output voltages of the frequency converter (42) and the line voltage connection (Netz), a switch-over signal (SP) can be sent to the switch-over installation (K1, K2).