Method and control unit for operating a drive train

A control unit configured to operate a drive train designed as pump or compressor, having a main drive designed as electric machine operable with a constant rotational speed, an auxiliary drive designed as electric machine operable with a variable rotational speed, and a speed modulation gearbox with a fixed mechanical transmission ratio connected between the two drives and the drive train,

• wherein the control unit for starting the drive train and for towing the drive train to a defined set rotational speed carries out the following on a control side:
• starting the main drive via a direct-on-line-start with direct coupling to a supply network;
• starting the auxiliary drive subject to an intermediate connection of a converter with indirect coupling to the supply network simultaneously with the main drive or at a time before the main drive or at a time after the main drive;
• and
• operating the main drive and auxiliary drive for towing the drive train to the defined set rotational speed which corresponds to a defined percentage of a rated rotational speed of the drive train,
• wherein the control unit operates the main drive motorically in forward mode at least at times, accelerating it to its rated rotational speed, wherein the control unit, parallel to this, operates the auxiliary drive motorically in reverse mode at least at times.