Well matched UPS and generator for IT mission-critical applications requires an on-line UPS that can accommodate most frequency variations. There should also be a properly sized generator that will absorb most voltage variations. Further insurance can be provided by an electronic governor on the generator and an in-phase monitor on the automatic transfer switch.
On-line UPS designs are ideal for use with generators because they accept input power with relatively wide variations in voltage and frequency.
An on-line UPS re-develops the waveform. The AC input from either the mains or a standby generator is first converted to DC by the rectifier. The DC current is used to charge the battery.
During an outage, DC current from the batteries is reformed by an inverter back to a perfect sine wave AC for the critical load. Newer models also use power factor correction to provide an even wider input voltage range.
Because of this double conversion (AC to DC, then DC to AC), variations in the input frequency are of little concern. Therefore the same frequency variations that would cause an off-line or a line-interactive UPS to transfer to battery power have no effect on the on-line UPS.