Almost all alternator manufacturers test the alternator at 0.8 power factor. But when installed at a site for power generation, the working power factor will be around 0.9 to 0.95, due to the power factor regulators used by the users. The manufacturers provide a data sheet that specifies how much is the maximum current at different power factors. It is observed that the current decreases as power factor improves. If so, will the output power also decrease? In case of a 3 phase 750KVA, 415 V,50Hz alternator, rated current is 1043 A at 0.8 pf. At unity pf, the full load current is 835 A as per the alternator manufacturer. If so, the output power at this condition is 600KVA i.e., 600KW. Is this calculation true?? If true, does it means that we can never load 750 KW load(without over loading) on a 750 KVA DG ???

  • \$\begingroup\$ In every case you get 600kW of true power, so I guess the generator is 750kVA/600kW. \$\endgroup\$ Commented Jun 3, 2016 at 11:33
  • \$\begingroup\$ Why the manufacturers are reducing the current by 20 to 25% as power factor improves to 0.9 or unity? \$\endgroup\$
    – Anthony
    Commented Jun 7, 2016 at 12:47

2 Answers 2


Yes. The ratings essentially say that the generator is rated 1043 amps maximum and the engine is rated 600 kW plus the generator losses. The engine and generator are matched for 600 kW at 0.8 pf, 1043 A. At any other operating point, either the generator or engine is working at more or less than its rated operating point.


Yes, your calculations are correct and 600 kW is all you can draw from the generator. There is, however, a simple configuration to run the generator at optimum.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Power factor correction on utility line only.

The only installations I'm familiar with have generator feed into the system after the power factor correction current transformers. This way, when running on generator, there is no current in the feed being monitored by the CT and the controller switches off all the capacitors.

Other options include powering the PF controller via an auxiliary contact of the LINE breaker. When the LINE breaker is opened the PF controller will switch off, drop its contactors and switch out the capacitors.

The final, "daft" option is to short out the CT when the BACKUP breaker is closed.


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