In short. Size from demand, not connected load, then test the transient. A generator must carry steady load within derated capacity, start the largest motor, absorb the largest load step with acceptable voltage and frequency dips, and cope with the nature of the load, such as UPS and drives.
Sizing sequence
- Demand. Maximum demand of the load to be supplied. See maximum demand.
- Power factor. kVA = kW ÷ PF; a typical generator rating assumes 0.8 power factor.
- Derating. Altitude, ambient temperature and fuel or ventilation constraints. See altitude derating.
- Motor starting. Largest motor start and sequence. See large-motor starting.
- Load steps. Largest sudden kW and kVAr change.
- Nonlinear loads. UPS, VFD and rectifiers add harmonics and may need larger alternators. See UPS and nonlinear loads.
- Margin and growth. Allow for future load and ageing.
- Redundancy. N+1 or other architecture. See multi-DG capacity.
Transient performance
When load is applied suddenly, the generator responds in stages. Voltage dips immediately by an amount set by the alternator’s reactance, and the AVR restores it over a fraction of a second. Speed dips because the engine must build up torque, and the governor restores it over seconds.
| Quantity | Depends mainly on |
|---|---|
| Instantaneous voltage dip | Alternator transient reactance and the step size in kVA |
| Voltage recovery time | AVR, excitation system and field time constants |
| Frequency dip | Step in kW relative to engine capability, inertia, turbocharger lag |
| Frequency recovery time | Governor type and tuning, fuel system response |
Engine capability to accept a step is typically given as a percentage of rated load in a single step. Check the manufacturer’s block-load data, because turbocharged engines accept smaller first steps than naturally aspirated ones.
Acceptance criteria
- Voltage dip and recovery within the limits of the most sensitive load.
- Frequency dip within limits for UPS inputs, drives and clocks.
- No protection operation during the step.
- Temperature and fuel limits respected in sustained operation.
Illustrative example
A site has 480 kW maximum demand at 0.85 power factor, about 565 kVA. Adding 15 percent growth gives about 650 kVA. The largest motor start and a 200 kW load step push the study toward a 750 kVA set rather than 650, because the voltage dip at the step exceeds the UPS input limit with the smaller alternator. The study shows the larger set passes with margin. See generator sizing.
Steady-state kVA is only the first question
A generator sized for the steady load may still fail on the first load step. A proper study answers four questions together.
| Question | Governing parameter |
|---|---|
| Can it carry the continuous load? | Engine kW and alternator kVA at site ambient |
| Can it start the largest motor? | Alternator X’d, AVR |
| Can it accept the largest block load? | Engine step-load capability, governor, turbocharger lag |
| Does it tolerate the nonlinear load? | Alternator X”d, derating for harmonics |
Derating factors for the site
- Ambient temperature, with a high summer ambient in Hyderabad.
- Altitude, small at about 540 m but still stated.
- Air filter restriction and cooling air recirculation in enclosed rooms.
See ambient temperature derating and altitude derating.
Transient performance outputs
- Voltage dip and recovery time.
- Frequency dip and recovery time.
- Peak current during the transient.
Typical acceptance targets (project specific)
Performance classes in ISO 8528-5 describe allowable voltage and frequency deviations for load steps. Select the class that matches the load sensitivity, and confirm the standard’s current edition.
See transient response and generator sizing.
Common mistakes
- Adding nameplates to get a size.
- Skipping the transient check.
- Ignoring derating.
What to send us
Send the load list and operating scenario. Send your application details. See also transient response.
Questions engineers ask
Why is the transient check needed if steady load fits?
A set can be thermally adequate but dynamically weak, causing dips that trip loads.
What loads are hardest on generators?
Large motors, UPS rectifiers, drives and sudden steps.
How much margin should I include?
Based on growth plans and measurement quality. State it clearly in the report.
Does a bigger alternator help?
Yes for voltage dips. The engine may also need to be larger to accept the step.
Values are illustrative. Sizing needs manufacturer data and project loads.