GENSET SIZING
Generator Set Sizing Calculations (Continued)
Step 5. Select a Generator Set
a.Establish the minimum size required
i.At this point the addition of future loads should be considered. The RkW and RkVA values that were highlighted or circled in Step 4 (previous page) should be multiplied by a factor representing your best judgement.
ii.Referring to the genset specification sheets, pick the generator set model having a kW/kVA rating that meets the highest RkW and RkVA totals highlighted or circled in Step 4. Use the values calculated for RkW and RkVA in
iii.In addition to the specification sheet, the motor starting curve should be referenced. Make sure to take into account any derating factors such as high altitudes or ambient temperature.
b.In applications where it is necessary to limit transient voltage dip to approximately 10 to 20 percent of nominal voltage, multiply the SkVA highlighted or circled in Step 4 by at least 1.25. Repeat the selection steps above.
A transient voltage dip of approximately 20 to 40% can be expected when the genset selected is only slightly greater than the maximum SkVA. The actual transient voltage dip is a function of several factors and is difficult to determine accurately.
c.In applications where GkW has been determined (Step
4)and where GkW is greater than the kW rating of the generator set that has been selected, an alternator (AC generator) must be picked for the set which has a kW rating equal to or greater than GkW.
i.See the alternator data sheet for the alternator temperature rise. Compare GkW to the alternator kW rating at the appropriate voltage. The greater the voltage, the greater the kW rating.
ii.If GkW is too high for the alternator selected to meet the temperature rise specifications (if any), find the alternator data sheet for the alternator specified for the next lower temperature rise. Compare GkW to the alternator kW rating at the appropriate voltage. Repeat the procedure with any other models. If there are no generator temperature rise specifications that have to be met, consider comparing GkW to the kW rating at the higher temperature rise rating of 125°C.
iii.If none of the alternators available for the generator set has a kW rating sufficient to meet GkW, refer to the specification sheet for the next larger size generator set and repeat the selection process.
The running load should not be
NOTE less then 30 percent of the generator set rating.
INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07) — PAGE 21
