GENSET SIZING — DETERMINING LOAD CHARACTERISTICS
Synchronous Motors
Although starting requirements for synchronous motors are lower, it is recommended to determine starting requirements in the same manner as induction motors previously covered.
Variable Frequency Drives
Variable Frequency Drives are nonlinear loads for which a calculation of GkW is made, in addition to RkW, RkVA, SkW, and SkVA.
Static UPS
Uninterrupted power supplies are nonlinear loads for which a calculation of GkW will be made, in addition to RkW, RkVA, SkW, and SkVA.
Calculate RkW as follows:
Calculate RkW as follows:
Assume 0.9 for EFF (drive running efficiency) unless otherwise known.
Calculate RkVA as follows:
Assume 0.9 for RPF (running power factor) unless otherwise known.
Since these drives are all current limiting:
Calculate GkW using the following formula, assuming a generator sizing factor of 2 unless otherwise known.
When sizing for a pulse width modulated (PWM) drive, consult the drive manufacturer to verify that the drive limits harmonic current is less than 10 percent THD on a high impedance source (e.g. a generator set), assume a sizing factor of 1.4.
Using these factors for GkW results in selecting a generator reactance low enough to limit voltage distortion caused by nonlinear loads to approximately 10 to 15%.
In the equation above:
1.Output kVA is the nameplate kVA capacity of the
UPS
2.Battery charging kVA is that required for battery charging, and can range from zero to fifty
3.If the RPF (Running Power Factor) for the UPS is unknown, assume 0.9 RPF.
4.If the EFF (Running Efficiency) for the UPS is unknown, assume 0.85 EFF.
Unless otherwise known:
Calculate GkW as follows:
Telecom DC Rectifiers and Battery Charging Equipment Telecom DC Rectifiers and battery charging equipment are nonlinear loads and similar to static UPS and should be sized using the same method.
PAGE 26 — INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07)
