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PAGE 26 — INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07)
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.
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%.
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:
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 (0-50%)
percent of the UPS kVA rating.
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.