appendix

Application Manual – Liquid Cooled Generator Sets
Whenever a reduced voltage starter is used for a fire pump motor, the user should
consider sizing for across–the–line starting because the fire pump controller includes
either a manual–mechanical, manual–electrical, or automatic means to start the pump
across–the–line in the case of a controller malfunction. GenSize will not disallow use of
reduced voltage starters for fire pumps, however.
RkW If HP entered: RkW = HP x 0.746 ÷ Running Efficiency
If kW entered: RkW = kW ÷ Running Efficiency
If Ramps entered: 1∅ RkW = (Ramps x voltage x RPF x Efficiency) ÷ 1000
3∅ RkW = (Ramps x voltage x RPF x Efficiency x 1.73)
÷ 1000
RkVA RkVA = RkW ÷ RPF
RPF Running power factor as entered or default from database
SkW High Inertia SkW = SkVA x SPF
Low Inertia SkW = SkVA x SPF x 0.6
SkVA SkVA = HP x (LRkVA/HP) x SkVA factor, where LrkVA/HP is the average kVA/HP
for the NEMA Code letter of the motor, and SkVA factor is 1.0 for full voltage
starting, or from reduced voltage starting table (see Reduced Voltage Starting
Method)
SPF As entered, or default values from database by HP and starting method
PkW PkW = SkW
PkVA PkVA = SkVA
AkW (non–VFD) AkW = RkW except solid–state starter where AkW = 2.0 x RkW
unless a bypass contactor is used, then AkW = RkW
AkW (VFD) Conventional AC Inverter: AkW = 2.0 x RkW
Pulse Width Modulated: AkW = 1.4 x RkW
DC Drive: AkW = 2.0 x RkW
Ramps 1∅ Ramps = (HP x 746) ÷ (voltage x Efficiency x RPF)
3∅ Ramps = (HP x 746) ÷ (1.73 x voltage x Efficiency x RPF)
UPS Load Calculations
A static UPS uses silicon controlled rectifiers (SCR) or another static device to convert
AC voltage to DC for charging batteries, and an inverter to convert DC to conditioned AC
power to supply the load. A UPS is a non–linear load and may require an oversized
alternator. Some incompatibility problems between generator sets and static UPSs have
led to many misconceptions about sizing the generator set for this type of load. Past
problems did occur, and the recommendation from UPS suppliers at that time was to
oversize the generator set from two to five times the UPS rating. Even then some
problems persisted, and since then those incompatibility problems have been addressed
by most UPS manufacturers. It is more cost effective to require generator compatibility of
the UPS supplier than to oversize the generator.
If the batteries are discharged when the UPS is operating on the generator set, the
generator set must be capable of supplying the rectifier for battery charging and the
inverter to supply the load. A second reason to use the full UPS rating is that additional
UPS load may be added in the future up to the nameplate rating. The non–linear load
sizing factors used by GenSize are based on the level of harmonics the UPS induces in
the generator output with the UPS fully loaded. Since the harmonics increase at lighter
loads, selecting the larger capacity alternator helps to offset this effect.
For multiple redundant UPS systems, size the generator set for the combined nameplate
ratings of the individual UPSs. Redundant system applications are those where one UPS
is installed to back up another and the two are on–line at all times with 50% or less load.
UPS equipment often has varying power quality requirements depending on the operating
mode. When the rectifier is ramping up, often relatively broad frequency and voltage
swings can occur without disrupting equipment operation. However, when the bypass is
enabled, both frequency and voltage must be very constant, or an alarm condition will
A–12 APPENDIX A
Rev. Jan 2011