appendix

APPENDIX A
Application Manual – Liquid Cooled Generator Sets
hand, since VFDs ramp the load on, the starting requirements will be reduced compared
to a motor started across the line. Select PWM if the VFD is of the pulse width modulated
type. PWM type VFDs require less oversizing than non–PWM types.
Motor starting requirements can be reduced by applying some type of reduced voltage or
solid state starter. Application of these devices can result in smaller generator set
recommendations. However, caution must be used when applying any of these starting
methods. First of all, motor torque is a function of the applied voltage and all of these
methods result in lower voltage during starting. These starting methods should only be
applied to low inertia motor loads unless it can be determined the motor will produce
adequate accelerating torque during starting. Additionally, these starting methods can
produce very high inrush currents when they transition from start to run if the transition
occurs prior to the motor reaching very near operating speed, resulting in starting
requirements approaching an across the line start. GenSize assumes the motor reaches
near rated speed before this transition, ignoring these potential inrush conditions. If the
motor does not reach near rated speed prior to transition, excessive voltage and
frequency dips can occur when applying these starters to generator sets. If unsure how
your starter and load will react, use across–the–line starting.
For across–the–line motor starting, select low inertia load if you know the load requires
low starting torque at low speeds. This will reduce the starting kW requirements for the
generator set and can result in a smaller set. Low inertia loads are typically centrifugal
fans and pumps. If unsure, use high inertia (leave low inertia unselected).
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
For loads that are designated to automatically cycle on and off:
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 RkWDC 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)
Fire Pump Load Calculations
GenSize will size the generator limiting the peak voltage dip to 15% when starting the fire
pump,with all other non–surge loads running. This is to meet North American fire code
requirements. The generator set does not have to be sized to provide the locked rotor
kVA of the fire pump motor indefinitely. That would result in an oversized generator set,
which could experience maintenance and reliability issues from being under–utilized.
Rev. Jan 2011
A–11