GENERATOR SELECTION CONTENTS - Diesel Generator

SECTION 4511
January 1990
Page 1
CONSTANT OR STEADY STATE LOADS - THREE PHASE, BALANCED
A balanced three phase load is one which demands identical power and current from each of the three phases of the
a.c. generator, i.e., the load is equally shared or balanced between three phases.
The total constant or steady state load is derived by adding together all the individual constant or steady state loads.
The same unit of load rating obviously must be used throughout, usually kW. Load ratings can be given in current (A,
amperes); horsepower (h.p.); kVA; or in kW. Conversion formulae are given below:-
kW genr = kW load
= kVA x p.f.
= 0.746 x h.p.
= √ 3 x volts x current x p.f.
1,000
where kW genr = output power from a.c. generator
kW load = input power to load in kW.
kVA = √ 3 x volts x current
1,000
p.f. = power factor derived from data given in 4501
or assume 0.8p.f. lag
h.p. = input power to load in h.p.
volts = rated line to line voltage
current = rated line current.
These formulae give the output power from an a.c. generator based on the input power required by the load. This is
true of all loads except motors. Motors are usually rated for their shaft output power. Therefore an extra ‘motor
efficiency’ term is required in these cases; thus:
kW genr = 0.746 x h.p (output)
η m
= kW (output)
η m
where h.p. (output) = motor shaft output power rating in h.p.
kW (output) = motor shaft output power rating in kW
η m = motor efficiency in per unit.
Having derived the total kW loading on the a.c. generator required to supply the load, then an estimate of prime mover
output can be made, thus:-
kW (prime mover output) = kW genr
η genr
where η genr = a.c. generator efficiency in per unit.
By this calculation the minimum kW rating of an a.c. generator has been determined. All the above assumes a
standard ambient temperature of 40°C and standard altitudes of less than 1000m. If higher ambient temperatures or
altitudes are required, then derate factors must be applied to this calculated minimum kW rating. A check through kW
rating is not listed then the next higher kW output frame size of machine should be selected. This will ensure that the
a.c. generator selected will supply continuously the full rated load within the temperature rise limits of the machine
design.