GENERATOR SELECTION CONTENTS - Diesel Generator

SECTION 4111
January 1990
Page 5
In this case the load applied causes the current waveform to lag behind the voltage waveform by
a constant amount, i.e., the voltage peaks and zeros occur at constant time before the current
peaks and zeros.
This is the normal situation in a.c. work where a phase or time difference exists between the
voltage and current waveforms. The instantaneous real power in watts is always the voltagecurrent
product for that instant in time. Since voltage and current are no longer in phase, the
summation of all of the instantaneous powers over one cycle will be less than the summation for
the in-phase case.
Hence the power factor figure will reduce from 1.0 to a figure proportional to the amount of
phase or time displacement. Consequently, as the phase difference between the voltage and
current waveforms increases, the power factor figure will reduce, as will the real power output
figure.
Since quite large powers are obtained the unit kW is preferred to the watt (W) where 1000W =
1kW.
Power Factor (p.f) The nature of the load in an a.c. circuit will determine if the current drawn is in phase or out of
phase with the generated voltage. The load again determines if that current waveform ‘leads’
the voltage waveform or ‘lags’ the voltage waveform. For normal industrial loads (e.g. motors)
the current will lag the voltage by some time interval or phase angle. See also kVA and Power.
The optimum situation is where current and voltage are in phase. This makes the power factor
unity (1.0) and hence the real power (kW) the same as the product of voltage and current (kVA).
Conventionally, a.c. generator work considers a lagging power factor of 0.8. In this case the
current will lag the voltage by an amount which causes the real power level supplied (kw) to fall
below the kVA level by a factor of 0.8 times.
It is possible for a load to demand a current which is almost totally out of phase with the generated
voltage. Also that current may be lagging the voltage (inductive or motor loads) or leading the
voltage (capacitive loads). This, therefore, completes the range of power factors for a.c.
generators from zero p.f. lagging through conventional 0.8 p.f lagging to unity (1.0) p.f. to zero
p.f. leading. One aspect of this is that even though only a small real power (kW) is demanded by
the load which is well within the machines capability, damage can easily result if the load is a
very low power factor load demanding a very high kVA level.
The diagram shows difference power factor-kVA loads all supplying the same real power (kW)
level. For normal constant voltage operation the length of the kVA line is directly proportional to
load current. The high current overloads at lower power factors can be easily seen from this
diagram.