GENERATOR SELECTION CONTENTS - Diesel Generator

SECTION 4111
January, 1990
Page 3
Frequency The frequency of the voltage from an a.c. generator depends on the driven speed and the
number of magnetic field poles, thus:-
Frequency (Hz) = Speed (rev/min) x Poles
120
Note: HZ = Hertz = cycles per second (see a.c.)
Therefore for standard frequencies, the most common combinations of speed and poles are
given below:-
50 Hz 3000 rev/min 2 pole
60 Hz 3600 rev/min 2 pole
50 Hz 1500 rev/min 4 pole
60 Hz 1800 rev/min 4 pole
50 Hz 1000 rev/min 6 pole
60 Hz 1200 rev/min 6 pole
h.p. Horsepower, a measure of the rate of doing work, use by primer mover manufacturers, but now
being replaced by the S.I. unit, kW - see power. A ‘metric horsepower’ C.V. (cheval vapeur) or
PS (pferdestarke) is common in Europe. The relationships are:
1 h.p. = 0.746kW = 33,000 ft lb/min
1 C.V. or PS = 0.735kW
kVA This unit is always the product of voltage and current in single phase a.c. systems. In the
three phase case an extra constant is involved (√ 3 ). It is the normal way of quoting a rating for
any a.c. generator. It is independent of power factor and is only used when considering a c
systems. It is also normal to specify a kVA level at specific power factortherefore defining the
level of real power capability of the machine and its primer mover:-
i.e. kW = kVA x power factor
= real power capability
This specific power factor is conventionally considered to be 0.8 p.f. lagging, although in practical
installations power factors of nearer 0.9 p.f. lagging are usually measured. As systems get
smaller, power factors approaching 0.95 p.f. lagging are obtainable in practice. Since quite
large outputs are obtained from a.c. generators it is conventional to divide the product of voltage
and current by one thousand to obtain the unit kVA.
i.e. 1kVA = 1000VA
kW See power.
V I
In single phase systems kVA = phase x phase
1000
V I
In three phase systems kVA = √ 3 line line
1000
V I
= 3 phase phase
1000
where √3 = 1.732
Magnetic Field A force set up around a magnet. The best known magnetic field is that of the earth, established
by the North and South magnetic poles.
The existence of a strong magnetic field is a pre-requisite of an a.c. generator. The magnetic
field can be produced by using a permanent magnet material or by electrical methods. A d.c.
supply is necessary for setting up a magnetic field electrically, commonly called an excitation
supply. The magnetic field strength can be varied by varying the d.c. excitation supply. The
number of magnetic field poles must be multiple of 2, as each ‘magnet’ comprises a North pole
and a South pole. The most common poleages for a.c. generators are 2 pole, 4 pole or 6 pole.
See also Frequency.
Nominal A range of operating conditions or values within which the machine can be safely and successfully
run. See also Rating.