Page 12 - Uninterruptible Power Supplies

Standby Power Generating Sets
10 Chapter One

■ A single-bearing generator with the frame spigot mounted directly to
the engine crankcase and the driven end of the generator shaft sup-
ported by the engine crankshaft via a coupling
■ For larger sizes, a two-bearing generator may be used. The engine and
generator are solidly mounted on a base frame and the generator is
driven through a flexible coupling.
Diesel engine–driven generators will run at engine speed (1000 or
1500 rpm for 50 Hz) and will have salient pole rotors. Gas turbine–
driven generators will usually run at 3000 rpm (for 50 Hz) and will
probably have cylindrical rotors.
The distribution voltages in common use within the United Kingdom
are 400 and 11,000 volts and generators will usually use one of these
voltages. It is not generally economical to manufacture a high voltage
machine for ratings below about 1 MVA, so below this size generation
may be expected to be at low voltage. The economics depend on the mate-
rial content of the stator windings, a low voltage machine with a high
rating would include an excessive amount of copper and a small amount
of insulation, whereas for a high voltage machine with a low rating the
reverse situation would apply.
However, a rating of 1 MVA at 400 volts results in a line current of
1443 amperes, and cables for such a current are quite large and may be
unmanageable. If cabling is likely to be troublesome due to heavy cur-
rents, consideration can be given to generating at low voltage and adding
a generator transformer. Generator transformers are discussed later in
this section.

Excitation Systems
Modern generators use brushless excitation systems, but there remain
in use many machines provided with dc exciters having commutators
and brushgear. The advent of semiconductor rectifiers made it possible
to replace the dc exciter with a much simpler ac exciter and a rectifier
mounted on the generator shaft. This arrangement dispenses with the
brushgear and its attendant maintenance problems and is achieved at
lower cost.
The ac exciters now fitted require a power supply to energize the stator
field and there are two methods in use:

■ A permanent magnet pilot exciter provides the field supply for the
main exciter as shown in Fig. 1.4.
■ The exciter field takes a supply from the generator output as shown in
Fig. 1.5. Note that for high voltage machines this involves the addition
of a step down transformer; it is therefore not recommended for high
voltage machines.

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