Page 171 - Uninterruptible Power Supplies

Source: Uninterruptible Power Supplies and Standby Power Systems

Chapter
6

Rotary UPS Systems

Definitions

This chapter covers rotating sets capable of providing a continuous, well-
regulated power supply to the load irrespective of the normal mains sup-
ply. There would appear to be various methods available to create such a
system, and they are probably best discussed under two headings: rotat-
ing transformer systems and close coupled diesel/clutch/generator sets.

Background

A very early system which one could hardly call a UPS system merely
consisted of motor/heavy flywheel/generator system. At this stage engi-
neers were trying to bridge small power transients. The design was not
very successful, the system would provide an enormous amount of
power. In most designs the output voltage held up for 5 to 10 s but
within 1 s the output frequency was out of tolerance. Note that this idea
of utilizing kinetic energy has been given a new lease on life due to
modern electronic circuitry (see Chap. 8).
For many years prior to the advent of the static system the circuit
illustrated in Fig. 6.1 was in use. It consisted of a rectifier and battery
charger supporting the load and also charging a battery. The dc supply
was used to drive a motor alternator set connected to the load. The fly-
wheel was present to provide kinetic energy when the system switched
from mains supply to battery supply. At this instant the battery would
go from float voltage instantaneously to discharge voltage a drop of
some 15 to 17 percent. The regulating device dynamic performance was
too poor to cope with this, and the kinetic energy from the flywheel
ensured a reasonable output performance.

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