Page 225 - Uninterruptible Power Supplies

Source: Uninterruptible Power Supplies and Standby Power Systems

Chapter
8

Kinetic Energy as an Alternative

Power Source

Introduction
Kinetic energy from flywheels has been in use for a variety of stabiliz-
ers and UPS units in the past.
Stabilizers with a short ride-through period consisting of a motor fly-
wheel and generator may provide an isolated power source with a short
ride through, the ride through being in the order of 1 s. The system con-
sists of three shafts—motor, flywheel, and generator. It is not practica-
ble economically to increase the ride-through time since the loss of the
incoming supply and dependence on the flywheel inertia will result in
a slowing down of the rotational mass and, thus, in the output fre-
quency being out of tolerance.

Circuit Developments
A further use of the flywheel occurs with the original design of rotary
UPS systems as illustrated in Fig. 8.1. Under normal operation the
mains supply drives the dc motor via the rectifier. On loss of mains the
system derives power from the battery. However, at the point of loss of
mains the dc machine sees an immediate drop in dc voltage corre-
sponding to approximately 15 percent. (Note this voltage drop will vary
slightly depending on the type of battery employed and the autonomy
period.)
The dc machine regulator is comparatively slow in response. Thus,
the flywheel enables the system to tolerate the dynamic change, result-
ing in the ac machine output remaining in tolerance.

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