Page 108 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
106 Chapter Three

Vibration
A detailed description of the effects of vibration is beyond the scope of
this book, advice should be sought from the engine maker or other spe-
cialist. However, this section may be helpful and is intended only as an
introduction to vibration control. It is concerned with the vibration of
the set in the vertical direction, but the set has other degrees of move-
ment such as, to use a nautical analogy, rolling, pitching and yawing.
For example, during an electrical fault or at load application, inertial
energy is extracted from the rotating parts and the resulting forces
cause a rolling motion of the set. Usually, only the engine maker has suf-
ficient information to take into account these more complex matters.

Diesel Engine–Driven Generating Sets on
Solid Foundations
A diesel engine has reciprocating parts which introduce out of balance
forces leading to vibration. In addition, there will be minor out of balance
forces due to dynamic unbalance of the generator or engine rotating
parts. The vibration cannot be prevented and has to be accommodated by
so-called antivibration mountings which allow the whole generating set
to move, under controlled conditions and in a vertical direction, relative
to the floor which supports it.
Within a building vibration must be kept within tolerable limits, and
if the building is occupied the need for vibration control is greater. For
sets having the generator solidly spigot-mounted on the engine body,
antivibration mountings are usually installed between the engine/gen-
erator mass and an underbase which rests on a solid floor. For sets
where the engine and generator are connected through a flexible cou-
pling, the two masses are usually solidly mounted on a rigid base frame
which rests on antivibration mountings resting on the specially pre-
pared floor pads. This arrangement is used only for larger sets and an
additional underbase is not normally provided.
The mountings may use coil springs in compression or rubber in
shear. Coil springs have a single linear characteristic but rubber is a
material which increases in stiffness when subject to vibration, its
dynamic and static characteristics therefore differ. When selecting
rubber mounts it is important to use the dynamic characteristics. Two
pairs of mountings, a pair on each side of the set, is the minimum num-
ber and if the disposition is symmetrical about the center of gravity the
loading on each will be equal. If more than four mountings are used,
attention must be given to the weight distribution.
From the vibration viewpoint, the best location for the generating set
is on a rigid solid floor at basement or ground level, the choice of mount-
ings then requires three basic rules to be followed:

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