Page 109 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 107

1. To avoid resonance each of the antivibration mounts and its share of
the supported mass must have a resonant frequency well below, say
one third of, the lowest forcing frequency which the engine produces.
2. To avoid rocking or pitching it is essential that the static deflection
of each mount on the generating set is the same. If the weight
distribution is unequal this will require the selection of different
mountings.
3. The antivibration mounts must be properly designed and capable of
supporting the applied load.

Provided that the weight resting on each mount and the correspond-
ing deflections are known, compliance with the first rule is fairly sim-
ple. The lowest forcing frequency should be ascertained from the
engine or set manufacturer, it will be associated with the speed and
number of cylinders. The natural frequency of vibration of the mass on
its mounting is dependent on the static deflection for a steel spring or
upon the dynamic deflection for rubber mountings in accordance with
the following formula:

15.8
f o Hz (3.4)

where f o the natural frequency
the deflection in mm

If the lowest forcing frequency is advised as 25 Hz, the natural fre-
quency of the mountings with their supported mass should not exceed
8 Hz and the deflection (static for springs, dynamic for rubber) should
not be less than 4 mm.
Antivibration mounts allow the set a freedom of movement and it is
important that all services to the set pass through flexible sections. The
services include electrical cables, fuel, oil and water pipes, ductwork,
and exhaust pipes.
The foregoing treatment covers basic concepts only and does not, for
example, consider transmissibility of forces to the floor, or the effect of
damping. Steel springs will be lightly damped but rubber mounts
more so. In general, specially damped mounts are not necessary for
generating sets.

Diesel Engine–Driven Generating Sets on
Suspended Floors
If the set is mounted on a suspended floor within a building, the selec-
tion of the mountings has to take into account the resilience of the floor
which will itself tend to act as one large antivibration mounting.

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