Page 247 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
Some System Failures: The Light of Experience! 245

in such cases the delay should be long enough to allow deceleration to,
say, half speed.

Restoration of Supply to an Inertially Loaded
Drive
This event concerns a large (probably 200-kW) squirrel cage induction
motor coupled to a centrifugal fan of welded sheet steel construction.
The large inertia of the fan resulted in a long run-on time after a sup-
ply failure, and it was found that restoration of supply, even after a long
interval, resulted in overload tripping.
This incident illustrates a different principle from that illustrated by
the previous example. An induction motor may be considered as a
transformer, the stator winding being the primary and the rotor wind-
ing the secondary. If a supply is restored to such a machine whilst it is
running at, say, synchronous speed, there is a sudden increase of sta-
tor rotating flux which is stationary with respect to the rotor. The rotor
winding will be seen as a short circuit and the stator flux is diverted
into the leakage flux paths, resulting in a large stator current which
causes the overload trip. If tripping does not occur, the flux transfers to
the rotor iron, and the stator current reduces, at a rate determined by
the rotor time constant.
The above description is a simplification of the effect, the motor is
likely to be running at a subsynchronous speed which will result in a
low-frequency current in the rotor, but unless the reader is already
familiar with the phenomenon it is easier to consider the behavior
when synchronous speed applies.
Large motors with large inertial loads need special attention if they are
likely to be energized at speeds above, say, one-half of the running speed.

Low Transformer Oil Level Due to Low
Ambient Temperature
This failure occurred at a conventional single-set installation located in
an exposed location well above sea level. The generator was connected
to a generator transformer which was located outdoors without
weather protection.
During a particularly cold spell of weather, the normal supply failed
and the standby set started, but it immediately shut down due to oper-
ation of the Buchholz relay which indicated low oil level.
The Buchholz relay had operated correctly, at normal ambient tem-
perature the oil level would not have been so low, but the reduction of
oil volume due to the low temperature had caused the oil level to fall
below the Buchholz float chamber. Most outdoor transformers are

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