are connected to both current and potential
FREQUENCY METERS
transformers to allow them to measure line
Frequency meters (fig. 5-26) measure the cycles
current and voltage. The kW meter shown in
per second rate of ac. The range of frequency
figure 5-27 is similar to the ones used on gas
meters found on gas turbine ships is between 55
turbine ships. Since each type of generator on gas
hertz (Hz) and 65 hertz. Frequency of the ac
turbine ships is rated differently, the scale will be
used on ships rarely varies below 57 Hz and
different on each class of ship.
seldom exceeds 62 Hz. A frequency meter may
The amount of power produced by a generator
have a transducer that converts the input
is measured in kilowatts. Therefore, when
frequency to an equivalent dc output. The
balancing the electrical load on two or more
transducer is a static device employing two
generators, you should ensure kW is matched.
separately tuned series resonant circuits that feed
Loss of the kW load is the first indication of a
a full-wave bridge rectifier. A change in frequency
failing generator. For example, two generators are
causes a change in the balance of the bridge. This
in parallel and one of the two units experiences
causes a change in the dc output voltage.
a failure. To determine which of the two units is
failing, compare the kW reading. Normally, the
KILOWATT METERS
generator with the lowest kW would be the
failing unit. However, you should know there is
Wattage is measured by computing values of
one case where this is not true. During an
current, voltage, and power factor. The kilowatt
overspeed condition, both units will increase in
meters used on ships automatically take these
frequency. But the failing unit will be the one with
values into account when measuring kilowatt
the higher kW load.
(kW) produced by a generator. Kilowatt meters
SYNCHROSCOPES
Before connecting a 3-phase generator to
bus bars already connected to one or more
other generators, you must ensure that certain
conditions prevail. A synchroscope is the device
you will use to find out if the following
conditions have been met:
1. Phase sequence must be the same for
generator and bus bars.
2. Generator and bus-bar voltage must be the
same.
3. Generator and bus-bar frequency must be
the same.
4. Generator frequency must be practically
Figure 5-26.--Frequency meter.
constant for an appreciable period of time.
5. The generator and bus-bar voltage must be
in phase. They must reach their maximum
voltages at the same time. This is so that when
connected, they will oppose excessive circulation
of current between the two machines.
Figure 5-28 shows a synchroscope. It is
basically a power factor meter connected to
measure the phase relation between the generator
and bus-bar voltages. The moving element is free
to rotate continuously. When the two frequencies
are exactly the same, the moving element holds
a fixed position. This shows the constant phase
relation between the generator and bus-bar
voltages. When the frequency is slightly different,
the phase relation is always changing. In this case,
the moving element of the synchroscope rotates
Figure 5-27.--Kilowatt meter.
5-20
