arbitrarily change phasing. The phase sequence gets
full output speed. Another is when the generator has
been shut down and/or slows below a safe output
changed when maintenance is performed on the unit
frequency. The generator may be connected or
and the cables are not reinstalled on the proper output
disconnected, as needed, through the use of any
terminals. Review records to determine how the
number of frequency-sensitive devices.
phasing might have been changed; if the cable was
replaced, do not attempt to correct the problem by
The simplest of frequency-sensitive devices
changing the cables at the generator (or vice versa).
employs a speed switch, which closes when the ac
Yes, it will remedy the problem for the aircraft power
generator's prime mover reaches a safe speed
rejection, but it will create further problems later when
(frequency). This speed switch simply opens or closes
tracing out wire numbers for other non-related
the circuit to the main ac power contactor relay. The
problems.
disadvantage of this speed switch is its sensitivity.
FREQUENCY CONTROL.--During certain
Where frequency protection must be limited to a
phases of ac generator operation, it is necessary to
narrower range of generator speed (frequency), the
prevent its output from being connected to the aircraft
electrical-type frequency-sensitive device is generally
electrical system. One of the most common times is
used. The operating principles of a commonly used
during a low-frequency output condition; this occurs
type of electrical frequency control device are similar
when the generator has been started but has not reached
to those of the circuits shown in figure 7-32. The
C1
R1
L
C
Ep
Ep
R
Ep
Ro
FREQUENCY
A . UNDERFREQUENCY
C1
R1
L
C
Ep
R
IRo
ERo
ERo
Ro
FREQUENCY
B.
ASf07032
OVERFREQUENCY
Figure 7-32.--Electrical frequency control circuits.
7-23
