Learjet 20 Series Pilot Training Manual page 41

AC POWER
INVERTERS
Alternating current for the AC electrical instru-
ments and electronic equipment is provided by
two or three 1,000-VA solid-state static invert-
ers located in the tailcone. Figure 2-13 illustrates
the typical inverter system. The auxiliary inverter
is an optional item. The primary and secondary
inverters power their respective buses through
a paralleling control box. Inverter output is 115
VAC, 400-Hz, single-phase. The 26-VAC power
is provided by two stepdown transformers lo-
cated, one on each side, in the cockpit aft of the
circuit-breaker panels. These two transformers
step down 115 VAC to 26 VAC. Other compo-
nents in the system include power relays, par-
alleling box, overload sensors, circuit breakers,
and inverter failure lights. The primary and sec-
ondary fail lights are red, and the auxiliary fail
light is amber. All fail lights are located on the
glareshield.
CONTROLS
Two (or three) inverter on-off switches (Figure
2-14), one for each inverter, are installed on
the lower center instrument panel. If the op-
tional auxiliary inverter is installed, another
switch labeled "LH" or "RH AC BUS" is used
to direct the auxiliary inverter output to the se-
lected bus.
INDICATORS
Two or three inverter warning lights (Appen-
dix B) labeled "PRI INV," "SEC INV," and
"AUX INV" are installed on the glareshield.
If an inverter fails, its respective warning light
illuminates.
On SNs 24-230 through -277, 25-070 through
25-134, and 25-136 through -140, turning off
the inverter switch extinguishes the inverter
fail light.
On SNs 24-278 and subsequent, and 25-135 and
-141 and subsequent, if the inverter fails and
the switch is turned to OFF, the PRI INV and
SEC INV fail lights remain illuminated. If the
2-14
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
Figure 2-14. Inverter Controls
auxiliary inverter fails, turning off the inverter
switch extinguishes the fail light.
A single AC voltmeter with a two-position se-
lector switch is provided to monitor the volt-
age on the selected bus.
AC OPERATION
When the primary inverter switch is set to
PRI, the control circuit is completed to the in-
verter power relay which closes and connects
the inverter to the generator bus through the
overload sensor. The inverter AC output is fed
to the paralleling control box and on to the left
AC bus through the 10-ampere L AC BUS
feeder circuit breaker. Operation of the sec-
ondary inverter is identical.
An overload on either 115-VAC bus causes
the affected AC BUS feeder circuit breaker to
open, activating the BUS TIE circuit breaker,
and thus isolating the bus.
If a red INV light illuminates with the inverter
turned on, refer to the "AC Power Loss" check-
list in the "Emergency Procedures" section of
the approved AFM. An illuminated INV light
does not necessarily indicate inverter failure.
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