Standby generator pulse excitation system and electric power generating system for use therein

A back-up electric power generating system comprises a first variable speed synchronous generator drivably coupled to a first prime mover, a second variable speed synchronous generator drivably coupled to a second prime mover, a converter having a first and a second input coupled to the first and second generators respectively, and an output, and a logic circuit in controlling communication with the converter for designating one of the first or second generators as a primary power source and one as a standby power source, exclusively. The system further comprises a primary voltage regulator selectably coupled to the primary power source generating a controlled current signal to maintain its output voltage at a given level, and a pulse exciter selectably coupled to the standby power source for verifying the operational readiness of the standby power source. This pulse exciter generates a current pulse signal to the exciter field of said standby power source and monitors the output voltage generated thereby. The converter produces a constant frequency voltage output. The method of verifying the operational readiness of a generator drivably coupled to a prime mover, the generator having an exciter field, a rotor, and a poly-phase wound stator forming an output, comprises the steps of a) providing a current pulse to the exciter field, b) monitoring the generator output, and c) indicating the operational status as ready upon detecting a voltage pulse on the generator output in response to the current pulse, or as not ready upon detecting the lack of a voltage pulse on the generator output in response to the current pulse.

FIELD OF THE INVENTION 
The instant invention relates generally to a back-up variable speed 
constant frequency (VSCF) electric power generation system, and more 
particularly to a pulse excitation system for a standby generator 
providing assurance of standby generator availability without a separate 
voltage regulator. 
BACKGROUND ART 
The airline industry continues to undergo an evolution leading to longer 
range, lower cost, greater capacity aircraft as the demand for air travel 
continues to rise. With the evolution of higher efficiency, higher thrust 
engines, it is possible for the aircraft manufacturers to produce twin 
engine planes with ranges and payloads far exceeding expectations. As the 
range of these new advanced aircraft increases opening up routes never 
before available, the greater the likelihood that the aircraft will be a 
long way from a suitable airport in case of emergency. To minimize the 
risk to these aircraft and their passengers and cargo, the Federal 
Aviation Administration (FAA) and other world regulatory authorities have 
imposed strict safety requirements for the certification and operation of 
twin engine planes. 
In the United States of America, the FAA has imposed a stringent set of 
requirements which must be met before certification of a twin engine 
aircraft is granted for extended twin engine operation (ETOPS). These 
increased requirements affect nearly every system on the aircraft and are 
designed to ensure that the plane can continue to fly safely to a suitable 
airport in case of an emergency, such as the loss of one of the two 
engines. One of the systems affected by these increased ETOPS requirements 
is the electric power generating system (EPGS). 
A typical EPGS has one generator mounted on each main engine to produce the 
required electric power for the aircraft. The electrical output from each 
of these generators, assuming more than one engine, is coupled through a 
plurality of relays or contactors to the various loads and systems which 
require electric power. Some of the larger electric power generating 
systems operate in parallel to allow the total system load to be shared 
equally by all of the generators, and to allow greater fault clearing 
capability. Other systems operate to maintain complete isolation between 
the generators to ensure that no single fault can cause the loss of all 
electric power. Regardless of its normal operating mode, parallel or 
isolated, all systems are required by the FAA to achieve or maintain at 
least two channel electrical isolation during certain flight phases, such 
as landing for example. With a two engine aircraft, the available sources 
of primary electric power only number two to start with, and the loss of 
an engine will not allow two isolated channels. In order to be certified 
to the higher ETOPS requirements, a third source of electric power capable 
of operating with either engine inoperative was needed. 
The solution to this problem, as illustrated in FIG. 1, was to include a 
small variable speed back-up generator 100, 102 on each engine in addition 
to the main integrated drive generators (IDGs) 104, 106 manufactured by 
the assignee of the instant invention. The output from each of these 
generators is coupled to a single back-up converter 108 which is available 
to supply certain loads during required periods of operation. Since the 
converter only requires power to be supplied from one of the two back-up 
generators (the primary generator), the other back-up generator (the 
standby generator) could be left de-energized to conserve power. However, 
since the operation of this standby generator must be verifiable and 
verified prior to and during each flight, a separate voltage regulator is 
required for this standby generator. This second voltage regulator is 
required to maintain output voltage regulation at a level below that of 
the primary generator to ensure that only the primary generator supplies 
the system loads. This requirement of a second voltage regulator increases 
the weight, cost, and complexity, and decreases the reliability of the 
system. 
The instant invention is directed at overcoming this problem by providing a 
system to verify the operational readiness of the standby generator prior 
to and during flight to ensure maximum system safety while reducing system 
weight, cost, and complexity, and increasing system reliability. 
SUMMARY OF THE INVENTION 
It is the principle objective of the instant invention to provide a new and 
improved back-up electric power generating system. More specifically, it 
is the principle objective of the instant invention to provide a 
verifiable third source of electric power for a two engine aircraft and a 
method of verification of the operational readiness of that third source 
which will allow extended twin engine operation certification. 
In a preferred embodiment of the instant invention a back-up electric power 
generating system comprises a first variable speed synchronous generator 
drivably coupled to a first prime mover, the first generator having an 
exciter field, a rotor, and a poly-phase wound stator forming a first 
output, the first generator producing a voltage signal on its output in 
response to a current signal applied to its exciter field. The system 
further comprises a second variable speed synchronous generator drivably 
coupled to a second prime mover, the second generator also having an 
exciter field, a rotor, and a poly-phase wound stator forming a second 
output, and producing a voltage signal on its output in response to a 
current signal applied to its exciter field. The system also comprises a 
converter having a first and a second input coupled to the first and 
second generators respectively, and an output. A logic circuit in 
controlling communication with this converter designates one of the first 
or second generators as a primary power source and the other as a standby 
power source. This embodiment includes a circuit selectably coupled to the 
exciter field of the primary power source, i.e. to the exciter field of 
the generator which has been designated as the primary power source, and 
in controlled communication with the logic circuit for regulating the 
output voltage of the primary power source. This regulating circuit, or 
voltage regulator, generates a controlled current signal to maintain the 
output voltage at a given level. Additionally, this preferred embodiment 
comprises a circuit selectably coupled to the exciter field of the standby 
power source, i.e. to the exciter field of the generator which has been 
designated as the standby power source, and in controlled communication 
with the logic circuit for verifying the operational readiness of the 
standby power source. The verifying circuit, or pulse exciter, generates a 
current pulse signal to the exciter field of the standby power source and 
monitors the output voltage generated thereby. The converter produces a 
constant frequency voltage on its output in response to the voltage signal 
from the primary power source. 
In an alternate embodiment of the instant invention, a method of verifying 
the operational readiness of a generator drivably coupled to a prime 
mover, the generator having an exciter field, a rotor, and a poly-phase 
wound stator forming an output, comprises the steps of a) providing a 
current pulse to the exciter field of the generator, b) monitoring the 
generator output, and c) indicating the operational status as "ready" upon 
detecting a voltage pulse on the generator output in response to the 
current pulse, or as "not ready" upon detecting the lack of a voltage 
pulse on the generator output in response to the current pulse. These 
steps may be repeated at regular intervals to continually verify the 
operational status of the generator. Additionally, the magnitude of the 
current pulse may be regulated to limit the voltage pulse on the generator 
output to a percentage of the generator's per unit voltage output. 
In a further alternate embodiment of the instant invention, a method of 
providing a verifiable third source of electric power on a two engine 
aircraft, the third source having a first variable speed generator 
drivably coupled to a first prime mover, the first generator having an 
exciter field, a rotor, and a poly-phase wound stator forming a first 
output, a second variable speed synchronous generator drivably coupled to 
a second prime mover, the second generator having an exciter field, a 
rotor, and a poly-phase wound stator forming a second output, and a 
converter having a first and a second input coupled to the first and the 
second generators respectively, and an output, comprises the steps of a) 
designating one of the first or the second generators as a primary power 
source and the other as a standby power source, b) generating a controlled 
current signal to the exciter field of the generator designated as the 
primary power source to maintain its output voltage at a given level, c) 
generating a current pulse signal to the exciter field of the generator 
designated as the standby power source, c) monitoring the output of the 
standby power source, d) indicating the operational status of the standby 
power source as "ready" upon detecting a voltage pulse on the output of 
the standby power source in response to the current pulse, or as "not 
ready" upon detecting the lack of a voltage pulse on the output of the 
standby power source in response to the current pulse, and e) converting 
the output voltage of the primary power source into a constant frequency 
voltage.

DETAILED OF THE PREFERRED EMBODIMENTS 
As illustrated in FIG. 2, a preferred embodiment of the instant invention 
comprises a first variable speed synchronous generator 100 drivably 
coupled by shaft 110 to a first prime mover, such as an aircraft engine 
(not shown). The generator 100 comprises a rotor 112 which is driven by 
shaft 110 and excited by an exciter field 114 to produce the main 
generator excitation to produce a poly-phase AC output on the poly-phase 
wound stator 116. Preferably, the generator 100 also comprises a permanent 
magnet generator 158 (PMG) drivably coupled by the shaft 110. The 
poly-phase output of the generator 100, illustrated as three phases 
although the number may vary, is coupled by generator feeders 118 to a 
first input to converter 120. The second input to converter 120 is coupled 
by a second set of generator feeders 122 to a second variable speed 
synchronous generator 102. This generator 102 is identical in construction 
as the first generator 100, and therefore it will not be described in 
detail herein. Each of these two inputs is coupled to a means for 
rectifying the poly-phase AC output, such as diode bridge rectifier 
circuits 124, 126. The rectified DC voltage signal outputs from each of 
the rectifiers 124, 126 are coupled to a DC link 128. This DC link 128 is 
coupled to a switching means 130 as is known in the art for converting the 
DC voltage signal to a constant frequency, poly-phase AC voltage. This 
constant frequency AC output may by coupled through an output filter 132 
to the main line feeders 134 which supply electric power to the various 
aircraft electrical loads (not shown). 
The preferred embodiment of the instant invention further comprises logic 
means 136 which controls the designation of either the first 100 or the 
second 102 generator as the primary power source, and the other as the 
standby power source. Means, illustrated as primary voltage regulator 152, 
are provided to regulate the output voltage of the primary power source by 
generating a controlled current signal to the exciter field of the 
generator (100 or 102) which has been designated as the primary source of 
power. Additionally, means illustrated as pulse exciter 150 are provided 
to verify the operational readiness of the standby power source by 
generating a current pulse signal to the exciter field of the generator 
(100 or 102) which has been designated as the standby source of power. 
This pulse exciter 150 monitors the output voltage of this standby power 
source, via lines 154 or 156. The appropriate couplings between the 
primary voltage regulator 152 and the exciter field power out lines (142 
or 144) and the permanent magnet generator power in lines (146 or 148) of 
the primary source of power, and between the pulse exciter 150 and the 
exciter field power out lines (144 or 142) and the permanent magnet 
generator power in lines (148 or 146) of the standby power source are made 
by the switchable coupling networks 138 and 140 once the control 
designation is communicated to them from the logic means 136. 
In a highly preferred embodiment of the instant invention, the logic means 
136, which may be implemented in a microprocessor, programmable logic 
array, or other appropriate logic circuitry, senses various system 
parameters to make its determination of the designation of the primary and 
standby power sources. Parameters such as engine shutdown status, 
operating status of the main IDGs 104, 105 (see FIG. 1 ), flight crew 
selections, etc., as well as various control laws which are programmed in 
the logic means 136 prior to operation are all considered in making the 
initial determination. This designation, preferably, may be changed after 
the initial designation based on changing system parameters. Once a 
designation is changed, the switchable coupling networks 138 and 140 
reconfigure to allow the primary voltage regulator 152 to control the 
output voltage of the newly designated primary power source. The pulse 
exciter 150 then begins to monitor the operational status of the newly 
designated standby power source. This reconfiguration also reconfigures 
the PMG power in so that the primary voltage regulator is using power from 
the PMG of the primary power source, and the pulse exciter 150 is using 
power from the PMG of the standby power source. 
The operation of the preferred embodiment of the instant invention is 
illustrated by the waveform diagram of FIG. 3. Waveform 160 illustrates 
the controlled current signal produced by the primary voltage regulator 
152 and coupled to the exciter field of the primary power source. In 
response to this controlled current signal, the primary power source 
generates a three phase AC output represented by waveforms 162. The 
magnitude of this waveform is controlled by the primary voltage regulator 
by increasing or decreasing the current pulse width, or changing the duty 
cycle of the waveform 160. As will be understood by one skilled in the 
art, other methods of output voltage magnitude control may be appropriate 
(such as having a constant exciter field drive and using the switching 
network 130 to control magnitude, etc.), and are within the scope of the 
instant invention. The pulse exciter output waveform is illustrated by 
waveform 164. As may be seen, the current pulse signal is controlled to a 
magnitude such that the output voltage generated by the standby generator, 
as illustrated by waveform 166, is less than the magnitude of waveform 
162. In this way, no power from the standby power source contributes to 
the power delivered by the DC link 128 to the switching network 130. This 
current pulse 164 may be preferably a periodic signal to continually 
verify the operational readiness of the standby power source, or may be 
generated only upon initiation of a system status check. 
A preferred method of verifying the operational readiness of a generator, 
therefore, comprises the steps of a) providing a current pulse 166 to the 
exciter field 114, b) monitoring the generator output via lines 154 or 
156, and indicating the operational status as "ready" upon detecting a 
voltage pulse 166 on the generator output, or as "not ready" upon 
detecting the lack of a voltage pulse on the generator output in response 
to the current pulse. Preferably, these steps would be repeated at regular 
intervals to continually verify the operational status of the generator. 
In a highly preferred method, the additional step of regulating the 
magnitude of the current pulse to limit the voltage pulse on the generator 
output to a percentage of the generator's per unit voltage output is 
included. 
Additionally, a preferred method of providing a third source of electric 
power on a two engine aircraft, wherein the third source comprises a first 
variable speed generator 100 and a second variable speed generator 102 as 
described above, comprises the steps of a) designating one of the first or 
the second generators as a primary power source and one of said first or 
said second generators as a standby power source, exclusively, b) 
generating a controlled current signal to the exciter field of the 
generator designated as the primary power source to maintain its output 
voltage at a given level, c) generating a current pulse signal to the 
exciter field of the generator designated as the standby power source, d) 
monitoring the output of the standby power source, e) indicating the 
operational status of the standby power source as "ready" upon detecting a 
voltage pulse on the output of the standby power source generated in 
response to the current pulse, or as "not ready" upon detecting the lack 
of a voltage pulse on the output of the standby power source in response 
to the current pulse, and f) converting the output voltage of the primary 
power source into a constant frequency voltage for use by the aircraft 
electrical loads. In a highly preferred method, the step of switching the 
designation of primary power source and standby power source upon failure 
or shutdown of the originally designated primary power source if the 
operational status of the standby power source is "ready" is included. 
Numerous modifications and alternative embodiments of the invention will be 
apparent to those skilled in the art in view of the foregoing description. 
Accordingly, this description is to be construed as illustrative only and 
is for the purpose of teaching those skilled in the art the best mode of 
carrying out the invention. The details of the structure may be varied 
substantially without departing from the spirit of the invention, and the 
exclusive use of all modifications which come within the scope of the 
appended claims is reserved.