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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an engine start type VSCF generating
system suitable for use as an electrical power generating system for
airplanes.
2. Description of the Prior Art
Recently, with the progress in the practical use of a socalled VSCF
(variable speed constant frequency) generating system which uses engine
power as a driving source, the need for using a brushless generator as a
brushless motor has been increased.
In a prior art VSCF generating system of the engine start type, it is usual
to include separately a starter for starting the engine, and as shown in
FIG. 1, such a system includes a starter 6 such as an air turbine or the
like, for starting an engine 1, and an AC generator 2 is coupled to the
engine 1. The AC output from the AC generator 2 is converted to desired AC
power, for example, of three-phase, 115 V at 400 Hz through a power
rectifier 3, a power inverter 4, and a filter 5.
Furthermore, a generating system of the DC excitation type is known, for
example, from "Brushless Generator for Aircraft" by A. W. Ford, the
Institute of Electrical Engineers Paper No. 3812 U, 1962, in which, as
shown in FIG. 2A, this system includes a main generator 2 having a field
winding 2a and an armature winding 2b, an AC exciter 9 having a field
winding 9a and a rotor winding 9b, a DC power source 71, a DC controller
81, and a rotary rectifier 10. A rotor assembly K1 includes the rotor
winding 9b, rotary rectifier 10, and field winding 2a.
In addition, a generating system of the AC excitation type is known, for
example, from "Brushless Excitation with Rotating Transformer", SHINKO
DENKI Technical Bulletin, Vol. 16, No. 2, 1971, in which as shown in FIG.
2B, and AC power source 72, an AC controller 82, and a rotary transformer
11 are provided to excite a field winding 2a of a main generator 2 by AC
power through the rotary transformer 11 and a rotary rectifier 10. In this
case, a rotor assembly K2 includes a secondary winding of the rotary
transformer 11, the rotary rectifier 10, and the field winding 2b.
Specifically, the AC power supplied from the AC power source 72 is
regulated by the AC controller 82 to an appropriate AC voltage according
to a required torque at the time of starting, and the AC voltage is
applied to the rotary transformer 11, the output thereof being rectified
by the rotary rectifier 10 to excite the field winding 2a. In generation
mode, AC power generated by a magnet generator (not shown) is regulated by
the AC controller 82 so that an AC voltage which enables the main
generator 2 to generate a constant voltage is applied to a primary winding
of the rotary transformer 11.
However, the following problems are involved in the prior art systems.
In the system shown in FIG. 1 in which the starter 6 constituted by an air
turbine or the like is separately provided, it is necessary to provide
such an additional device (starter) as compared to the system used in
airplanes wherein the DC power is primary electrical power and a generator
for supplying power to various facilities in the airplane is used also as
a DC motor serving as a starter for starting the engine. The necessity of
such an additional device in particular poses a serious problem when the
generating system is to be used on airplanes in which the reduction of
weight is required.
In the DC excitation system shown in FIG. 2A, there has been the problem in
that when the rotational speed of the rotor assembly K1 is zero, the
electrical power is not generated in the exciter rotor winding 9b and
therefore magnetic flux is not generated in the field winding 2a of the
main generator 2.
On the other hand, in the AC excitation type system shown in FIG. 2B, since
the rotary transformer 11 has no power amplifying capability, the stator
and rotor require substantially equal capacity. As a result, although the
field magnetic flux can be obtained at the time of starting, it is
necessary to supply large electrical power to the rotary transformer 11 as
compared to the exciter. Therefore the drawback is involved in that the
size and weight is large as compared to the DC excitation type system.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a VSCF generating system of the
engine start type which requires no separate starter, which is compact and
light in weight as compared with the prior art generating systems of the
AC excitation type, and which is capable of generating field magnetic flux
at the time of starting.
In order to achieve the above object, a VSCF generating system of the
engine start type in the present invention comprises a main generator
coupled to an engine, an AC exciter coupled to the engine for exciting the
main generator, a power rectifier and a power inverter for converting
output power of the main generator, a position sensor for detecting a
position of a rotor of the main generator, and a distributor responsive to
a signal of the position sensor for phase controlling the power inverter,
wherein at the time of starting the engine, the main generator is operated
as a no-commutator motor by using the position sensor and the distributor
to obtain a starting torque, and a field winding (stator winding) of the
AC exciter is selectively connected to an AC power source or a DC power
source by a switch.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a prior art VSCF generating system;
FIGS. 2A and 2B are respectively circuit diagrams of prior art DC
excitation type and AC excitation type generating systems;
FIGS. 3A and 3B, connected as shown in FIG. 3C, comprise a block diagram of
a VSCF generating system of an embodiment of the present invention;
FIGS. 4A and 4B, connected as shown in FIG. 4C, comprise a circuit diagram
of the distributor in FIG. 3; and
FIGS. 5A and 5B, connected as shown in FIG. 5C, comprise a circuit diagram
of the DC field controller and the voltage regulator in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIGS. 3A to 5B, the present invention will be described
by way of an embodiment of the invention. In the Figures, like reference
numerals designate like or corresponding parts to those in FIGS. 1 and 2.
In FIG. 3A and 3B a permanent magnet generator 12 is mounted to a rotating
shaft 1a of an engine 1, and it functions as an excitation power source
for a field winding 9a of an AC exciter 9. AC output power from an output
winding 12a of the permanent magnet generator 12 is converted by devices
described later, and supplied to the field winding 9a for excitation by
DC. The output of the permanent magnet generator 12 is also used to detect
the rotational speed of the rotating shaft 1a of the engine 1, and for
this purpose, the output is supplied to a speed detection circuit 13
having a frequency/voltage converter 13a and a comparator circuit 13b. The
comparator circuit 13b includes, as shown in FIG. 5A and 5B differential
amplifiers M11.about.M13, transistors Tr4 and Tr5, resistors
R27.about.R30, and a Zener diode TZ3.
A position sensor 14 is mounted to the rotating shaft 1a to detect a
rotational position of the rotating shaft 1a, and it provides a control
signal for commutation to a distributor 19 which will be described later.
A first switch 15A performs switching of the supply of an output of a
power inverter 18 (described later) between an armature winding 2b of a
main generator 2 and a load depending on whether the system is in starter
mode (including low speed running) or normal running (generator) mode. On
the other hand, a second switch 15B operates to switch sources of power
supply to a power rectifier 17 between an AC output of the main generator
2 and an external AC power source 16 depending on whether the system is in
the starter mode or in the normal running mode. The power inverter 18
converts the DC output of the power rectifier 17 into an AC output. The
distributor 19 performs phase control of the power inverter 18 so as to
supply to the armature winding 2b an armature current of a phase
corresponding to the signal from the position sensor 14. The distributor
19 includes, for example, as shown in FIG. 4A and 4B, AND circuits
A1.about.A2, NOT circuits N1.about.N2, differential amplifiers
M1.about.M4, diodes d1.about.d3, capacitors C1.about.C6, resistors
R1.about.R11, a Zener diode TZ1, and rectifiers S1. A filter 20 removes
noise contained in the AC output of the power inverter 18.
A third switch 21 and a fourth switch 22 both responsive to a rotational
signal from the speed detection circuit 13 perform switching of supply of
a current to the field winding 9a of the AC exciter 9 between a DC output
from a DC field controller 8A and an AC output from an AC field controller
8B depending on whether the system is in the normal running mode or the
starter mode (including operation at a low speed rotation). In each of the
first, second, and fourth switches 15A, 15B and 22 shown in FIGS. 3A and
3B, the character "S" designates a switching terminal of the start side,
and "G" designates a switch terminal of the normal running (generating)
side. In the third switch 21, "L" designates a low speed side terminal,
and "H" designates a high speed side terminal.
A DC power source 7 includes, for example, as shown in FIGS. 5A and 5B a
full-wave rectifier 7a constituted by rectifier elements such as
thyristors or the like which are adapted to be phase controlled, and the
AC output supplied from the output winding 12a of the permanent magnet
generator 12 is rectified to obtain a DC output. In this case, the output
of the full-wave rectifier 7a is phase controlled based on the output of
the frequency/voltage converter 13a.
On the other hand, a second DC power source 7C supplies DC power by
rectifying AC output of an external AC power source 16, and for example,
as shown in FIGS. 5A and 5B, a diode d4, transistors Tr1.about.Tr3, and
resistors R10.about.R12 are included.
The DC field controller 8A includes, for example, as shown in FIGS. 5A and
5B, differential amplifiers M4.about.M7, resistors R13.about.R20, and a
capacitor C7, and the DC power from the second DC power source 7C is
regulated to DC power of a desired level.
The AC field controller 8B, as shown in FIGS. 3A and 3B and 5A, 5B
regulates the AC power (this power corresponds to the power of the AC
power source 72 in FIG. 2B) supplied from the external AC power source 16
to a desired level, and as shown in the Figures, this may include a
transformer.
A voltage regulator 23 maintains the output voltage of the main generator 2
at a predetermined value regardless of the engine speed in the normal
running (generator) mode, and it includes, for example, as shown in FIGS.
5A and 5B, a diode d4' transistors Tr1'.about.Tr3' resistors
R10'.about.R12', differential amplifiers M8.about.M10, Zener diodes TZ2
.about. TZ3, resistors R21.about.R26, capacitors C8 .about. C9, a
rectifier S2, and a transformer T.
In the embodiment described above, the rotor winding 9b of the AC exciter 9
is shown as having a three-phase winding, however, the winding is operable
if it has not less than two phases. Also, the number of the phases of the
rotary recrifier 10 may be applicable to a half wave and a full wave
rectification. The switching operation of the first to fourth switches
15A, 15B, 21, and 22 are performed automatically by detecting that the
output of the speed detection circuit 13 has reached a predetermined value
as shown in FIGS. 3A and 3B, however, this switching may be carried out
manually.
With the arrangement described above, at the time of starting the engine 1,
that is, the rotor assembly K is stopped, by switching the second switch
15B to the side of the external AC power source 16, the VSCF generating
system is driven as a no-commutator motor. Specifically, the AC power
supplied from the external AC power source 16 is converted to DC power by
the power rectifier 17, and the DC power is converted to AC power by the
power inverter 18 there by to supply power to the armature winding 2b of
the main generator 2. As a result, the main generator 2 is driven as a
no-commutator motor and the engine 1 is driven and accelerated. In this
case, the distributor 19 receives the position signal representative of a
rotor position of the main generator 2 detected by the position sensor 14,
and controls the power inverter 18 so that the commutation thereof is
appropriate.
On the other hand, in such a starting time of the engine 1 and during the
time in which the generated voltage of the AC exciter 9 is low due to low
engine speed, this state is detected by the speed detecting circuit 13 and
the speed signal is supplied to the third switch 21. As a result, the
third switch 21 is switched to the side of the AC field controller 8B, and
therefore to the external AC power source 16.
Accordingly, in this case, the field winding 9a of the AC exciter 9 is
supplied with the AC power, and an AC voltage is generated in the rotor
winding 9b due to a transformer action. On the other hand, when the rotor
is rotating, the AC voltage is generated in the rotor winding 9b of the AC
exciter 9 due to both the transformer action and the generator action. In
either case, a DC current flows in the field winding 2a of the main
generator 2, and desired field magnetic flux is generated.
When the rotational speed of the engine 1 reaches a predetermined speed
during the starter mode, the third switch 21, responsive to the signal
from the speed detecting circuit 13, is automatically switched to the side
of the DC field controller 8A. Consequently, the field winding 9a of the
AC exciter 9 is supplied with DC power from the second DC power source 7C
through the DC field controller 8A. Thus, the field winding 2a of the main
generator 2 is excited by the AC exciter 9 through the rotary rectifier
10, and predetermined field flux is generated. In this case, the
predetermined speed for switching from AC excitation to DC excitation is
at a level sufficient to produce a required field current by the AC
voltage generated in the AC exciter 9 even by the DC excitation thereof.
Next, when the engine 1 is running and the engine speed reaches a normal
running speed or larger, the fourth switch 22, in response to a speed
signal from the speed detection circuit 13, is automatically switched to
the side ("G") of the voltage regulator 23. Furthermore, the second switch
15B is switched from the external AC power source 16 to the side of the
main generator 2, and the first switch 15A is disconnected from the side
of the filter 20. Consequently the system is operated as the VSCF
generating system. In this case, the switching operation of the first and
second switches 15A and 15B is performed based on the speed signal from
the speed detection circuit 13. Specifically, the main generator 2
generates AC power at a variable frequency corresponding to a variable
speed of the engine 1, and after the AC power is once converted to DC
power of a constant voltage by the power rectifier 17, the DC power is
again converted to AC power at a low frequency. The resultant AC power is
sine wave shaped including removal of noise by the filter 20, and supplied
to the load as predetermined 3-phase AC power, for example, 115 V, 400 Hz.
The commutation of the power inverter 18 is controlled by an oscillator
25.
The voltage regulator 23 receives the output from the armature winding 2b
of the main generator 2 and responsive to a signal supplied through he DC
power source 7 from the speed detection circuit 13 representative of a
speed variation of the rotating shift 1a of the engine 1, regulates the
voltage generated by the main generator 2 to a predetermined constant
voltage by controlling the current supplied through the fourth switch 22
to the field winding 9a of the AC exciter 9 by setting the Zener voltage
of the Zener diode TZ2 included in the voltage regulator 23 to a
predetermined voltage.
In the present invention, in order to operate the VSCF generating system as
an engine starting apparatus, that is, as a no-commutator motor, a power
rectifier and a power inverter are used also at the time of starting the
engine 1, and at the same time, an AC power source, a DC power source, and
switches for supplying the power to the AC exciter are provided so as to
perform the excitation by switching between AC excitation and DC
excitation. According, the following advantages are provided.
(1) The starter such as an air turbine driven by a high pressure air source
which has been provided separately in the prior art system becomes
unnecessary. Thus, the weight is reduced, the apparatus associated with
the engine is simplified, and the maintenance of the system is improved.
For this purpose, the equipment which is required additionally includes
merely two switches.
(2) As regard the AC exciter, since it is started by AC excitation to
generate the field magnetic flux at the time of engine start, it is
applicable to brushless starting. After the engine start is completed, and
when the engine is running normally and in the geneator mode, by switching
the excitation of the AC exciter to the DC excitation, the power required
for the excitation of the AC exciter can be made minimum.
(3) Since the rotary transformer is not necessary as compared with the
prior art AC excitation system, the size of the overall system can be made
compact, and the weight is reduced to a great extent. Accordingly, an
exciter suitable for use in airplanes can be provided.
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