Note: Descriptions are shown in the official language in which they were submitted.
Q7~
This invention pertains to an electrical propulsion
system for use in transportation applications which have dedi-
cated guid~way, for example, a street ca~ on a trackl a subway
system.
Presently, transportation systems in service supplied
with electrical power utilize rotating elect:rical machines as
the prime moti~e power source. The torque cleveloped by the ro-
tating machine is transmitted, via a gearbox in normal circum-
stances, to the vehicle driving wheels, which in normal circum-
stances also serve to support the opexational vehicle from itstrack guideway. In addition to the motor, gearbox if required,
and wheel, all of these types of propulsion systems have one
further component which can be termed the power control unit,
the type of which is dependant upon ~he type of elec~rical mo~or
used in the application, i.e. direct current (DC) or alternating
curre~t (AC).
Where DC motors are used the power contxol unit can be
either a variable resistance and switch arrangement or a DC to
DC solid state power converter ~chopper) or an AC to DC solid
state power converter ~controlled rectifier). The firat two of
these power control units are utilized when the power source i~
direct aurrent. The las~ type can be utilized when the power
source is alternating current.
Where alternating current motors are used the power
control unit can be either a DC ~ AC solid state po~ar converter
(inverter~ or an AC to AC solid state converter (voltage regulator
or cycloconverter).
The former type is utilized when the power souxce is
DC. ~he latter type is utilized when the power source i~ AC~
3~ Systems of the first type, i.e. comprised of a solid
state DC to DC power converter (chopper), DC machines, gearboxes
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and drive~ wheels and systems of the second type, i~e. comprise~
of a solid state DC to AC power converter (inverter) AC electrical
machines, gearboxes and driven wheels have c:ertain common charac-
teristics.
These common characteristics include manual or auto-
matic means which provide a demand signal to the propulsion system.
This demand which may be a signal corresponding to velocity, is
normally compared with the appropriate state of the ~ehic:le in
order to produce an error signal. This error signal is processed
by the power conversion unit in such a manner that the electrical
power rom the propulsion system power supply is conver~ed to a
form which is suitable for the electxic motors and o the appro-
priate magnitude. The power fed to the motoræ i5 convexted into
mechanlcal power which is then tran~mitted via the gearbo~e~ and
the wheels to the guideway in the appropriate manner. The action
of this total power transferred is to reduce the operational error
until the point is reached where the demand is equalled. ~;
The linear induction motor has de irable characteris-
tics in transportation system applications that stem from the
manner in which they apply thrust directly to the track. This
a~oids reliance on adhesion characteristics between wheel and
track/ eliminateG the need for a gearbox and generally offers a
more reliable, lighter and easier to maintain system. However,
to date a satisfactory method of supplying control~ed propulsion
power to linear induction motors in a tra~sportation sy~tem
has not been devised. Consideration has been given to po~er
supply from a DC source through a pulse width modulated inverter
but the inverter is too large and too heavy to be practical for
most transportation systems. Con~ideration has also been given to
3Q power supply from an AC source through a cycloconvertex. In this
case, the power fac or of the load is of proportions that make it
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S
objecti~nable or use in transportation systems~
This invention provides a compact, light-weight method
for supplying power ~o a linear induction motor for a trackea
vehicle that uses a DC power source. It avoids the problems
of the pulse width modulated inverter and cycloconverter as
power supply devices. The result is a practieal propulsion
system that is a substantial improvement over the presently
used rotaxy induction mot,.or and gsarbox.
The propulsion system described h~erein uses a l)C to
AC power converter (inverter), including a control unit and a
linear induction motor whose secondary member is laid in the
guideway portion of the transportation system. The inverter is
of a special type termed a chopper commutated inverter. The
system described uses the combination of a chopper commutated
inverter, an appropriate control unit and a linear induction
motor. The advantages that the system provides over the priox
art include:
i) Reliability and maintainability are improved by the
use of this type o~ propulsion systemO
ii) There is no longer a reliance on adhesion character-
istic~ between the wheel and the guideway because power is no
longer transmitted via the ~eelq. This leads to signi~:icant
advantages in the capability of the transit ~ystem as a whole.
iii) ~he combination of the chopper commutated inverter
and the linear induction motor can provide the minimum weight to
volume solution as a specified power level.
iv) The combi~ation of chopper commutated inverter and
linear induction motor with appropriate controls provides a
significant degree of ruggedness in terms of response to fault
and overload conditions when compared with the alternate propul-
sion system~, notably those which utilize DC to DC power converters
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~choppers) and DC to AC powsr converter~ (inverters) of the
voltage controlled,pulss width modulated types.
In accordance with this invention, a propulsion sy~tem
for a vehicle having a dedicated guideway comprise~ a m~
phase linear induction motor having a stator mountable on said
vehicle and a ~econdary incorporatable into said guidewayi a
multipha~ gate controlled inverter connect:able to a source
of DC power ~or supplying multiphase AC power ~o said stator
of ~aid linear induction motor; each phase of ~aid inverter
having means for controllably ~upplying DC current in termin~
able pulses ~o said inverter including commutating capaaitors,
free wheel loop diode3 and gate controlled rectifiers~ means
ox controlling the frequQncy of operation of said g,ate aon-
txolled rectifiexs to control the magnitude of the DC current
~upplied by said terminable pulses, means or cyclically
forcing current through said mean~ for supplying DC current
to zero whereby to form it into terminable pulse~ as aforesaid
whereby to control the output ~requency of each pha~e of said
gate controlled inverter, and a control unit having a feedback
related to vehicle velocity in use and sensitive to the dema~ds
of the propul~ion 3y~tem adapted to operate the gates of ~aid
gate controlled inver~er to supply power to said stator of said
linear induction motor to propel the vehicle and to operate the
gates of said gate controlled rectifiers by controlling the
fre~uency of their operation in accordance with the demands
to the propul~ion system. ~ '.
The inven~ion will be understood after reference to the
following detailed specification read in conjunction with the
drawings, wherein
Fig~ a ~hematic illustra~on of a propulsion system;
Fi~. 2 is a ~chematic illustra~on of one phasle of the
chopper commutated inverter; and
Fig. 3 is a ~chematic illustration of ar appropriate
:' ~ control unit.
~7~7~i
Figure 1 illustrates schematically the basic parts of
a propulsion system which include a control unit 1, a solid state
chopper commutated inverter 2 and a linear induction mstor 3.
The stator of the motor 3 is carried by the vehicle and the
secondary is incorporated into the vehicle g~ideway or trackO
Linear induction motors of themselves are well h~own but have
lacked suitable control systems. The operation of this sytem
as a whole is described as follows.
It i8 a well known principle of operation in the use
of induction motors in propulsion systems generally to adjust
the current supply to the motor in accordance with the demand
for propulsion (say accelerate, decelerate or maintain speed)
and, at the same time, adjust the frequency of the current supply
on the ba~is of mainta~ning a constant slip requency. This
involves measuring the velocity of the moving member,relating
it to a corresponding synchronous frequency, adding the slip
frequency to it and supplying the current to the stator at the
frequency so devised. With such a system, one can achieve good
torque or thrust characteristic from an induction motor and meet
all requirements of a train or street car propulsion system in
respect of acceleration, braking and maintaining speed.
The invention provides for the adjustment of stator
frequency and supplies current in accordance with propulsion
demand in accordance with this principle of operation from the
output of a chopper commutated inverter. Figure 1 is a simple
block diagram of a propulsion system in accordance with the
invention wherein the numeral 3 refers to a linear induction
motor, numeral 2 refers to a chopper commutated inverter and
numeral 1 refers to a control unit for the chopper commutated
3~ inverter.
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7~5
The stator of the motor 3 is caxried by a wheeled
vehicle and the secondary of the motor 3 is associated with a
track for the vehicle. Direct current power is supplied to the
inverter 2 as at 9 and alternating current power flows from the
inverter to the motor 3 through the link 6. Propulsion demand
to the system (i.e. accelerate, brake or maintain speed) is ;~
transmitted to the control unit as at 4. The output of the
control unit communicates with the converter as at 5. Feedback
from the inver~er to the c:ontrol unit for inverter operation is
through link 8 and a signal representative of the speed of the
vehicle and coxresponding synchronous frequency i~ transmitted
to the control un~t, through link lO as the means of controlling
frequency of power supplied to the motor. A11 of the foregoing
oomponents are mounted on the vehicle and, in use, one can by
manually manipulating the propulsion demand in the case o a
hand controlled vehicle, operate the motor to accelerate by
increasing current, to maintain speed, or to decelerate by
causing the motor to operate in a regenerative mode and feedback
power into the line.
Figure 2 is a power circuit schematic of one phase of
the chopper commutated inverter. Three single phase circuits,
as shown, comprise a three phase inverter the ou~put of which is
utilized with a linear induction motor load for propulsion of
a vehicle on a track. Each sinyle phase inverter section is
comprises of two chopper su~sections and one inverter subsection.
The chopper section commutates a chopped DC current
which is subsequently converted to an AC output in the inverter
section.
The chopper subsection in Figure 2 comprises a plurality
of silicon controlled rectifiers hereafter identified as SCRI 5
11 to 18 and free wheel loop diodes 23 and 24. Normal commutation
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is achieved in the chopper stage by commutating capacitors 25
and 2~. SCR's 10, l4, 15 and 18 are triggered together during
one half cycle o chopper operating frequency while SCR's 13, 12,
17 and 16 are triggered toge~her during the alternate half cycle.
The chopper is preferably operated at high frequency in the
order of from 600 to 4000 Hertz to reduce the size and weight
of conunutating capacitors 25 and 26. Power output is a direct
function of frequency.
The capacitor current i8 monitored by sensors 30 and 31
to detect the in~tant of current zero. After z~ro curren~ flow
through one group vf SCR' 8 terminates and the control unit
initiates flow through the other groups, flow alternates be-
tween the group 10, 14, 15 and 18 to 13, 12, 17 and 16.
During one half cycle of the chopper the current path
i8 27-10-25-14-29-19-load-22-15-26-18-28, while dur:Lng the
other half cycle the current ~lows through 27-13-25-12-29-
20-load-21-17~26-16-28.
Capacitor~ 25 and 26 are charged in one directiDn during
one half cycle and in opposite direc~ion during the;~iLternate
half cycle. When fully charged the curren~ through capaci~or~
25 and 26 has allen to zero. At this instant, the inductive
nature of the load maintains the current cons~ant which, there-
fore, i9 allowed to flow through diode~ 23 and 24 back ints:~ the
DC supply. Such flow o~ current amounts to instantaneous regen-
eration of power during which the chopper SCR'~ are lcept off~
The pre~ens:e of capacitors in serie~ with the load enables nat-
ural con~nutation to be achieved, the zero capa~:itor current turns
off the conducting SCR's. Reactors 27 and 28 axe used to main-
tain the rate o~ rise of current within the SCR rating.
rrhe frequency of chopper operation is controlled by a
voltage controlled oscillator (VCO) in the control Ullit such
S
that the peak DC output current is a direct function of this
frequency.
The average output voltage of the chopper can be con-
trolled between positive and negative supply magnitudes while
the output current is always positive. The chopper, therefore,
is a two quadrant regenerative type capable of passing power back
intot~he DC supply. The chopper is designe~d for operation at
a high frequency and, therefore, exhibits a fast response charac-
teristic. To enable satisfactory operation at auch frequency
fast switching SCR's and diodes are used.
Chopper operating frequency determines the size of
the capacitors 25 and 26 and it is preferable to keep it high
to minimize the size of the components. Frequency of between
600 and 4000 Hertz has been indicated preferable but other
~requencies are feasible.
As mentioned earlier, the inverter section comprises
four steering SCR's l9, 20, 21 and 22 used to provide AC curxent
in the load at controlled frequency usually in the range 0.1
to 100 Hertz depending on the m~tor. Whenever the polarity of
the load current is to be changed the current is forced to zero
in the chopper whereby the need for commutating components in
the inverter i9 obviated. This result~ in signi~icant reduction
in weight and volume for the overall system. Inverter SCR's
19 and 22 are triggered during one half cycle of the desired
output frequency while SCR's 20 and 21 are triggered during the
alternate half dycle.
It will be appreciated that the inverter section con-
trols the frequency of current to the motor and operates ~t a
very much xeduced frequency to the chopper sections.
The load thus experiences AC current drive. The in-
verter SCR triggering and hence the inverter frequency is control-
-8-
,~ .
- ', ." : ,, , :
s
led by a voltage controlled oscillator (VCO) in unit 1 of Figure
1~ The VCO output responds to the sum ox difference of the slip
frequency setting and the vehicle speed feedback signal through
line 10 on Figure 1.
Reactors 27~ 28 and 29 are for protection purposes.
As there is no commutation requirement placed on the
inverter, phase control SCR's are usedO These SCR's have the
necessary power rating for propulsion systems.
It will be noted that there is no requirement or es-
tablishing ~Iy initial conditions across any components which
resul s in simple start up procedure.
Three single phase inverter units as described above
are used in a three phase system. It will be appreciated that
while re~erence herein is to a three phase system other multi-
phase systems are possible depending on induction motor design.
The control unit 1 shown in Figure 1 is described in
schematic form in Figure 3.
~ arious blocks shown comprise plurality of digital
linear integrated circuit chips as well as discrete components
2n~ suitable for operational and environmental conditions specified
for the system. The control unit c~nsists o~ the following
main functional blocks.
One - Propulsion Controller - 40
Three - Chopper/Inverter Logic Units - 42,44,46
One - Inverter Logic Unit - 48
Three - Pulse Amplifier Units - 50,52,54
Three - Pulse Tran~fo~mer Units - 56,58,60
One - DC Power Supply 62
The propulsion controller 40 processes input signals
to gi~e output signals to control inverter current magnitude,
inverter frequency, direction o~ travel by phase sequence control
6;37~
and inhibition of the ir.verter SCR gating in the event of a faulty
operation. The system thus has features of controllable cruise
velocity, acceleration, deceleration. In the embodiment il-
lustrated the input signals to the controller are velocity
o vehicle, accelerate, decelerate, stop and start.
In case of faulty operation, the propulsion controLs
initiate various inhibitsi resets and mode displays.
The chopper~inverter logic 4 for each phase 42, 44 and
46 comprises a plurality of inteyrated ciscuits -and discrete
components. The VCO's as previously stated determine the fre-
quency of operatio~ of the chopper which will vary according
to the output current demand and the actual load curre~t feed-
back from each phase. The VCO output is suitably processed and
steexed to respective pulse amplifiers and transformers for sub-
sequent triggeriny of chopper SCR's.
The common inverter logic 48 for the three inverter
output phases consists of a VCO, a phase reversing mea~s for
direction control and a ring counter for generation of si~nals
120 apart for balanced three-phase triggering of invertes-SCR's.
The VCO responds to an analogue sisnal which is the sum or dif-
erence of slip frequency setting and the speed feedback signal
fro~ the moving element of the motor, in this case a vehicle
carrying the motor.
Each pair of inverter SCR's conducts for 120C of the
output cycle in accordance with standard practice to remoYe the
objectionable triplen harmonic.
Protection features incorporated in the control unit
- illustrated include inhibition of the chopper VC0 ~Id, therefore,
triggering of all inverters and chopper SCR's in the event of:
a) overcurrent condition
b) overlap due to all chopper SCR's conducting.
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il5
The following resets are provided in the control circuit:
a) overlap reset
b) overcurrent reset
c) turn off time reset.
The pulse amplifiers 50, 52 and 54 are used to interface
digital outputs with the pulse transformers 56, 58 and 60. These
pulse transformers isolate the high voltage power circuit from
the voltage control circuit.
The DC power supply 62 isahigh noise immunity DC/DC
conver~er producing stabilized output voltages for various con~
trol wnit blocks.
The operation of the linear induction motor as an elec-
trical machine i5 well known and needs no additional description.
In use the linear induction motor, lt~ chopper com-
mutated inverter and propulsion system control unit are mounted
on a tracked vehicle with the stator of the motor in operative
relationship with the secondary which is mounted in the vehicle
track. The vehicle is manually controlled as to propulsion by
operation of the accelerate, decelerate, stop and start inputs
to the propulsion controller 40. There are many practical
variations. For example, a plurality of motors may be used,
the manual inputs to the propulsion controller could be auto-
matic. However, all variations use the chopper commutated
inverter and would have the above noted advantage`s.
There ha~ been disclosed a propulsion system utilizing
a chopper commutated inverter with an appropriate control unit
and a linear induction motor, that does not rely upon adhesion
for pxopulsion effort, is lighter and lower in volume than prior
art equipment for equivalent rating, is inherently more rugged
than alternative static inverter fed schemes and doe~s not insig-
nificantly deviate from the reliability and maintainability that
can be expected from such systems.
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