Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Case 2407
50~L~3
This invention relates to a control system, and in
particular it rel~tes to a control system for controlling a
rectifier fed direct current motor drive system.
In electric drive systems having a direct current
motor supplied by a direct current generator, reduction of the
generator output in order to slow the motor will tend to cause
a reversal of armature current which will provide regeneration
back to the generator. In electric drive systems having an
alternating current generator and using a thyristor arrangement
to provide direct current for a motor, it is also possible to
arrange for regeneration when it is desired to decelerate the
motor. However, in electric drive systems where an alternating
current generator or generators are used with non-controlled
rectifiers to provide power to a direct current motor drive,
there can be no regeneration back to the generators because
current cannot be reversed through the rectifiers. There are
problems in the design of such a system, particularly where
the motor must be slowed quickly or reversed such as in a
propulsion drive system.
When a power source feeding a motor i5 reduced the
motor tends to act as a generator and, unless the motor load
energy is so low that ~he power generated by the decelerating
motor can be absorbed by the natural resistance of the loop, it
is neces~ary to introduce into the loop a power ab~orbing
device. In most propul~ion drive systems a decelerating motor
will have considerable inertia and it is necessary to absorb
or dispose of the ~tored energy in order to stop the motor. In
addition, it is frequently necessary, particularly in a
propulsion drive, to develop a counter torque in order to reverse
the rotation of the motor. In a direct current motor, torque
is produced by interaction of the fields caused by armature
current and motor field current. The armakure current cannot
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conveniently be reversed in a rectifier fed system and in
this situation the motor field current must be reversed to
develop a counter tor~ue. ~he reversal of the fie~d current
will introduce another problem.
When a direct current motor is running normally it
develops a counter electromotive force (emf) and the counter
emf opposes the em~ applied to the motor. Reversing the motor
field current will reverse the direction of the counter emf.
In a rectifier fed motor system where the motor i5 decelerating
and the inertia of the motor and its connected load maintain
rotation of the motor, a reversal of the motor field will
result in a counter emf which acts in the same direction as the
rectifier output~ This may cause armature currents which are
unacceptably large. It is necessary to limit or control these
armature currents while providing as much reverse torque as
practical to stop or reverse the motor as quickly as possible.
In the past there have been developed systems which
aid in the braking of rectifier fed direct current motors, buk
the systems have not been completely satisfactory particularly
where reversal of the drive must be considered.
In one prior art system using a rectifier fed direct
current motor in a vehicle drive system, a resistance grid is
carried in the vehicle and as soon as braking action is initiated
the traction motors are connected to the grid so that any
electrical power generated by the motor is dissipated or absorbed
in the resistance grid.
It is also known in a diesel-electric locomotive to
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vary the dynamic ~ g b~ changing the motor field excitation.
In one such arrangement the motor field is directly connected
to the generator output and the excitation to the generator
varied to control the output and thus control the motor field
and the braking effort.
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Neither of these prior art arrangements are concerned
with a reversal following the stopping of the motor and do not
describe a control which will permit rapid motor reversal.
It is a feature of the present invention to provide
a control system for a rec~ifier fed direct current motor
drive which achieves improved deceleration and when required
rapid reversal of the motor drive.
According to the present invention there is provided
a control system for controlling a drive system having an
alternating current genera~or, rectifier means for rectifying
the output of said generator and a direct current motor supplied
by tha rectified output of said generator, said generator and
said motor each having a field winding, ~aid control system
comprising an energy absoxbing device for connection in the
armature circuit of said motor to absorb energy from said motor
when said motor is being decelerated, sensor means for sensing
the speed of said motor and providing a speed signal representing
the ~ensed speed, circuit means connected to said sensor means
and including a counter emf determination control, said counter
emf determination control receiving said speed signal and
determining a computed counter emf using said speed signal
and a maximum motor field current and being responsive to said
computed counter emf exceed.ing a predetermined level of counter
em~ to connect 3aid energy absorb~ng. device into said armature
circuit.
Also according to the invention in another form there
is provided in the control ~ystem a generator field current
control means and a motor ~ield current control means each
being responsive to a control signal to control the current in
the re~pective field winding, means providing an armature
current reference signal and a motor field reference signal
representing respectively a desired armature current and a
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desired motor field current, sensor means for sensing a~mature
current in said motor armature and providing an armature
current signal representing the sensed current, first and
second comparing means, said irst compa.ring means receiving
said armature current reference signal and said armature current
signal and providing an armature current control signal related
to a comparison of the received signals, switching means for
receiving said armature current control signal and having a
first position where said armature current control signal i5
directed ~o said generator field current control means to
control current in said generator field winding tending to
cause in armature current in accordan~e with s aid armature
currant reference ~ignal, and a second position where said arm-
ature current control signal i9 directed to said ~econd
comparing mean~, ~aid second comparing means receiving said
motor field reference signal and with said switching means
in said second position said armature current control signal
for comparing the received signals and providing a motor field
control signal based on the compari~on, said motor field
control signal being applied to said motor field current
control means providing a controlled value of armature current
permitting maximum deceleration with said switching means in
said second position, ~aid switching means normally being in
said first position and responsive to a reversal of direction
of motor field current with said motor field current below
maximum to switch to said second position and responsive to
said motor ~ield current reading maximum ~ return to said
first position.
1~e invention will be described with reference to
the accompanying drawings, in which:
Figure 1 is a block diagram in greatly 3implified
form, useful in explaining the operation of the control system
of the invention, and
Case 2407
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Figure 2 is a block diagram showing one form of the
invention.
m e control system as shown in greatly simplified form
in Figure 1 will be described first, together with its operation,
and then a description will be given of the control system of
Figure 2 which is a more complete block diagram. The control
system is particularly useful on a ship, such as an ice-breaker
for example, where rapid reversal of the drive is often re~uired.
However, the control system of the invention may be used on any
drive system which requires braking and reversal, and which may
in addition have several generator rectifiers supplying a motor.
Referring to Figure 1, block 10 represents a system
control having an input 11~ The input 11 sets the output
requirements of the drive and may be a control for actuation
by an operator. Input 11 may require the motor to provide a
certain steady speed, or it may require braking or reversal.
The system control lQ provides two reference signals.
One reference is on conductor 12 and the other is on conductor
14. The conductor 12 is connected to circuitry represented by
block 27 as will be described hereinafter. The conductor 14
is connected to a motor field current control represented by
block 16. The two reference signals on conductors 12 and 14
represent levels of generator voltage and motor field current
respectively, required by system control 10.
The drive portion of the system includes an alternating
current generator 17, driven by a prime mover ~not shown) such
as a diesel engine, a rectifier system 18 and a direct current
motor 20. The generator 17, rectifier 18 and motor 20 are
connected in the usual manner so the generator 17 provides
alternating current, rectified by rectifier 18, to drive motor
20. An energy absorbing device 21 is included in a series
arrangement in the circuit of the drive system and it may
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be switched into and out of the circuit by a switching control
22a and switch 22.
In Figure 1 there are four parameters sensed. Armature
current is sensed and a signal representing armature current i~
on conductor 23. Motor speed or rpm is sen~ed and a motor speed
signal i5 provided on conductor 24, and a signal representing
the motor field is on conductor 25. Motor armature voltage i~
sensed and appears on conductor 29. m ere are two circuits which
receive the signals representing the sensed parameters. One of
these is rspresented by block 26 which receives the ~ignal from
conductor 24 and provides an output which controls switch control
22a to operate switch 22 to switch the energy absorbing device 21
into and out of the motor armature circuit. The other is
represented by block 27 which receives the signals from conductors
12, 23, 25 and 29 and provides an output which control~ the
operation of a switch control 28a to control switch ~8, and also
provides a control signal to switch 28. The control signal
i9 switched by switch 28 so that it either appears on conductor
30 and is applied to a generator field control 15 or it appears
on conductor 3I and is applied to motor field control 16~
The circuitry of block 26 is re~ponsive to motor speed
and direction and makes what may be referred to for convenience
as a first decision. It will be recalled that a reversal of the
motor fie}d current will reverse the counter emf, and that
because the reversed counter emf will then be in the same
direction as conduction through the rectifier there could be
exces~ive armature current. The circuitry of block 26 receives
a signal representing motor speed and, if there i9 a reversal of
the motor field current, it derives a value for maximum possible
counter emf (based on rpm and max permissible field current) and
if this value exceeds a predetermined value, then a signal is
applied to switch control 22a which operates switch 22 to switch
Case 2407
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energy absorbing device 21 into the armature circuit. This may
be referred to as a counter emf determination control. In this
case, the circuitry of block 26 will sense actual reversal of
the motor shaft rotation and apply a signal to control 22a to
switch 22 to switch energy absorbing device 21 out of the armature
circuit.
It will be apparent that more than one energy absorbing
device 21 could be switched into or out of the armature circuit
depending on circumstances, or in other words, a variable energy
absorbing load can be placed in the armature circuit in accordance
with signals from the circuitry o~ s~itch control 22a.
It should be noted that the energy absorbing load is
not switched into the armature circuit in response to ev~ry degree
of deceleration, but only when the deceleration is such that the
motor counter emf will exceed a predetermined limit.
The circuitry of block 27 is responsive mainly to two
parameters to provide a control decision. One is counter emf
in the motor (i.e. motor armature voltage) and the o~her is the
motor field current. These two parameters may be derived in a
number o~ ways and the simplified diagram of Figure 1 shows a
signal input representing motor armature voltage on conductor 29
and a signal input representing field current on conductor 25.
The parameters may be determined from other signals. The
circuitry of block 27 makes what may be re~erred to as a
second decision.
The circuitry o~ block 27 normally receives a
reference signal on conductor 12 from system control 10, and
a signal representing armature current on conductor 23. These
signals correspond to a desired armature current and an actual
armature current. The difference or error signal resulting
from a comparision of the two signals is provided on conductor
Case 2407
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30 to control the generator field and thereby control armature
current as is well known in the art. Also in normal operation,
the motor field control 16 receives a control signal on conductor
14 from system control 10 and this will control the motor field.
However, when certain conditions exist, block 27
makes a second decision as referred to previously and redirects
the control signals. When the circuitry of block 27 determlnes
there has been a reversal of the counter emf and the motor
field is below its maximum permissible level, then switch 28
is activated by switch control 28a and the control signal
which is normally on conductor 30 is switched to conductor 31.
The signal on conductor 31 representing an armature circuit
control is applied over conductor 31 to oo~b~ with the signal
on conductor 14 from system control 10 to determine the motor
field. The ~ignal representing armature current control
remains on conductor 31 until maximum motor field current is
reached. At this point the counter emf cannot increase further
and therefore cannot maintain full armature current. Thus,
when maximum motor field current is reached, the circuitry of block
27 actuates switch control 28a causing switch 28 to restore
normal operation. The generator voltage can now be increased in
the normal manner to provide maximum armature current as re~uired.
It will be seen that a maximum torque is available
for stopping or reversing the motor. The control system permits
modification of the control of the torque to meet various needs.
Referring now to Figure 2, there i5 shown a block
diagram which represents in more detail one form of the invention.
The generator 17, rectifier 18, energy absorbing device 21,
switch 22 with switch control 22a, motor 20, and generator
and motor field control 15 and 16 have the same designation
numbers for convenience.
In Figure 2 an input 11 is provided on a system
Case 2407
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control and decision circuitry 33 to enable required operational
values to be achieved. System control and decision circuitry
provides the necessary output reference signals as well be
described. A motor speed sensor 34 detects or senses motor
speed and direction and is connected by a conductor 39 to the
system control and decision circuitry 33 to provide another input.
The system control and decision circuitry 33 provides output
signals on conductors 35 - 38. The signal on conductor 35
represents a desired generator output voltage. The signal on
conductor 36 represents the "second decisi.on" signal referred
to in connection with Figure 1. The signal on conductor 37
represents the "first decision" signal referred to in connection
with Figure 1. The signal on conductor 38 represents a desired
motor field current.
A voltage sensor 40 detects motor volta~e and provides
a signal representing motor voltage on conductor 41. Conductors
35 and 41, carr~ing signals respectively representing desired
voltage and sensed voltage, are connected to a device 42 which
provides an output signal which is a function of the difference
of the two voltage signals. This difference signal or "error"
signal is applied to voltage control and current reference 43.
An output signal is provided from voltage control and current
reference 43 which is related to its input signal but which
may have maximum or minimum values (for example, it may have
a maximum limit to avoid an excessive rectifier output current)
and this is applied as one signal to device 44, This is a
current reference signal. A current sensor 45 senses armature
current and provides on conductor 46 a signal representing
armature current. Conductor 46 is connected to device 44 to
provide the armature current signal to device 44. Device 44 pro-
vides an output which is a function of the difference of the cur~
rent reference signal and the actual armature current signal. The
Case 2407
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difference signal or error signal from device 44 i5 applied to
armature current regulator 47. The current error signal is
intended in normal operation to alter the generator field to
change the armature current in such a manner that the armature
current signal on conductor 46 will approach the current reference
signal from current reference 43~ The armature current regulator
47 then acts to control the armature curren~ to the desired
level. The control signal from armature current regulator 47
is applied to a control and routing amplifier 48 which routes
the control signal over either conductor 50 to generator field
regulator 15 or conductor 51 to device 52. In other words, the
control and routing amplifier 48 i9 responsive to a signal on
conductor 36 (the "second decision" signal) to switch the
control signal to conductor 50 or 51. When the control signal
is on conductor 50 it actuates generator field control 15 to
regulate the generator field thereby controlling armature current.
~hen the control slgnal is on conductor 51 there has been a
reversal of the counter emf in motor 20 but the motor field is
below its maximum level. The control signal is applied to device
52 where it is added to the motor field reference on conductor
38. The resulting signal is applied via a reference limiter 53
to regulate the motor field thereby controlling armature current.
Reference limiter 53 prevents the motor field from exceeding a
permissible level. mis gives maximum braking effect and the
energy i5 absorbed by energy absorbing device 21.
At some point in the deceleration the motor speed
will decrease to a level where its counter emf cannot develop
full armature current even when the motor field current is at
its maximum. As the maximum field is reached the control and
decision circuitry 33 provides the signal on conductor 36 which
routes the currenk control back to conductor 50 restoring normal
operation. It should be noked that this can take place before
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there is actual reversal of the motor. With the motor field
at a maximum the generator field will increase according to the
control signal from armature current regulator 47 until maximum
armature current is achieved. The motor torque is now maximum
and will provide maximum braking and acceleration in the opposite
direction.
~ he operation of switch control 22a and switch 22
to switch energy absorbing device 21 into and out of the armature
circuit is the same as was described in connection with Figure 1.
It is believed that the operation of the Figure 2 form
of the invention should be clear. The control system operates
to achieve a greater dynamic braking and a rapid reversal of a
dr,ive motor.
