Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
653
073073-BWL ~ 1 -
STABILIZING CIRCUIT FOR CONTROhLED
INVERTER-MOTOR SYSTEM
Description
This invention rela~es to a control system
for stabilizing the operation of an inverter-motor
system during transients or disturbances, thereby
greatly improving its dynamic behavior, and during
steady state when the operating conditions are
appropriate to cause system oscillation.
- 10 Inverter-motor systems have a tendency to
oscillate at low frequencies under certain operating
conditions, at which time the inverter appears as
a negative resistance to the d-c bus over which
the inverter receives its energizing voltage~
This invention serves to make the inverter appear
as a more positive resistance, thus reducing the
tendency to oscillate.
In ad~dition, the inverter-motor systems may
have load torques which change abruptly, causing
significant deviations from the desired steady
state d-c bus voltage. This invention also serves
to reduce the effect that the sudden load torque
change has on the d-c bus voltage, thereby reducing
the deviations ~rom the desired d-c bus voltage.
The cause of the undesired bus voltage change
is attributable to the presence o~ the filter,
comprising a series-connected choke and a shunt-
connected capacitor, which is usually included in
the d-c power supply that produces the bus voltage.
s~
079073-BWL - 2 -
During normal steady state operation no average
current flows to the filter capacitor and all of
the current flows to the inverter and then to the
motor windings. If the~e is a sudden drop, for
example, in the load torque on the motor; the
motor cuxrent abruptly decreases and the inductor
current flows into the filter capacitor, charging
- it to a considerably higher voltage. Since the d
G bus is coupled across the capacitor, the bus
voltage likewise increases significantly. The
high bus voltage, in addition to deleteriously
affecting the operation of the motor, may also
- destroy the switching devices (which may be transistors~
in the inverter. To prevent such destruction,
oversized switching devices are customarily employed
having performance characteristics sufficient to
withstand the highest bus voltage that could occur
in the event of a very sudden load change.
The control system of the present invention
achieves significantly improved dynamic performance
and overcomes the instability problem by effectively
regulating the d-c bus voltage to provide motor
stability, while at the same time avoiding the
necessity oE oversizing the switching devices.
With the present invention, the negative resistance
of the inverter is made more positive to eliminate
the tendency to oscillate. At the same time, the
invention effects the way the motor torque changes
with time to reduce the d-c bus voltage deviation
3~ from the desir~d value.
The control system of the invention controls
the operation of an a-c motor driven by the output
S6S3
a-c voltage from an inverter which in turn is energiæea
by an adjus-table d-c bus voltage received over a d-c bus
from a controlled d--c power supply. The control system
comprises means for providing a set point voltage
representing a desired steady state of amplitude and a
desired steady state f.requency for the a-c voltage proa--
uced by the inverter. There are first utilizing means
for utilizing the set point voltage to ad~ust the d-c
~us voltage to establish the inverter output voltage
at the desired steady state amplitude~ and including a
frequency insensitive feedback circuit, having a pr~de-
term.ined first gain factor, from the d-c bus for determin-
ing the ratio of steady state d-c bus voltage to steady
state inver-ter fre~uency. Second utilizing means are
provided for utilizing the set point voltage to establish
the frequency of the inverter output voltage at the
desired steady state level. Stabilizing means are also
provided including a frequency insensitive circuit having
a predetermined second gain factor, responsive to a
deviation in the d-c bus voltage from the desired steady
state amplitude for varying the inverter frequency to
change the electrical load across the d-c bus to maintain
the bus voltage constant under changing conditions, the
second gain factor determining the ratio of d-c bus
voltage deviation to inverter frequency deviat~on under
changing conditions, thereby permitting the employment of
different ratios for steady state and changing conditionsO
The features of the invention which are believed
to be novel are set forth with particularity in the
appended claims. The invention, together with further
advantages and features thereof, may hest ~e unaerstood,
however, by reference to the following description in
conjunction with the accompanying drawing in which:
FIGURE 1 schematically illustrates a control
system, constructed in accordance with one embodiment of
the present invention, and the manner in which that control
system is coupled to an inverter-motor system to control
and to
bm~ ^
,
J\~ . ~
~s~s,~
stabili~e the operation thereof; and
FIGURES 2 and 3 show portions of the control
sys-tem oE FIGURE 1 modified in acc~rdance with two other
embodiments of the invention~
bm~ - 3a -
~' ~
`' ~
~L~ilS~i~i3
079073-BWL - 4 -
Referring to FIGURE 1~ l.ine conductors Ll, L2
and L3 connect to a conventional three-phase a~c
power system an~ thus provide three-phase a-c
voltage, namely three alternating voltages that
$ are phase-displaced with respect to each other by
120 and have a frequency of 60 hertz. Each of
the three phase voltages is a line-to-line voltage
and appears on one of the line conductors Ll, L2
and L3 relative to another one of the line conductors.
The a-c energy received over the line conductors
is converted to d-c power by a controlled rectifier
bridge 10 which is of well-known construction.
Specifically, the bridge has a family of six
silicon controlled rectifiers or SCR's 11-16
~hich, when fired into conduction by gate current
from recti:Eier controller 17, rectify the applied
a-c voltage and develop across the bridge's positive
and negative output terminals (designated 13 and
19, respectively) rectified vol-tage of a magnitude
determined by the conduction angles of the SCR's
during each half cycle of the applied a-c voltage~
To explain, each SCR in bridge 10 can conduct,
during each positive polarity half cycle of the
voltage applied thereto from the a-c power system,
when the SCR's anode is positive relative to i.ts
cathode. However, conduct.ion will not occur
during a positive half cycle ~mtil gate current is
supplied to the SCR's gate from rectifier controller
17. At that instant, the SCR fires into conduction,
or turns on, and permits load current to flow
therethrough until the end of the positive half
cycle. The greater the phase angle or time delay
079073~BWL ~ 5 -
between the start of a positive half cycle and the
firing of the SCR into conduction, the less the
conduction angle and the less average voltage will
be rectified and supplied to the load. Of course,
the rectified voltage will be of positive polarity
at terminal 18 with respect to terminal 19.
A filter 20 comprising series-connected choke
21 and shunt-connected capacitor 22 filter the
rectified voltage from the bridge to develop a
filtered d-c voltage for application to inverter
25 over the d-c bus provided by lines 26 and 27.
A positive-polarity d-c bus voltage will therefore
appear on line 26 with respect to line 27 which is
connected to a ground plane of reference potential
or circuit common, zero volts in the illustratad
embodimentO By controlling the conduction angles
of SCR's l:L-16 the d-c bus voltage applied to
inverter 2'i, via bus 26, 27, is controlled.
Invert:er 25 has a well-known circuit con-
figuration. It includes three pairs of NPN bipolar
: power transistors 31-36, each pair being series-
connected across the d-c bus 26, 27. The circuit
junctions 37, 38 and 39 of the three transistor
pairs connect to the windings of a-c induction
motor 41, whose output shaft 42 drives some mechanical
load 43. By supplying drive current to the bases of
the six bipolar transistors 31-36 at prescribed times,
the d-c vol-tage across the d c bus is effectively
changed to a-c voltage as applied to the windi.ngs of
the motor, thereby delivering alternating current to
the windings to effect rotation of motor 41 at a speed
53
079073-BWL - 6 -
determined hy and directl~ proportional to the frequency
of t~e inverter output a-c voltage~ For example, if
base drive current îS simultaneously supplied to
transistors 31 and 35 to drive those transistors into
their saturation modes, current will flow from line 26
and through, in the order named, the emitter-collector
conduction path of transistor 31, junction 37, a winding
of motor 41, junction 38 and the emitter-collector
conduction path of transistor 35 to line 27 or ground.
If transistors 31 and 35 are then cut off and tran-
sistors 32 and 34 are turned on instead, current will
flow through the same motor winding in the opposite
direction. Of course, control circuitry for supplying
base drive current to switch the transistors 31-36 on
and off in the correct sequence, at the correc~ times
and at the correct frequency in order to provide the
required a-c energy for rotating motor 41 and driving
load 43 in the desired manner is well understood by
those skilled in the art. In the illustrated embo
diment this control circuitry is shown by the block 45
labelled "inverter control circuit". Usually, such a
control circuit will include logic circuitry and a
controlled oscillator whlch, in response to a control
voitage received over line 47, supplies programmed
periodically recurring timing pulses, or base drive
signals, to transistors 31-36 tc establish the fre-
quency of the inverter output voltage and consequently
the speed of motor 41. The pulse repetition requency
of the timing pulses will be a function of the control
voltage on line 47. Hence, the inverter frequency is
controlled by varying the magnitude of that control
voltage.
~8~i~S3
079073-BWL - 7 -
The emitter-collector conduction path of each of
the six power transistors 31-36 is shunted by a res-
pective one of a series of six oppositely poled feed-
back diodes 51-56 which are employed to circulate the
motor reactive current back to filter capacitor 22O
Feedback diodes 51-56 are also effective to cla:mp the
motor terminal voltage so that it will never exceed the
d-c bus voltage.
Although not shown in the drawing, prefera.bly each
of transistors 31-36 will also be shunted by a ,-on-
ventional snubber network to prevent the inductive
energy in the motor windings from damaging the tran-
sistors when they are switched off by inverter control
circuit 45 during normal operation.
While each of power bipolar transistors 31--36 is
illustrated as a conventional NPN transistor (to
simplify the drawing), in reality it would preerably
take the fo:rm of a well-known power darlington tran-
sistor. In the darlington arrangement, each of tran-
sistors 31-36 comprises a combination of two trcm-
sistors, while still having only three connections,
i namely base, emitter and collector connections clS is
the case in the drawing. Of course, while power
bipolar transistors are shown as the switching clevices,
other devices can be used such as thyristors, field
effect transistors, etc.
To control the operation of rectifier controller
17 in order to adjust the amplitude of the inverter
output a-c voltage and to control the operation of
control circuit 45 to :regulate the frequency of the
inverter voltage, an adjustable set point or reference
5653
079073-BWL - 8 -
d~c voltage is provided at the junction 57 of fixed
resistor 58 and adjustab,le resistor 59. As will be
made apparent, the set pOillt voltage from voltage
divIder 58, 59 determines the s~eady state operating
conditions and represents a desired amplitude and a
desired frequency for the a-c voltage produced by
inverter 25 for application to motor 41. Preferably,
the set point will be selected to satisfy the speed
demanded by mechanical load 43. Comparator 61 compares
the set point voltage with a voltage proportional to
the d-c bus voltage (due to the presence of gain factor
K62) to provide an error voltage which is a fwnction of
- the difference between the compared voltages. K62 sets
the gain between the d-c bus voltage and the set point
voltage. Rectifier controller 17 responds to the error
voltage to produce properly timed gate current pulses
for application to the gates of SCR's 11-16 to control
the conduction angles of the SCR's as required to
establish the d-c bus voltage across lines 26 and 27 at
the magnitude necessary to establish and to maintain
the inverter output voltage at the desired amplitude.
If the d-c bus voltage tends to vary from the requisite
steady state level, the error voltage changes and
causes rectifier controller 17 to vary the conduction
angles as necessary to adjust the d-c bus voltage until
the correct steady state amplitude level is re-
established.
In the meantime, the control voltage for regu-
lating the operation of control circuit 45 is developed
by adding, in summing circuit 63, the set point voltage
and a voltage proportional to the error voltage. K64
represents a transfer function and indicates the gain
S65~
079073-BWL - g
in the feedhack loop. The value of K64 is selected for
~est system performance. Too low a value will defeat
the positive featllre of the control system, whereas too
high a value for K64 can cause instability. The
control voltage will thus be a function of the error
voltage and the set point voltage, as a result of which
the programmed timing or base drive signals supplied to
transistors 31-36 will have the ~requency required to
set the frequency of the inverter output voltage at the
desired steady state level. Since both the amplitude
and frequency of the inverter output vcltage ,are
determined by the set point voltage, those two
- characteristics of the inverter voltage will have a
ixed ratio with respect to each other when no
error voltage i5 present. This steady state ratio can
~e set by gain factor 62. A fixed ratio is desirable
in order to avoid overheating of motor 41 and to
provide the motor with a constant torque output
capability xegardless of motor speed.
When there is a normal variation in load demand,
thereby dictating a different steady state motor speed,
resistor 59 may be adjusted to vary the set pcint
- voltage to the extent necessary to change the control
voltage as required to establish the frequency of the
inverter voltage at the level necessary to drive motor
~1 at the new desired speed. At the same time, the new
set point voltage varies the d-c bus volta~e as needed
to maintain the same fixed ratio of the amplitude and
frequency characteristics of the inverter output
voltage.
Of course, while the motor speed may be changed by
manually adjusting resistor 59, the set point voltage
3LlB5653
079073-BWL - 10 -
may be derived by sensing some parameter or character
istic of the system~ in which the controlled inverter-
motor system is inco~porated, in order to automatically
control the motor speed in response to that sensed
informationO
It is also to be recognized that the d-c power
supply (namely rectifier bridge 10 and filter 20) may
operate in response to single-phase rather than three-
phase a-c energy. In the single-phase environment,
line conductor L3 and SCR's 13 and 16 would be omitted.
: Of course, a d-c bus voltage of positive polarity will
- still be produced on line 26 with respect to line 27.
It should also be appreciated that the constructions of
~he d-c power supply and the inver-ter may take a
variety of different forms. For example, the con-
trolled d-c bus could be provided by a DC-DC converter
supplied by a d-c bus.
In the event that the mechanical load on motor 41
~: undergoes a sudden change, in the absence of the
disclosed stabilizing circuit the d-c bus voltage would
change. To explain, assume that the load torque on
motox 41 i.s abxuptly decreased, or removed entirely,
causing sharply decreased average d-c bus current to
the inverter. The ~ilter choke current thereupon flows
into filter capacitor 22 to charge that capacitor to a
voltagQ significantly higher than the desired d-c bus
voltage. The tendency of the bus voltage to increase
will be compensated, however, by the present invention
by increasing the motor speed in response to increasing
~us voltage so that the inverter-motor system will not
~35~i53
079073-B~L - 11 -
be deleteriously affected and stable operation will
ensue. In effect, the invention operates to maintain,
in the presence of sudden abnormal mechanical load
changes on motor 41, an electrical load across d-c bus
26, 27 which causes the d-c bus voltage to tend to
remain relatively fixed.
This immunization to abrupt or transient load
torque ahanges is obtained ~y controlling the inverter
frequency so that it i6 a function ofl and will varv
directly with, the d-c bus voltage. If the bus voltage
tends to increasè, a~ the load torque drops, the error
voltage will increase and cause the control voltage to
increase, whereupon inverter control circuit 45 will
efect an increase in the frequency of the inverter
output voltage. As a consequence, the motor speed
increases and this effectively increases the electrical
load on the d-c bus to compensate for the decreased
electrical load which occurred when the mechanical load
decreased. In short, the motor is accelerated anytime
the mechanical load drops, thereby presenting a reduced
change in electrical load to the d-c bus. The error
voltage is then decreased to zero by the d-c bus
re~ulating circuit and the system is once again
operative at the frequency and voltage called
for by the set point voltage. The gain for the ratio
of the transient change or deviation in the bus voltage
relative to the resulting frequency deviation for the
inverter will be adjusted by gain factor K64 and can be
adjusted differently than, and independently rom, the
steady state gain factor 62.
6~3
079073~BWL - 12 -
Of course, if motor 41 experiences a sudden
increase in load torque resulting in an increased
electrical load and a reduced d-c bus voltaye, the
error voltage will drop and cause the inverter
frequency and motor speed to decrease so that the
electrical load on the d-c bus wïll drop and the
bus voltage will increase.
To summarize the operation of the invention
j as embodied in Figure 1, under normal steady state
conditions rectifier controller 17 serves to
reduce the error voltage to zero. The d-c ~us
- voltage then is reIated to the set point voltage
by the gain factor K62. At that time, the control
voltage, which determines the inverter frequency,
is equal to the set point voltage. Hence, the
ratio of d-c bus voltage to inverter frequency can
be adjustecl by K62. Under transient conditions,
the error voltage will not be zero. Gain factor
K64 determines the amount of control voltage
deviation from the set point that will occur for a
given error voltage. As a consequence, K64 determines
the magnitude of the frequency deviation from the
steady state frequency value that will occur under
transient conditions. The ratio of the d-c bus
voltage deviation from the desired steady state
value to the deviation of the inverter frequency
from the desired steady state value is, therefore,
adjusted by gain factor K64. As a result, the
ratio of steady state d-c bus voltage to inverter
frequency can be set independently of the ratio of
d-c bus voltage deviation to inverter frequency
deviation under trans-ient conditions.
35i6~3
079073-BWL - 13 ~
It should be understood ~hat the invention
may be practiced in a variety of di.fferent manners
ïn order to cause a devi.ati.on of the frequency
control voltage proportional to the dev;ation o~
the d-c bus voltage from its desired value. In
the Figure 1 em~odiment, transient bus voltage
changes result in trans.ient error voltage ch.anges
which are then used to vary the inverter frequency.
The sta~ilizing means: of the invention can, of
~: 10 course, respond directly to a transient variation
in the d~c bus: voltage. This is shown in the
embodiment of Figure 2. There, the feedback loop
from the d-c bus. to the inverter excludes the
comparator 61, unlike the case in Figure 1. Also,
in accordance with the Figure 2 embodiment, K64 is
pxeceded by a band-pass filter 65 to r~move any
steady state ef~ect of the control system. With
such a filter, the stabilizing circuit will respond
only to transie.nt bus voltage changes. In other
20: words, for normal operation when the inverter-
~:~ motor system is operated at different stea.dy state
conditionsv no vol-tage is applied, through ~ilter
6~, to sur~ning circuit 63 and the inverter frequency
is established entirely by the set point voltage.
However, when the hus voltage experiences an
undesired transient change, that change will pass
: through filter 65 to summing circuit 63. In
effect, summing circuit 63 is made unresponsive to
relatively slow changes in the bus voltage but
3G responsive to relatively fast changes~ The control
voltage, produced by the summing circuit, will
~ 35~;~3
079073-BWL - 14 -
-therefore be ~ari~ed and the inverter frequency
will be altered, in accordance with the invention,
to CO~QenSate for the sudden bus voltage change.
Of ~ourse, any transient change in the d-c
bus voltage will also appear as a transient change
in the motor voltage or the inverter output voltage,
which are essentially the same. ~ence, the
invention may be practiced ~y providing a feedback
loop from a-c motor 41 to the inverter 25 for
adjusting the inverter frequency in response to
transient changes in the inverter output voltage.
T~is is shown in t~e embodiment of Figure 3. The
three-phase voltage produced by inverter 25 is
rectified by rectifier 67 to develop a d-c voltage
which will have su~stantially the same magnitude
as the d-c bus voltage applied to the inverter.
Moreover, any transient change in the d-c bus ~
voltage will manifest itself as a transient change
în the d-c output voltage of rectifier 67O Hence,
such a d-c voltage may be applied to comparator 61
and the system will operate substantially the same
as described in connectïon with Figure 1.
It is thus apparent that the present invention
~; not only stabilizes the operation Gf an inverter-
~ 25 motor system, but by preventing the d-c bus voltage
; from increasing, in response to a sudden drop in
load torque, the transistors in the inverter need
not be oversized.
While a particular embodiment of the invention
has been shown and described, modif;cations may ~e
made, and it îs intended ;n the appended claims to
cover all such modifications as may fall within
the true spirit and scope Gf the inventïon.
