Note: Descriptions are shown in the official language in which they were submitted.
076176-A-BWL
Large capacity air conditioning systems have used
centrifugal compressors with guide vanes at the compressor
inlet, which vanes are adjustable to regulate both the
direction (or "swirl") of the incoming gas and also produce
a pressure drop which is a function of the vane position.
These vanes, sometimes termed pre-rotation vanes (PRV), are
thus adjusted to vary the capacity of the compressor. In
the condition of wide-open vanes (WOV) a small change in the
vane position does not have a substantial effect on the
compressor head or capacity. When the PRV are nearly
closed a slight change has a very substantial effect on the
compressor capacity, and could even send the compressor into
surge if care is not taken in adjusting the vane position.
Because capacity control solely by changing PRV position is
an inefficient method of capacity control, there have been
attempts to regulate system capacity only by governing the
speed of the electrical motor driving the compressor. If
speed control is utilized as the only means of regulating
the capacity, the compressor can only be operated down to ~ -
about 70% of its full load. This is economically impractical
for large installations, since for substantial periods of
time the load is below 70% of the full load value.
Accordingly various attempts have been made to
combine adjustment of the PRV with regulation of the motor
speed, which can successfuly reduce the load down to about
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t6176-A-BWL llllS.~
10% of full load. One significant early effort in this
direction is taught in U.S. Patent No. 3,335,906, entitled
'Refrigeration System Inlcuding Control for Varying Compressor
Speed", which issued December 5, 1967, and is assigned to
the assignee of this invention. In that arrangement the
motor speed was adjusted as a function of the ratio between
the compressor suction pressure and discharge pressure, and
the PRV were adjusted in relation to a signal derived from
the temperature of the heat exchange medium at the discharge
of the evaporator. Most similar efforts since that time
have used the pressure ratio of the compressor to regulate
the motor speed, and derivation of this pressure information
has proved difficult and expensive in practice. In addition
it has been found that the regulation of the pre-rotation
vanes is a complex function, which has not been effectively
controlled to avoid surge and provide most efficient operation
by sensing of only a single variable condition.
It is therefore a primary object of the present
invention to provide a control system for regulating a
refrigeration system, such as one including a centrifugal
compressor, in which not only are the motor speed controlled
and PRV position adjusted, but this speed control and vane
adjustment are achieved over an opti~um path of control with
minimum energy expended.
A related important aspect of the invention is the
- provision of such a control system which achieves such
energy effective system operation, and at the same time
avoids surge.
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~76176-A-BWL
Of prime importance is the provision of such a
control system, which is not only capable of installation
with new equipment, but is reatily installed on existing
systems to achieve the optimum control path while avoiding
surge in those systems.
Still another important object of the invention is
linearization of the PRV action, through a controller with a
non-linear duty cycle to produce more uniform system control.
Another important object of the invention is to
achieve such precise system operation over a wide load range
without the necessity of sensing compressor pressures to
derive the compressor head value.
The control system of the invention is useful to
regulate a refrigeration system including a compressor, a
condenser and an evaporator, all connected in a closed
refrigeration circuit. Inlet guide vanes are positioned
between the evaporator and the compressor to regulate the
capacity of the compressor, and some means, such as an
electrical motor, is connected to adjust the inlet guide
vanes' position. Means, such as another electrical motor,
drives the compressor. The control system of the invention
is connected to regulate both the means for adjusting the
guide vanes' position and the means for driving the compressor.
A first temperature sensing means is positioned adjacent the
condenser to provide a first signal related to the refrigerant
condensing temperature, and a second sensing means is
positioned in the evaporator to provide a second signal
related to the refrigerant evaporating temperature. A
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. . .
S.~P~t
summation means is connected to receive both the first
and second signals, and to provide a resultant signal
which connotes the head of the compressor. The control
system uses the resultant signal to regulats the re-
frigeration system. A third temperature sensing means
is positioned adjacent the chilled water discharge of
the evaporator to provide a third signal. An adjustable
means is provided in the contrcl system for establishing
a temperature set point signal. Means is also provided
for combining the third signal and the temperature set
point signal to produce another regulating signal, for
use in controlling the refrigeration system. -
An additional means is coupled to the inlet
guide vanes for providing an electrical signal which in-
dicates the physical position of the inlet guide vanes.
Means is provided for combining the inlet guide vane posi-
tion signal with the resultant signal to produce a regu-
lating signal for use in system regulation.
According to one aspect of the present invention
there is provided a control system for a refrigeration
system including a compressor, a condenser, and an evap-
orator, all connected in a closed refrigeration circuit,
which compressor includes an adjustable capacity control
means, means for regulating the adjustable capacity con-
trol means, and an electrical prime mover connected to
drive the compressor, which control system comprises:
means for providing a timing signal; means for providing
a position signal which varies as a function of the set-
ting of the adjustable capacity control means; and a
drive control circuit, having a first input coupled to
the means for providing a timing signal, a second input
coupled to the means for providing the position signal,
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r~``` l ~ l S.~
and an output coupled to the means for regulating the
adjustable capacity control means, which drive control
circuit produces an output drive signal having at least
one characteristic which varies as a function of the
position signal, thus driving the means for regulating
the adjustable capacity control means as a function of
the position of the adjustable capacity control means.
According to one aspect of the invention there ~
is provided a control system for a refrigeration system :
including a compressor, a condenser, and an evaporator,
all connected in a closed refrigeration circuit, which
compressor includes adjustable capacity control means,
means for regulating the adjustable capacity control means,
and an electrical prime mover connected to drive the com-
pressor, which control system is characterized by a prime
mover control circuit, including first circuit means for
deriving a signal which varies as the compressor head,
and means for utilizing the head-indicating signal to
regulate the speed of the electrical prime mover such
that combined control of the prime mover speed and the
adjustable capacity control means is effected in an
energy-conservation manner and without sending the
compressor into surge.
According to a second aspect there is provided
a method of controlling a refrigeration system havinga o~
~, ~
pc/ ~ - 4A ~
" 1 1.1S.~R~
pressor, a condenser, and an evaporator, all connected in a
closed refrigeration circuit, which compressor includes
adjustable inlet guide vanes to vary the compressor capacity,
and an electrical adjustable speed motor connected to drive
the compressor, such that motor speed adjustment also varies
the capacity, comprising the steps of: continually establish-
ing a compressor head signal as a function of the condensing
refrigerant and evaporating refrigerant temperatures; deriving
a functional signal related to the instantaneous position of
the inlet guide vanes; combining the head-indicating signal and
the functional signal to produce an intermediate signal; provid-
ing a signal related to the actual motor speed; combining the
actual motor speed signal and the intermediate signal to provide
a first signal for use in regulating the speed of the compressor
drive motor; and deriving a temperature error signal, related
to the difference in temperature between the cooling medium at
the evaporator outlet and the desired temperature set point,
and employing the temperature error signal as a second signal
for use in regulating both the speed of the compressor drive
motor and the position of the inlet guide vanes.
In the accompanying drawings:
FIGURE 1 is a block diagram illustrating the incorpora-
tion of the control system of this invention into a coolins
system which includes a centrifugal compressor;
FIGURES 2, 3 and 4 are graphical illustrations useful
in understanding operation of the invention;
FIGURE 5 is a diagram showing the principal signal
paths in the control system of the invention;
FIGURES 6A, 6B and 6C are schematic diagrams which,
taken together, illustrate the circuit details of the control
system of this invention;
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,76176-A-BWL ~ t
FIGURE 7 is a graph depicting the relationship of
compressor capacity to PRV opening for different head values;
FIGURES 8A-8F are graphical illustrations useful
in explaining one aspect of the invention; and
FIGURE 9 is a graph of compressor speed dependence
on PRV position, for a fixed head value.
Detailed Description of the Invention
FIGURE 1 depicts certain conventional components
of a cooling system, such as a centrifugal compressor 20 for
passing refrigerant (such as R-ll or another suitable
medium) through line 21 to a condenser 22. In the condenser
the water from the cooling tower passes from line 23 into
the condenser, and is returned over line 24 to the cooling
tower, or to the other head rejection means when different
systems are used. The refrigerant at the discharge side of
condenser 22 is passed over line 25, through a fixed orifice
26, and line 27 to the refrigerant inlet connection of the
evaporator. The refrigerant passes through the evaporator
and out the duct 30, which includes a plurality of inlet
guide vanes 31 positioned as shown. In this description the
inlet guite vanes are termed PRV, or pre-rotation vanes, and
the position of the PRV is regulated by a small motor 32
which receives a control signal over a plurality of conductors,
here represented as a single line 33. Those skilled in the
art will readily appreciate that a plurality of conductors
are represented by the single lines of FIGURE 1. The higher
temperature water from the building (or other cooling load~
,76176-A-BWL
is returned over line 34, cooled in the evaporator 28, and
the chilled water is returned to the building over line 35.
An induction motor 36 is coupled over shaft 3~ to
the centrifugal compressor 20, and this motor 36 is itself
driven from an inverter 37. The inverter receives a d-c
input voltage over line 38, thus determining the amplitude
of the inverter output voltage. A voltage control circuit
40 is provided between a voltage supply line 41 and line 38
which passes the voltage to the inverter. This can be a
conventional circuit, such as a phase-controlled rectifier
circuit, which receives an input a-c voltage on line 41 and
provides a d-c voltage on line 38 which is regulated in
accordance with the signal received over line 42. If no
regulation is necessary, a d-c voltage can be supplied over
line 38 to the inverter from batteries, a transformer-
rectifier, or any suitable source. The frequency of the
inverter output voltage is regulated by the periodicity of
the timing signals, or gating signals, supplied over line 43
from a logic circuit 44. This is a well-known circuit which
receives a regulating signal on line 45 and utilizes this
regulating signal to govern the frequency of the pulses
supplied on line 43. One well-recognized arrangement
receives a d-c voltage as a control signal on line 45, and a
voltage-controlled oscillator in the logic circuit 44
provides pulses at a frequency determined by the amplitude
of the signal on line 45. The logic circuit generally
includes a ring-counter type circuit to distribute the
pulses to as many thyristors or other switches as are used
16176-A-BWL
in the inverter circuit.
In accordance with the present invention, the
control system 50 is utilized to regulate not only the speed
of induction motor 36 but also the physical position of the
pre-rotation vanes 31, by a speed controi signal supplied
over line 51 and a vane position tdrive open or drive
closed) signal supplied over line 33. The circuit arrangement
of the inven~ion insures that surge is avoided, and that
the compressor is regulated in the most energy-efficient
manner. In this embodiment the speed control signal is a d-
c voltage supplied from an integrating circuit 52, and the
vane control signal can be either an "open vanes" signal on
line 53 or a "close vanes" signal on line 54, or no signal
("hold vanes"). These output control signals are derived
from different input signals, including a first signal on
line 55 which is provided by a thermistor or other temperature
sensing unit 56 positioned to contact the refrigerant in the
condenser discharge line as shown. A second signal is
provided on line 57, obtained from a second sensing means or
thermistor 58 which is exposed to the saturated refrigerant
vapor leaving the ev~porator. The first and second signals
are combined in a summation means 59, which can be a differential
amplifier circuit, to provide a resultant signal on line 60
which connotes the head of the compressor. The circuit
means in the control system which utilize this signal, and
the other input signals, will be described hereinafter in
connection with FIGURES 6A-6C. For the present it is
important to emphasize that the provision of the compressor
head information from this simple temperature difference
determination is an important aspect of the present invention.
Without appreciating that the compressor head is a virtually
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076176-A-BWL
linear function of this temperature difference, more expensive
pressure transducers would have to be positioned in or
adjacent the compressor itself to provide some signal
related to the compressor head. Accordingly the realization
that the compressor head can be inferred in this manner is
an important part of the inventive contribution .
A potentiometer 61 is shown with its movable arm
or wiper mechanically coupled to the P~V, or to the output
shaft of motor 32 which drives the PRV. Thus the electrical
signal on line 62 indicates the physical position (fully
open 3/4 open, and so forth) of ~he inlet guide vanes in a
continuous manner. Subsequently the circuitry will be
described which combines this inlet guide vane position
signal with the compressor-head-indicating signal to assist
in regulating the compressor operation.
A third temperature sensing means, which can be
another thermistor 63, is positioned to sense the temperature
of the chilled water discharged from the evaporator 28.
~hermistor 63 thus provides a third signal, which is applied
over line 64 to another differential amplifier stage 65,
which also receives a temperature set point signal from a
potentiometer 66 or another suitable unit, such as a thermostat
in the building space. Thus the output signal on line 67
represents the difference, if any, between the condition
called for (denoted by the signal derived from component 66)
and the instantaneous load condition (represented by the
signal on line 64).
An unload control stage 68 is shown coupled to the
movable arm of the potentiometer 66. For the purposes of
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1~.5~
076176-A-BWL
this invention, stage 68 represents any means for limiting
the consumption of electrical energy by changing the temperature
set point, or effectihg a different circuit adjustment, to
change the load on the compressor and reduce the rate of
energy consumption. Those skilled in the art will appreciate
that this is similar to physically changing the thermostat
setting, but in large installations this can be done auto-
matically by a control system which monitors the rate of
power consumption in successive time periods (such as half-
hour intervals), and prevents the equipment from consuming
more than a pre-set amount of power in each given time
period.
Of the additional signal paths shown in FIGURE 1,
line 70 represents means for supplying a shut-down signal to
the logic circuit 44 when an overload condition is sensed by
the control system. To determine this, and other operating
signals, a signal related to the amplitude of the current
flowing through the windings of motor 36 is passed over line
71 to the control system, and another signal related to the
motor speed is passed over line 72 to the control system.
The motor current signal is derived from a current trans-
former in a well-known manner, and is not illustrated
herein. The motor speed signal can be derived from a
conventional tachometer (not shown), to provide a d-c
voltage signal on line 72 which denotes the motor speed. It
is not requisite to have an external set of conductors, as
represented by line 72, to return a signal related to the
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076l76-A-BWL
motor speed. Instead the control system can use an internal
line, tied to the output of stage 52 which provides the
motor speed control regulating signal, and use a portion o~
this d-c speed signal to signify motor speed. With this
general perspective of the complete refrigeration arrange-
ment and the control system, a more detailed explanation
will now be set out.
FIGURE 2 is a graphical illustration of operating
characteristics of a conventional compressor, taken from
test block data. The compressor head is shown plotted
against flow (or capacity), characteristics that will be
described in more detail in connection with FIGURE 3. The
three surfaces depicted on the graph of FIGURE 2 represent
regions of system operation at a constant Mach number or
compressor speed. For example, the uppermost surface
depicts the possible variations in head and the capacity
while operating at a Mach number of 1.5. If the operating
parameters change and go beyond the longer of the upper
boundary lines for the 1.5 surface, the surge region is
entered, indicating the system will be unstable and the
compressor will surge. The upper right line termination of
this 1.5 surface represents operation with wide open vanes,
and the three dash-dot lines are used to indicate the
regions of operating at 3/4 open vanes, 1/2 open vanes and
1/4 open vanes. These regions are depicted as discrete
lines but the system can be continuously adjusted over the
entire vane-opening range, affording operation at any vane
opening. For operating at a lower speed or Mach number of
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176L76-A-B~
1.4, the system "drops down" to the intermediate surface
depicted in FIGURE 2. Similarly, with a further reduction
of operation to a 1.3 Mach number, the system drops down to
the lowest of the three surfaces illustrated. It is also
understood that in a system in which the compressor speed is
continually adjustable, the Mach number changes are not made
in large increments as shown, but the adjustment is continuous
in the "space" between the surfaces depicted in FIGURE 2 as
the compressor speed is reduced. With this perspective of
the compressor head-flow for regions of constant speed, a
description of the compressor characteristics in another
format will now be set out.
FIGURE 3 is a graph illustrating the "minimum Mach
number" Mo along the left ordinate, the extent of the PRV
opening along the abcissa, and the compressor capacity
r, specified at selected points, all related to the compressor
head Q which is depicted by the series of curves shown on
the graph. The "minimum Mach number" Mo can be considered
to represent the motor speed. More accurately, it is the
ratio of the impeller tip speed to the suction stagnation
acoustic velocity (this velocity is hereinafter termed "A").
Because the induction motor is coupled to the compressor,
the motor speed can be considered directly related to the
impeller tip speed. For this analysis it is assumed that A,
for a given refrigerant, does not change significantly over
the typical operating range of the evaporator, and hence Mo
can be considered a function of the motor speed alone. The
PRV opening is represented in increments from fully closed
to the wide open vane (~OV) condition. The capacity,
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076176-A-BWL
depicted as 9,is technically the ratio of the suction (in
cfm) times 2.4, and this term is divided by the product of A
times the square of the impeller diameter. The compressor
head Q, represented by the curves in FIGURE 3, is technically
the measurement of the head (in feet) times the constant
32.2, and this product is divided by A2. Because A is
considered not to change significantly, as noted above, it
simplifies the consideration of the capacity ~ and the
compressor head value Q.
Each of the different curves in FIGURE 3 represents
a constant head value Q, and the minimum speed and PRV
necessary to accommodate a given capacity without entering a
surge region. For example the curve in the upper right
portion with a head value of 1.2 indicates the motor speed
and PRV opening changes which must be made to reduce the
capacity at this head value. The arrow 75 shows the direction
of decreasing capacity, and if only the motor speed (and
thus the compressor speed) is reduced, the capacity is
reduced as shown by the decreasing value of ~ along the
right side of the graph. However if the speed is reduced to
the point referenced a, and it is still desired to further
reduce the capacity without decreasing the compressor head
below 1.2, at this point the motor speed must be maintained
constant and the PRV gradually started to close, thus
reducing capacity as indicated to the point b, where the
value of 3 is .058 and the vanes are approximately 3/4 open.
The system will surge if, at point b, the speed more further
reduced. It is important to note that for a further
capacity reduction at the same head, the vane closure
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76176-A-BWL
lli'~1.
continues but the motor speed must now be increased, up to
the point marked c.
In a similar manner, if the head value is 1.0, the
vanes can be maintained wide open whiie the speed is decreased
down to the value where ~ equals 0.0385, at reference point
d. From this point the speed is maintained constant and the
vanes are gradually closed down, until they are about 3l4
open as shown at the point e. Hereafter the motor speed is
again gradually increased while the vane closure is continued,
to the point marked f. The points a-f are also shown on
FIGURE 2. Thus it is apparent that a complex control
function is required to coordinate regulation of both the
compressor speed (by regulating motor speed) and the opening
of the PRV. The load line for a given system, when drawn on
FIGURE 3, would cut across the different head curves. Thus
it is necessary to consider the change in head, as well as
the present position of the PRV and the instantaneous motor
speed, whenever a change is made in the system, to be certain
that surge is avoided and the most efficient operation is
achieved.
The control path for combining motor speed control ;'
and PRV opening control, found effective in achieving such
regulation without going into surge, is depicted by the
curve 80 in FIGURE 4. The constant-head Q curves are shown
as background for the ideal control path 80. Suppose that
the system is originally operating at full capacity with
wide open vanes; this is represented at the point designated
g. When it is desired to decrease the load, the capacity is
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~6176-A-BWL
reduced by reducing the motor speed, following the curve
which passes through point h and eventually straight down to
point j, at which time the control algorithm must be modified
to include adjustment of the PRV position to maintain the
optimum control path. The vanes are then gradually c'osed
to about an 80% open position, while the motor speed is
increased from the level indicated at j ~o that represented
at the point k. From this point the vane closure is continued,
but the motor speed is again reduced, until the control path
80 reaches the nadir at point m. At this part of the curve
the vanes are about 35% open, and as the vane closure is
continued, the speed is then increased up to the point n.
When it is desired to increase the load, considering
the system is now operating under conditions represented at
the point n, the PRV opening is gradually increased while
the motor speed is decreased, until the point m is reached.
i Thereafter, as the vane opening continues, the motor speed
is increased up to the point k. In ac~ordance with an
important aspect of this invention, the motor speed increase
is continued, as the vanes are further opened, along the
segment kh of the control curve; the other two legs, kj and
jh, are not followed in restoring load. Instead the control
path is directly from k to h, and then up to g after the
vanes are fully opened.
Because it has been determined that this path
avoids surge and achieves a very high operating efficiency,
it is important that the control system "know" not only the
amount of change required, but the direction in which the
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076176-A-BWL
lli.S;.~F~
change is being effected. This requires both combinational
logic and stored information regarding compressor performance
in the control system, as will be described, to achieve the
optimum control path 80 depicted in FIGURE 4.
FIGURE 5 is an illustrative diagram setting out
general signal flows of the more detailed circuits depicted
in FIGURES 6A, 6B and 6C. In general the chilled water
outlet sigral from thermistor 63 (FIG. 5), and the temperature
set point signal, derived either from the potentiometer 66
or from any other means for establishing the desired temper-
ature control level, are combined to produce a temperature
error signal on line 67. A "dead-band" network 81 is
provided to produce separate output signals on line 82
(which represents a plurality of conductors), to avoid
giving switching commands to the PRV control logic circuit
96, and to the inverter speed logic circuit 83 when operating
in the PRV control region, until the temperature error
signal has exceeded the amount determined by the dead band.
The output of the logic arrangement 83 is passed through
integrating stage 52 to provide on line 51 the motor speed
control signal, to regulate the inverter operating frequency
and hence the motor and compressor speeds.
The vane position information of the PRV is taken
from potentiometer 61 and, over line 84, a portion of this
signal is applied to the inverter speed logic circuit 83.
Another part of the PRV position signal is passed over line
85 to a duty cycle control circuit 86. In addition the PRV `
position signal is passed through a network 87 and combined
with the minimum Mach number Mo signal on line 88. As
described previously, this minimum Mach number for wide open
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vanes is derived from the condensing temperature as sensed
by thermistor 56, and the evaporating temperature as sensed
by the thermistor 58. These two signals are combined to
provide the minimum Mach number Mo signal (for wide open
vanes) on line 89. When this signal is passed over line 88
and combined with the output of network 87, it provides
a composite signal on line 90 which is then combined with
the actual motor speed signal received over line 72 to
produce an error signal on line 91. This error signal is
the error in speed set point when the system is controlling
the PRV position, in addition to regulating the motor speed.
The motor speed signal is also compared with the Mo signal
on line 89 to produce on line 92 a logical signal indicating
whether the actual motor speed is above or below Mo~ This
signal is then applied to the PRV control logic circuit 96
and, over line 99, to the inverter speed logic arrangement
83.
The duty cycle control circuit receives both the
PRV position control signal over line 85 and, over line 93,
another signal from an overload circuit 94. The output of
circuit 86 is passed over line 95 to the PRV control logic
circuit 96, which determines whether the vane position
should be changed, the direction in which the vanes should
be moved, and the amount o movement which should occur.
The overload circuit 94 receives a signal over line 71
proportional to motor current and, in addition to providing
a signal on line 93 to the duty cycle control circuit 96,
provides another signal over line 97 to the PRV control
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~076176-~-BWL
logic circuit 96, and another signal over line 98 to the
inverter speed logic control circuit 83. With this general
perspective of the system, those skilled in the art will
more readily correlate the showing in FIGU~ES 6~-6C with the
entire apparatus arrangement depicted in FIGURE 1.
In the showing of FIGURES 6A-6C, the chilled water
outlet water temperature was derived from thermistor 63 and
passed over line 64 and one of the 24K resistors to one
input terminal of the differential amplifier ~. The
signal from the set point potentiometer 66 is passed over
the other 24K resistor to the other input connection of
stage ~, providing the temperature error signal on the
output line 67. To assist those skilled in the art the IC
component identifications are given hereinafter, and the
operating voltages of the different stages are shown in a
circle. The + sign in a circle indicates the B+ voltage of
12 volts is applied to that point. The chilled water
temperature and set point signals are also combined in
another comparator stage 101, to provide a signal on line
102 to the inverter logic when a low-water temperature
condition is sensed. The application of this signal, and
other overload signals, are not shown in the logic circuit,
but those skilled in the art will readily understand the
application of this signal to shut down the inverter operation
when the chilled water temperature is too low.
The stages 103 and 104 are connected in a lead-lag
compensation circuit for the system. This network provides
a phase lead at .007 hertz and a phase lag at .02 hertz.
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76176-A-BWL ~ ;3~.
Thus the error signal on line 67 passes over line 69 to this
network, so that the output signal o~ the network on line
105 is a compensated temperature signal for application to
the inverter speed logic arrangement.
The signal on line 67 is passed through another
comparator circuit 106 to provide on line 107 a logic
signal, which is a logical 1 (or high voltage) when the
chilled water outlet temperature is greater than the set
point temperature, and is a logical zero (low voltage) ~hen
the chilled water temperature is less than the set point
temperature. This is applied to one input connection of the
~OR gate 108 as shown. Stages 110 and 111 are connected to
provide logic signals on lines 112, 113 when the difference
between the set point temperature and the chilled water
temperature exceeds the amount of the dead band, or the dead
zone. In the illustrated embodiment, the dead zone was set
to include a temperature difference of + 0.15F, electrically
represented by the difference between 5.12 volts and 5.86
volts applied to the two stages 111 and 110. A reference or
center-band voltage of 5.5 volts is utilized as the reference.
NOR stage 114 and an inverter stage 115 are connected as
shown to provide a logic signal for use in the inverter
speed logic control circuit. These stages, and the additional
stages 116, 117, 118, 120 and 121 will be identified at the
end of this description to enable those skilled in the art
to use the invention with a minimum of experimentation. In
addition another inverter 122 and the gates 123, 124 and 125
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'6176-A-BI~L ~ t
which regulate the application of signals to the integrating
stage 52, will also be identified.
The signal from the PRV position potentiometer 61
is derived and supplied on line 62 as shown. A portion
of the signal is supplied over line 84 to one connection of
a comparator stage 130 which also receives a d-c reference
signal at its other input connection. The wiper of potentio-
meter 61 is at the zero resistance (top) position in the
wide open vane condition. This setting, and the other
circuit components and voltages shown in the drawing,
cooperate to produce a logical one signal on conductor 131
at the output side of stage 130, when the inlet guide vanes
are in the wide open position. The signal becomes a logical -
zero when the vane opening is reduced from the wide open
vane condition. The signal is applied to one input connection
of the NAND gate 120, and is also inverted in the inverter
stage 132. The inverted signal is passed over line 133 to
the NAND gate 117 and to the NAND gate 134, the output of
which provides one input to the NAND gate 135 in the PRV
control logic circuit 96.
The vane position signal on line 62 is also
supplied to the n~gative input connection of each of the
comparator stages 136-140. The outputs of these comparators
are respectively coupled to the gate circuits 141-145, as
shown. These outputs from the gates are summed in stage 146
to provide a signal indicating the speed deviation from the
minimum Mach number based on the actual vane position.
The refrigerant condensing temperature sensed by
thermistor 56 provides a signal on line 55, and the refrigerant
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evaporating temperature sensed by thermistor 58 provides
another signal on line 57. These two signals are combined
in differential amplifier 147 to provide on lines 88 and 89
a signal related to the minimum Mach number Mo for wide open
vanes. At this point in the circuit the signal is related
only to the compressor head, and does not include any factor
related to the vane position. A portion of the evaporator
signal on line 57 is passed to another comparator 148, to
provide on line 150 a shut-down signal for the inverter
logic when the evaporator temperature drops below a pre-set
value, which value is established by the reference voltage
applied to the other input connection of this stage.
The Mo signal on line 88 is combined with the
speed change signal at point 90, and applied to the negative
input connection of op amp 151. Thus this connection
receives a composite input signal which is a function both
of the speed change signal and the minimum Mach number Mo~
The other input connection of stage 151 receives an actual
motor speed indicating signal over line 72, as described
previously. Thus the output signal from stage 151 which is
provided on line 91 to the inverter speed logic arrangement
repre~ents the error in the speed set point signal when the
refrigeration system is operating in the PRV control region,
where both the motor speed and the vane position must be
regulated to follow the optimum control path 80 depicted in
FIGURE 3.
In FIGURE 6B, the speed signal on line 91 is also
applied to the + input connection of another comparator 153,
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~76176-A-BWL
which receives a reference voltage at its other input
connection. Thus the output from stage 153 is a logic
signal, passed over line 154 to the NAND circuit 120,
signifying whether the actual motor speed is above or below
the desired motor speed for the given operating conditions.
In brief this logic signal is used to allow temperature
error information ~o modify the motor speed control command
and obtain faster system response, when such modification
does not pose the danger of sending the system into surge.
The duty cycle control circuit 86 operates to
limit the time of contact closure in the drive-open and
drive-closed portions of the PRV control, as a function of
the instantaneous PRV position. This is an important aspect ~
of the present invention, because, when the vanes are closed ~-
down to leave only a small opening, it prevents a "drive
closed" signal from being applied at a rate faster than the
system response rate. The duty cycle control circuit 86 can
also be considered a drive control circuit, in that it
determines the percentage of the time, in a given time
interval, that a drive signal is actually applied to the PRV
motor 32, or to any suitable capacity control means which is
adjustable to vary the capacity of the compressor. Hence
PRV motor 32 is a means for regulating the adjustable
capacity control means, the PRV themselves.
The problem posed by previous PRV control systems
can be better understood in connection with FIGURE 7. The
several curves there shown illustrate variations in compressor
capacity as a function of the PRV opening, for different
constant values of head (n). For example the curve 300
indicates the capacity-vane-opening relationship for a
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`'6176-A-B~L
compressor head value of 1.2. The succeeding curves 301,
302, 303 and 304 depict decreased values of compressor head,
down to an ~ of 0.8 for curve 304. As the compressor head
drops to 1.0 and values below that level, the slope of the
curve portions to the right of a PRV opening of about ~
become increasingly smaller. However for low values of PRV
opening, irr~spective of the head value, the slopes of the
curves are very great. Thus it is manifest that a very
slight physical displacement of the inlet guide vanes, when
the vanes are closed or almost closed, effects a very large
change in the system capacity. It would be desirable if the
curve shown in FIGURE 7 where more linear, and it is toward
a linearization of the system operation that the duty cycle
control circuit 86 is directed.
The duty cycle control circuit or drive control
circuit 86 (FIGURE 6B) basically comprises an integrator
stage 156, a NOR gate 157, and a comparator stage 160. This
drive control circuit receives two different input signals.
A first input of the drive control circuit can be considered
conductor 200, which receives a timing signal from the
oscillator 155. This timing signal, at 0.033 hertz in the
illustrated embodiment, is applied over the 1.3M resistor to
the upper input terminal of integrator stage 156, and is
also applied to the upper input terminal of NOR gate 157.
This signal is represented generally by the waveform 201 in
FIGURE 8A. NOR gate 157 will provide a logical 1 output
signal when both inputs are low, or 0. When the oscillator
signal represented by the waveform 201 goes low, the 5.5
volts reference voltage on the other input terminal of
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'6'76-A-BWL ~ t
integrator 156 allows this stage to begin charging at time
to~ and produces an output voltage such as 202 (FIGURE
8B) over line 161 to the upper input terminal of comparator
160. This comparator also receives a position signal over
line 85; this position signal varies as a function of the
setting of the adjustable capacity control means, which in
this embodiment is the PRV of the compressor. When the PRV
is virtually fully closed, the d-c voltage level on line 85
is approximately 2 volts, represented by the line 203 in
FIGURE 8B. When the increasing signal from integrator 156
over line 161 intersects the 2 volt line at point A, com-
parator 160 switches at time tl, represented by waveform 204
(FIGURE 8C). Thus for the time between to and tl, the
signal from comparator 160 was low, and the signal depicted
by curve 201 (FIGURE 8A) from the integrator 156 was also
low. Hence the output of NOR gate 157 was high at this
time, as shown by the pulse near the beginning of curve 205
in FIGURE 8D. So long as the comparator output remains
high, shown by curve 204, there is no further output from
the NOR gate 157. The signal 204 goes low again when the
decreasing slope of curve 202 intersects the 2 volt line, at
time t5. However at this time the oscillator output is
high, and hence there is no rurther output from NOR gate 157
until the increasing-slope portion of the integrator curve
202 again intersects the 2- volt line 203. Thus the drive
signal to the PRV, or to the means for regulating the
adjustable capacity control means, is clearly a function of
the vane position, represented by the level of the signal on
input line 85.
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6176-A-BWL
Supposing that the PRV are virtually wide open, a
10 volt signal is provided over line 85; this signal is
represented by the line 206 in FIGURE 8B. It is thus
apparent the increasing-slope portion o~ curve 202 will
intersect the 10 volt line at point B, or at time t3 shown
in curve 207 of FIGURE 8E. At this time the output of
comparator 160 goes high, as shown on curve 207, preventing
any output from NOR gate 157. However prior to that switch-
ing, between times to and t3 as shown on curve 208 in FIGURE
8F, the output of NOR gate 157 was high because both inputs
to this gate were low. This provides a much longer duration
pulse in the curve 208 for driving the PRV or any other
adjustable capacity means. In the illustrated embodiment,
the circuit components provided a pulse of approximately 1
second on the waveform 205, and approximately 5 seconds in
the waveform 208. Intermediate positions of the PRV provide
corresponding intermediate pulse lengths. Of course other
drive control circuits could be employed to provide a
variation of some other signal characteristic, such as
amplitude, to regulate the change in the adjustable capacity
control means in accordance with the instantaneous position
of the adjustable capacity control mèans.
The capacity control signal is passed from NOR
gate 157 into the NAND gate 162, and also passed upwardly
into the other NAND gate 135. The operation of the logic
arrangement for driving the vanes open or closed is achieved
with the additional stages 163-168, and 170, connected as
shown.
For example, a logical zero signal from NOR stage
165 (FIGURE 6C) provides no gate drive for the NPN-type
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`~176-A-BWL
transistor, energizing the light-emitting diode from B+ and
the lK resistor, and providing gate drive to the thyristor,
which is energized and provides a close-vane signal on line
. 0~ ~1
54. The terminal below the 0.~2 microfarad capacitor,
marked X, corresponding to the other similarly marked
terminal in the open-vane circuit, represents a common
electrical connection for the PRV motor. Of course, the
system can be manually operated by displacing the mode
selector switch 171 from the automatic position, in which it
is illustrated, to either of the other three positions.
When the movable contact is displaced to engage the "hold"
contact, then the vane motor cannot be driven open or closed
but remains in the present position. When displaced to
contact the "open" contact, an energizing circuit is completed
;, to gate on the thyristor and provide a signal on line 53 to
open the vanes. Similarly when the switch is further
displaced to engage the "close" contac~, operation of the
logic circuit provides the gate drive signal necessary to
energize the thyristor and provide the close-vane signal on
line 54.
The motor current level signal is received over
conductor 71 and divided over the illustrated 3K potentio-
meter, with a portion of this signal being diverted over
line 172 to the first current limit stage ffl . When the
level of the motor current reaches a previously determined
value representing 100% of the current rating, stage 173
; switches and provides an output signal which is passed both
` to the PRV logic control circuit and to the inverter speed
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` - '
6176-A-B1~
logic control circuit as illustrated. In the inverter speed
control portion, this means that the inverter motor cannot
be driven any faster while this 100% current level is
maintained, and the speed will in fact be reduced. In
addition, the operation o~ the PRV logic control circuit
under these conditions is such that the vanes cannot be
driven open, but when a decrease-load signal is provided,
the vanes can be driven closed to reduce the loading on the
system. If the current level increases further, to a level
indicating 103% of the rated current is flowing in the motor
windings, stage 174 is switched to provide a signal to the
PRV logic control circuit which begins to drive the vanes
toward the closed position, to reduce the load. If the
current increases further to the level of 106% of rated
current, this signal is sensed over line 175 and stage 176
switches to provide on line 70 a signal which shuts down th~
inverter logic and thus correspondingly removes the motor
energization.
FIGURE 9 illustrates a pair of curves depicting
the variation of compresQor speed as a function of the
opening of the PRV, for a fixed compressor head value.
Curve 307 depicts a surge curve line developed from actual
data, so that operation in the lower left portion of this
curve would cause compressor surge. To avoid encountering
surge, an actual functional 308 was derived, representing a
mathmatical function to regulate operation of the control
system of this invention. By regulating the speed of the
electrical prime mover 36 and the extent of opening of the
PRV to follow the functional 308, not only is surged avoided,
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076176-~-BWL
.q~
but the system is operated in substantially the most energy
efficient manner. That the control system can regulate
operation along curve 308 is due in part to the effective
derivation of the minimum Mach number Mo on line 89 (FIGURE
5) from the condensing and evaporating temperatures. There-
after this minimum Mach number, or head-indicating signal,
is passed to the output side of network 87. The signal
derived from PRV position potentiometer 61 is modified by
the network 87 to produce a modified or functional signal
for combination with the minimum Mach number signal on line
90. This combination of signals on line 90 produces a
signal which is then combined with the actual motor speed
signal on line 72 to produce a "speed boost" signal for the
inverter speed control portion of the control system. The
term "speed boost" refers to the speed correction desired
for the induction motor driving the compressor, considering
the miminum Mach number Mo~ the functional signal at the
output side of network 87, and the actual motor speed signal.
The resultant speed boost signal provides an efficient
corrective value for regulating the induction motor speed in
an optimum manner.
Technical Advantages
The present invention provides effective control
for refrigeration systems using compressors having adjustable
inlet guide vanes, in which the compressor is driven by a
variable speed electrical motor. The system and method of
the invention were successfully tested, and proved that
automatic adjustment of compressor speed and the inlet guide
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76176-A-BWL
vane position could match the compressor head and flow
requirements for a given evaporator load, while maintaining
the chilled water at a constant temperature. The path of
control, as shown in FIGURE 4, tends to minimize system
energy requirements while at the same time avoiding compressor
surge. In addition the control system and the control
method of this invention have shown a capability of maintaining
the chilled water within 0.15F. of its set point, through-
out the entire feasible load range. In particular the
maximum possible use of motor speed control is effected,
before beginning to adjust the PRV position to vary the
compressor load. This is the most ~nergy-efficient method
for operating an arrangement with adjustable inlet guide
vanes, and has proved significantly more efficient than
systems using a combination of PRV control and hot gas
bypass to prevent surge. The present arrangement has
eliminated the need for a gear box, and allowed operation
with smaller, more efficient, higher speed motors. In
addition the compressor is quieter at part load operation,
it is driven more slowly, at a vane angle more suited to
noise reduction.
The invention is in part based on the appreciation
of the compressor characteristics, and the derivation of the
minimum Mach number Mo~ which is limited by the head on the
compressor. This use of the Mach number for wide open vanes
is continually monitored in the control system, to match the
load requirements to the capacity of the compressor at that
time. It is very important that the maximum range of speed
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~76176-A-BWL
control be utilized, before switching to the region of PRV
control as described in connection with FIGURE 4. The
derivation of the head information from the two temperatures
is a significant aspect of the invention. This provides
accurate control with minimum transducer cost, because
thermistors can be used in place of the more expensive
pressure sensors frequently used in such arrangements.
The system and method of the invention regulates
the refrigeration system in both the "speed control'~ region
and in the "PKV control" region. In the speed control
region, the concept of control is simply to adjust compressor
speed by regulating the frequency of the inverter output
voltage either up or down, depending on whether system
capacity needs to be increased or decreased. The error in
chilled water temperature, with respect to the set point, is
used to define needed system capacity changes. The technique
is easily understood and, after defining the response,
stability, and loop compensation requirements, can be
readily implemented with low cost electronic circuitry. In .-
the PRV control region, chilled water error changes require
a more complicated set of control changes and continuous
monitoring of system variables. Not only does the PRV
position need to be adjusted, but in order to avoid surge,
the compressor speed must be concomitantly regulated according
to head measurements and a pre-stored function of PRV
position (networ~ 87). Superimposed on these two regions of
control characteristic, and its attendant boundary definition
problem, is the requirement that the path of control chosen
for compressor speed and PRV position adjustment must be
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176176-A-BWL
energy conservative.
In the method and system of the invention, the
observation that the compressor head is a nearly linear
function of the difference between the refrigerant condensing
and evaporating temperatures was of prime importance.
Furthermore, experimental data on the compressor used
demonstrated that the minimum allowable Mach number, to
avoid surge, was linearly related to compressor head.
Thereafter it was discovered that compressor head, in
combination with the PRV position9 could be used ~o define
surge locii of the compressor at part vanes as well as at
wide open vanes, and thereby produce the desired system
operating path shown in FIGURE 4. Thus signals from re-
latively low cost transducers, indicating condensing and
evaporating temperatures, compressor speed, and PRV position,
together with the chilled water temperature, form the basis
for the effective control system and method of this invention.
The method of system control can be readily
understood in connection with FIGURE 5. In the method,
the compressor head signal is continually established as a
function of the condensing refrigerant and evaporating
refrigerant temperatures; this head signal appears on line
89. A functional signal, related to the instantaneous
position of the inlet guide vanes, is derived at the output
side of network 87. The head-indicating signal and the
functional signal are then combined to produce an inter-
mediate signal on line 90. A signal related to the actual
motor speed is provided on line 72; this can come from the
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.- , ..
0? 617 6 -A- B~
motor, or from line 51, or some other source. The actual
motor speed signal and the intermediate signal are combined
to provide a first signal, on line 91, for use in regulating
the speed of the compressor drive motor. A temperature
error signal, related to the difference in temperature
between the cooling medium at the evaporator outlet and the
desired temperature set point, is produced on line 67, and
the temperature error signal is used as a second signal for
regulating both the speed of the compressor drive motor and
the position of the inlet guide vanes.
Another way of understanding the method of the
invention is in connection with FIGURE 4. As there shown,
the system is considered to be initially operating at a
first operating condition, referenced g. Then the compressor
drive motor speed is reduced, while the inlet guide vanes
are kept wide open, from the first operating condition (g)
through a second operating condition (h) to a third operating
condition (j). Then a gradual closing of the inlet guide
vanes begins, and simultaneously the compressor drive motor
speed is increased , to reach a fourth operating condition
(k). Closure of the inlet guide vanes is continued while
the drive motor speed is simultaneously reduced, to reach
the fifth operating condition (m). Closure of the inlet
guide vanes is continued, while simultaneously increasing
the drive motor speed, until a sixth operating condition (n)
is reached.
It is important to note that the control sequence
to again restore capacity to the system is significantly
different from that just described (g, h, j, k, m, n) if the
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076176-A-BWL
most energy-efficient path is followed. First the inlet
guide vanes are gradually opened while the compressor drive
motor speed is decreased as the system moves from the sixth
operating condition (n) to the fifth operating condition
(m). Then the inlet guide vanes are further opened while
simultaneously increasing the motor speed, while the system
moves from the fifth operating condition (m). Then the
inlet guide vanes are further opened while simultaneously
increasing the motor speed, while the system moves from the
fifth operating condition (m) to the fourth operating
condition (k). The opening of the inlet guide vanes i9
increased, and simultaneously the compressor drive motor
speed is increased, as the system moves from the fourth
operating condition (k) directly to the second operating
condition (h), without going through the third operating
condition (j). The inlet guide vanes are fully open at the
second operating condition (h). Lastly, the compressor
drive motor speed is increased to increase the system
capacity, until the first operating condition (g) is again
reached. More succintly, the method of the invention
regulates the position of the inlet guide vanes and the
speed of the compressor drive motor substantially according
to the functional variations illustrated in FIGURE 4 and
described above, to avoid surge while operating in an
energy-efficient manner.
Those skilled in the art will appreciate that the
control system of the invention is similarly applicable to
regulate a heat pump arrangement, in which both the speed of
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076176-A-BWL ~ l~ .S.~ I
the compressor and the amount of gas admitted to the com-
pressor are varied to follow an optimum control path.
To assist those skilled in the art to implement
the invention, below is a list of the integrated circuit
(IC) types, and identification of other components not
already specified on FIGS. 6A-6C.
IC TyPe Stages (FIGS. 6A-6C)
MLM2902P 59, 140, 148, 149
MLM2902P 136, 137, 138, 139
MC14016BCP 141, 142, 143, 144
MLM2902P 65, 101, 103, 104
MLM2902P 106, 110, 111, 156
MLM2902P 130, 146, 151, 153
MC14016BCP 123, 124, 125, 145
CA3160S 52
MLM2902P 160, 173, 174, 176
MC14572BCP 114, 115, 116, 117, 122, 132
MC14023BCP 120, 135, 162
MC14001BCP 108, 121, 165, 167
MC14011BCP 134, 164, 166, 170
MC14001BCP 118, 157, 163, 168
MC14541BCP 155
T2310A All Triacs
NSL5057 All led's
2N3415 All transistors
IN914 All diodes
;
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076176-A-BWL
With this explicit and detailed exposition of the
inventive principles and a preferred embodiment of the
control system, it is evident that such a system can be
utilized both with existing equipment, by way of retrofit,
and with newly installed systems.
In the appended claims the term "connected" means
a d-c connection between two components with virtually zero
d-c resistance between those components. The term "coupled"
indicates there is a functional relationship between two
components, with the possible interposition of other elements
between the two components described as "coupled" or "inter-
coupled".
: 34
