[0001] The present invention relates to a control method and control apparatus for a displacement-type
fluid machine to control the number of revolutions of a drive motor by means of an
inverter or the like in operating a displacement-type fluid machine such as a Roots-type
blower or a vane pump.
[0002] A displacement-type fluid machine for handling fluid such as a displacement-type
pump is, for example, employed for lowering or decreasing pressure on an intake side
of a displacement-type pump, increasing the pressure on a discharge side of the pump,
or transporting liquid across the pump. A displacement-type pump is normally used
together with a sealable container such as a tank, and processing values such as pressure
and liquid level within the tank, etc., are detected and controlled so as to be within
a predetermined range, by actuating or stopping the displacement pump. When an inverter
or the like is employed to adjust the number of revolutions of the motor, the frequency
is gradually increased or decreased to avoid abrupt acceleration or deceleration upon
driving or stopping, or the number of revolutions is selected according to fluctuations
in process values.
[0003] In a so-called ON/OFF control method, a displacement-type pump is actuated when an
allowable limit of a process value is detected and the pump is stopped when a predetermined
process value is detected. In this control method, however, the pump may be actuated
too frequently depending on the operating conditions, resulting in damage to the motor
and related equipment and a decrease in the working life of the equipment. In order
to restrain the actuation frequency to within allowable times, a sealable container
such as a tank must have sufficient capacity, which leads to increased facility costs.
Further, since abrupt changes in process values are unavoidable in the ON/OFF control
method, great fluctuations in the pressure or liquid level on the intake side or discharge
side of the pump are caused, preventing stable operation of the system. Moreover,
since the aforementioned control method greatly relies on detectors for detecting
a pressure and liquid level, a proper operation of the apparatus is often prevented
by the malfunctioning of these detectors.
[0004] The present invention has been made in the light of the aforementioned problems,
and the object thereof is to provide a method and an apparatus for controlling a displacement-type
fluid machine which enables to keep the process values within an allowable limit without
effecting repeated actuation and stopping of the pump.
[0005] In order to accomplish the object of the invention stated above, according to a first
aspect of the invention, in a method for controlling a displacement-type fluid machine
which handles fluid including gas and/or liquid for increasing or decreasing pressure
of the fluid or transporting the fluids, the method comprises: provision of an alternating
current motor for driving the displacement-type fluid machine; and provision of a
frequency converter which is capable of conducting frequency conversion up to a range
higher than the power source frequency to adjust the number of revolutions of the
aforementioned motor; wherein the number of revolutions is adjusted so that an input
current to the motor is kept constant, regardless of any change in operating pressure
of the displacement-type fluid machine.
[0006] According to a second aspect of the invention, in a method for controlling a displacement-type
fluid machine according to the first aspect, an input current value to the motor is
detected either within the frequency converter or at the primary or secondary side
thereof, a current setting device is provided to set a constant current value according
to the motor rating, and wherein input frequency to the motor is adjusted so that
the input current value to the motor is maintained constant, based on an output signal
from a comparator/adjuster device which compares the input current value with the
set current value.
[0007] According to a third aspect of the invention, in a method for controlling a displacement-type
fluid machine according to the first or second aspect, an upper limit is provided
for the input frequency to the motor, whereby the number of revolutions of the motor
and displacement-type fluid machine is maintained at a predetermined value or lower.
[0008] According to a fourth aspect of the invention, in a method for controlling a displacement-type
fluid machine according to any of the first aspect to the third aspect, the motor
and displacement-type fluid machine are stopped when the input frequency to the motor
reaches a predetermined minimum value, and the reduction in pressure difference or
liquid level difference between the upstream side and downstream side of the displacement-type
fluid machine is measured, and the motor and displacement-type fluid machine are actuated
when the reduction reaches a predetermined value.
[0009] According to a fifth aspect of the invention, in an apparatus for controlling a displacement-type
fluid machine which handles fluid including gas and/or liquid for increasing or decreasing
pressure of the fluid or transporting the fluids, the aforementioned apparatus comprises:
an alternating current motor for driving the displacement-type fluid machine; a frequency
converter which is capable of conducting frequency conversion up to a range higher
than the power source frequency to adjust the number of revolutions of said motor;
and control means for adjusting the number of revolutions of the motor so that input
current to the motor is constant, regardless of any change in operating pressure of
the displacement-type fluid machine.
[0010] According to a sixth aspect of the invention, in an apparatus for controlling a displacement-type
fluid machine according to the fifth aspect, further comprises: means for detecting
the input current value to the motor either within said frequency converter or at
the primary or secondary side thereof, a current setting device for setting a constant
current value according to the motor rating; and a comparator/adjuster device which
compares the input current value with the constant current value set at the current
setting device; wherein the control means adjusts input frequency to the motor so
that the input value to the motor is maintained constant based on the output signal
of the comparator/adjuster device.
[0011] According to a seventh aspect of the invention, in an apparatus for controlling a
displacement-type fluid machine according to the fifth or sixth aspect, means for
providing an upper limit for the input frequency to the motor is provided, thereby
maintaining the number of revolutions of the motor and displacement-type fluid machine
at a predetermined value or lower.
[0012] According to eighth aspect of the invention, in an apparatus for controlling a displacement-type
fluid machine according to a fifth to seventh aspect, further comprises: means for
stopping the motor and displacement-type fluid machine when the input frequency to
the motor reaches a predetermined minimum value, and means for measuring the reduction
in pressure difference or liquid level difference between the upstream side and downstream
side of the displacement-type fluid machine, and actuating the motor and displacement-type
fluid machine when said reduction reaches a predetermined value.
[0013] The aforementioned displacement-type fluid machine may comprises two-lobe or three-lobe
Roots-type vacuum pump or compressor, gear pump, rotary vane-type pump or compressor,
water sealing vacuum pump or compressor, reciprocating liquid pump or compressor,
or reciprocating vacuum pump.
[0014] with the arrangement according to the invention, since the number of revolutions
is adjusted so that the input current to the drive motor of the displacement-type
pump is kept constant regardless of any change in operating pressure of the displacement-type
pump, when the operating difference pressure or operating pressure of the displacement-type
pump decreases and the required motive power decrease, the number of revolutions is
increased, and thus the intake flow rate increases proportionally. On the other hand,
when the operating pressure of the displacement-type pump increase and the required
motive power increases, the number of revolutions is decreased so as to maintain the
input current to the motor at a constant level, and thus the intake flow rate decreases
proportionally.
[0015] In general, the maximum operating pressure and flow rate of the displacement-type
pump driven by an alternating current motor is achieved at the rated number of revolutions
at the power source frequency. However, the operation stated above can be realized
by means of a frequency converter which is capable of conducting frequency conversion
up to a range higher than the power source frequency, to enable the motor speed to
be increased even when the operating pressure is decreased.
[0016] By selecting the capacity of the displacement-type pump around the average value
with time for the fluctuating demand, the displacement-type pump is not actuated and
stopped repeatedly, but is rather continuously driven in such a way that the number
of revolutions is increased or decreased according to fluctuation on demand, resulting
in a simple control mechanism and lower costs.
[0017] An apparatus including a displacement-type pump is actuated either manually or automatically
upon detection of a value lower than the predetermined operating pressure difference
or liquid level difference across the displacement-type pump. However, since the pressure
or liquid level exerts little load on the displacement-type pump upon actuation, the
number of revolutions of the motor and the flow rate are increased rapidly in the
early operating stage, thereby providing a predetermined pressure and liquid level
in a short time.
[0018] When the input current value to the motor is detected either within the frequency
converter or at the primary or secondary side thereof, and a comparator/adjuster device
compares the input current value with a constant current value set at the current
setting device, it is possible to maintain an input value to the motor at the constant
current value based on the motor rating.
[0019] When an upper limit is provided for the input frequency signal, it is possible to
prevent an excessive increase in the number of revolutions.
[0020] And when a lower limit is provided for the aforementioned input frequency signal,
it is possible to prevent an excessive load by detecting this lower limit and stopping
the drive motor.
[0021] The above and other objects, features and advantages of the present invention will
become more apparent from the following description when taken in conjunction with
the accompanying drawings in which preferred embodiments of the present invention
are shown by way of illustrative examples.
[0022] Fig. 1 is a block diagram illustrating the control apparatus of the displacement-type
pump of the first embodiment of the present invention.
[0023] Fig. 2 is a graph illustrating the performance of the displacement-type pump according
to the control method of the first embodiment of the present invention.
[0024] Fig. 3 is a block diagram illustrating the control apparatus of the displacement-type
pump of the second embodiment of the present invention.
[0025] Fig. 4 is a graph illustrating the performance of the displacement-type pump according
to the control method of the second embodiment of the present invention.
[0026] Fig. 5 is a block diagram illustrating the control apparatus of the displacement-type
pump of the third embodiment of the present invention.
[0027] Fig. 6 is a graph illustrating the performance of the displacement-type pump according
to the control method of the third embodiment of the present invention.
[0028] Fig. 1 illustrates a first embodiment of the present invention applied to a vacuum
blower for a vacuum-type sewage collection system. This system is provided with a
vacuum tank 1 at a vacuum pumping station, and effects continuous collection of sewage
via connected piping 2 by maintaining the tank under a vacuum state.
[0029] An alternating current motor 4 which drives the vacuum blower 3 is supplied with
electrical power from an inverter (frequency converter) 5. A current detector 6 is
provided at the primary side of the inverter 5, and the detected current is input
to a comparator/adjuster device 7. On the other hand, a signal from a current setting
device 8 which sets the current value according to the rating of the motor 4 is compared
with the detected current value in the comparator/adjuster device 7, from which a
frequency increase/decrease signal based on the deviation of the above comparison
is input to the frequency setting portion of the inverter 5, thereby increasing or
decreasing the number of revolutions of the motor 4, i.e., the vacuum blower 3. The
reference numeral 9 denotes a frequency detector for measuring the secondary side
frequency of the inverter 5, which can be used for setting the upper limit of frequency.
[0030] Fig. 2 is a diagram describing the change in performance of the vacuum blower 3 shown
in Fig. 1 in the event that the input current to the motor 4 is controlled so as to
be a constant value. This figure illustrates the theoretical performance of a displacement-type
pump when it is operated at different numbers of revolutions. Namely, when the theoretical
flow rate Q at each constat number of revolutions of the motor is represented by the
ordinate in the upper half of Fig. 2, and the degree of vacuum P is represented by
the abscissa, the flow rate of the displacement-type pump is proportional to the number
of revolutions, and the flow rate at each of the number of revolutions is constant
value as represented by horizontal lines Q 100%N, 120%N, ... On the other hand, the
required motive power changes according to the degree of vacuum P. The required motive
power is represented as being 100% when the rated flow under the rated number of revolutions
is taken to be 100% and the rated degree of vacuum is taken to be P
0 at which the required motive power reaches a maximum. When the operation of the vacuum
pump is taken to be adiabatic compression, the required theoretical motive power at
each of the number of revolutions regarding the degree of vacuum P is represented
by a group of curves; L 100%N, 120%N ... The points of intersection a1, a2, a3 ...
between these curves and the horizontal line L100% representing constant motive power
indicate degrees of vacuum which provide a constant value of 100% theoretical motive
power at each of the number of revolutions. The flow rate corresponding to these degrees
of vacuum at each number of revolutions can be obtained from the points of intersection
b1, b2, b3 ... between these degrees of vacuum and the horizontal lines of the flow
Q100% , 120% ... corresponding to each number of revolutions. Thus, by controlling
the number of revolutions of the motor so that the primary current to the motor or
the input motive power to the motor is made constant under a constant power source
voltage, the pump exhibits flow rate to vacuum degree properties as indicated by the
curved line Q-P (L const) in the figure.
[0031] Although in Fig. 2, the adiabatic efficiency of the vacuum pump and the mechanical
efficiency are taken as being constant, and further, the efficiency of the motor and
inverter are also taken as being constant, the fluctuations of these efficiencies
in practical apparatus are relatively small even when the number of revolutions or
degree of vacuum fluctuates, so that the relation between the degree of vacuum P and
flow rate Q under a constant input motive power indicates a tendency shown by the
curve of Q-P (L const). As can be clearly seen from Fig. 2, when the degree of vacuum
P drops, i.e., when the intake absolute pressure increases, the motor power source
frequency, i.e., the number of revolutions increases so that the input current is
kept at constant value and the intake flow rate is remarkably increased. For example,
in Fig. 2, when the degree of vacuum reaches P
1, the number of revolutions increases to 160% of the rated revolution number and the
flow rate is increased accordingly.
[0032] Conventionally, with vacuum sewage collection systems, etc., the vacuum pump is operated
at a constant speed, and when the degree of vacuum drops to an intermediate degree
of vacuum such as P
1, the pump is actuated, and when the degree of vacuum reaches the maximum P
0, the pump is stopped, thereby repeating this actuation and stopping. The vacuum tank
pressure is normally operated at a value between the maximum degree of vacuum P
0, and an intermediate degree of vacuum P
1. In this invention, by setting the vacuum pump capacity to a predetermined air capacity
which is most frequently used, and by controlling the number of a revolutions so that
the required motive power is kept at constant value, the vacuum pump is not needed
to be turned on and turned off during this process, but can be continuously operated
at a number of revolutions corresponding to the degree of vacuum. Further, a conventional
vacuum tank having a great capacity to avoid the frequent actuation of the vacuum
pump is not needed.
[0033] Since the degree of vacuum of the vacuum tank is low when starting up the facilities,
the vacuum pump operates at a high speed, thereby obtaining the predetermined degree
of vacuum in a short time. Subsequently, continuous operation is maintained while
the number of revolutions is automatically adjusted according to demand. The vacuum
pump may be arranged in such a way that the maximum degree of vacuum P
0 is detected and the pump is shut down when the facilities are inoperative, such as
at night, and the pump is actuated by detecting the intermediate degree of vacuum
P
1 upon start-up of the facilities.
[0034] In order to prevent an excessive increase in speed of the vacuum pump when the degree
of vacuum in the vacuum tank is low, by detecting the frequency at the secondary side
of the inverter 5, and by setting an upper limit in the frequency detector 9, the
vacuum pump can be operated at all times at a number of revolutions which is within
an allowable limit.
[0035] With frequency converters employing general-use inverters, the ratio between a secondary
voltage and secondary frequency is constant, but at frequencies higher than the power
source frequency, the secondary voltage is limited by the power source voltage and
consequently is the same value. Therefore, by controlling the primary current to be
constant, the motor current becomes approximately constant around the rated value,
thereby avoiding problems such as an increase in the temperature of the motor and
excessive load, etc.
[0036] As a second embodiment, Fig. 3 illustrates an apparatus which pressurizes fluid and
accumulates the pressurized fluid in a pressure tank 12 by means of a displacement-type
liquid pump 11, for applying the pressurized fluid to various processing. Automatic
ON/OFF operation of the pressure-oil pump 11 is generally conducted to maintain the
pressure or liquid level in the pressure tank within a predetermined range.
[0037] Fig. 4 illustrates a theoretical performance of the displacement-type liquid pump
when it is controlled in the apparatus shown in Fig. 3. At rated-speed operation,
the flow rate Q is constant regarding the operating pressure P and is represented
by a horizontal line Q (100%N). The required motive power L
p at rated speed operation increases proportionally to the pressure P and is represented
by a straight line L
P 100%N.
[0038] By controlling the number of revolutions so that the input current to the displacement-type
pump driving motor is kept at a constant level according to the present invention,
the relation between the flow rate and operating pressure becomes such as that represented
by the curve Q-P (L
p const). Consequently, the number of revolutions is increased with a drop in operating
pressure, and the flow rate is remarkably increased. An upper limit Nmax is set for
the number of revolutions.
[0039] Upon starting up the apparatus, the displacement-type pump 11 is actuated after having
detected pressure of P
1 or lower. Since the flow rate after actuation of the pump is great, the pressure
or liquid level of the predetermined level can be attained in a short period. Further,
by appropriately selecting the capacity of the displacement-type pump, the pump is
operated continuously between pressures P
1 and P
0 so that the frequency of actuation can be reduced. Thus, a pressure tank having a
great capacity, which was needed to cope with the frequent activation of the pump
in the conventional ON/OFF operation under the fixed motor speed becomes unnecessary.
The displacement-type pump is stopped upon detection of lower limit value Nmin of
the number of revolutions, i.e., minimum frequency. This Nmin is set so that the secondary
side current of the frequency converter 5, i.e., motor current, does not exceed the
allowable value.
[0040] As a third embodiment, Fig. 5 illustrates an apparatus which pressurizes gas and
accumulates pressure in a pressure tank 14 by means of a displacement-type compressor
13, for applying the pressurized gas to various processing. Fig. 6 illustrates a theoretical
performance of an adiabatic compression of the displacement-type compressor with a
rated compression ratio of P
2/P
1=2.5. When the apparatus is operated at the set speed or rated speed, the intake flow
rate Q
1 is constant regarding the operating compression ratio P
2/P
1, and is represented by a horizontal line Q
1 100%N. On the other hand, the required motion power Lad increases with the compression
ratio P
2/P
1, and is represented by a curve Lad 100%N.
[0041] By controlling the number of revolutions so that an input current to the motor driving
the displacement-type compressor is kept constant according to the present invention,
the relation between the intake flow rate and the compression ratio is represented
by a curve Q
1 (Lad const), so that the intake flow rate is remarkably increased with a decrease
of the compression ratio. An upper limit Nmax of the number of revolutions is set
in the frequency detector 9, which detects the frequency of the secondary side of
the frequency converter apparatus, and limits the speed of the motor.
[0042] By appropriately selecting the capacity of the displacement-type compressor, taking
into consideration the operation under variable speed, the compressor can be operated
in continuous basis between (P
2/P
1)
0 ∼ (P
2/P
1)
1 during operation of the apparatus. When the minimum number of revolutions Nmin is
detected from the input frequency to the motor, the motor is stopped, thereby preventing
an excessive load on the motor.
[0043] As described above, according to the present invention, by controlling the number
of revolutions of a displacement-type pump so that the input electrical power to the
drive motor of the pump is made constant, it is possible to continuously operate the
pump unit in an automatic manner according to the operating pressure or liquid level,
and to exhibit the utmost capability as a pump unit including a drive motor. Also,
while relatively large-scale pressure or decompression containers were needed in the
conventional ON/OFF operation to restrain the actuation frequency of the machinery
within a allowable Limit, with the present invention such large-scale containers are
unnecessary or can be made quite small. Further, when starting up the apparatus, the
number of revolutions is increased to the upper limit of mobile power, so that the
pressure, vacuum level, liquid level, compression ratio, etc. created by the displacement-type
pump can be increased to a usable level in a short period. Moreover, continuous operation
is made possible according to demand while avoiding excessive activation and stopping,
by selecting the rated capacity of the displacement-type pump to be compatible to
the demand.
[0044] According to its broadest aspect, the invention relates to a method for controlling
a displacement-type fluid machine which handles fluid including gas and/or liquid
for increasing or decreasing pressure of fluid or transporting fluid, said method
comprising: provision of a motor driving the displacement-type fluid machine; and
provision of a frequency converter to adjust the number of revolutions of said motor.
1. A method for controlling a displacement-type fluid machine which handles fluid including
gas and/or liquid for increasing or decreasing pressure of fluid or transporting fluid,
said method comprising: provision of an alternating current motor for driving the
displacement-type fluid machine; and provision of a frequency converter which is capable
of conducting frequency conversion up to a range higher than the power source frequency
to adjust the number of revolutions of said motor;
wherein the number of revolutions is adjusted so that an input current to the motor
is kept constant, regardless of any change in operating pressure of said displacement-type
fluid machine.
2. A method for controlling a displacement-type fluid machine according to Claim 1, wherein
the input current value to said motor is detected either within said frequency converter
or at the primary or secondary side thereof, a current setting device is provided
so as to set a constant current value according to the motor rating, and wherein input
frequency to said motor is adjusted so as to maintain the input value to said motor
constant, based on the output of a comparator/adjuster device which compares said
input current value and set current value.
3. A method for controlling a displacement-type fluid machine according to Claim 1 or
2, wherein an upper limit is provided for said input frequency to said motor, thereby
maintaining the number of revolutions of said motor and displacement-type fluid machine
to a predetermined value or lower.
4. A method for controlling a displacement-type fluid machine according to any of Claims
1 through 3, wherein said motor and displacement-type fluid machine are stopped when
the input frequency to said motor reaches a predetermined minimum value, and the reduction
in pressure difference or liquid level difference between the upstream side and downstream
side of the displacement-type fluid machine is measured, and the motor and displacement-type
fluid machine are actuated when said reduction reaches a predetermined value.
5. An apparatus for controlling a displacement-type fluid machine which handles fluid
including gas and/or liquid for increasing or decreasing pressure of the fluid or
transporting the fluid, said apparatus comprising: an alternating current motor for
driving the displacement-type fluid machine; a frequency converter which is capable
of conducting frequency conversion up to a range which is higher than the power source
frequency to adjust the number of revolutions of said motor; and control means for
adjusting the number of revolutions so that input current to said motor is constant,
regardless of change in operating pressure of said displacement-type fluid machine.
6. An apparatus for controlling a displacement-type fluid machine according to Claim
5, further comprising: means for detecting the input current value to said motor either
within said frequency converter or at the primary or secondary side thereof, a current
setting device for setting a constant current value according to the motor rating;
and a comparator/adjuster device which compares said input current value with the
constant current value set at said current setting device; wherein said control means
adjusts an input frequency to said motor so that the input value to said motor is
maintained constant based on the output signal of said comparator/adjuster device.
7. An apparatus for controlling a displacement-type fluid machine according to Claim
5 or 6, wherein means for providing an upper limit for input frequency to said motor
is provided, thereby maintaining the number of revolutions of said motor and displacement-type
fluid machine to a predetermined value or lower.
8. An apparatus for controlling a displacement-type fluid machine according to any of
Claims 5 through 7, further comprising means for stopping said motor and displacement-type
fluid machine when the input frequency to said motor reaches a predetermined minimum
value, and means for measuring the reduction in pressure difference or liquid level
difference between the upstream side and downstream side of said displacement-type
fluid machine, and actuating said motor and displacement-type fluid machine when said
reduction reaches a predetermined value.
9. An apparatus for controlling a displacement-type fluid machine, wherein said displacement-type
fluid machine comprises two-lobe or three-lobe Roots-type vacuum pump or compressor,
gear pump, rotary vane-type pump or compressor, water-ring vacuum pump or compressor,
reciprocating liquid pump or compressor, or reciprocating vacuum pump.
10. A method for controlling a displacement-type fluid machine which handles fluid including
gas and/or liquid for increasing or decreasing pressure of fluid or transporting fluid,
said method comprising: provision of a motor driving the displacement-type fluid machine;
and provision of a frequency converter to adjust the number of revolutions of said
motor.