Note: Descriptions are shown in the official language in which they were submitted.
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Apparatus and Method for Maintaining an Operating Condition for a Blower
Technical Field and Background Art
The present invention relates to devices that move air by spinning such as
blowers
and/or fans including draft inducers. Such device may be used with HVAC
systems, furnaces
and hot water heaters for venting air or gases from an inlet to an outlet.
Prior art ventilation systems for HVAC (heating, ventilation, and air
conditioning),
furnaces and hot water heaters are designed to extract a requisite amount of
gas from the
system regardless of the output impedance. Stated in another way, a
ventilation system for a
hot water heater that includes a draft inducer is designed such that the
system will operate
assuming a maximum impedance at the output of the draft inducer. As such, if
the impedance
at the output of the draft inducer is less than the maximum, heat that could
be used for
heating is removed from the system making the system less than ideal in terms
of energy
efficiency and cost. If too little heat is removed the system will operate
inefficiently and
create possible negative emissions. Negative emission may be levels of exhaust
gas above
that specified by a manufacturer or by a government body. If the flow rate is
not high
enough, the exhaust may stagnate and extinguish the heating source. AC motors
and forward
curved impeller blades were used by prior art systems in draft inducers for
hot water heaters
because AC motors exhibit a flat torque/speed curve. As such, even if the
torque drops, the
speed of the motor remains nearly constant. This characteristic was viewed as
desirable,
since the impedance on the draft inducer from application to application
varies. For example,
the length of the exhaust pipe that the draft inducer is used to drive may
vary from
approximately 1 foot to lengths of 45 feet or more. As such, the draft inducer
with an AC
motor would be able to drive the gases through the exhaust piping regardless
of the length of
the exhaust pipe.
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Summary of the Invention
An apparatus for maintaining a predetermined flow rate in a ventilation system
having a motor driven blower is disclosed. In one embodiment, the blower is a
draft inducer.
The apparatus contains a module for determining the impedance at the outlet of
the blower.
The impedance results from the length of pipe through which the blower blows
the exhaust
from a heating device, such as a hot water heater along with any obstructions.
The
impedance is determined by using a look-up table to compare the measured
rotation rate
(RPMs) of the motor shaft to values in the look-up table. The RPMs are
measured by a
sensor that is coupled to the rotating shaft of the motor. The sensor provides
a sensor signal
to the impedance module. Once a substantially matching value is found, the
corresponding
impedance is obtained from the look-up table. The impedance is then passed to
a module for
setting the motor speed. The motor speed module and the impedance module may
be part of
a single processor. In other embodiments the motor speed module and the
impedance module
may be part separate processors. In other embodiments the motor speed module
and the
impedance module may be firm ware that is partially hardware and partially
software.
The motor speed module adjusts the speed of the motor based on the impedance
to
maintain a constant flow rate. A look-up table is accessed which contains RPM
values for
given impedances that will maintain the preferred flow rate. In one embodiment
the preferred
flow rate is between 26 and 27 cubic feet per minute (cfm). Based on the
difference between
the measured RPM value and that found in the look-up table the motor speed is
adjusted until
the two RPM values are equal. The motor speed can be adjusted by changing the
pulse-width
modulated signal that is sent from the processor to the motor.
In other embodiments, the blower is part of a heating system, such as an HVAC
(heating, ventilation, and air conditioning) system. When the impedance is
determined, the
module for determining the impedance, may output a signal that indicates the
length of pipe
that is coupled to the outlet of the blower.
The apparatus performs the following methodology in order to adjust the flow
rate to
a desired flow rate after the blower has started. First, the rate of rotation
of the motor is
measured. The measured rate of rotation of the motor is compared to an
empirically
3o measured rate that is in a look-up table. The rate of rotation is then
adjusted until the
measured rate of rotation matches the empirically determined rate from the
look-up table.
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The rate of rotation of the motor is adjusted by changing the duty cycle of a
pulse width
modulated signal that is provided to power the motor. The look-up table that
is used has an
association between empirically determined rotation rates and impedance
attached to the
outlet of the blower. In one embodiment, the flow rate is adjusted such that
the flow rate
produces a substantially optimal energy efficiency for the heating device that
is coupled to
the inlet of the blower. The flow rate allows impurities to be removed from
the hot water
heater without having the exhaust become stagnant, while not drawing out an
excessive
amount of heat from the heating device. Thus, the system is energy efficient.
In certain embodiments of the invention, prior to adjusting the rotational
rate to
to obtain a desired flow rate, the system determines the impedance that is
attached to the
system. For example, the impedance may be the length of pipe that is.attached
to a draft
inducer and may include any other impedance, such as any blockage that is
within the pipe.
The system can automatically determine the impedance that is attached based
upon
previously determined empirical information. Without knowing the length of
pipe/impedance
that is coupled to the system, a processor will provide a power signal to a
motor. The power
signal may be in the form of a PWM signal. The processor receives a sensed
rotational
signal from the sensor attached to the motor that measures the rotational rate
of the shaft of
the motor. The rotational rate is then compared to a look-up table that
associates rotational
rates with impedances for the power signal. Based on the measured rotational
rate, the
2o impedance is determined. In certain embodiments if the rotational rate is
between a first and
a second value that are found in the look-up table, the processor will select
the rotational rate
associated with the larger impedance. This is done so as to provide a slightly
larger flow rate
than the ideal flow rate rather than a flow rate that is less than the ideal,
thus the system at
least meets any government flow requirements or heating manufacturer's
suggested flow
requirements. In preferred embodiments the motor is a DC motor.
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Brief Description of the Drawings
The foregoing features of the invention will be more readily understood by
reference
to the following detailed description, taken with reference to the
accompanying drawings, in
which:
Fig. 1 is schematic drawing of a ventilation system using a blower;
Fig. 2 is a schematic diagram showing a draft inducer.
Fig. 3 is a block diagram showing an impedance module and a motor speed
module;
Fig. 4 is a graph showing torque speed curves and impedance curves for a
motor;
Fig. 5 is an exemplary look-up table showing the values in the look-up table
for a
60% duty cycle;
Fig. 6 is a flow chart showing the steps that are taken by the module
impedance
module; and
Fig. 7 is a flow chart showing the steps that are taken by the module for
setting the
motor speed.
Detailed Descriution of Specific Embodiments
Definitions. As used in this description and the accompanying claims, the
following
terms shall have the meanings indicated, unless the context otherwise
requires: the term
'backward curved blade' is a term of art as understood by those in the art of
impeller
construction. As used herein, the term 'blower' shall imply all devices that
move air by
spinning including fans.
In one embodiment, the invention automatically determines an impedance that is
coupled to the output of a blower that is~part of a ventilation system. After
determining the
impedance, the system uses the impedance to determine the duty cycle for a
motor that is
powered using pulse-width-modulated (PWM) waveforms, such that the flow rate
through
the ventilation system to the outside environment will be nearly optimal in
terms of energy
conservation given that the ventilation system must exhaust a minimal amount
of fluid. The
minimal amount of fluid that the ventilation system must exhaust is determined
either by
statute or a manufacturer's specification for a heating element, such as a hot
water heater,
furnace, or heating component of a I-1VAC system.
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Fig. 1 is schematic drawing of a ventilation system 10 using a blower. In this
example, the ventilation system is coupled to a hot water heater 20 for
collecting the output
of the hot water heater and moving the output 25 from an inside environment 30
to an
outside environment 40. It should be recognized that a hot water heater is
being employed as
an exemplary embodiment of a heat source and that the invention may be
employed with
other systems that need to vent an exhaust of a heating or cooling source to
an outside
environment. As shown, the inside environment 30 could be a structure such as
a building
and the outside environment 40 may be the atmosphere. The inside environment
30 is
therefore a space that is volumetrically smaller than the outside environment
40. The outlet
of the hot water heater is coupled to the inlet of a draft inducer 50. The
draft inducer 50
includes a motor with an attached blower within a housing. An example of a
draft inducer is
shown in U.S. patent application no. 10/847,207 entitled "Draft Inducer Having
a Backward
Curved Impeller" that was filed on May 17, 2004 and that is incorporated
herein by reference
in its entirety. The draft inducer 50 draws air in from the inside environment
30 into a mining
chamber where it mixes with the exhaust output of the hot water heater. During
mixing, the
air from the inside environment 30 cools the exhaust output. The draft inducer
50 also causes
the mixed fluid to be directed toward the outside environment through outlet
piping 60. The
voltage supply that is provided to the motor of the draft inducer is
preferably regulated. By
regulating the voltage supply, the motor performance is immune to input
voltage variations.
By maintaining a constant DC voltage to the motor, the proportionality between
the duty
cycle of the PWM waveform that is provided to the motor and the resulting RPM
of the
motor are very accurately related.
If the draft inducer draws in too much of the exhaust into the mixing chamber,
energy
is removed from the hot water heater that could be used for heating the water.
If too little
exhaust is removed from the hot water heater, the exhaust will stagnate and
potentially
extinguish the heating source. In order to balance the amount of heat that
must be removed
from the hot water heater as prescribed by law or as designated by the
manufacturer of the
hot water heater and to increase energy efficiency, the motor of the draft
inducer is
controlled by a processor.
3o The impedance that is coupled to a draft inducer at its outlet varies from
application
to application. The draft inducer may be installed in a system having exhaust
piping 5 feet in
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length and in other embodiments the exhaust piping may be 50 feet in length.
In order to
properly regulate the flow rate through the ventilation system so as to
increase the energy
efficiency over prior art system, the impedance is determined by the
processor. The
processor performs a routine in combination with a sensor that is attached to
the motor of the
draft inducer to automatically determine the impedance of the system.
Fig. 2 is a schematic diagram showing a draft inducer 200. The draft inducer
200
includes an impeller 210 that spins about the shaft 215 of a motor 220. The
motor 220 is
powered by the processor 235. In a preferred embodiment a DC motor is used
with the draft
inducer because a DC motor exhibits a steep torque/speed curve and therefore
variations in
speed can be equated to changes in torque. In such an embodiment, the
processor is coupled
to the motor by way of two connections. The processor receives a sensor signal
through a
sensor lead 225 from a sensor that is attached to the motor 220. This sensor
signal provides
information regarding the rotational speed of the shaft of the motor 220. The
processor uses
this information to determine the duty cycle of the pulse width modulated
signal that is to be
employed in powering the motor through the power lead 230. As shown in Fig.2,
the exhaust
240 of the heating device enters the draft inducer the inlet 245 of the draft
inducer 200. The
impeller blades 250 of the draft inducer spin drawing in the exhaust 240 and
mix the exhaust
with ambient air 255, thereby cooling the exhaust. The impeller blades 250
direct the mixture
to an outlet of the draft inducer 260 that is coupled to an exhaust pipe that
leads to the
outside environment. The spinning of the blades of the impeller creates a flow
of fluid to the
outside environment and therefore the flow rate is controlled by the power
that is supplied to
the motor.
The processor 235 as shown in Fig. 3 uses two modules that may be either
circuitry,
software code or a combination of the two (firmware). If the modules are
hardware based,
the modules could be part of the processor. The processor using the impedance
module 310
first determines the impedance that results from the length of pipe that is
attached to the draft
inducer along with any obstructions within the length of pipe. The impedance
is
automatically determined preferably prior to the heating source being turned
on.
The impedance module 310 causes the processor 235 to set the duty cycle of the
PWM waveform for the motor. This duty cycle could be any value, however it is
preferable
to begin the PWM waveform at approximately a 50% duty cycle in order to create
enough air
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flow through the ventilation system prior to turning on the heating source to
avoid any
problems with the energy source due to back ignition. The sensor signal
produced by the
rotation of the shaft of the motor is received by the processor 235 and this
measured
rotational speed is compared to values in a look-up table 330 that are stored
in memory 340
that is associated with the processor 235. The values in the look-up table 330
are empirically
determined values and are the intersection points of the torque-speed curves
for a DC motor
and the impedance curves. The look-up table can include three associated
values: 1. the duty
cycle (i.e. 60%, 70% etc.), 2. the RPM value (i.e. 2000 RPM, 3000 RPM etc.)
and 3. the
impedance (i.e. the equivalent length of exhaust pipe lOFt., 20Ft. etc.).
Thus, by knowing the
duty cycle and measuring the RPM value the length of pipe that is attached to
the draft
inducer can be determined, which is the equivalent impedance.
Fig. 4 shows a series of torque speed curves and impedance curves for a motor
and
exhaust piping having a known diameter. For each diameter of exhaust piping, a
different set
of curves would exist. The torque speed curves are relative to the duty cycle
of the PWM
waveform and are shown with respect to 10% increments from 0 to 100% duty
cycle. These
curves are provided to show how the three data values are determined in the
look-up tables.
The units of the x-axis are the percentage of the duty cycle of the PWM
waveform and the
units of the y-axis are RPMs. As shown in Fig. 4, for a duty cycle of 50% and
a measured
RPM value of 2000 RPM, the impedance is equivalent to 15 feet of pipe.
2o Fig. S shows an exemplary look-up table showing the values in the look-up
table for a
60% duty cycle. The data within the table is empirically determined by
attaching a known
length of exhaust pipe to the draft inducer and then cycling through all of
the PWM values
and measuring the RPM level. The table can then be used with a similar draft
inducer having
the same diameter exhaust pipe to determine the length of pipe that is
attached to the draft
inducer. The unknown length of exhaust pipe is determined by measuring the RPM
value and
correlating that with the duty cycle and the impedance. If the measured RPM
value is
between two RPM values in the table, the system can interpolate between the
two to
determine the impedance. For example, if the measured RPM value was 1750 RPM
then the
system would interpolate between the impedances associated with 1500RPMs and
2000RPMs to determine that there is 12.5 feet of pipe that is causing an
impedance at the
output to the draft inducer. In other embodiments, if the RPM value is not
exactly the same
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as the empirically determined intersection point, the processor could select
the larger of the
two impedances that are closest to the measured RPM value. In the provided
example, the
module would select the 2000 RPM group and the associated 15 feet of pipe. The
larger
impedance provides a margin of error when the processor finally sets the PWM
signal for the
motor. Thus, by selecting the higher impedance the flow rate is guaranteed to
be above the
minimum that is required either by the manufacturer of the heating/cooling
device or by law.
The second module within the processor is the module for setting the motor
speed
(the motor speed module) 320. This module 320 accesses the memory associated
with the
processor and finds a look-up table 330 that contains a listing of PWM
settings that are each
associated with a different impedance for the empirically determined near
optimal flow rate.
This look-up table 330 can be part of the look-up table that is employed by
the impedance
module or may be a separate look-up table. In one embodiment, the optimal flow
rate is
approximately 2627 CFM for a 4 inch diameter outlet pipe. Based upon the
determined
impedance the PWM signal is selected and the processor provides the PWM signal
to the
motor. For the given PWM signal, the motor speed module expects that the
sensor will sense
a signal that is equivalent to an expected RPM value. If the measured RPM
value is not
equivalent to the expected RPM value the module will adjust the duty cycle of
the PWM
waveform. If the RPM value is less than expected RPM value, the duty cycle
will be
increased. If the RPM value is greater than the expected RPM value, the duty
cycle will be
decreased. The motor speed module continues this process until the measured
RPM value
equals the expected RPM value. Even after the measured RPM value and the
expected RPM
value are equal, the system will continue to measure and adjust for any
fluctuations that
occur.
Fig. 6 is a flow chart showing the steps that are taken by the module
impedance
module. First the module sets the duty cycle of the PWM signal (600).
Preferably, the duty
cycle is set between 40% and 60% i.e. 50%. The module then receives the sensor
signal from
the sensor that is attached to the motor (610). The sensor may be built into
the motor and
may be part of the stator or rotor. The sensor senses the revolutions of the
shaft of the motor.
The number of revolutions per minute are determined within the module based on
the signal
(620). The revolutions per minute and the duty cycle are then used to
determine the
impedance based upon a look-up table that contains the intersection points of
Fig. 4 (630). If
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the RPM value falls between empirically measured RPM values within the look-up
table and
therefore between two impedance levels, the greater impedance level is
selected. It should be
understood by one of ordinary skill in the art that rotational measurement
signal may be
converted into any format that provides information about the rotational speed
and that
revolutions per minute are used in this application for convenience.
Fig. 7 is a flow chart showing the steps that are taken by the module for
setting the
motor speed. After the impedance is determined by the impedance module, the
impedance is
received by the motor speed module (700). The motor speed module accesses a
look-up table
in memory and locates the duty cycle for the impedance that will produce a
near optimal
flow rate that removes a minimal amount of energy from the heating element
(710). As
previously stated the heating element may be a hot water heater, a furnace or
a heating
element in an HVAC system for example. Also associated with the duty cycle is
the expected
RPM rate of the shaft. The processor provides the appropriate duty cycle to
the motor and the
sensor senses the revolutions of the shaft. The sensor signal is provided to
the motor speed
module (720). The motor speed module compares the RPM rate of the shaft to the
expected
RPM rate. The motor speed module inquires if the measured RPM rate is equal to
the
expected RPM rate (730). If it is the process ends. In certain embodiments,
the process can
continue to check and compensate for any deviations in RPM. The motor speed
module then
checks to see if the RPM rate is lower than the expected RPM rate (732). If it
is low, the
motor speed module increases the duty cycle (733). If the measured RPM rate is
not lower
than the expected RPM rate, then the motor speed module decreases the duty
cycle (735). As
previously stated, the measured RPMs will be provided to the motor speed
module on an
ongoing or periodic basis to compensate for any fluctuations that may occur
during the
operation of the blower and the heating element in the ventilation system. It
should be
understood by one of ordinary skill in the art that the steps of the previous
flow charts may
be implemented in a different order without deviating from the purpose of the
invention.
Additionally, the processor can include an alarm function in the event that
the
measured RPM value exceeds a predetermined upper threshold or is less than a
separate
predetermined lower threshold. If either condition occurs, the processor will
set off an alarm
3o that may be audible and/or visual. The alarm will also shut down the hot
water heater or
other heating device and turn off the motor associated with the draft inducer.
As the RPM
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level passes the upper threshold level the impedance is greater than expected
and this
indicates that a blockage exists in the exhaust piping. Similarly, if the RPM
value is too low
and goes below the lower threshold, the system assumes that either there is a
hole in the
exhaust pipe or the exhaust pipe has been removed, since the impedance is much
lower than
expected and again the processor will shut down the heating element and will
turn off the
motor.
In an alternative embodiment, the invention may be implemented as a computer
program product for use with a computer system. Such implementation may
include a series
of computer instructions fixed either on a tangible medium, such as a computer
readable
to media (e.g., a diskette, CD-ROM, ROM, or fixed disk), or transmittable to a
computer
system via a modem or other interface device, such as a communications adapter
connected
to a network over a medium. The medium may be either a tangible medium (e.g.,
optical or
analog communications lines) or a medium implemented with wireless techniques
( e.g.,
microwave, infrared or other transmission techniques). The series of computer
instructions
embodies all or part of the functionality previously described herein with
respect to the
system. Those skilled in the art should appreciate that such computer
instructions can be
written in a number of programming languages for use with many computer
architectures or
operating systems. Furthermore, such instructions may be stored in any memory
device,
such as semiconductor, magnetic, optical or other memory devices, and may be
transmitted
using any communications technology, such as optical, infrared, microwave, or
other
transmission technologies. It is expected that such a computer program product
may be
distributed as a removable media with accompanying printed or electronic
documentation
(e.g., shrink wrapped software), preloaded with a computer system (e.g., on
system ROM or
fixed disk), or distributed from a server or electronic bulletin board over
the network (e.g.,
the Internet or World Wide Web).
Although various exemplary embodiments of the invention have been disclosed,
it
should be apparent to those skilled in the art that various changes and
modifications can be
made which will achieve some of the advantages of the invention without
departing from the
true scope of the invention. These and other obvious modifications are
intended to be
3o covered by the appended claims.
00917/00195 327857.1
