Title: PSC motor system for use in HVAC applications with improved start-up
Abstract: A motor control system for use in heating, ventilation and air conditioning applications including a blower and a motor coupled to drive the blower, an inverter coupled to provide energization to the motor, a relay capable of coupling the motor to the output of the inverter or to line power, and a controller providing signals to control the output of the inverter, where, upon startup and transfer of the motor from the line to the inverter, the output of the inverter is controlled by the inverter to provide for an enhanced startup or transfer.
Patent Number: 6,952,088 Issued on 10/04/2005 to Woodward, et al.
| Inventors:
|
Woodward; Arthur E. (Manchester, MO);
Shahi; Prakash B. (St. Louis, MO);
Furmanek; Ralph D. (Wildwood, MO)
|
| Assignee:
|
Emerson Electric Co. (St Louis, MO)
|
| Appl. No.:
|
266238 |
| Filed:
|
October 8, 2002 |
| Current U.S. Class: |
318/430; 318/807; 318/808; 318/810; 318/816; 318/817 |
| Intern'l Class: |
H02P 001/04 |
| Field of Search: |
318/430,432,772,776,777,807,808,812,816,817
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
U.S. Appl. No. 10/266,793, filed Oct. 8, 2003, Woodward et al.
U.S. Appl. No. 10/266,451, filed Oct. 8, 2003, Woodward et al.
|
Primary Examiner: Leykin; Rita
Attorney, Agent or Firm: Locke Liddell & Sapp LLP
Claims
1. A motor control system for use in heating, ventilation, and air conditioning
applications comprising:
a blower and a motor coupled to drive the blower;
an inverter coupled to provide energization to the motor, the inverter receiving
line power at line frequency and
a controller coupled to the inverter, the controller providing signals to control
the output of the inverter and wherein, upon startup of the motor the controller:
(i) controls the inverter to drive the output of the inverter to a predetermined
voltage level and to a predetermined frequency, the predetermined frequency being
greater than line frequency; (ii) the controller controls the inverter to maintain
the output of the inverter at or near the predetermined voltage and the predetermined
frequency for a predefined period of time; and (iii) upon the conclusion of the
predetermined period of time, the controller controls the inverter to drive one
output parameter of the inverter to a desired set point value.
2. The motor control system of claim 1 wherein the predetermined voltage level
is between 30% and 70% of the voltage available to the inverter.
3. The motor control system of claim 1 wherein the predetermined frequency is
approximately 74 Hz.
4. The motor control system of claim 1 wherein the controller provides signals
to control the output of the inverter in response to received input control signals,
wherein the input control signals received by the controller can define a first
operating state and a second operating state and wherein, in response to the input
control signals defining the first operating state, the controller controls the
output of the inverter in accordance with a first volts vs. hertz relationship
and wherein, in response to the input control signals defining the second operating
state, the controller controls the output of the inverter in accordance with a
second volts vs. hertz relationship, the first volts vs. hertz relationship being
different than the second volts. vs. hertz relationship.
5. The control system of claim 1 wherein each operating state corresponds to
a desired current level in the motor.
6. The control system of claim 1 wherein the controller provides output signals
to control the output of the inverter in response to received input control signals
and in response to field adjustment signals, wherein the input control signals
can define a first and a second operating state of the controller, each of the
first and second operating states corresponding to a desired operating state of
the system; and a first field adjustment circuit for providing a first field adjustment
signal to the controller, the first field adjustment signal, in combination with
the input control signals, defining a desired output power level for the inverter
for the first operating state.
7. The control system of claim 6 wherein, in response to the input control signals
defining the first operating state, the controller controls the output of the inverter
in accordance with a first volts vs. hertz relationship and wherein, in response
to the input control signals defining the second operating state, the controller
controls the output of the inverter in accordance with a second volts vs. hertz
relationship, the first volts vs. hertz relationship being different than the second
volts. vs. hertz relationship.
8. A motor control system for use in heating, ventilation and air conditioning
applications comprising:
a blower and a motor coupled to drive the blower;
an inverter coupled to provide energization to the motor; and
a controller coupled to the inverter, the controller providing signals to control
the output of the inverter;
a relay having a first input coupled to receive line power, a second input coupled
to receive the output of the inverter, an output coupled to the motor; and a control
input coupled to receive a control signal controlled by the controller wherein
the controller controls the relay to cause the motor to be energized by line power
when the desired frequency for energization of the motor as defined by the controller
in response to the input control signals is at or near line frequency; and
wherein, prior to switching the relay from a state where the motor is being energized
by the inverter to a state where the motor is being energized by the line the controller
will: (i) first control the output of the inverter such that the voltage level
of the inverter output is approximately equal to the maximum voltage level available
to the inverter and the frequency output of the inverter is approximately equal
to the line frequency; (ii) thereafter control the output of the inverter such
that the inverter does not provide any power to the motor for a defined period
of time; and (iii) thereafter generate a signal to cause the relay to change states
so as to couple the motor to line power.
9. A motor control system for use in heating, ventilation and air conditioning
applications comprising:
a blower and a motor coupled to drive the blower;
an inverter coupled to provide energization to the motor; and
a controller coupled to the inverter, the controller providing signals to control
the output of the inverter;
a relay having a first input coupled to receive line power, a second input coupled
to receive the output of the inverter, an output coupled to the motor; and a control
input coupled to receive a control signal controlled by the controller wherein
the controller controls the relay to cause the motor to be energized by line power
when the desired frequency for energization of the motor as defined by the controller
in response to the input control signals is at or near line frequency; and
wherein in switching the relay from a state where the motor is being energized
by the line to a state where the motor is being energized by the inverter the controller
will: (i) before switching the relay to couple the inverter output to the motor,
generate control signals to cause the output of the inverter to be at a defined
frequency that is approximately equal to the line frequency and at a defined voltage
level between 30% and 70% of the normal output voltage of the inverter at the defined
frequency; (ii) thereafter switching the relay to couple the motor to the output
of the inverter; and (iii) control the inverter to cause the voltage level of the
inverter output to increase from the defined voltage level to the normal output
voltage of the inverter at the defined frequency.
10. The motor control system of claim 9 wherein the defined voltage level is
approximately one-half of the normal output of the inverter.
11. The motor control system of claim 9 further including:
a generally L-shaped bracket assembly;
a lid element adapted to be coupled to the curved exterior of a blower housing,
the generally lid element being hingedly coupled to the generally L-shaped bracket
assembly; and
a control module containing circuit components for an inverter and an electronic
controller, mounted to the generally L-shaped bracket assembly such that, when
the L-shaped bracket assembly is in a first position wherein the L-shaped bracket
assembly makes contact with the lid assembly, access to the control module is blocked
by the bracket assembly, and when the L-shaped bracket assembly is swung open along
the hinged connection access to the control module is enabled.
Description
BACKGROUND OF THE INVENTION
The present disclosure relates to motor control systems and, more particularly,
to permanent split capacitor ("PSC") motor control systems for use in heating,
ventilation, and air conditioning ("HVAC") applications.
Conventional HVAC applications often utilize multi-tapped PSC type motors.
In general, a multi-tapped PSC motor is a motor that has a multi-tapped main winding
where all or part of the main winding is coupled in parallel with an auxiliary
starting winding that is coupled in series with a capacitor. Such multi-tapped
PSC motors are used in HVAC applications, such as furnace blower and air handler
applications, because the multi-tapped winding can produce variable output torque
and, therefore, variable output speed for the purpose of delivering different amounts
of air flow for different applications. For example, one tap setting may be provided
to provide a relatively low amount of air flow to provide for air circulation when
there is no heating or cooling activity. Another tap setting could be provided
to increase the air flow when cooling is desired. By using multiple taps, various
operating states can be established for a tapped PSC motor, such as heating, cooling,
and air. In general, each tap point on the multi-tapped PSC motor is coupled to
an input line and relays are energized in response to control signals from, for
example, a thermostat to provide energization to one of the tap points at any given time.
One characteristic of multi-tapped PSC motors when used with air blowers, such
as a squirrel cage blower, is that the Speed vs. Torque curves for such systems
are not constant, but have a generally "reverse C shape" wherein the torque will
increase with speed up to a maximum point but, thereafter, as the speed increases
the torque will begin to decrease. FIG. 1 generally illustrates the Speed vs. Torque
characteristics for a conventional multi-tapped PSC motor for low, medium, medium
high and high settings with each setting having its own Speed vs. Torque curve.
As the figure illustrates, for each Speed vs. Torque curve, as speed increases
the output torque will initially increase from a minimum value at or near zero
speed to a maximum value and then decrease to near or zero torque at a maximum speed.
In addition to having non-linear Speed vs. Torque characteristics, the operation
of conventional multi-tapped PSC motors can be significantly impacted by the static
pressure of the environment in which the system is operating. This is reflected
by FIGS. 1 and 2, where FIG. 1 was described above, and FIG. 2 illustrates Static
Pressure (in inches of water) vs. Air flow (in cubic feet per minute (CFM)) for
the various taps of a conventional multi-tapped PSC motor. Lines reflecting average,
low and high static pressures are illustrated in FIGS. 1 and 2.
As will be appreciated from FIGS. 1 and 2, for a given tap setting, as the static
pressure is increased above the average static pressure value, the speed of the
motor will increase. This speed increase will, therefore, result in a decrease
in the output torque of the blower and accordingly a decrease in the output airflow
from the blower. The reverse may occur if the static pressure drops below the average
value. Because of this influence of the static pressure on the output airflow,
in most HVAC systems using a multi-tapped PSC motor, the operation of the system
will vary (perhaps significantly) from day to day, month to month as the static
pressure within which the system operates changes. Such variations provide for
unstable and inconsistent operation which is undesirable.
The present disclosure describes several embodiments a motor control system for
a PSC motor that are designed to address the described and other limiting characteristics
to conventional systems to provide an improved motor control system.
SUMMARY OF THE INVENTION
In accordance with one exemplary embodiment constructed in accordance with certain
teachings of the present disclosure, a motor control system for use in heating,
ventilation, and air conditioning applications is provided that includes a blower
and a motor coupled to drive the blower, an inverter coupled to provide energization
to the motor, the inverter receiving line power at line frequency and a controller
coupled to the inverter, the controller providing signals to control the output
of the inverter and wherein, upon startup of the motor the controller: (i) controls
the inverter to drive the output of the inverter to a predetermined voltage level
and to a predetermined frequency, the predetermined frequency being greater than
line frequency; (ii) the controller controls the inverter to maintain the output
of the inverter at or near the predetermined voltage and the predetermined frequency
for a predefined period of time; and (iii) upon the conclusion of the predetermined
period of time, the controller controls the inverter to drive one output parameter
of the inverter to a desired set point value.
In accordance with yet another exemplary embodiment constructed in accordance
with certain teachings of this disclosure a motor control system for use in heating,
ventilation and air conditioning applications is provided that includes a blower
and a motor coupled to drive the blower; an inverter coupled to provide energization
to the motor; and a controller coupled to the inverter, the controller providing
signals to control the output of the inverter; a relay having a first input coupled
to receive line power, a second input coupled to receive the output of the inverter,
an output coupled to the motor; and a control input coupled to receive a control
signal controlled by the controller wherein the controller controls the relay to
cause the motor to be energized by line power when the desired frequency for energization
of the motor as defined by the controller in response to the input control signals
is at or neat line frequency; and wherein, prior to switching the relay from a
state where the motor is being energized by the inverter to a state where the motor
is being energized by the line the controller will: (i) first control the output
of the inverter such that the voltage level of the inverter output is approximately
equal to the maximum voltage level available to the inverter and the frequency
output of the inverter is approximately equal to the line frequency; (ii) thereafter
control the output of the inverter such that the inverter does not provide any
power to the motor for a defined period of time; and (iii) thereafter generate
a signal to cause the relay to change states so as to couple the motor to line power.
Other aspects of the present disclosure will be apparent from a review of the
disclosure, the figures and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The description is presented with reference to the accompanying drawings in which:
FIG. 1 generally illustrates the Speed vs. Torque characteristics for a conventional
multi-tapped PSC motor for low, medium, medium high and high settings with each
setting having its own Speed vs. Torque curve.
FIG. 2 illustrates Static Pressure (in inches of water) vs. Air flow (in cubic
feet per minute (CFM)) for the various taps of a conventional multi-tapped PSC
motor as illustrated in FIG. 1.
FIGS. 3A and 3B generally illustrate an exemplary permanent split capacitor
("PSC") induction motor control system constructed in accordance with certain teachings
of this disclosure for use, for example, as a blower drive for an HVAC application
FIG. 4 generally illustrates an exemplary non-tapped (single speed) PSC motor
including a main winding that is coupled in parallel with a series connection of
an auxiliary winding and a capacitor for use in the exemplary system of FIGS. 3A
and 3B.
FIG. 5 generally illustrates an exemplary embodiment of the input conversion
circuitry 8 of FIGS. 3A and 3B for converting relatively high voltage level
signals 10 (e.g., 24V or 115V signals) in one format into logic level signals
(e.g., 5V signals) of another format for use in determining the operating state
of the control system.
FIG. 6 generally illustrates an exemplary lower leg current monitoring circuit
for monitoring the current in the lower leg of the inverter 4 of FIGS. 3A
and 3B.
FIG. 7 generally illustrates representative volts/hertz curves for six exemplary
operating states of the exemplary control system of FIGS. 3A and 3B.
FIG. 8 generally compares exemplary CFM/Static Pressure curves for one exemplary
embodiment of the system of FIGS. 3A and 3B operating in the CURRENT CONTROL MODE
with exemplary curves for a conventional multi-tapped PSC motor.
FIGS. 9A-9C illustrate in greater detail one exemplary configuration of the
field adjustment circuits of FIGS. 3A and 3B.
FIG. 10 illustrates an exemplary control scheme that may be implemented by the
controller 18 of FIGS. 3A and 3B.
FIG. 11 generally illustrates an exemplary during a start-up operation that
may be implemented by the controller 18 of FIGS. 3A and 3B where the PSC
motor 2 goes from an off state to a running state, and where the voltage
and frequency output of the inverter 4 are controlled from a predetermined
frequency to provide optimum starting of the motor.
FIG. 12 generally illustrates an alternate exemplary start-up process in which
the frequency output of the inverter is driven to above line frequency during start-up.
FIG. 13 generally illustrates an exemplary embodiment of the system of FIGS.
3A and 3B wherein the relay 6 is configured such that the relay, in its
unenergized state, couples the PSC motor 2 to the inverter 4.
FIG. 14 generally illustrates an exemplary embodiment of an upper current trip
circuit than may be used with the system of FIGS. 3A and 3B to monitor the current
in the upper leg of the inverter.
FIG. 15 generally illustrates alternate embodiments of the system of FIGS. 3A
and 3B where the inverter operates off of a single DC buss obtained by full wave
rectifying the input line voltage.
FIGS. 16A-16C generally illustrate an exemplary mounting structure that may
be used with the control system of FIGS. 3A and 3B wherein a control module of
the system 1 (which includes all major components of the system except for
the motor) is mounted within a bracket like device that may be readily secured
to a blower enclosure.
DESCRIPTION OF EMBODIMENTS
Turning to the drawings and in particular to FIGS. 3A and 3B, a permanent
split capacitor ("PSC") induction motor control system
1 for use, for example,
as a blower drive for an HVAC application is illustrated.
The illustrated PSC inverter system
1 comprises six primary components
and/or componant systems: (i) a pennant split capacitor motor
2; (ii) a
variable frequency inverter
4 coupled to provide ouput power to the motor
2; (iii) a switching relay
6, configured to couple the input of the
PSC motor
2 to either the output of the variable frequency inverter
4
or line power; (iv) an input converter
8 that receives input signals
10
in one form from, for example, a furnace board or a thermostat and converts the
same to logic level control signals
12; (v) a field adjustment system
14
that can be set in the field to provide variable field adjustment signals
16
to adjust the effect of the control signals
12 on the operation of the motor
control system; and (vi) a control circuit
18 that receives control signals
12 and field adjustment signals
16 and controls the variable frequency
inverter
4 and the switching relay
6 to drive the motor
2
in a desired manner. The componants of the system are illustrated in a block form
in FIG.
3A and in more detail in FIG.
3B.
In general, AC line power is provided as an input to the variable frequency inverter
4. The variable frequency inverter
4 converts the AC line power to
a DC bus voltage and then converts the DC bus voltage to a single-phase synthesized
sinusoidal waveform of variable voltage and frequency for application to the motor.
AC line power is also provided to a first input contact point for the relay
6
which, in the illustrated embodiment of FIGS. 3A and 3B, is a single pole, double
throw relay. The output from the variable frequency inverter
4 is provided
to a second input for the relay
6. The output of the relay
6 is coupled
to one input terminal of the PSC motor
2. In the example above, the other
input to the PSC motor
2 is coupled to one of the ac input lines.
In operation, the controller
18 controls the relay
6 to couple
the
input of the PSC motor
2 to either the output of inverter
4 or to
the line power. In general, the controller
18 determines the operating state
of the system in response to the control signals
12 and the field adjustment
signals
16. Depending on the operating state defined by the control signals
provided to the controller
18, the controller will either: (a) generate
signals to switch the relay
6 to couple the motor
2 to AC line power,
thus operating the motor
2 at a substantially fixed speed corresponding
to the line frequency; or (b) generate signals to switch the relay
6 to
couple the motor
2 to the output of the inverter
4 and also generate
control signals to control the inverter
4 to provide a single phase output
voltage having appropriate voltage and frequency characteristics to drive the PSC
motor
2 in a desired manner.
In many applications of the illustrated system the inverter
4 will be
driving
the motor
2 when the HVAC system is performing active heating and/or cooling
operations. In such applications, the controller
18 may be configured to
operate in multiple operating states for each operation state. For example, the
controller
18 may be configured to provide differing output currents at
different settings or differing output frequencies or to control the power output
of the blower to provide different CFM outputs. These precise configuration and
settings for the controller
18 may be software and/or field programmable.
As a result, the installer of a product containing the illustrated PSC Inverter
System may adjust the operation of the system depending on the comfort level of
the consumer.
Controller
18 may be a microprocessor-based, software driven controller
that receives input commands and generates switching signals for the relay
6
and the inverter
4 to control the motor
2 in an optimized manner.
In general, the controller
18 controls the relay
6 based on the desired
output frequency of the inverter. At desired output frequencies around line frequency,
the controller will typically switch the relay
6 to couple the PSC motor
2 to the line. The precise speed threshold level at which such a switching
of the relay
6 occurs may vary. This variation may depend on the specific
mode in which the controller is operating or whether the motor is going from a
higher speed to a lower speed or vice versa.
At desired output frequencies below the threshold level, the controller
18
will switch the relay
6 to couple the PSC motor
2 to the output of
the inverter
4. The controller will also generate signals to control the
switching of the power switches in the inverter
4 to provide an output having
the desired voltage and frequency to achieve the desired output speed. Again, the
threshold level where the controller switches to the inverter
4 output can
be fixed or can vary with the operating mode of the controller or other conditions.
In one embodiment, the controller
18 will switch the relay
6 to
drive the motor
2 from the inverter
4 when the frequency of the voltage
to be applied to the motor
2 is below some fixed percentage of the line
frequency (e.g. 80%, 90% 95% or some other percentage). In that embodiment, the
controller
18 will switch the relay
6 to provide line voltage when
the frequency of the voltage to be applied to the motor is over the fixed percentage.
Still further embodiments are envisioned where the frequency selected for a line
to inverter transition is different from the frequency required for an inverter
to line transition.
Further, details and alternate constructions of the various components of
the system of FIGS. 3A and 3B are provided below.
In the example of FIGS. 3A and 3B, the PSC motor
2 is a single phase PSC
motor of a size that is commonly found in circulation blowers for HVAC applications
(e.g., an approximately ⅓ hp to 1 hp). The PSC motor
2 may be a conventional
multi-tapped PSC motor or may be a specially constructed, non-tapped PSC motor
having only two motor input leads. Generally, a run capacitor as used on conventional
PSC motors should be used since the embodiments described herein utilize signal
phase power to drive the motor. If a multi-tap PSC motor is used, only the highest
speed tap should typically be used in connection with the illustrated system.
In a preferred embodiment, a non-tapped (single speed) PSC motor
2 is
provided
that includes a main winding that is coupled in parallel with a series connection
of an auxiliary winding and a capacitor. A schematic representation of such a motor
is illustrated in FIG. 4 where a main winding
20 is coupled in parallel
with a series connection of a auxiliary winding
22 and a capacitor
24.
To control the amount of noise and/or vibration produced by operation of the
motor
2, it may be desirable to select the winding pattern for the windings
20
and
22 to produce the lowest average noise over the range of expected operating
frequencies. Alternately, in embodiments where it is anticipated that the motor
2 will be operating predominately in response to an excitation signal of
a given frequency (e.g., a frequency corresponding to an operating state of the
system where the blower is in a FAN or CIRCULATION mode), it may be desirable to
wind the motor so that the noise/vibration produced at the anticipated predominate
operating frequency is minimized. Additionally, in a motor specifically constructed
for use with the motor control system described herein, the amount of copper in
the main winding can be increased to increase the overall efficiency of the system.
While the exemplary system of FIGS. 3A-3B may be used with PSC motors accross
a large range of horsepower ratings, it is anticipated that the described systems
will be used with PSC motors having ratings of between ⅕ to 1 Hp.
As illustrated in FIGS. 3A-3B, the motor control system
1 receives input
command signals
10 that determine the operating state of the system
1.
In the illustrated embodiment, the operating state of the system
1 is determined
based on three logic level control signals
12 that are developed and provided
by an input converter circuit
8 based on up to five relatively high voltage
level signals
10. The relatively high voltage signals
10 may be provided
by, for example, a conventional thermostat or an ignition control board in a furnace
that was designed to control a PSC motor having a multi-tapped winding. The use
of the input converter circuit
8 allows for the motor control system
1
to be used in retrofit applications where the control system
1 will replace
a conventional system that operates in response to signals from a conventional
thermostat or from control signals provided by an ignition control board in a furnace.
Certain existing HVAC systems operate in response to voltage signals provided
by a conventional, e.g., wall-mounted, thermostat. In general, such conventional
thermostats provide output control signals at a level of approximately 24 Volts
AC. Although the precise nature of the signals provided by such conventional thermostats
will vary from thermostat to thermostat, there is typically an output signal "FAN,"
that is energized with 24VAC when the fan is to operate in a circulate mode; a
"HEAT" output that is energized with 24VAC when the thermostat is indicating that
the system is to operate in a heating mode; a "COOL" signal that is energized with
24VAC when the thermostat is indicating that the system needs to operate in a cooling
mode. Certain types of thermostats also have a HIGH HEAT and a HIGH COOL signals.
The precise manner in which the 24VAC signals described above are provided by a
thermostat will vary from thermostat to thermostat. For some thermostats, only
one of the output signals (e.g., HEAT) will be active high at any given time. For
other thermostats, multiple signals may be simultaneously active high (e.g., FAN
and HEAT). As described below, the construction of the input interface circuit
is such that the system can properly function with a wide variety of thermostats
and thermostat signals.
In most furnaces a furnace control board or an ignition control board uses these
24VAC signals to control various relays located on the ignition board. These relays
are typically switched to provide 115VAC output power that is applied directly
to one tap of a multi-tap motor. In such systems, only a single output is typically
active in a given operating mode, as that will be the output used to power the
motor coupled to the HVAC blower at the desired speed. Such ignition control board
systems typically are capable of providing from three to five different outputs,
with the outputs generally corresponding to FAN (Circulate); HEAT; HIGH HEAT; COOL
and HIGH COOL. As described below, the input interface
8 is constructed
to be able to properly process such 115VAC signal outputs as input commands. The
input interface
8 can also properly interpret the 24VAC input signals with
some component value changes that will be apparent to those of ordinary skill having
the benefit of this disclosure.
FIG. 5 generally illustrates an exemplary embodiment of the input conversion
circuitry
8 for converting relatively high voltage level signals
10
(e.g., 24V or 115V signals) in one format into logic level signals (e.g., 5V signals)
of another format for use in determining the operating state of the control system
1.
Referring to FIG. 5, the input conversion circuitry
8 includes an
interface board which directly receives the signals
10A-
10E from
either a thermostat (24V) or a furnace ignition control board (115 VAC). Each signal
10A-
10E is then applied to processing circuitry that includes: (1)
a return path for allowing for some of the current flowing from the ignition control
board to flow through the processing circuit and back to the source of the signal;
and (2) a active path that, depending on the state of the input signals
10,
will pass through one or more optocouplers to set the states of the three logic
level output signals
12. The optocouplers are configured to provide outputs
signals at logic levels suitable for processing by the digital controller
18.
Each of the return paths for each of the five high voltage level signals
10A-
10E
includes an initial input resistor (
30a for signals
10A,
30b
for signals
10B, etc.), coupled in series with a parallel connection
of a resistor
31a-
31e and capacitor
32a-
32e
coupled to a common return path. In the illustrate embodiment of FIG. 5, two
common return paths are provided such that the same circuit
8 can handle
input signals at 24V or 115VAC levels. A first return
33 is provided for
handling 24V control signals. A second return
34 is provided that includes
a drop-down resistor
35 that may be used when an ignition control provides
very high output voltage signals at, for example, a 115VAC level.
In addition to being provided to the first return path described above, the signal
from the ignition control board is applied to a secondary processing circuitry
that combines and converts the signal to three digital logic level signals. The
secondary processing is slightly different for each signal from the ignition control
board and combines the five high-level voltage signals
10A-
10E to
produce three logic level control signals
12A-
12C.
It should be understood that the precise nature of the secondary processing circuitry
may vary depending on the precise form that the input signals
10 from the
thermostat or ignition control board take. In general, because the input conversion
circuitry
8 provides three logic level output signals
12A-
12C,
there are eight possible operating states of the system. In the exemplary embodiment
described herein, however, only five of these states are utilized with the five
utilized states corresponding to: HI COOL, LOW COOL, HIGH HEAT, LOW HEAT OR FAN/RECIRCULATION.
In general, the nature of the secondary processing circuitry is such that the input
signals
10 produce the combination of the logic level signals
12
that corresponds to the operating mode commanded by the thermostat or furnace board.
For example, if the combination of logic level signals
111 corresponds to
FAN/RECIRCULATION the second circuitry should be configured such that the receipt
of the 24V or 115VAC signal(s) corresponding to the circulate mode would produce
the desired logic level output signal
111.
In the exemplary embodiment of FIG. 5, the three logic level signals
12A-
12C
are provided, respectively, as outputs from three optocouplers
35A-
35C.
The optocouplers
35A-
35C provide a mechanism for converting the high-level
voltage signals to logic level signals and for isolating the high voltage side
of the input conversion circuit
8 from the logic level side of the circuit,
thus providing some degree of intrinsic safety.
Each of the optocouplers
35A-
35C has, on the input side, two input
terminals and, on the output side, two output terminals. In FIG. 5, the upper output
terminals of the optocouplers
35A-
35C are tied to the logic supply
voltage Vcc. The lower output terminals of the optocouplers
35A-
35C
provide the logic level signals
12A-
12C. Such lower output terminals
are all coupled to a common ground point
36 through parallel connections
37A-
37C of a resistor and a capacitor. These resistor-capacitor networks
thus normally provide logic low levels on the signals
12A-
12C when
the optocouplers are off. However, when one of the optocouplers
35A-
35C
is turned on, it will pull its associated output signal to the high logic state.
The value of the logic level output signals from the optocouplers
35A-
35C
will be determined by the value of the input signals
10A-
10E. In
the exemplary embodiment of FIG. 3, the logic level signal
12A will be determined
by the input to the upper input terminal of optocoupler
35A which corresponds
directly to the input signal
10A. Thus, whenever the level of the
10A
signals is at a high level, current will flow from input
10A, through a
diode
38A, through optocoupler
35A and through zener diode
39
to one of the two return paths (
33 or
34). The zener diode
39
should be selected to control the voltage threshold for the 24VAC inputs and the
resistor
305 should be selected to control the amount of current flowing
through the optocoupler with 115VAC inputs to ensure that the optocoupler
35A
is not damaged or overloaded. The current signal flowing through the input terminals
of the optocoupler
35A will be controlled by the precise nature of the secondary
circuitry and, as described above, should properly map the input signals
10A-
10E
to the appropriate logic level signals
12A-
12C.
While the above discussion focuses on the impact of signal
10A on output
12A, the impact of the other input signals
10A-
10E on the
logic level output signals
12A-
12C would be apparent to one of ordinary
skill in the art having the benefit of this disclosure.
Thus, in the manner described above, the input conversion circuit
8
can convert five conventional high level voltage signals from the ignition control
board are converted into three logic level digital signals for application to the
microprocessor-based controller
18.
As described above, the microprocessor-based controller
18 receives the
logic control signals
12A-
12C and in response to these signals—and
other signals as described below—controls the switching of the inverter to
operate the PSC motor
2 in one of several possible operating modes.
Referring to FIGS. 3A and 3B, the microprocessor-based controller
18
may be any suitable controller such as, for example, the MCU MC68HC908JK3 available
from Motorola. The controller should include an interface for receiving the logic
level control signals
12A-
12C as well as the field adjustment signals
16, to be discussed in more detail below.
In general, the logic level signals
12A-
12C determine the operating
state of the microcontroller
18. As described above, the logic level signals
that define the operating state may come from the input conversion circuit
8
or directly from a thermostat designed to provide such logic level outputs. Such
a thermostat may use, for example, serial communication through the optocouplers
for isolation or an RF communications link.
In addition to being able to provide control capability to multiple operating
states, the controller
18 of the present disclosure may be programmed to
operate in one or more operating modes. For example, the controller may be configured
to operate in a CURRENT CONTROL MODE, where each operating state in such mode corresponds
to a desired motor current. Alternately, the controller may be configured to operate
in a FREQUENCY CONTROL MODE, where each operating state in such mode defines a
desired output voltage frequency. Still further, the controller
18 may be
configured to operate in a SPEED CONTROL MODE where the output speed of the motor
is controlled or a POWER CONTROL MODE where the power output of the inverter coupled
to the PSC motor is controlled. Still further embodiments are envisioned where,
depending on the types of inputs received by the controller
18, the controller
may be configured to switch among any of the described—or other possible—operating modes.
The operating of the controller
18 in the CURRENT CONTROL MODE will be
initially discussed.
In the CURRENT CONTROL MODE, each of the eight possible operating states (as
defined
by the logic level inputs
12A-
12C) will correspond to a desired current
level in the PSC motor
2. In this mode, a current feedback signal will be
provided to the controller
18 to provide an indication to the controller
of the magnitude of the current in the motor winding. The current feedback signal
may be obtained from a current sensor coupled to the windings of the PSC motor
2 or derived from a current sensor or sensor positioned within one or both
legs of the inverter
4.
In one exemplary embodiment, the current feedback signal provided to the controller
18 is taken from a current sensor in the lower leg of the DC bus in the
inverter
4. This embodiment is illustrated generally in FIG.
3B and
FIG.
6.
Referring to FIG. 3B, it may be noted that there exists a shunt resistor
60 that is positioned in the lower leg of the DC bus. A voltage reading
from this shunt resistor is provided as an input to a lower leg current monitoring
circuit, that is illustrated in more detail in FIG.
6. Because the voltage
across the shunt resistor
60 will vary with changes in the current flowing
in the lower leg of the inverter and because the current flowing in the lower leg
of the inverter will correspond to the current in the PSC motor
2, the voltage
from the shunt resistor
60 provides an indication of the current in the
PSC motor
2.
Referring to FIG. 6, the voltage from the shunt resistor
60 is provided
as an input to two differential amplifiers
63 and
64. Differential
amplifier
63 is configured as a comparator and it compares the detected
voltage value to a reference value and generates a lower current trip signal on
line
65 in the event that the voltage value exceeds a predetermined value.
As described in more detail below, the lower current trip signal may result in
a resetting of the controller
18.
As reflected in FIG. 6, the voltage from the shunt resistor
60 is also
applied as an input to differential amplifier
64. Differential amplifier
64 is configured to perform some filtering and voltage level adjustment
of the signal from the shunt resistor to product an output voltage signal on line
66 that varies with, and corresponds to, the voltage from the shunt resistor
60 and, therefore, that varies with and corresponds to the current flowing
in the PSC motor
2. Differential amplifier
64 should be configured
to produce an output voltage that varies in response to the input voltage but where
the maximum expected output voltage on line
66 will be less than the maximum
input voltage of the A to D converter and the logic supply voltage supplied to
the controller
18.
While FIGS. 3B and 6 illustrate the use of a shunt resistor to generate a signal
representative of the PSC motor current, other forms of current detection may be used.
In the current control mode, the microcontroller
18 will compare the value
of the current feedback signal with the desired current level for the selected
operating state. If the comparison indicates that the motor current is less than
the desired setpoint current, then the controller
18 will increase the output
voltage and frequency applied to the windings of motor
2 so as to tend to
increase the current in the motor
2 by increasing the speed of the blower
motor. If the comparison indicates that the motor current is above the desired
setpoint current, then the controller will decrease the voltage and frequency of
the output voltage to tend to cause the current in the motor to decrease by reducing
the speed of the blower motor. This comparison and adjustment of the output voltage
and frequency will regularly occur in an effort to maintain the current in the
motor at the desired setpoint level. The comparison and adjustment may be done
in software, hardware or firmware and the implementation of such functionality
will be within the level of one of ordinary skill in the art having the benefit
of this disclosure.
In one embodiment, the relationship between the output voltage and the output
frequency will vary depending on the specific operating state of the system. In
this embodiment, each operating state—in addition to defining a particular
desired current setpoint—will also define a desired volts/hertz curve such
that the relationship between the output voltage and the current may vary from
operating state to operating state. In such an embodiment, the volts/hertz curve
may take any appropriate form. In one desired embodiment, linear volts/hertz curves
are used.
FIG. 7 generally illustrates representative volts/hertz curves for six exemplary
operating states A, B, C, D, E and F. Note that the desired current setpoints for
such operating states are not reflected in FIG.
7.
Referring to FIG. 7, it may be noted that each of the volts/hertz curves
is linear in that the rate of change of the output frequency is constant when compared
to the rate of change of the output voltage. In the illustrated example, each volts/hertz
curve also has a minimum output frequency and a maximum output frequency. In one
embodiment, the minimum output frequency during normal operation of the control
system is 26 Hz and the maximum output frequency is 57 Hz (corresponding to a speed
range for the PSC motor
2 of 500-1100 RPMs). Alternate embodiments are envisioned
wherein different ranges of output frequency are possible, including embodiments
wherein the maximum output frequency during normal operation is 60 Hz (the typical
line frequency) or even higher. In such embodiments when the desired output frequency
is at or near 60 Hz, the controller
18 may be programmed to generate a control
signal to switch a relay to cause the motor to operate off line power.
In addition to having minimum and maximum operating frequencies, the voltz/hertz
curves of FIG. 7 also define minimum and maximum voltage values. Notable, while
the minimum and maximum frequency values are shared by the curves for the different
operating states, the minimum and maximum voltage levels may be different. In the
example of FIG. 7 each operating state defines a different minimum voltage value
and different maximum voltage value.
The precise nature of the volts/hertz curves for the various operating states
should be set to maximize a desired operating characteristic of the system such
as, for example, efficiency, noise, vibration, etc. In the embodiment illustrated
in FIG. 7, the volts/hertz curves were selected to provide for maximum operating efficiency.
This use of differing volts/hertz curves for each operating state in the CURRENT
CONTROL mode produces PSC motor tap-like performance, in that, the energization
characteristics of the motor at the different operating states causes the motor
to operate differently.
Unlike a tapped PSC motor, however, the use of the CURRENT CONTROL mode as
described herein allows for operational advantages that are not obtainable with
a conventional PSC motor and control system. For example, if the volts/hertz curves
are selected to control the slip of the motor, the present system can provide for
highly efficient operation, even at low operating speeds, provided that the volts/hertz
curves are selected to maintain a slip within, for example, the range of 100-200
RPM for all of the operating states and for all static pressures. Alternate embodiments
are envisioned where the slip is even less. Typically the slip will be at least
50 to 75 RPM for a conventional six pole PSC motor. Additionally, because the operating
characteristics of the PSC motor
2 are adjusted depending on the operating
state of the system and because it is the current in the motor
2 that is
being controlled, excess airflow at high speeds and low static pressures can be eliminated.
FIG. 8 provides a general comparison of the performance of the current system
operating in the current control mode with the performance of a conventional, multi-tapped
PSC motor. Specifically, FIG. 8 illustrates CFM/Static Pressure curves for a system
as described herein operating in the CURRENT MODE and a conventional multi-tapped
PSC motor. The CFM/Static Pressure curves for the system of the present invention
are illustrated in bold for six different operating states and CFM/Static Pressure
curves for the conventional PSC motor are illustrated in the light lines for four
different taps. As the figure illustrates, for all of the illustrated operating
states or tap settings: (1) the curves associated with the system described herein
are straighter (meaning that the CFM output of the system is more constant); and
(2) the system described herein allows for airflows at a lower CFM level than is
available with the tapped PSC system. Moreover, although not reflected directly
in FIG. 8, the system described herein uses less energy for the same airflow for
all operating states/taps except for the highest speed tap.
In addition to being capable of operating in CURRENT CONTROL mode, the controller
18 of the present disclosure can operate in a FREQUENCY CONTROL mode. In
the FREQUENCY CONTROL mode, each operating state (as defined by the input signals
12A-
12C) defines a desired output voltage operating frequency. Each
output operating frequency will also correspond to a desired output voltage, with
the voltage varying linearly with changes in the desired output frequency. Thus,
in this FREQUENCY CONTROL mode, the input signals
12A-
12C will define
a desired operating frequency which will have a corresponding desired output voltage.
The controller will then drive the inverter to provide the desired output frequency
and voltage and the motor current will not be directly controlled.
In the FREQUENCY CONTROL mode, the frequency output for the inverter will correspond
roughly to the rotational speed of the motor and, thus, roughly to the blower output.
In one embodiment, the controller
18 may be configured to drive the inverter
to produce one of eight possible output frequencies. For example, the controller
may be configured to provide output operating frequencies of 60 Hz, 55 Hz, 50 Hz,
45 Hz, 40 Hz, 35 Hz, 30 Hz and 25 Hz with the higher frequency output corresponding
to higher blower speeds and generally higher CFM outputs and the lower frequency
outputs corresponding to generally lower speeds and lower CFM outputs.
One potential issue with operating the system in the FREQUENCY CONTROL mode is
that the output parameter of most consequence to the user of the HVAC system in
which the motor system is used is not inverter output frequency but rather the
CFM moved by the blower. In general—at a given static pressure—the
CFM moved by the blower will correspond to the rotational speed of the blower motor,
which will correspond to the frequency of the inverter output voltage. However,
for a given output frequency, the actual CFM moved by the motor will vary significantly
depending on the static pressure against which the blower is working. Thus, in
the FREQUENCY CONTROL mode, the CFM produced from a HIGH HEAT setting will vary
depending on the static pressure of the system which can be affected by, for example,
the ambient atmospheric pressure, the number of doors in a house that are opened
or closed, the position of the return ducts, etc. As such, controlling the inverter
to produce a set frequency does not necessarily result in good CFM control.
To overcome some of the limitations of the FREQUENCY CONTROL mode, a POWER CONTROL
mode may be provided in which each operating state corresponds to a desired POWER
OUTPUT of the inverter. Because the actual work done by the blower will generally
correspond to the CFM moved by the blower—regardless of the static pressure—this
form of control may more accurately control the CFM and provide enhanced control
of the system. Accordingly, under this control scheme, while the output voltage
magnitude and frequency of the inverter may vary for a given operating mode (e.g.,
HIGH HEAT), the actual CFM for the mode will be relatively constant irrespective
of changes in the static pressure.
The work output of the motor can be accomplished by sensing the voltage applied
to the motor and the current drawn by the motor, which will indicate the power
applied to the motor. Once the power actually being drawn by the motor is detected,
the inverter can be controlled to adjust the voltage and/or frequency output of
the inverter until the desired power is being drawn by the motor, and therefore,
the desired amount of work and CFM circulation is being done by the motor. Under
such a control scheme, the setpoints for the various operating modes would correspond
to desired workloads (or even desired CFM outputs).
The various operating modes described above may be implemented through software,
hardware and/or firmware within the controller
18 or an external memory
may be provided to determine the functionality of the controller
18 and,
therefore, the functionality of the system. In one exemplary embodiment, the software
that determines the functionality of the controller
18 and, thus, the system,
is stored in flash memory located within the controller
18. In such an embodiment,
a data exchange port may be provided to allow for updating and modification of
the software within the controller
18 and for changing the operating mode
of the controller. In some embodiments, the data exchange port may also be used
for monitoring the operation of the controller
18 and receiving diagnostic
data about the over system.
As described above, the field adjustment circuit
14 allows for field adjustment
of the setpoints that correspond to the operating states defined by the control
signals
12A-
12C. In general, for each possible operating state, some
form of circuitry may be provided in the field adjustment circuit
14 to
allow for modification or adjustment of the set point corresponding to that operating
state. Thus, if the controller
18 is operating in the CURRENT CONTROL MODE
and the input signals can define five valid operating states, with each operating
state corresponding to a specific current setpoint, the field adjustment circuit
may allow for modification of the current set points corresponding to the various
operating states. If the controller
18 is operating in the FREQUENCY CONTROL
MODE, then the field adjustment circuitry will allow for adjustment of the frequency
setpoints corresponding to the various operating states.
Because the controller
18 will, in certain embodiments, be a digital
controller, the field adjustment circuitry may take the form of a digital communications
interface that would allow an installer, technician or user to couple a digital
communications device (e.g., a laptop computer) to the interface. This embodiment,
however, requires that the installer, technician or user have access to relatively
sophisticated equipment and an understanding of how to use such equipment. Accordingly,
for some applications a lower cost, simpler approach is desirable where few—if
any—tools will be required on the part of the installer, technician or user
to provide field adjustment of the setpoints corresponding to the operating states.
One embodiment for illustrating such an elegant, essentially tooless, approach
for providing field adjustability of the setpoints is illustrated in FIG.
3B
and FIG.
9A. In the exemplary embodiment of FIG. 3B, there are five possible
operating states. Accordingly, there are five dedicated field adjustment circuits
91,
92,
93,
94 and
95, one for each of the possible
operating states. As described above, each operating state may correspond to a
specific setpoint which—depending on the operation mode of the controller
18—can be a current setpoint, a frequency setpoint, a speed setpoint
or a CFM setpoint. For purposes of the present discussion, it will be assumed that
the controller is operating in FREQUENCY CONTROL MODE although it will be appreciated
that the setpoints could, for example, refer to a desired current setpoint of the
controller
18 operating in the CURRENT CONTROL MODE.
Referring to FIG. 3B, each of the field adjustment circuits
91,
92,
93,
94 and
95 comprises a string of series connected
resistors coupled across a defined voltage and a set of jumpers that include taps
coupled at various points in the resistor chain. FIG. 9A illustrates in greater
detail one of the field adjustment circuits.
Referring to FIG. 9A, the exemplary field adjustment circuit includes three
series connected resistors
96,
97 and
98 coupled across a
5V bus. The voltage level at one point of resistor
98 is output on line
99 as the output voltage of the field adjustment circuit. The points where
the resistors are coupled together are provided as inputs to a jumper box
100
that provides, in the illustrated example, five access points to which jumpers
may be coupled.
