BACKGROUND A typical power vented water heater has a blower at or near the top of the flue for pulling combustion gases through the flue and pushing those gases through an exhaust or outlet conduit or pipe. See, for example, U. S. Patent Nos. 5,255,665 and 4,672,919, both of which are incorporated herein by reference. The size, length and path of the outlet conduit can vary greatly from one installation to another.

SUMMARY OF THE INVENTION The invention eliminates some of the complications in the installation of a power vented water heater. Since each installation is different and requires varying lengths and sizes of inlet and exhaust pipe, the installer must modify the airflow with dampers, change pipe diameters or even relocate the appliance in order to provide the airflow required for proper combustion. The invention allows the manufacturer to set the proper airflow for combustion and a variable speed control for the blower motor automatically compensates for installation variation.

Thus, the invention provides a power vented water heater with a variable speed control for the blower motor. The blower can be controlled, for example, with the control of U. S. Patent No. 5,656,912, which is assigned to the assignee hereof and which is incorporated herein by reference, or with the control of U. S. Patent Application Serial No.

09/204, 594, which is assigned to the assignee hereof and which is incorporated herein by reference.

The variable speed system adapts the blower speed to provide the optimum airflow through the combustion chamber to compensate for static pressure variations due to inlet and outlet pipe length, diameter and number of elbows or fittings.

The control can be applied to a blower on either the inlet (pushing) or outlet (pulling) of the outlet pipe.

The control can also be used to compensate for other environmental parameters that affect the combustion process, such as altitude or temperature, which affect air density.

The control can be set up to periodically purge the air in the combustion chamber even when the burner is off in order to prevent low-oxygen make-up air from reaching the pilot, potentially extinguishing it. This could be caused by a cold-air charge, wind down- draft, or normal convection. The purging can be controlled by a timer (open-loop) or a sensor for negative flow or low oxygen.

Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of a first power vented water heater embodying the invention.

Fig. 2 is a graphical illustration showing the relationship between the stator current and the frequency of the electrical stimulus used to energize the motor of the power vented first water heater.

Fig. 3 is a graphical illustration showing the relationship between the stator voltage and the frequency of the electrical stimulus used to energize the motor of the power vented first water heater.

Fig. 4 is a graphical illustration showing the relationship between the desired fluid flow rate and the corresponding motor energization current for the first water heater.

Fig. 5 is a schematic illustration a power vented second water heater embodying the invention.

Fig. 6 is a schematic illustration of a first speed logic and timing circuit of the fixed speed drive controller.

Fig. 7 is a schematic illustration of a second speed logic and timing circuit of the fixed speed drive controller.

Fig. 8 is a schematic illustration of a third speed logic and timing circuit of the fixed speed drive controller.

Before various embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or

being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of"including"and"comprising"and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

DETAILED DESCRIPTION Fig. 1 schematically illustrates a first power vented water heater 1 embodying the invention. The water heater 1 comprises a water tank 2, a combustion chamber 3 below the tank 2, and a flue 4 extending upwardly from the combustion chamber 3 through the tank 2. A gas burner 5 is located in the combustion chamber 3, and the flue 4 conducts combustion products from the chamber 3 and transfers heat from the combustion products to water in the tank 2. A blower 6 has an inlet communicating with the upper end of the flue 4 and an outlet communicating with an outlet pipe 7 that conducts combustion products out of the building in which the water heater 1 is located. A water heater as thus far described is known in the art and need not be described in greater detail.

The blower 6 is driven by a variable speed motor 14. The motor 14 is controlled by a controller 10 that is substantially identical to the controller of U. S. Patent No.

5,656,912 (the"912 Patent"), which is incorporated herein by reference. Fig. 1 uses the same reference numerals as the'912 Patent, and the controller 10 of Fig. 1 differs from the controller of the'912 Patent only in that the input for the decoder 42 is provided by a water heater control 100 that determines when the blower 6 should operate. The water heater control 100 also controls the burner 5 and includes a thermostat 8 that monitors the temperature of the water in the tank, as is known in the art. The water heater control 100 can make the motor 14 and the blower 6 operate only simultaneously with the burner 5, or can make the motor 14 and blower 6 operate at other times. The water heater control 100 further includes a timer 9 and/or a sensor 11. The timer 9 is connected to the thermostat 8 and generates a control signal to periodically cause operation of the blower to purge flue gases. The sensor 11 is either a flow sensor for measuring negative air flow in the flue, or an oxygen sensor for detecting a low oxygen condition in the flue. In response to the sensed condition, the sensor 11 informs the water heater control 100 to purge the air in the combustion chamber 3.

The motor 14 includes a stator (not shown) having three phase windings, and a rotor (not shown) mounted for rotation about a rotor axis (also not shown). As is commonly known in the art, energization of the stator phases causes rotation of the rotor.

The motor 14 also includes an arbor 18 connected to the rotor for rotation therewith. The blower 6 is mounted on the arbor 18 so that as the blower 6 rotates, air is forced out of the flue 4 into the outlet pipe 7 to be exhausted or vented from the building. A series of switches 26 selectively electrically connect the motor 14 to electrical power (typically direct current derived from standard alternating current line voltage) in response to control signals produced by the controller 10.

The controller 10 includes a microprocessor 34 connected to the water heater control 100 to receive therefrom control signals. The microprocessor 34 is also connected to the switches 26 supplying power to the motor 14 to control the switches 26 and energize the motor 14 so that the fan 22 delivers a constant flow rate volume of air despite any differences in the load conditions experienced by the motor 14. Typically, such load differences occur as a result of variations in the installation of the outlet pipe 7 as is necessary to install the unit in different buildings. As is commonly known in the art, a series of drivers 38 are connected between the power switches 26 and the microprocessor 34.

The microprocessor 34 includes a decoder 42 for receiving the control signals and includes drive signal means or energizing means connected to the decoder 42 for producing an electrical drive signal or electrical stimulus resulting in current flow in the stator phase. While various means for producing the electrical drive signal are appropriate, the drive signal means of the preferred embodiment includes a current convertor 46 connected to the decoder 42 and a current command calculator 50 connected to the current convertor 46.

The microprocessor 34 also includes change signal means for producing a change signal related to changes in stator current flow. While various means for generating the change signal are appropriate, in the preferred embodiment the change signal means includes a comparator 54 connected to the current command calculator 50.

The microprocessor 34 also includes manipulation means connected to the comparator 54 and to the drive signal means for changing the electrical drive signal in response to the output from the comparator 54. While various means for changing the electrical drive signal are appropriate, the manipulation means of the preferred embodiment includes a current regulator or integrator 58 connected to the comparator 54 and a summation node 62 connected to the current regulator 58.

The summation node 62 has an output which is fed back through a delay element 66 to an input of the summation node 62 and to the current command calculator 50. The

output of summation node 62 is also connected to a frequency-to-voltage convertor 70. A pulse width modulator 72 is connected to the frequency-to-voltage convertor 70. The pulse width modulator 72 is connected to the switch drivers 38 to output signals thereto and selectively connect the phases of the motor 14 to electrical power.

The controller 10 also includes monitoring means for monitoring the current flow in the stator phase. Any known means for monitoring or measuring the stator current is appropriate. In the preferred embodiment, the monitoring means is a current sensor 74 connected to at least one of the motor phases to detect motor phase current. The current sensor 74 is connected to the comparator 54 to transmit the phase current to the comparator 54.

In operation, the microprocessor 34 controls the motor 14 using the relationship between stator current, stator frequency and air flow rate shown in FIG. 2. This relationship has been empirically determined and, as clearly shown in FIG. 2, for a given air flow rate, the stator current versus the stator frequency relationship is generally linear, i. e., can be defined by the linear equation: y=nvnc b ; where, desired stator command current for current time period (I) ; x=stator command frequency for previous time period (co,) ; m=slope of current frequency curve (the slope is determined by the blower characteristics, for example, cage size, number of blades, etc.); and b=the zero frequency or steady state no-load stator current (I2).

By knowing the desired air flow rate at which the combustion gases are to be vented from the system, the zero frequency stator current I2 at that air flow rate and the stator command frequency cal for the previous time period, the microprocessor 34 can easily calculate the desired stator command current I at which the motor 14 must be energized to generate the desired air flow rate output. If the desired stator command current I differs from the actual stator current I1, then the stator command frequency col can be adjusted to compensate for the difference, which is assumed to be the result of a change in the load on the motor 14. In a broad sense, the controller 10 can be used to

control any motor where the relationship between the electrical signal used to energize the motor and the output of the motor is known.

More specifically, and referring to FIG. 1, the decoder 42 receives the speed inputs and generates in response to the speed inputs an output that is indicative of a desired cubic feet per minute flow output (CFMdesired) for the motor blower 6. The current convertor 46 receives the CFM desired signal and generates in response to the CFMdesired signal the zero frequency stator current value (I2). The current convertor 46 can generate I2 using a real time calculation, however, in the preferred embodiment, the current convertor 46 is simply a memory based look-up table that stores a separate zero frequency stator current value for a number of different flow rates. The relationship between CFMdesired and I2 is shown in FIG. 4. The current convertor 46 transmits the zero frequency stator current to the current command calculator 50.

At approximately the same time, the command frequency wu, i. e., the command frequency from the previous 0.6 second time period, is fed back to the current command calculator 50 from the output of the summation node 62. In response to receipt of the zero frequency stator current I2 and the command frequency signal cl., the current command calculator 50 generates a command current I, i. e., the current at which the motor 14 should be energized for a given blower output. As stated above, the relationship used for this determination is shown in FIG. 2.

The command current I is fed to the comparator 54 and compared against the actual phase current Il as measured by the current sensor 74. The current comparator 54 outputs a current error value (AI) that represents the difference between the actual stator phase current Il and the desired stator phase current I2 for the desired air flow rate deseed The current error (AI) is transmitted to current regulator 58 which integrates the current error signal AI to generate a manipulation output (ace). The manipulation output Aco is added to the previous command frequency col to generate an updated command frequency (1) The updated command frequency tO2 represents an updated frequency signal which is required at existing motor current Il to maintain the desired blower air flow rate output CFM. The command frequency 02 is transmitted to the frequency-to-voltage convertor 70 which generates an updated command voltage. The frequency-to-voltage convertor 70 uses the relationship shown in FIG. 3 to generate the command voltage and this voltage is input to the pulse width modulator 72 along with the updated command frequency. The function performed by the frequency-to-voltage convertor 70 can be conducted using a real time software based calculation based on the equation:

where, V is the updated command voltage, Kv is a constant to convert the frequency units to a voltage units, and (02 is the command frequency.

In the preferred embodiment, the results of the function are precalculated and, like the functions of the current convertor 46 and the current command calculator 50, the frequency-to-voltage convertor function is stored in a memory based look-up table. The command frequency (@2) is also fed back to the current command calculator 50 through the delay element 66 which causes a transmission delay of approximately 0.6 seconds.

In response to the updated command frequency zu and the updated command voltage V, the pulse width modulator 72 generates control signals for the drivers 38 which operate the switches 26 to generate an updated current output for the motor 14 to maintain the desired air flow rate output. The current sensor 74 will continue to measure the stator phase current. If the blower motor load remains the same from one 0.6 second interval to the next, then the stator phase current Il will not change, and there will be no resulting current error signal AI generated. As a result, the command frequency (02 output at the summation node 62 will not change. Alternatively, if the blower motor load changes from one 0.6 second interval to the next, then a new current error signal AI will be generated to cause a recalculation of the command frequency 032 as described above.

Fig. 5 schematically illustrates a second power vented water heater 105 embodying the invention. The power vented water heater 105 shares many common elements with the power vented water heater 1, and common elements are designated with the same reference numerals in Figs. 1 and 5. The blower 6 is driven by a variable speed motor.

For the water heater 105, the variable speed motor is an induction motor 110. While any single phase induction motor can be controlled using the controller 114, the induction

motor 116 shown in the drawings and referred to in the description is a permanent split capacitor ("PSC") motor.

The motor 116 is controlled by a controller 114 that is substantially identical to the controller of U. S. Patent Application No. 09/204,594 (the"'594 Application"), which is incorporated herein by reference. The controller 114 of Fig. 5 differs from the controller of the'594 Application only in that the input for the thermostat logic and timing is provided by the water heater control 100 controlling when the blower 6 should operate. In other words, for this application, the thermostat logic and timing of the'594 Application is a speed logic and timing 166.

Similar to power vented water heater 1, the water heater control 100 also controls the burner 5. The water heater control 100 can make the motor 14 and the blower 6 operate only simultaneously with the burner 5, or can make the motor 14 and blower 6 operate at other times. The control 100 further includes a thermostat 8, and a timer or sensor 11 for measuring negative flow or low oxygen.

The controller 114 includes power input terminals 118 and 122 adapted to be connected to a source of electrical power (shown as VAC Input). The power input terminals are selectively connected directly to the motor through relay RLY2. The controller 114 also includes a full wave bridge rectifier 126 and an EMI filter 130 connected to the power input terminals 118 and 122. The EMI filter 130 is connected to the inverter 134, which is, in turn, selectively connected to the motor through relay RLY1.

A micro-controller 138, and pulse width modulator PWM 142 are connected to the inverter 134 to control the output of the inverter 134. The relays RL41 and RL42 are controlled by inputs from the thermostat, various embodiments of which are shown in Figs. 6-8.

As shown in Fig. 5, the inverter 134 includes positive and negative voltage busses 150 and 154. Capacitors Cl and C2 are serially connected to one another between the voltage busses 150 and 154. Power switches IGBT1 and IGBT2 are serially connected to one another in a"half-bridge"configuration between the positive and negative busses 150 and 154 and in parallel with the capacitors Cl and C2. Power switches IGBT1 and IGBT2 each include a gate 158 connected to a gate driver 162. The gate driver 162, pulse width modulator 142 and micro-controller 138 control operation of the power switches IGBT1 and IGBT2. The inverter 134 is designed to operate efficiently at only one or perhaps only a few fixed, predetermined speeds that are less than the rated full operating speed at full line voltage. At these speeds, the micro-controller 138 calculates a quadratic relationship

between applied voltage and frequency rather than the constant voltage to frequency ratio of the prior art. The use of a quadratic control relationship between the applied voltage and the frequency reduces the torque output matching the fan law torque curve, resulting in a more efficient controller that requires fewer and lower cost, lower power rated parts.

As a result of the quadratic voltage-to-frequency control relationship, the motor requires approximately only half the voltage normally supplied during full speed operation.

Due to the nature of this reduced speed/reduced voltage requirement, the inverter 134 uses fewer components and lower power, lower voltage components that are less expensive than the components required by the prior art. Ultimately, these two factors reduce the cost of the inverter 134 over prior art full frequency/full voltage range inverters.

The controller shown in Fig. 5 also includes speed logic and timing circuitry 166 (see Fig. 6) having speed inputs Y1 and Y2. Speed inputs Y1 and Y2 connect to conventional 24VAC inputs. The speed inputs Yl and Y2 are used to select low speed or high speed operation. Of course, additional speed inputs may be added and more than two speeds may be used for controlling the blower 6.

Fig. 7 illustrates another speed logic and timing circuit 206. Like parts are identified using like reference numerals. The speed logic and timing circuit 206 also includes timing means for ensuring that, when switching from low speed operation to full speed operation, a"break-before-make"condition exists whereby one relay is disabled (breaks) for a period of time necessary to let the motor's magnetic field collapse and before energizing the other relay (make). From experimental data, the time required for the magnetic field to decay is on the order of several hundred milliseconds, and is a function of motor size and design. The"break-before-make"timing of thermostatic logic and timing circuit 206 is equal to seven hundred milliseconds.

The timing circuit 206 allows the controller 114 to deactivate its outputs before switching to the line in an attempt to protect the controller 114 from possible switching transients in the relay. Switching transients can potentially occur during switching as current is interrupted from the inductive load (i. e., the motor). The interruption of current flow will usually result in arcing when using a mechanical switching means such as a relay. This arcing may damage the power switching output devices of the controller 114.

Fig. 8 illustrates another speed logic and timing circuit 306 that switches the relays RLY1 and RLY2 with"break before make"timing equal to one hundred milliseconds timing, at lower cost, and space than the circuit shown in Fig. 7. Like parts are identified using like reference numerals.

A secondary problem that needed to be overcome occurs when switching from the line driven (high speed) operating mode to the inverter driven (low speed) operating mode.

The motor 110 must slow down to at least the speed of the lower speed drive. If not, the motor 110 will act as a generator, charging the bus capacitors and perhaps exceeding the capacitor voltage ratings. This condition may result in permanent damage to the capacitors. To eliminate the potential for this damage to occur, the drive is informed by the circuit shown in Fig. 8 that the motor 110 is being switched from the line to the drive.

A timing means in the form of a software delay was created to wait three seconds before starting the drive, allowing the fan load to slow the motor below the inverter drive frequency thus preventing the generating condition from occurring.

The speed logic and timing circuit 306 can be combined with the controller 114 to provide a total system solution to run a PSC motor in a power vented water heater application in both high and low speed operation, selectable by thermostat controls, in a very efficient manner.

Referring specifically to Fig. 8, circuit inputs Y1 and Y2 are 24VAC signals from a water heater control 100 controlling two speed operation. Input Y1 is energized if low speed operation is called for, and input Y2 is energized (usually along with Y1) when high speed operation is called for. Input Y2 will take precedence over input Y1, if active.

Logic Truth Table: Y1 Y2 Motor speed off off off on off low speed off on high speed on on high speed In operation, inputs Y1 and Y2 are half wave rectified by diodes D5 and D4, respectively, and are filtered by RC filters formed by R4/C4 and R1/C5, respectively, to create a DC signal representing the state of the signal. Component IC1 is a single package containing seven separate, open collector (Darlington transistor) inverters with recirculation diodes connected to the collectors and common emitters connected to ground.

Each DC signal is routed first through two inverters in IC1. The output of the second inverter is connected to a seventy-five Kohm resistor and a ten-microfarad capacitor. At first, the capacitor does not carry a charge. When a DC signal becomes present, the output

of the second inverter goes"high"and the ten-microfarad capacitor charges. After about one-tenth of a second the capacitor reaches the threshold of the third inverter thereby turning on the third inverter and energizing the corresponding relay RLY1. When the inputs Y1 or Y2 both switch to"low,"the DC signal quickly decays, allowing the first inverter output to be pulled high, and thereby causing the second inverter to short (i. e. discharge) the ten-microfarad capacitor quickly. This turns off the third inverter so that the third inverter no longer energizes the relay coil and the relay RLY1 opens quickly.

This timing scheme allows the relays RLY1 and RLY2 to be delayed by about one hundred milliseconds when turning on. However, relays RLY1 and RLY2 turn off much quicker, i. e., almost immediately.

In order to have input Y2 take precedence over input Yl, diode D3 discharges capacitor C4 (of input Yl's input filter) when every input Y2 is present, thereby turning off quickly and keeping off the relay controlling input Yl. To inform the inverter that it is being selected to run, the signal to energize the relay RLY1 is optically coupled through optical coupler Kl. Capacitor C7 provides noise filtering. Relays RLY1 and RLY2 switch both terminals of the motor 110 from line to the FSD. Diode D6, capacitors Cl and C6, and voltage regulator U4 provide a DC supply for the thermostatic logic and control circuit 206 from the same 24VAC transformer which supplies power to the thermostat.

In situations where the load on the motor changes frequently, the controllers 10 and 114 will adjust the speed of the motor to ensure constant in-flow. In the case of a power vented water heater, the load will change rarely and the advantage of the controllers 10 or 114 is that the controllers 10 or 114 adjusts the motor speed to generate a consistent air flow rate regardless of the differing configurations of the outlet pipe 7 as it is installed in different buildings. This eliminates the need for the installer to perform adjustments to the motor speed upon installation of the system.

Various features and advantages of the invention are set forth in the following claims: