Abstract:
A method and apparatus for controlling a motor. The apparatus includes a voltage input; a half-bridge inverter connected the voltage input and to the motor to provide low speed excitation to the motor; and a circuit for selectively electrically connecting the voltage input to the motor and for selectively electrically disconnecting the inverter from the motor.

Description:
RELATED APPLICATIONS 
     This application claims priority, under 35 U.S.C. Section 119, of prior-filed, co-pending provisional application Ser. No. 60/090,721 filed Jun. 26, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to induction motors, and particularly to a method and apparatus for controlling an induction motor. 
     Previous methods and apparatus of controlling an induction motor incorporate the changing of speed taps or the use of triac controls, which simply provide an effective reduction in voltage or flux so as to cause the motor to run at a reduced speed by the nature of slip. Slip is generally a measurement of how much the movement of the rotor follows the excitation field, and is defined as the difference between the frequency of the excitation energy and the speed of the motor. While these controls provide adequate speed control, they do so at the expense of efficiency as the motor runs at a higher slip which is proportional to rotor conduction loss. 
     U.S. Pat. No. 5,252,905 a shows and describes such a controller for a motor. The controller uses a single phase pulse width modulated inverter to control the speed of the motor at speeds that are less than the full operating speed of the motor. The voltage applied to the motor is adjusted in direct proportion to the frequency output from the pulse width modulator. That is, a constant voltage to frequency ratio is maintained. This constant voltage to frequency ratio results in a constant torque output regardless of the speed of the motor. 
     SUMMARY OF THE INVENTION 
     It has been determined that the reduction of input power to a blower motor has a dramatic effect on increasing the Seasonal Energy Efficiency Ratio or (“SEER”) of heating, ventilating, and air conditioning (“HVAC”) equipment. It has also been determined that blower applications exist that do not require the full torque output (typically measured in cubic feet per minute (“CFM”)) that an induction motor excited by sixty cycle voltage would provide. The purpose of the invention is to provide a low cost, “power efficient” way of reducing the indoor blower speed, and consequently, the indoor blower CFM and input power so as to realize an increase in overall system efficiency. 
     Accordingly, the invention provides a controller having a variable speed drive that provides an optimum reduced speed setting for the operation of the blower motor when less than full speed and full torque output are required. The controller includes an inverter and pulse width modulator connected to the inverter to control the inverter. The inverter includes a microprocessor to calculate 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. 
     The controller also incorporates the use of relays local to the controller. The relays allow the motor to be run using the variable speed excitation voltage (for reduced speed operation) and allow the inverter to be bypassed to excite the motor using sixty cycle line excitation voltage (for full speed operation). The ability to run the motor at full speed directly from the line voltage further reduces the power requirements of the components in the inverter. Because set reduced speed operating points are selected using a quadratic voltage to frequency relationship, the electronic components of the inverter can be sized for approximately ¼ the full rated horsepower. 
     This control concept is particularly effective in applications which follow a “cubed-law” power characteristic such as fans and pumps. In these applications, the power demanded by the load is a cubed function of the speed, that is: y=CX 3 ; wherein y is the power, and X is the speed of the motor. By reducing the speed in half, you reduce the power required by ⅛. 
     Various other features and advantages of the invention are set forth in the following drawings, description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram form schematic illustration of an induction motor and a fixed speed drive controller embodying the invention. 
     FIG. 2 is a block diagram form schematic illustration of the inverter used in connection with full range motor speed controllers of the prior art. 
     FIG. 3 is a schematic illustration of one embodiment of a thermostat logic and timing circuit of the fixed speed drive controller. 
     FIG. 4 is a schematic illustration of another embodiment of a thermostat logic and timing circuit of the fixed speed drive controller. 
     FIG. 5 is a schematic illustration of yet another embodiment of a thermostat logic and timing circuit of the fixed speed drive controller. 
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates schematically an induction motor  10  and a controller  14  embodying the invention connected to the motor. While any single phase induction motor can be controlled using the controller, the induction motor  10  shown in the drawings and referred to in the description is a permanent split capacitor (“PSC”) motor. 
     The controller includes power input terminals  18  and  22  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 RLY 2 . The controller also includes a full wave bridge rectifier  26  and an EMI filter  30  connected to the power input terminals  18  and  22 . The EMI filter  30  is connected to the inverter  34 , which is, in turn, selectively connected to the motor through relay RLY 1 . A micro-controller  38 , and pulse width modulator PWM  42  are connected to the inverter  34  to control the output of the inverter  34 . The relays are controlled by inputs from the thermostat, various embodiments of which are shown in FIGS. 4-6. 
     As shown in FIG. 1, the inverter  34  includes positive and negative voltage busses  50  and  54 . Capacitors C 1  and C 2  are serially connected to one another between the voltage busses  50  and  54 . Power switches IGBT 1  and IGBT 2  are serially connected to one another in a “half-bridge” configuration between the positive and negative busses  50  and  54  and in parallel with the capacitors C 1  and C 2 . Power switches IGBT 1  and IGBT 2  each include a gate  58  connected to a gate driver  62 . The gate driver  62 , pulse width modulator  42  and micro-controller  38  control operation of the power switches IGBT 1  and IGBT 2 . The inverter  34  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  38  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 circuit of FIG. 1 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 drive of FIG. 1 over prior art full frequency/full voltage range inverters such as the one illustrated in FIG.  2 . The prior art circuit shown in FIG. 2 includes a second stage of power switches IGBT 3  and IGBT 4 . Furthermore, because the inverter of FIG. 2 is designed to operate over the full range of frequency and voltage, the components of the inverter of FIG. 2 must be rated for higher power and higher current, and are therefore more expensive than the components used in the controller shown in FIG.  1 . 
     The controller shown in FIG. 1 also includes thermostat logic and timing circuitry  66  (see FIG. 3) having thermostat inputs Y 1  and Y 2 . Thermostat inputs Y 1  and Y 2  connect to conventional 24 VAC inputs. The thermostat inputs Y 1  and Y 2  are used to select low speed or high speed operation. 
     FIG. 4 illustrates another thermostat logic and timing circuit  106  embodying the invention. Like parts are identified using like reference numerals. The thermostat logic and timing circuit  106  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&#39;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  106  is equal to 700 msec. 
     The timing means allows the drive to deactivate its outputs before switching to the line in an attempt to protect the drive 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 drive. 
     FIG. 5 illustrates another thermostat logic and timing circuit  206  that switches the relays RLY 1  and RLY 2  with “break before make” timing equal to 100 msec timing, at lower cost, and space than the circuit shown in FIG.  4 . 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 must slow down to at least the speed of the lower speed drive. If not, the motor 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. 5 that the motor is being switched from the line to the drive. A timing means in the form of a software delay was created to wait 3 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 thermostat logic and timing circuit  206  can be combined with the controller  14  to provide a total system solution to run a PSC motor in an HVAC application in both high and low speed operation, selectable by thermostat controls, in a very efficient manner. 
     Referring specifically to FIG. 5, circuit inputs Y 1  and Y 2  are 24 VAC signals from a thermostat controlling two stage heating. Input Y 1  is energized if low speed operation is called for, and input Y 2  is energized (usually along with Y 1 ) when high speed operation is called for. Input Y 2  will take precedence over input Y 1 , 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 Y 1  and Y 2  are half wave rectified by diodes D 5  and D 4 , respectively, and are filtered by RC filters formed by R 4 /C 4  and R 1 /C 5 , respectively, to create a DC signal representing the state of the signal. Component IC 1  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 IC 1 . The output of the second inverter is connected to a 75 Kohm resistor and a 10uf 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  10 uf capacitor charges. After about {fraction (1/10)}th of a second the capacitor reaches the threshold of the third inverter thereby turning on the third inverter and energizing the corresponding relay RLY 1 . When the inputs Y 1  or Y 2  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 10 uF capacitor quickly. This turns off the third inverter so that the third inverter no longer energizes the relay coil and the relay RLY 1  opens quickly. This timing scheme allows the relays RLY 1  and RLY 2  to be delayed by about 100 msec when turning on. However, relays RLY 1  and RLY 2  turn off much quicker, i.e., almost immediately. 
     In order to have input Y 2  take precedence over input Y 1 , diode D 3  discharges capacitor C 4  (of input Y 1 &#39;s input filter) when every input Y 2  is present, thereby turning off quickly and keeping off the relay controlling input Y 1 . To inform the inverter that it is being selected to run, the signal to energize the relay RLY 1  is optically coupled through optical coupler K 1 . Capacitor C 7  provides noise filtering. Relays RLY 1  and RLY 2  switch both terminals of the motor  10  from line to the FSD. Diode D 6 , capacitors C 1  and C 6 , and voltage regulator U 4  provide a DC supply for the thermostatic logic and control circuit  206  from the same 24 VAC transformer which supplies power to the thermostat. 
     Various features and advantages of the invention are set forth in the following claims.