Abstract:
A controller for an electric machine. The controller includes a first voltage input that directly provides a first voltage to the electric machine, a second voltage input configured to receive a second voltage, and an inverter coupled to the second voltage input. The inverter is configured to be activated by the second voltage, to frequency-regulate the second voltage to generate a frequency-regulated voltage, and to provide the frequency-regulated voltage to the electric machine.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to induction motors, and particularly to a method and apparatuses for controlling an induction motor. 
     Many methods and apparatuses are used to control an induction motor. Exemplary methods and apparatuses include speed tab changing, triac controls, and fixed speed drive. In changing the speed tabs, an effective reduction in voltage or flux is provided to cause the motor to run at a reduced speed by the nature of a slip. The slip is 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. 
     SUMMARY OF THE INVENTION 
     In the fixed speed drive, a series of signals are entered from the thermostat. These signals are then interpreted by a thermostat logic and timing control that is enabled by an inverter circuitry and a micro-controller. While the fixed speed drive is an inexpensive and efficient product that provides half speed settings, a number of components will have to be replaced when an original equipment manufacturer (“OEM”) chooses to use the fixed speed drive. For example, the OEM may have to replace the induction motor with optional taps, and relays that are used to select voltage and speed. Replacing these components can be costly. 
     Accordingly, there is need for an interface between a thermostat and an induction motor that also provides high motor efficiency at low-speed. In a first embodiment of the invention, a fixed speed drive (“FSD”) interface that includes a controller is configured to determine at which speed to operate a motor based on an AC input. The FSD interface receives electrical power from a tapped winding relay that has two AC input connections: a high-speed connection and a low-speed connection. The high-speed connection directly drives the motor. The low-speed connection powers the motor through an inverter or a capacitor and inverter sub-circuit. The FSD interface also includes an analog-to-digital converter (“ADC”) that detects the difference in voltage between the high-speed and low-speed voltages. Based on this voltage difference from the ADC, a controller determines whether to power the motor from the high-speed connection or from the low-speed connection via the inverter. 
     The present invention also provides a controller for an electric machine. The controller includes a first voltage input, a second voltage input, and an inverter. The first voltage input is configured to receive a first voltage, and is operable to directly provide the first voltage to the electric machine. The second voltage input similarly configured to receive a second voltage. The inverter is coupled to the second voltage input, and is activated by the second voltage. The inverter is also configured to frequency-regulate the second voltage to generate a frequency-regulated voltage, and to provide the frequency-regulated voltage to the electric machine. 
     The present invention also provides a controller for an electric machine. The controller includes a voltage input that is configured to receive a first voltage, and a relay module that is coupled to the voltage input. The relay module is configured to relay the first voltage and to generate a second voltage. The controller also includes a half-bridge inverter that is coupled to the relay module, and that is configured to be activated by the second voltage, and to generate a frequency regulated voltage. The controller further includes a micro-controller that is coupled to the first and the second voltages, and that is configured to generate a soft control signal. The controller also includes a second relay that is coupled to the micro-controller. The second relay is also configured to select an electric machine operating voltage from the first voltage and the frequency regulated voltage using the soft control signal. 
     The present invention also provides a method of controlling an electric machine. The method includes the step of providing one source of unregulated electrical power selectively connected to the electric machine through a relay when a first speed is selected. The method includes the step of generating a second source of regulated electrical power when a second speed is selected, the second source is selectively connected to the electric machine through the relay. The method includes the step of selectively switching the relay to connect the electric machine to the one source corresponding to the first speed, and to the second source for operation of the electric machine corresponding to the second speed. 
     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 
       In the drawings: 
         FIG. 1  shows a block diagram of a fixed speed drive interface according to the present invention; and 
         FIG. 2  shows a circuit diagram of the interface shown in  FIG. 1  according to the present invention. 
       Before any 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 arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being 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. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a FSD interface  100  coupled to an induction motor  104  according to the present invention. The interface  100  includes a thermostat relay or tapped winding relay  108  that receives a selection input from a thermostat  112 , and power from an alternating current (“AC”) power source line-in  113 . The interface  100  also receives a second AC power source line-in at input  114 . The tapped winding relay  108  has two speed outputs  116 ,  120  representing a high-speed signal and a low-speed signal, respectively. More specifically, the tapped winding relay  108  will generate a high-speed signal when the motor  104  is run at a full-speed mode, whereas the tapped winding relay  108  will generate a low-speed signal when the motor  104  is run at a low-speed mode. The low-speed signal output  120  is fed into an electromagnetic interference (“EMI”) filter  124  to attenuate electromagnetic interference to generate a filtered low-speed output  126 . The filtered low-speed output  126  and the high-speed signal output  116  are coupled to a rectifying circuit  128  to generate different levels of direct current (“DC”). The relay  108  includes a plurality of inputs that are opto-isolated from the high voltage side of the interface  100 . The inputs can be configured for inputs, such as from a thermostat, to turn the interface  100  on for high speed operation at an operating frequency, for example, between 50 Hz and 60 Hz, or to turn the interface on for a preset low speed operation. Furthermore, these inputs can also be configured for pulse width modulation control for running the motor  104  at low speed. 
     Before proceeding further, it should be apparent from the figures that the high-speed output  116  and the low-speed output  120  are a high-speed input and a low-speed input to the FSD interface  100 . These inputs are also referred to herein as the first and second voltage inputs. The high-speed output  116  and the low-speed output  120  are also summed individually in a summing module  136  to provide different analog voltages each representing a particular speed. For example, the high-speed output  116  is summed into a 10KΩ resistor via a 1MΩ resistor. Meanwhile, the filtered low-speed output  126  is summed into the 10KΩ resistor via a 499KΩ resistor to generate a summed voltage. The summing module  136  thus provides a summed voltage that represents either a high-speed signal or a low-speed signal. The summed voltage is further conditioned at a filter module  148  to filter out undesirable noise or to clean the summed voltage so that the summed voltage is detectable by an A/D converter (“ADC”)  140 . The ADC  140  can be embedded in a micro-controller  144  as shown in  FIG. 1 , or the ADC  140  is external to the micro-controller  144 . Examples of micro-controller include embedded micro-controller, such as PIC 16C717I/SS from MicroChip, and ST micro-controller from ST Microelectronics. The micro-controller  144  reads in the summed voltage, and then generates a software control or selection signal based on the summed voltage. The micro-controller  144  further includes an internal memory (not shown) that stores a plurality of codes and associated parameters. Although the memory is described as internal to the micro-controller  144 , external memory can also be used in the interface to store data such as customer-specific parameters. 
     In the embodiment shown, the rectifying circuit  128  includes a high-speed rectifier and a low-speed rectifier. Each of the rectifiers includes a full wave bridge rectifier that has four diodes. Additionally, the high-speed rectifier and the low-speed rectifier share a common pair of diodes, and a common rectified or DC output. The DC output is frequency regulated in a capacitor-inverter circuit or an inverter module  132 . Particularly, the inverter module  132  includes a first capacitor that is serially connected to a second capacitor. The inverter module  132  also includes a plurality of power switches that are arranged in parallel with the first and the second capacitors. The inverter module  132  is configured to provide one or perhaps only a few fixed, predetermined speeds that are less than the rated full operating speed at full line voltage at input  116 . At low speed, in order to reduce the torque output to match a fan law torque curve, the micro-controller  144  is configured to determine a quadratic voltage-to-frequency control relationship between an applied voltage and the operating frequency. As a result of the quadratic relationship, the motor  104  requires approximately only half the voltage normally supplied during full speed operation. Although voltage-to-frequency control relationship described in the embodiment is quadratic, other forms of motor applied voltage/operating frequency relationship can also be used such that the voltage and the frequency can be controllably applied. In the embodiment shown, the output frequency of the inverter module  132  ranges from about 32 Hz to about 45 Hz. A potentiometer can also be used to allow a user to adjust the output frequency in the same range. 
     Although the high-speed signal output  116  generally indicates the motor  104  is to be run at full speed (for example, between 50 Hz and 60 Hz), it will be appreciated that multiple full speed scenarios can be installed in systems where a multiple-tapped motor is used. Specifically, in an embodiment where a single speed single phase (“SSSP”) permanent split capacitor (“PSC”) motor is used the high-speed signal output  116  will have a single high-speed value at a fixed operating frequency, such as 60 Hz. In yet another embodiment, the interface  100  can be coupled to a multiple-tapped, single phase (“MTSP”) PSC motor. In such case, the high-speed signal output  116  will have multiple speed values all running at a single operating frequency, such as 60 Hz. Furthermore, the interface  100  will have multiple outputs coupled to the MTSP PSC motor. 
     Referring back to  FIG. 1 , the interface  100  also includes a switching module  149 . The switching module  149  selects either the low speed signal or voltage from the inverter module  132  based on a pair of software generated control or selection signals from the micro-controller  144 . For example, when it is desired to run the motor  104  at high speed, the high speed output  116  of the tapped winding relay  108  will relay direct AC power from input  113 , while the low speed output  120  is essentially open, deactivated or having a null value. The high-speed signal is also summed into the 10KΩ resistor via the 1MΩ resistor at the summing module  136 , filtered at the filtering module  148 , and fed to the ADC  140 . Thereafter, the micro-controller  144  processes the summed voltage and switches the switching module  149  to relay the high-speed voltage to the motor  104 . When it is desired to run the motor  104  at low speed, the low speed output  120  is EMI filtered, rectified at the rectifying circuit  128 , and frequency regulated at the inverter module  132 . Efficient and frequency regulated low speed voltage is thereafter provided to the switching module  149 . The frequency regulated low speed voltage can then be selected by the micro-controller  144 . 
     A detailed circuit diagram  200  of the interface  100  is shown in  FIG. 2 . Like parts are identified using like reference numerals. The low-speed output  120  is filtered at the EMI filter  124  to yield a filtered low-speed output  126 . The filtered low-speed output  126  and an EMI filtered AC power line-in  115  are coupled to a low-speed rectifier of the rectifying circuit  128  to generate a low-speed DC signal. The low-speed rectifier includes a first pair of forward-biased diodes  150 ,  154 , and a second pair of reversed-based diodes  158 ,  162 . Meanwhile, the high-speed signal output  116  together with the EMI filtered AC power line-in  115  are coupled to a high-speed rectifier to generate a high-speed DC signal. Specifically, the high-speed rectifier includes a pair of forward-biased diodes  154 ,  166 , and a pair of reversed-biased diodes  158 .  170 . In other words, both the low-speed rectifier and the high-speed rectifier share the diodes  154 , and  158 . A DC bus feedback monitor  182  couples the rectifying circuit  128  to the inverter module  132 . The DC bus feedback monitor  182  is configured to monitor the rectified voltage (sometimes referred to as a DC bus voltage or a DC speed signal) of the rectifying circuit  128 . When the monitored DC speed signal changes, the DC bus feedback monitor  182  will alert the micro-controller  144  to adjust the output duty cycle of the inverter module  132  accordingly. The DC bus feedback monitor  182  is also configured to detect different types of full speed AC voltages, such as  115  VAC and  230  VAC. The DC speed signal is then fed to the inverter module  132  to provide one or perhaps only a few fixed, predetermined speeds that are less than the rated full operating speed at full line voltage at input  116 . 
     Furthermore, either the high-speed signal output  116  is summed into a 10KΩ resistor via a 1MΩ resistor, or the filtered low-speed output  132  is summed into the 10KΩ resistor via a 499KΩ resistor, to generate a summed voltage that represents an analog high-speed signal or low-speed signal. The summed analog voltage is then filtered via a resistor-capacitor type filter before being fed into the ADC  140  of the micro-controller  144 . In the embodiment shown, a 49KΩ resistor is arranged in parallel to a 1 μF capacitor. It should be noted that other filter combinations, and other resistors and capacitor values can also be used to clean up the undesirable noise in the analog summed voltage. 
     The switching module  149  ( FIG. 1 ) includes two switches  174  and  178  to select between a high-speed input voltage and a low-speed input voltage, as described earlier. Specifically, the micro-controller  144  sends a pair of software control signals or soft control signals as selection signals to the switches  174  and  178  based on the summed voltage. Meanwhile, the inverter module  132  and the high-speed voltages from inputs  114  and  116  provide a low-speed voltage input and a high-speed voltage input to the switches  174  and  178 , respectively. When the selection signals from the micro-controller  144  represent high-speed signals, the switches  174  and  178  will couple the high-speed voltages from inputs  114  and  116  to the motor  104 . At this time, the inverter module  132  is disabled, while a small amount of power is supplied to the micro-controller  144 . When the selection signals from the micro-controller  144  represent low-speed signals, the switches  174  and  178  will couple the low-speed voltages from the inverter module  132  to the motor  104 . 
     Generally, power demanded by a load is typically a non-linear function of the operating speed or frequency. Specifically, when P is the power demanded by a load, C is a constant and S is the motor speed, P=CS 3 . That is, reducing the motor speed or the operating frequency of the motor by half will reduce the power demanded by the load to ⅛ of the original power when run at full speed. That is, the interface  100  can efficiently deliver power to the motor  104  using the inverter module  132  after a low-speed voltage has been detected. More specifically, when the low-speed voltage has been detected, the inverter module  132  will be activated. The inverter module  132 , which includes a DC capacitor and a plurality of inverters, then regulates the low-speed voltage such that the low-speed voltage has a regulated or a pre-determined operating frequency. For example, when the input  116  is run at 60 Hz, the inverted output voltage from the inverter module  132  can be configured to generate an operating frequency of 30 Hz, that is, half of the original frequency. Since the power is torque times speed, if the motor is run at half speed and the power applied is ⅛ of its original value, the torque is thus ¼ of its original value. 
     Furthermore, the interface  100  is also configured to detect feedback from the motor  104  when the high-speed or low-speed voltage outputs  116 ,  120  is disconnected via the tapped winding relay  108 . This condition can cause both a high-speed and a low-speed voltage to be fed to the interface  100 . More specifically, when the tapped winding relay  108  switches from high speed to low speed, there is normally a time delay for the motor  104  to switch from high speed to low speed. Ideally, there should be no voltage at the high-speed output  116  when switching from high speed to low speed. In practice, however, the motor  104  is still spinning at high speed while the tapped winding relay  108  is switching from high-speed voltage to low-speed voltage. At this time, the summing module  134  generates an unusually high summed voltage representing a sum of the high-speed voltage and the low-speed voltage. When both the low-speed voltage and the high-speed voltage, or a unusually high summed voltage, are detected by the ADC  140 , the micro-controller is configured to disconnect the switching module  149  from the motor  104 , or to disable the inverter module  132  such that no frequency-regulated voltage is generated. Thereafter, the micro-controller  144  is configured to read the voltage output from the tapped winding relay  108 , and to determine an appropriate soft control signal to generate to control the switching module  149  as described earlier. In an alternative embodiment, the micro-controller  144  can also be configured to detect the presence of the condition where both voltages are fed to the FSD. Upon detecting this condition, the micro-controller  144  can operate the motor  104  at a third speed, a low-speed, a high-speed, or a zero speed setting. 
     Various features and advantages of the invention are set forth in the following claims.