Patent Publication Number: US-9853588-B2

Title: Motor drive control using pulse-width modulation pulse skipping

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. patent application Ser. No. 13/963,317, filed Aug. 9, 2013 (now U.S. Pat. No. 9,240,749), which claims the benefit of U.S. Provisional Application No. 61/682,149, filed on Aug. 10, 2012, U.S. Provisional Application No. 61/697,079, filed on Sep. 5, 2012, U.S. Provisional Application No. 61/729,229, filed on Nov. 21, 2012, and U.S. Provisional Application No. 61/755,230, filed on Jan. 22, 2013. The entire disclosures of these applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to electric motor control systems and methods. 
     BACKGROUND 
     Electric motors are used in a wide variety of industrial and residential applications including, but not limited to, heating, ventilating, and air conditioning (HVAC) systems. For example only, an electric motor may drive a compressor in an HVAC system. One or more additional electric motors may also be implemented in the HVAC system. For example only, the HVAC system may include another electric motor that drives a fan associated with a condenser. Another electric motor may be included in the HVAC system to drive a fan associated with an evaporator. 
     SUMMARY 
     A control system for a motor includes a pulse-width modulation module, a mode determination module, a pulse skip determination module, a duty cycle adjustment module, a pulse module, and an inverter power module. The pulse-width modulation module generates three duty cycle values based respectively on three voltage requests and based on a bus voltage. The mode determination module selectively enables a pulse skipping mode based on a speed of the motor. The pulse skip determination module, in response to the pulse skipping mode being enabled, serially generates pulse skipping numbers. The pulse skipping numbers are selected randomly from a group consisting of zero, one, and two. The duty cycle adjustment module, for each switching period of a plurality of switching periods, selectively sets the three duty cycle values to a zero value in response to a corresponding one of the pulse skipping numbers being nonzero. The pulse module, for each of the switching periods, generates three pulse waveforms in response to the three duty cycle values as modified by the duty cycle adjustment module. The inverter power module controls three phases of the motor based on the three pulse waveforms, respectively. 
     A control system for a motor includes a pulse-width modulation module, a pulse skip determination module, and a duty cycle adjustment module. The pulse-width modulation module generates three duty cycle values based on three voltage requests, respectively. A plurality of solid-state switches control three phases of the motor in response to the three duty cycle values, respectively. The pulse skip determination module generates a pulse skip signal. The duty cycle adjustment module selectively prevents the plurality of solid-state switches from switching during intervals specified by the pulse skip signal. 
     In other features, the pulse-width modulation module generates each of the three duty cycle values based on a ratio of each of the three voltage requests, respectively, to a voltage of a bus, wherein the bus provides power to the motor via the solid-state switches. The system also includes a mode determination module that generates a mode signal in response to at least one motor operating parameter. In response to the mode signal being in a first state, the duty cycle adjustment module prevents the plurality of solid-state switches from switching during intervals specified by the pulse skip signal. The motor operating parameter is a speed of the motor. The mode determination module sets the mode signal to the first state in response to the speed of the motor being less than a predetermined threshold. 
     In further features, the system also includes a pulse module that generates three pulse waveforms using duty cycles set by the three duty cycle values, respectively. The plurality of solid-state switches are controlled based on the three pulse waveforms. The duty cycle adjustment module selectively prevents the plurality of solid-state switches from switching by causing the pulse module to generate the three pulse waveforms using duty cycles of zero percent. 
     In other features, the pulse skip determination module generates the pulse skip signal based on a series of integer values. The duty cycle adjustment module prevents the plurality of solid-state switches from switching in response to the pulse skip signal having a first state. The pulse skip determination module generates the pulse skip signal having the first state in response to a present one of the series of integer values being nonzero. The duty cycle adjustment module sets the three duty cycle values to the zero value for a number of consecutive switching periods, where the number is equal to the nonzero one of the pulse skipping numbers. The series of integer values is a predetermined sequence. The system also includes a random number generator that generates the series of integer values. 
     A method of controlling a motor includes generating three duty cycle values based on three voltage requests, respectively. A plurality of solid-state switches control three phases of the motor in response to the three duty cycle values, respectively. The method further includes generating a pulse skip signal, and selectively preventing the plurality of solid-state switches from switching during intervals specified by the pulse skip signal. 
     In other features, the method includes generating each of the three duty cycle values based on a ratio of each of the three voltage requests, respectively, to a voltage of a bus, wherein the bus provides power to the motor via the solid-state switches. The method includes generating a mode signal in response to at least one motor operating parameter, and in response to the mode signal being in a first state, preventing the plurality of solid-state switches from switching during intervals specified by the pulse skip signal. 
     In further features, the motor operating parameter is a speed of the motor, and the method includes setting the mode signal to the first state in response to the speed of the motor being less than a predetermined threshold. The method includes generating three pulse waveforms using duty cycles set by the three duty cycle values, respectively. The plurality of solid-state switches are controlled based on the three pulse waveforms. The selectively preventing the plurality of solid-state switches from switching is performed by causing the three pulse waveforms to be generated using duty cycles of zero percent. 
     In other features, the method includes generating the pulse skip signal based on a series of integer values, and preventing the plurality of solid-state switches from switching in response to the pulse skip signal having a first state. The method includes generating the pulse skip signal having the first state in response to a present one of the series of integer values being nonzero. The series of integer values is a predetermined sequence. The method includes randomly generating the series of integer values. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an example refrigeration system; 
         FIG. 2  is a functional block diagram of an example drive controller and an example compressor; 
         FIGS. 3A-3C  are simplified schematics of example inverter power modules and example motors; 
         FIG. 4  is a functional block diagram of a motor control module; 
         FIG. 5  is a functional block diagram of a pulse-width modulation (PWM) module; 
         FIG. 6A  is a functional block diagram of an example PWM control module; 
         FIG. 6B  is a chart showing traces of example pulse-width modulated signals. 
         FIG. 7  is a functional block diagram of an open-loop torque module; 
         FIG. 8A-8B  are flow diagrams of example methods for PWM pulse skipping; and 
         FIG. 9  is a flow diagram of an example method for open-loop torque control. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a functional block diagram of an example refrigeration system  100  including a compressor  102 , a condenser  104 , an expansion valve  106 , and an evaporator  108 . According to the principles of the present disclosure, the refrigeration system  100  may include additional and/or alternative components. In addition, the present disclosure is applicable to other types of refrigeration systems including, but not limited to, heating, ventilating, and air conditioning (HVAC), heat pump, refrigeration, and chiller systems. 
     The compressor  102  receives refrigerant in vapor form and compresses the refrigerant. The compressor  102  provides pressurized refrigerant in vapor form to the condenser  104 . The compressor  102  includes an electric motor that drives a pump. For example only, the pump of the compressor  102  may include a scroll compressor and/or a reciprocating compressor. 
     All or a portion of the pressurized refrigerant is converted into liquid form within the condenser  104 . The condenser  104  transfers heat away from the refrigerant, thereby cooling the refrigerant. When the refrigerant vapor is cooled to a temperature that is less than a saturation temperature, the refrigerant transforms into a liquid (or liquefied) refrigerant. The condenser  104  may include an electric fan that increases the rate of heat transfer away from the refrigerant. 
     The condenser  104  provides the refrigerant to the evaporator  108  via the expansion valve  106 . The expansion valve  106  controls the flow rate at which the refrigerant is supplied to the evaporator  108 . The expansion valve  106  may include a thermostatic expansion valve or may be controlled electronically by, for example, a system controller  130 . A pressure drop caused by the expansion valve  106  may cause a portion of the liquefied refrigerant to transform back into the vapor form. In this manner, the evaporator  108  may receive a mixture of refrigerant vapor and liquefied refrigerant. 
     The refrigerant absorbs heat in the evaporator  108 . Liquid refrigerant transitions into vapor form when warmed to a temperature that is greater than the saturation temperature of the refrigerant. The evaporator  108  may include an electric fan that increases the rate of heat transfer to the refrigerant. 
     A utility  120  provides power to the refrigeration system  100 . For example only, the utility  120  may provide single-phase alternating current (AC) power at approximately 230 Volts (V) root mean squared (V RMS ) or at another suitable voltage. In various implementations, the utility  120  may provide three-phase AC power at approximately 400 V RMS  or 480 V RMS  at a line frequency of, for example, 50 or 60 Hz. The utility  120  may provide the AC power to the system controller  130  via an AC line. The AC power may also be provided to a drive controller  132  via the AC line. 
     The system controller  130  controls the refrigeration system  100 . For example only, the system controller  130  may control the refrigeration system  100  based on user inputs and/or parameters measured by various sensors (not shown). The sensors may include pressure sensors, temperature sensors, current sensors, voltage sensors, etc. The sensors may also include feedback information from the drive control, such as motor currents or torque, over a serial data bus or other suitable data buses. 
     A user interface  134  provides user inputs to the system controller  130 . The user interface  134  may additionally or alternatively provide the user inputs to the drive controller  132 . The user inputs may include, for example, a desired temperature, requests regarding operation of a fan (e.g., the evaporator fan), and/or other suitable inputs. The system controller  130  may control operation of the fans of the condenser  104 , the evaporator  108 , and/or the expansion valve  106 . 
     The drive controller  132  may control the compressor  102  based on commands from the system controller  130 . For example only, the system controller  130  may instruct the drive controller  132  to operate the compressor motor at a certain speed. In various implementations, the drive controller  132  may also control the condenser fan. 
       FIG. 2  is a functional block diagram of an example implementation of the drive controller  132 . An electromagnetic interference (EMI) filter  202  reduces EMI that might otherwise be injected back onto the AC line by the drive controller  132 . The EMI filter  202  may also filter EMI carried on the AC line. 
     A power factor correction (PFC) module  204  receives AC power from the AC line as filtered by the EMI filter  202 . The PFC module  204  rectifies the AC power, thereby converting the AC input power into direct current (DC) power. The generated DC power is provided at positive and negative terminals of the PFC module  204 . The PFC module  204  also selectively provides power factor correction between the input AC power and the generated DC power. 
     The PFC module  204  selectively boosts the AC power to a DC voltage that is greater than a peak voltage of the AC power. The term selectively means that the PFC module  204  is configured to boost the AC power under some conditions and to not boost the AC power under other conditions. For example only, the PFC module  204  may operate in a passive mode, where the DC voltage generated is less than a peak voltage of the AC power. The PFC module  204  may also operate in an active mode, where the DC voltage generated is greater than the peak voltage of the AC power. A DC voltage that is greater than the peak voltage of the AC power may be referred to as a boosted DC voltage. 
     AC power having an RMS voltage of 230 V RMS  has a peak voltage of approximately 325 V (230 V multiplied by the square root of 2). For example only, when operating from AC power having an RMS voltage of 230 V RMS , the PFC module  204  may generate boosted DC voltages between approximately 350 V (which may also be represented as 350 VDC or 350 V DC ) and approximately 410 V. For example only, the lower limit of 350 V may be imposed to avoid unstable operating regimes of the PFC module  204 . The limits may vary, such as with the voltage of the AC input. In various implementations, the PFC module  204  may be capable of achieving higher boosted DC voltages than 410 V and/or lower boosted voltages than 350 V. However, an upper limit, such as 410 V, may be imposed to improve long-term reliability of components that would experience greater stress at higher voltages, such as components in a DC filter  206 . In various implementations, the upper and/or lower limits may be dynamically varied. 
     The DC filter  206  filters the DC power generated by the PFC module  204 . The DC filter  206  minimizes ripple voltage present in the DC power that results from the conversion of AC power to DC power. In various implementations, the DC filter  206  may include one or more series or parallel filter capacitors connected between the positive and negative terminals of the PFC module  204 . In such implementations, the positive and negative terminals of the PFC module  204  may be connected directly to positive and negative terminals of an inverter power module  208 . 
     The inverter power module  208  (described in more detail with reference to  FIGS. 3A, 3B, and 3C ) converts the DC power, as filtered by the DC filter  206 , into AC power that is provided to the compressor motor. For example only, the inverter power module  208  may convert the DC power into three-phase AC power and provide the phases of the AC power to three respective windings of the motor of the compressor  102 . In other implementations, the inverter power module  208  may convert the DC power into more or fewer phases of power. Further, the principles of the present disclosure apply to motors having more or fewer windings than three. 
     A DC-DC power supply  220  may also receive the filtered DC power. The DC-DC power supply  220  converts the DC power into one or more DC voltages that are suitable for various components and functions. For example only, the DC-DC power supply  220  may reduce the voltage of the DC power to a first DC voltage that is suitable for powering digital logic and a second DC voltage that is suitable for controlling switches within the PFC module  204 . For example only, the second DC voltage may be selectively applied to gate terminals of the switches. In various implementations, DC power may be provided by another DC power source (not shown)—for example, rectifier connected via a transformer to the main AC input. 
     In various implementations, the first DC voltage may be approximately 3.3 V and the second DC voltage may be approximately 15 V. In various implementations, the DC-DC power supply  220  may also generate a third DC voltage. For example only, the third DC voltage may be approximately 1.2 V. The third DC voltage may be derived from the first DC voltage using a voltage regulator. For example only, the third DC voltage may be used for core digital logic and the first DC voltage may be used for input/output circuitry of a PFC control module  250  and a motor control module  260 . 
     The PFC control module  250  controls the PFC module  204 , and the motor control module  260  controls the inverter power module  208 . In various implementations, the PFC control module  250  controls switching of the switches within the PFC module  204 , and the motor control module  260  controls switching of switches within the inverter power module  208 . The PFC module  204  may be implemented with 1, 2, 3, or more phases. 
     A supervisor control module  270  may communicate with the system controller  130  (not shown in  FIG. 2 ) via a communications module  272 . The communications module  272  may include an input/output port and other suitable components to serve as an interface between the system controller  130  and the supervisor control module  270 . The communications module  272  may implement wired and/or wireless protocols. 
     The supervisor control module  270  provides various commands to the PFC control module  250  and the motor control module  260 . For example, the supervisor control module  270  may provide a commanded speed to the motor control module  260 . The commanded speed corresponds to a desired rotational speed of the motor of the compressor  102 . 
     In various implementations, the commanded compressor speed may be provided to the supervisor control module  270  by the system controller  130 . Additionally or alternatively, the supervisor control module  270  may determine or adjust the commanded compressor speed based on inputs provided via the communications module  272  and/or parameters measured by various sensors (i.e., sensor inputs). The supervisor control module  270  may also adjust the commanded compressor speed based on feedback from the PFC control module  250  and/or the motor control module  260 . 
     The supervisor control module  270  may also provide other commands to the PFC control module  250  and/or the motor control module  260 . For example, based on the commanded speed, the supervisor control module  270  may command the PFC control module  250  to produce a commanded bus voltage. The supervisor control module  270  may adjust the commanded bus voltage based on additional inputs, such as operating parameters of the inverter power module  208 , information concerning load on the motor  400 , and the measured voltage of the incoming AC line. 
     The supervisor control module  270  may diagnose faults in various systems of the drive controller  132 . For example only, the supervisor control module  270  may receive fault information from the PFC control module  250  and/or the motor control module  260 . The supervisor control module  270  may also receive fault information via the communications module  272 . The supervisor control module  270  may manage reporting and clearing of faults between the drive controller  132  and the system controller  130 . 
     Responsive to the fault information, the supervisor control module  270  may instruct the PFC control module  250  and/or the motor control module  260  to enter a fault mode. For example only, in the fault mode, the PFC control module  250  may halt operation of the switches of the PFC module  204  while, in the fault mode, the motor control module  260  may halt operation of the switches of the inverter power module  208 . In addition, the motor control module  260  may directly exchange fault information with the PFC control module  250 . In this way, the PFC control module  250  can respond to a fault identified by the motor control module  260  even if the supervisor control module  270  is not operating correctly, and vice versa. 
     The PFC control module  250  controls switches in the PFC module  204  using pulse-width modulation (PWM). More specifically, the PFC control module  250  may generate PWM signals that are applied to the switches of the PFC module  204 . The duty cycle of the PWM signals is varied to produce desired currents in the switches of the PFC module  204 . The desired currents are calculated based on an error between a measured DC bus voltage and a desired DC bus voltage. In other words, the desired currents are calculated in order to achieve the desired DC bus voltage. The desired currents may also be based on achieving desired power factor correction parameters, such as the shapes of current waveforms in the PFC module  204 . The PWM signals generated by the PFC control module  250  may be referred to as PFC PWM signals. 
     The motor control module  260  controls switches in the inverter power module  208  using PWM in order to achieve the commanded compressor speed. The PWM signals generated by the motor control module  260  may be referred to as inverter PWM signals. The duty cycle of the inverter PWM signals controls the current through the windings of the motor (i.e., motor currents) of the compressor  102 . The motor currents control motor torque, and the motor control module  260  may control the motor torque to achieve the commanded compressor speed. 
     In addition to sharing fault information, the PFC control module  250  and the motor control module  260  may also share data. For example only, the PFC control module  250  may receive data from the motor control module  260  such as load, motor currents, estimated motor torque, inverter temperature, and duty cycle of the inverter PWM signals. The PFC control module  250  may also receive data from the motor control module  260 , such as the measured DC bus voltage. The motor control module  260  may receive data from the PFC control module  250  such as AC line voltage, current(s) through the PFC module  204 , estimated AC power, PFC temperature, and commanded bus voltage. 
     In various implementations, some or all of the PFC control module  250 , the motor control module  260 , and the supervisor control module  270  may be implemented on an integrated circuit (IC)  280 . For example only, the IC  280  may include a digital signal processor (DSP), a field programmable gate array (FPGA), a microprocessor, etc. In various implementations, additional components may be included in the IC  280 . Additionally, various functions shown inside the IC  280  in  FIG. 2  may be implemented external to the IC  280 , such as in a second IC or in discrete circuitry. For example only, the supervisor control module  270  may be integrated with the motor control module  260 . 
     Referring now to  FIG. 3A , a simplified schematic of a motor  400  and an example implementation of the inverter power module  208  are presented. The motor  400  is a component of the compressor  102  of  FIG. 2 . However, the principles of  FIGS. 3A-3C  may apply to other motors, including a motor of the condenser  104 . 
     The inverter power module  208  includes a switch block  402 . The switch block  402  receives the filtered DC voltage from the DC filter  206  via a positive DC terminal  404  and a negative DC terminal  406 . The switch block  402  includes a first inverter leg  410  that includes first and second switches  420  and  422  and first and second diodes  424  and  426 . 
     In this example, a first terminal of the first switch  420  is connected to the positive DC terminal  404 , while a second terminal of the second switch  422  is connected to the negative DC terminal  406 . A second terminal of the first switch  420  is connected to a first terminal of the second switch  422 . An anode of the first diode  424  is connected to the second terminal of the first switch  420  and a cathode of the first diode  424  is connected to the first terminal of the first switch  420 . An anode of the second diode  426  is connected to the second terminal of the second switch  422  and a cathode of the second diode  426  is connected to the first terminal of the second switch  422 . 
     The control terminals of the switches  420  and  422  receive generally complementary signals from the motor control module  260 . The motor control module  260  controls the switches  420  and  422  using PWM in order to achieve the commanded compressor speed. The duty cycle of the inverter PWM signals controls the current through the windings of the motor  400 . The motor currents control motor torque, and the motor control module  260  may control the motor torque to achieve the commanded compressor speed. 
     In various implementations, each of the switches  420  and  422  may be implemented as an insulated gate bipolar transistor (IGBT). In such implementations, the first, second, and control terminals may correspond to collector, emitter, and gate terminals, respectively. Alternatively, the switches  420  and  422  may be implemented as other forms of solid-state switch, such as metal-oxide semiconductor field-effect transistors (MOSFETs) or power MOSFETs. 
     The switch block  402  may include one or more additional inverter legs. In various implementations, the switch block  402  may include one inverter leg for each phase or winding of the motor  400 . For example only, the switch block  402  may include second and third inverter legs  430  and  440 , as shown in  FIG. 3A . The inverter legs  410 ,  430 , and  440  may provide current to windings  450 ,  452 , and  454  of the motor  400 , respectively. The windings  454 ,  452 , and  450  may be referred to as windings a, b, and c, respectively. Voltage applied to the windings  454 ,  452 , and  450  may be referred to as Va, Vb, and Vc, respectively. Current through the windings  454 ,  452 , and  450  may be referred to as Ia, Ib, and Ic, respectively. 
     For example only, first ends of the windings  450 ,  452 , and  454  may be connected to a common node. Second ends of the windings  450 ,  452 , and  454  may be connected to the second terminal of the first switch  420  of the inverter legs  410 ,  430 , and  440 , respectively. 
     The inverter power module  208  may also include a shunt resistor  460  that is associated with the first inverter leg  410 . The shunt resistor  460  may be connected between the second terminal of the second switch  422  and the negative DC terminal  406 . In various implementations, respective shunt resistors may be located between each of the inverter legs  430  and  440  and the negative DC terminal  406 . For example only, current through the first winding  450  of the motor  400  may be determined based on the voltage across the shunt resistor  460  of the first inverter leg  410 . 
     In various implementations, the shunt resistor of one of the inverter legs  410 ,  430 , or  440  may be omitted. In such implementations, current may be inferred based on the measurements of the remaining shunt resistors. The third current may be determined based on an assumption that Ia+Ib+Ic=0. 
     Additionally or alternatively, a resistor  462  may be connected in series with the negative DC terminal  406 , as shown in  FIG. 3B . Current through the resistor  462  may therefore indicate a total current consumed by the inverter power module  208 . Current through each of the inverter legs  410 ,  430 , and  440  may be inferred from the total current based on the known phase timing of the current through the inverter legs  410 ,  430 , and  440 . Further discussion of determining currents in an inverter can be found in commonly assigned U.S. Pat. No. 7,193,388, issued Mar. 20, 2007, the entire disclosure of which is hereby incorporated by reference. 
     Any method of measuring or sensing current through any or all of the inverter legs  410 ,  430 , and  440  may be used. For example, in various implementations, the current through the first inverter leg  410  may be measured using a current sensor  487 , as shown in  FIG. 3C . For example only, the current sensor  487  may be implemented between the first inverter leg  410  and the first winding  450 . Current through the inverter legs  430  and  440  may also be measured using associated current sensors  488  and  489 , respectively. In various implementations, current sensors may be associated with two of the inverter legs  410 ,  430 , and  440 . The current through the other one of the inverter legs  410 ,  430 , and  440  may be determined based on an assumption that the current in the motor windings sums to zero. 
     Referring now to  FIG. 4 , an example implementation of the motor control module  260  of  FIG. 2  is shown. The motor control module  260  controls switches within the inverter power module  208  to control voltages applied to the windings  454 ,  452 ,  450  (hereinafter, “windings a-c ”) of the motor  400 . This may also be referred to as controlling the inverter power module  208  or as controlling the motor  400 . 
     For example, when the motor  400  includes a three-phase motor, the motor control module  260  applies voltages V a-c  to windings a-c , respectively. Voltages V a-c  may collectively be referred to as output voltages. Currents I a-c  are generated in the windings a-c , respectively, when voltages V a-c  are applied to the windings a-c . Currents I a-c  may collectively be referred to as winding currents. Currents in the windings a-c  produce magnetic flux about the windings a-c , and vice versa. The motor control module  260  generates the output voltages to control the winding currents and/or to control magnetic flux. The motor  400 , for example, may be a three-phase internal permanent magnet (“IPM”) motor or a switched reluctance (“SR”) motor. 
     The motor  400  includes a rotor (not shown) that rotates in response to the winding currents. The motor control module  260  controls the amplitude, duty cycle, and/or frequency of the output voltages to control the torque and speed of the rotor. The motor control module  260  may control the output voltages based on a commanded motor speed, which represents a desired rotational speed of the rotor. 
     The motor control module  260  may implement field-oriented control of the motor  400 . Accordingly, the motor control module  260  may map motor driving variables onto various frames of reference. Motor driving variables may include requested current/voltage values used to control the motor  400  as well as measured currents/voltages. For example, motor driving variables may include measured currents I a-c  through the windings a-c  and voltage requests used by the motor control module  260  to apply voltages V a-c  to the windings a-c . 
     The motor control module  260  may map motor driving variables in an abc frame of reference (FoR), an αβ FoR, and a qdr FoR. The abc FoR represents a three-phase stator frame based on the windings a-c . Each of the measured currents I a-c  may be mapped onto respective axes a, b, and c of the abc FoR. Additionally, the motor control module  260  may map requested voltages corresponding to voltages V a-c  to the abc FoR. 
     The αβ FoR includes stationary, stator-based x and y coordinates onto which the motor driving variables are projected. The qdr FoR is a rotating FoR that corresponds to the rotor and rotates in sync with the rotor. Accordingly, the qdr FoR is based on an angle of the rotor. 
     The motor control module  260  may transform motor driving variables from one FoR to another FoR. For example, the motor control module  260  may transform currents represented in the abc FoR into currents represented in the αβFoR, and vice versa. The motor control module  260  may transform motor driving variables from the abc FoR to the αβFoR using a numerical transformation. The motor control module  260  may transform motor driving variables from the αβFoR to the qdr FoR based on the angle of the rotor. 
     The motor control module  260  controls the inverter power module  208  based on the commanded speed from the supervisor control module  270  of  FIG. 2 . In various implementations, a filter module  501  may filter the commanded speed from the supervisor control module  270  of  FIG. 2 . In these implementations, the output of the filter module  501  is referred to below as the commanded speed ω v . 
     In open-loop mode, the actual speed of the rotor will generally follow the commanded speed ω v , assuming that the commanded speed ω v  does not change too quickly. As a result, the coefficients of the low-pass filter of the filter module  501  may be chosen so that the rotor acceleration can keep up with changes in the commanded speed ω v  output from the filter module  501 . Otherwise, rotor synchronization may be lost. In various implementations, the filter module  501  may implement a ramp function, which updates the commanded speed ω v  by up to a maximum increment during each predetermined interval of time. 
     The motor control module  260  may control the motor  400  based on a commanded FoR (e.g., a qdv FoR) when operating in open-loop mode. The qdv FoR is associated with the commanded speed ω v  of the rotor and a commanded angle (θ v ) of the rotor. A commanded angle generation module  502  may determine the commanded angle θ v , such as by integrating the commanded speed ω v . 
     The motor control module  260  may operate in various modes, such as an open-loop mode or a closed-loop mode. For example only, the motor control module  260  may operate in open-loop mode when starting the motor  400  and later transition to operating in closed-loop mode. When operating in open-loop mode, the rotor will tend to synchronize with the commanded speed ω v , especially when the motor control module  260  is operating the rotor at slower speeds. However, the actual rotor angle may differ from the commanded angle θ v  because of a load applied to the motor  400 . For example, a change in load while operating in open-loop mode may change a phase difference between the commanded angle θ v  and the actual rotor angle. 
     A transition module  503  determines when to transition the motor control module  260  from open-loop mode to closed-loop mode. For example only, the transition module  503  determines when to transition based on at least one of the commanded speed ω v , an operating time of the motor  400 , a load on the motor  400 , at least one driving variable of the motor  400 , a commanded acceleration of the rotor, and/or feedback from an estimator module  504 . 
     For example, the transition module  503  may predict the speed of the rotor based on the commanded acceleration and/or the elapsed operating time of the motor  400 . The transition module  503  may transition from open to closed-loop when the predicted speed is greater than a speed threshold. Additionally or alternatively, the transition module  503  may transition from open to closed-loop when the commanded speed ω v  is greater than the speed threshold. For example only, the speed threshold may be 1400 revolutions per minute (RPM). In various implementations, the transition module  503  may transition from open-loop mode to closed-loop mode when an elapsed time from when the motor  400  was started exceeds a predetermined period. 
     The estimator module  504  estimates the speed (ω est ) and angle (θ est ) of the rotor. The estimator module  504  may determine the estimated speed ω est  based on the estimated angle θ est . For example, the estimator module  504  may differentiate and filter the estimated angle θ est  over a period of time to determine the estimated speed ω est . The transition module  503  may transition from open to closed-loop mode when the estimator module  504  has achieved stable estimates of the estimated angle θ est  and the estimated speed ω est . In various implementations, the transition module  503  may transition from open-loop mode to closed-loop mode when convergence in the estimator module  504  has occurred, which may be indicated by, for example, flux estimates. 
     The estimator module  504  may determine the estimated angle θ est  based on various motor driving variables. For example, the motor driving variables may include voltages V a-c  to be applied to the windings a-c  and currents I a-c  measured in the windings a-c . Additionally, the estimator module  504  may determine the estimated angle θ est  based on the commanded speed ω v . The estimator module  504  may implement a state observer (e.g., a Luenberger observer) to determine the estimated angle θ est  and the estimated speed ω est  based on the motor driving variables. Further description of sensorless control systems and methods can be found in U.S. Pat. No. 6,756,757, issued Jun. 29, 2004, U.S. Pat. No. 7,208,895, issued Apr. 24, 2007, U.S. Pat. No. 7,342,379, issued Mar. 11, 2008, and U.S. Pat. No. 7,375,485, issued May 20, 2008, the entire disclosures of which are incorporated herein by reference. 
     The estimator module  504  may receive actual voltages in addition to or in place of the voltage commands. The estimator module  504  may receive a filtered and limited version of the estimated speed ω est . In various implementations, the filtered and limited version may be received from the angle/speed determination module  508 , and may correspond to ω r . A current determination module  506  may measure the currents I a-c  of the windings a-c  (hereinafter “measured currents”). The estimator module  504  may use the measured currents to estimate θ est  and ω est . 
     An angle/speed determination module  508  generates an output angle θ r  and an output speed ω r  based on the currently enabled mode, such as open-loop mode or closed-loop mode. The angle/speed determination module  508  may set the output angle θ r  equal to the commanded angle θ v  when operating in open-loop mode and may set the output angle θ r  equal to the estimated angle θ est  when operating in closed-loop mode. 
     When the transition module  503  instructs a transition from open-loop mode to closed-loop mode, the angle/speed determination module  508  gradually adjusts the output angle θ r  from the commanded angle θ v  to the estimated angle θ est . This gradual adjustment may minimize transient current demands when transitioning from open-loop mode to closed-loop mode, which may prevent disruption of current control (described below) and/or estimation of the estimated angle θ est . The gradual adjustment may therefore improve stability during transitions and allow for starting the motor  400  more reliably, especially under higher loads. 
     The angle/speed determination module  508  sets the output speed ω r  equal to the commanded speed ω v  when operating in open-loop mode and sets the output speed ω r  equal to the estimated speed ω est  when operating in closed-loop mode. In various implementations, the angle/speed determination module  508  may immediately switch the output speed ω r  from the commanded speed ω v  to the estimated speed ω est  when the transition module  503  instructs a transition from open-loop mode to closed-loop mode. 
     The transition module  503  may also instruct a change from closed-loop mode back to open-loop mode. For example only, a transition back to open-loop mode may be performed when error conditions, such as a lost rotor, or abnormal operating conditions, are observed. The angle/speed determination module  508  may therefore also switch the output speed ω r  from the estimated speed ω est  back to the commanded speed ω v , and switch the output angle θ r  from the estimated angle θ est  back to the commanded angle θ v . In various implementations, similarly to the transition from open-loop mode to closed-loop mode, switching the output speed ω r  may be performed immediately, while switching the output angle θ r  may be performed gradually. 
     In various implementations, additional modes may be supported. For example only, three, four, or more modes may be supported. The transition module  503  may instruct the angle/speed determination module  508  to transition from one of the modes to another. During each transition, the angle/speed determination module  508  may switch the output speed ω r  immediately to a speed corresponding to the selected mode. Alternatively, the output speed ω r  may be ramped toward the speed of the selected mode. Further, the angle/speed determination module  508  ramps the output angle θ r  toward an angle corresponding to the selected mode. The transition module  503  may instruct the angle/speed determination module  508  to transition from one of the modes to another using a transition signal. For example, the transition signal may specify a target mode to which the angle/speed determination module  508  should transition. 
     A speed loop control module  510  outputs a closed-loop demanded torque signal calculated to match the output speed ω r  to the commanded speed ω v . In closed-loop mode, the output speed ω r  is equal to the estimated speed ω est  of the motor  400 . Therefore, the speed loop control module  510  may generate the closed-loop demanded torque signal in order to keep the speed of the motor  400  approximately equal to the commanded speed ω v . For example only, when the output speed ω r  is less than the commanded speed ω v , the speed loop control module  510  may increase the closed-loop demanded torque, and vice versa. 
     An open-loop torque module  511 , described in more detail in  FIG. 7 , outputs an open-loop demanded torque signal designed to increase the speed of the motor toward a desired speed on which the commanded speed ω v  is based. In various implementations, the commanded speed ω v  is ramped toward the desired speed. 
     A multiplexer  513  receives the open-loop demanded torque signal from the open-loop torque module  511  and the closed-loop demanded torque signal from the speed loop control module  510 . In response to the transition signal from the transition module  503 , the multiplexer  513  outputs a demanded torque signal based either on the closed-loop demanded torque or the open-loop demanded torque. 
     In response to the transition signal indicating that the motor control module  260  is operating in closed-loop mode, the multiplexer  513  sets the demanded torque signal equal to the closed-loop demanded torque signal. In response to the transition signal indicating that the motor control module  260  is operating in open-loop mode, the multiplexer  513  sets the demanded torque signal equal to the open-loop demanded torque signal. 
     An Idr injection module  512  generates a d-axis current (Idr) demand based on the DC bus voltage, the demanded torque signal, and the commanded speed ω v . The Idr demand is used by current control, described below, for Idr injection, which may also be referred to as field weakening or phase advance. In various implementations, the Idr injection module  512  may adjust the Idr demand based on an out-of-volts (OOV) signal, described below, and measured current. 
     A torque mapping module  514  generates a q-axis current (Iqr) demand based on the demanded torque signal. Because the Idr demand may affect generated torque, the torque mapping module  514  may determine the Iqr demand based also on the Idr demand. For example only, the torque mapping module  514  may implement a maximum current limit. In various implementations, the torque mapping module  514  may compare a combination of the Idr demand and the Iqr demand to the maximum current limit, and reduce one or both of the demands when the combination exceeds the maximum current limit. In various implementations, the torque mapping module  514  may limit only the Iqr demand. For example only, the maximum current limit may be a root mean square limit, such as 25 Amps RMS . 
     When the torque mapping module  514  is limiting the Iqr demand to meet the maximum current limit, the torque mapping module  514  may output a limit signal to the speed loop control module  510 . When the limit signal is received, the speed loop control module  510  may temporarily suspend increasing the closed-loop demanded torque. In various implementations, the speed loop control module  510  may take similar action to temporarily suspend increasing the closed-loop demanded torque in response to the OOV signal. 
     For example only, the speed loop control module  510  may attempt to match the output speed ω r  to a reduced version of the commanded speed ω v . Alternatively or additionally, the speed loop control module  510  may selectively suspend error summing and/or integrating operation that would lead to increasing the closed-loop demanded torque. In other words, when the torque mapping module indicates, via the limit signal, that the maximum current limit is reached, the present demanded torque cannot be achieved within the maximum current limit. Therefore, the speed loop control module  510  may stop increasing the closed-loop demanded torque to prevent demanding even more unachievable torque. 
     A current control module  516  determines q-axis voltage command Vqr and d-axis voltage demand Vdr, in the qdr FoR, based on the current demands Iqr and Idr. In various implementations, the current control module  516  may determine the voltage commands Vqr and Vdr based also on the measured currents. In various implementations, the current control module  516  may attempt to match the measured currents to the Iqr and Idr demands by adjusting the voltage commands Vqr and Vdr. In various implementations, the current control module  516  may also receive the output speed ω r  (not shown in  FIG. 4 ). 
     An abc to qdr module  520  maps the measured currents I a-c  onto the qdr FoR based on the output angle θ r  from the angle/speed determination module  508 . The resulting mapped current may be referred to as Iqdr, and may include Iqr and Idr components. Components of the motor control module  260 , such as the current control module  516 , may therefore use the Iqdr representation of the measured currents. 
     A qdr to αβ module  522  may transform the voltage commands Vqr and Vdr from the qdr FoR to the αβFoR, thereby generating a voltage request in the αβFoR (hereinafter “voltage request”). The voltage request may indicate the voltages to be applied to the windings a-c . The qdr to αβ module  522  may perform the transformation based on the output angle θ r , and, in various implementations, may perform the transformation based on the output speed ω r . 
     A pulse-width modulation (PWM) module  524 , described in more detail in  FIG. 5 , generates duty cycle values to control the inverter power module  208  using PWM. For example only, the PWM switching frequency may be approximately 5 kHz or approximately 10 kHz. In various implementations, the inverter power module  208  and the motor  400  have three phases, and the PWM module  524  therefore generates three duty cycle values, one for each inverter leg. The PWM module  524  may also receive a mode signal from a PWM control module  528 . 
     The PWM control module  528 , described in more detail in  FIG. 6A , controls the inverter power module  208  by converting the duty cycle values from the PWM module  524  into driving waveforms according to the duty cycle values. The PWM control module  528  may operate in different modes in response to the output speed ω r  from the angle/speed determination module  508  and/or based on other inverter/compressor parameters (such as compressor torque/current or load). The PWM control module  528  may provide corresponding mode information to the PWM module  524 , and the PWM module  524  may alter operation in response to the mode information, as described below. 
     In various implementations, each leg of the inverter power module  208  includes a pair of complementary switches, and each of the duty cycle values is therefore converted into a pair of complementary duty cycle values for the respective complementary switches. For example only, the switch  420  and the switch  422  of the first inverter leg  410  in  FIG. 3A  may be controlled with complementary duty cycle values. 
     In various implementations, deadtime is introduced to prevent a temporary short circuit condition, in which both complementary switches (such as switches  420  and  422 ) are at least partially conducting. Introducing deadtime involves adjusting when signals based on the complementary duty cycle values are applied to a switch so that the switch is not turned on when the complementary switch has not yet finished turning off. In other words, the off-times of the two switches are partially overlapped. Deadtime may be introduced by the PWM control module  528 . Introduction of deadtime is also applicable to other PWM control, and so may be used for PFC control by the PFC control module  250  of  FIG. 2 . 
     Introducing deadtime may affect the time during which current is flowing and may therefore cause the actual PWM waveform produced to not match the instructed duty cycle value. Therefore, deadtime may be introduced with knowledge of in which direction current is flowing and whether the off-going transistor (the transistor turning off) will control the flow of current or whether the on-coming transistor (the complementary transistor turning on) will control the flow of current. The deadtime can be introduced in each instance so that the controlling transistor transitions at the time that will result in the instructed duty cycle value. The transition time of the other transistor is adjusted accordingly to produce the desired deadtime. 
     Instead of using adaptive deadtime introduction, the duty cycle values could be pre-compensated based on an understanding of how the deadtime will be introduced. In other words, the duty cycle value provided for deadtime introduction can be increased or decreased so that, once deadtime is introduced, the actual current generated in the inverter power module  208  matches the instructed duty cycle value. In the context of  FIG. 5 , this could mean that the PWM module  524  pre-compensates the duty cycle values provided to the PWM control module  528  based on how the PWM control module  528  is expected to introduce deadtime. 
     Referring now to  FIG. 5 , an example implementation of the PWM module  524  includes an αβ to abc module  604 , which transforms the voltage requests from the qdr to αβ module  522  of  FIG. 4  into the abc FoR, resulting in three voltage demands (i.e., Vr a , Vr b , and Vr c , collectively Vr a-c ), one corresponding to each of the three windings of the motor  400 . The three voltage demands represent the instantaneous voltages to be applied to the respective windings to generate desired currents. 
     In order to effectuate the voltage demands, a duty cycle module  608  converts the three voltage demands into three duty cycle values. Because the inverter power module  208  is powered by the DC bus, in various implementations, the duty cycle values are calculated by dividing the voltage demands by the DC bus voltage. For purposes of illustration only, when the DC bus voltage is 400 V, and a voltage demand is 320 V, the calculated duty cycle value is 80% (320 V/400 V). 
     In various implementations and in various operating regimes, the calculated duty cycle values may violate one or more constraints imposed on duty cycle values. For example, a maximum duty cycle limit cannot be greater than 100% by definition, and a minimum duty cycle limit cannot be less than 0% by definition. In some specific PWM implementations, a duty cycle value of 50% may be represented with the number 0, while duty cycle values of 0% and 100% are represented by −0.5 and 0.5, respectively. In other specific PWM implementations, a duty cycle value of 50% may be represented by the number 0, while duty cycle values of 0% and 100% are represented by −1 and 1, respectively. Translating between these representations is trivial mathematics and duty cycle values will be represented as percentages in the following description. 
     In some implementations, the maximum duty cycle limit is set to less than 100%, such as to 96%, 95%, or 92%. The maximum duty cycle limit may be set based on requirements for measurement of the winding currents I a-c . For example, if a duty cycle of 100% were applied to one of the switches, the complementary switch would never turn on and current would not pass through a current-measuring resistor corresponding to the complementary switch. If the position of the current-measuring resistor were changed, a maximum duty cycle limit of 100% might be allowed, but the minimum duty cycle limit would be set greater than 0% to allow for current measurement. For example only, the minimum duty cycle limit could be set to 4%, 5%, or 8%. In various other implementations, the minimum duty cycle limit may be set equal to one minus the maximum duty cycle limit. 
     In various implementations, the motor  400  may respond not to the winding voltages themselves, but instead to differences between the winding voltages. As a simplistic example, applying 50 V to a first winding and 150 V to a second winding is generally equivalent to applying 0 V to the first winding and 100 V to the second winding. Therefore, even if one of the voltage demands may exceed an available voltage, the PWM module  524  may shift the voltage demands when generating the duty cycles. In other words, a duty cycle that exceeds the maximum duty cycle limit may be corrected by shifting all of the duty cycles down until the highest duty cycle no longer exceeds the maximum duty cycle limit. 
     A scaling module  612  determines whether shifting is necessary and shifts the duty cycles accordingly. In various operating regimes, the scaling module  612  may perform shifting even if no duty cycle falls outside of the duty cycle limits, as described in more detail below. Because the calculated duty cycles may be modified by the scaling module  612 , they can be referred to as preliminary duty cycle values. Note that in various implementations the scaling module  612  shifts the preliminary duty cycle values even if all of the preliminary duty cycle values are between the minimum duty cycle limit and the maximum duty cycle limit. 
     For example only, the scaling module  612  may shift the preliminary duty cycle values so that the highest and lowest preliminary duty cycle values are, once shifted, centered about a predetermined value, such as 50%. This shifting technique is referred to as center-based control. In an alternative implementation of center-based control, the scaling module  612  may shift the preliminary duty cycle values so that an average of all the shifted duty cycle values is equal to a predetermined value, such as 50%. In various implementations, the scaling module  612  may implement both types of center-based control, and dynamically choose which to use, or may be pre-configured to use one of the types of center-based control. 
     When the maximum and minimum duty cycle limits are asymmetrical (such as 95% and 0%, respectively), center-based control may prevent the entire range of possible duty cycle values from being used. In other words, center-based control about 50% may effectively limit the possible duty cycle values to 5%-95% as a result of combining the center constraint with the maximum duty cycle limit. This limitation may be mitigated in implementations where center-based control is used with low voltage demands, because the duty cycle values will remain closer to 50% and not run into the 5% limit. 
     According to another technique called bus clamping, the scaling module  612  shifts the preliminary duty cycle values so that the lowest of the preliminary duty cycle values is shifted to a minimum allowed duty cycle, such as 0%. This is referred to as lower bus clamping. The scaling module  612  may alternatively shift the preliminary duty cycle values so that the highest of the preliminary duty cycle values is shifted to the maximum duty cycle limit, such as 95%. This is referred to as upper bus clamping. 
     As a numerical example, consider preliminary duty cycle values of −30%, −10%, and 40%. The first implementation of center-based control would shift the preliminary duty cycle values by 45% and result in shifted duty cycle values of 15%, 35%, and 85%. The highest and lowest shifted duty cycle values, 15% and 85%, are then equally spaced about 50%. The second implementation of center-based control would shift the preliminary duty cycle values by 50% and result in shifted duty cycle values of 20%, 40%, and 90%. The average of these three shifted duty cycle values is 50%. 
     Meanwhile, lower bus clamping (with a minimum allowed duty cycle of 0%) would shift the preliminary duty cycle values by 30% and result in shifted duty cycle values of 0%, 20%, and 70%. Alternatively, upper bus clamping (with a maximum allowed duty cycle of 100%) would shift the preliminary duty cycle values by 60%, resulting in shifted duty cycle values of 30%, 50%, and 100%. In various implementations, a hybrid clamping approach may be used, where control alternates between lower bus clamping and upper bus clamping depending on which approach is preferable at any moment. 
     The scaling module  612  may implement one or more approaches to scaling and shifting, including but not limited to those described above. The scaling/shifting approach used may be preconfigured at manufacturing time or may be selected later, such as during a first run of the motor or during each run of the motor. In various other implementations, two or more of these approaches may be used at various times by the scaling module  612 . For example only, the scaling module  612  may use center-based control when the motor  400  is operating below a predetermined speed and may use a form of bus clamping, such as lower bus clamping, when the motor  400  is operating above the predetermined speed. Bus clamping may reduce switching losses because one of the inverter legs remains off for that each PWM cycle and therefore only the switches of the other inverter legs need to change state during that PWM cycle. 
     It is possible that the maximum and minimum duty cycle limits cannot both be met through shifting the preliminary duty cycle values. In other words, the difference between the largest of the preliminary duty cycle values and the smallest of the preliminary duty cycle values is greater than the difference between the maximum and minimum duty cycle limits. This condition is referred to as operating in an out-of-volts (OOV) state. The OOV state may be determined by a scaling determination module  616  of the scaling module  612 . 
     For an implementation where the minimum duty cycle limit is zero, a test for OOV state can alternately be formulated as follows: operation in the OOV occurs when a difference between any two of the three voltage demands is greater than an available voltage—where the available voltage is equal to the DC bus voltage multiplied by the maximum duty cycle limit. 
     To respond to the OOV state, the scaling module  612  may scale the preliminary duty cycles so that they fit within the confines of the maximum and minimum duty cycle limits. In various implementations, the scaling module  612  may scale the duty cycle values or voltage demands as little as possible, such that the lowest one of the duty cycle values is set to the minimum duty cycle limit, and the highest one of the duty cycle values is set to the maximum duty cycle limit. 
     Scaling may be performed consistently across the three voltage demands or duty cycle values with the intent of keeping the applied voltage vector (such as the voltage vector in the αβFoR) pointed in the same direction. In other words, the ratios of each voltage difference (Vr a −Vr b , Vr b −Vr c , and Vr c −Vr a ) to each of the other voltage differences remain the same. 
     When the OOV state is present, scaling and shifting may both be necessary to meet the maximum/minimum duty cycle limits, and the order of scaling and shifting can be interchanged through simple mathematical transformation. For computational simplicity, when using lower bus clamping (with a minimum allowed duty cycle of 0%), shifting may be performed before scaling, as the lowest duty cycle value would remain fixed at 0% after the shifting. When using center-based control, scaling may be performed before shifting. Otherwise, scaling may change the center point of the shifted duty cycle values, requiring additional shifting. 
     The amount by which the duty cycle values need to be scaled can be referred to as a scaling factor. The scaling module  612  may multiply each of the duty cycle values by (1—scaling factor). For example, if the duty cycle values need to be scaled by 10% to fit within the constraints of the maximum/minimum duty cycle limits, each of the preliminary duty cycle values may be multiplied by 90% (i.e., 1-10%). 
     An adjustment module  620  performs any necessary scaling and shifting and outputs commanded duty cycle values to the inverter power module  208 . 
     The scaling factor may be used as an indication of how far OOV the drive controller  132  currently is. The scaling factor may be referred to as OOV magnitude, and may be included in an OOV signal used by other components of the drive controller  132 . Meanwhile, an OOV flag can be implemented to indicate whether scaling is presently being performed (in other words, that the OOV condition is present). In various implementations, the OOV flag may be set to an active value (such as 1 in an active-high environment) when scaling is being performed and set to an inactive value (such as 0 in an active-high environment) otherwise. The OOV flag may also be included in the OOV signal used by other components of the drive controller  132 , including other components of the motor control module  260  of  FIG. 4 . 
     For purposes of illustration only, OOV operation may be thought in terms of a 2-dimensional circular balloon placed within a 2-dimensional rigid hexagon, where the hexagon represents the operating limits of the drive controller  132  (for the currently available DC bus voltage) and the balloon represents voltage demands. As the balloon expands, the balloon will eventually contact the hexagon at a single point on each side of the hexagon. As the balloon expands further, more and more of the balloon flattens out against the sides of the hexagon. The flattening of the balloon against the inside of the hexagon represents clipping (also referred to as OOV, and indicated by the OOV flag). In other words, the voltage demand cannot be satisfied by the drive controller  132 . 
     An OOV amount, distinct from the OOV magnitude, may be determined based on the OOV flag. The OOV amount may represent the proportion of the time that the drive controller  132  is spending in the OOV state. The OOV amount may be determined by a filter module  624 , which may determine the OOV amount by applying a digital low-pass filter to the OOV flag. For example only, the OOV amount may be determined by applying a moving average to the OOV flag, such as the following weighted moving average:
 
 y ( k )=α· y ( k− 1)+(1−α)· x ( k )
 
where x(k) is the input at sample interval k and the value of a sets the rate at which the contribution of older samples decreases.
 
     If the OOV flag assumes values of either 0 or 1, the OOV amount will range between 0 and 1, inclusive. When multiplied by 100, the OOV amount represents the percentage of time the drive controller  132  is spending in the OOV state. A value closer to 1 will indicate that the OOV state is occurring frequently, and when the OOV amount reaches 1, the OOV state will have been present continuously for as long as a filter window of the filter module  624  extends back. Similarly, when the OOV amount reaches 0, the OOV condition will have been absent for the length of the filter window. 
     The motor control module  260  may use multiple approaches to minimize OOV operation, or to maintain OOV operation below a predetermined threshold. In various implementations, the Idr injection module  512  of  FIG. 4  may use the OOV amount in determining how to adjust the Idr demand. In addition, the speed loop control module  510  of  FIG. 4  may use the OOV amount to determine when to suspend increases in the demanded torque. Further, the current control module  516  of  FIG. 4  may suspend increases to one or both of the Vqr and Vdr commands based on the OOV flag. 
     Referring now to  FIG. 6A , a functional block diagram of an example implementation of the PWM control module  528  is shown. The PWM control module  528  selectively adjusts a plurality of duty cycles based on an operating mode of the motor control module  260 . The PWM control module  528  includes a duty cycle adjustment module  626 , which includes first, second, and third multiplexers  630 - 1 ,  630 - 2 , and  630 - 3  (collectively, multiplexers  630 ). 
     The PWM control module  528  also includes a pulse skip determination module  634  and a mode determination module  638 . The mode determination module  638  generates a mode signal to determine whether the duty cycle adjustment module  626  will perform pulse skipping, which is described below. Pulse skipping may also be referred to as zero vector injection and, in short, creates a difference between each of the phases of the motor of approximately zero. Zero vector injection may be accomplished in some implementations by controlling each leg of the inverter using the same pulse width. The mode signal may also be used by the adjustment module  620  of  FIG. 5  to determine when to switch from center-based control to bus clamping. 
     For example only, in response to the mode signal being in a first state (referred to as an active state), the duty cycle adjustment module  626  performs pulse skipping and the adjustment module  620  uses center-based control. Continuing the example, in response to the mode signal being in a second state (referred to as an inactive state), the duty cycle adjustment module  626  ceases pulse skipping and the adjustment module  620  uses bus clamping. 
     The mode determination module  638  receives the output speed ω r , indicating the speed of the motor  400 . The mode determination module  638  then selects a pulse skipping mode in response to the output speed ω r . For example only, the mode determination module  638  sets the mode signal to the active state (enabling pulse skipping) in response to the output speed ω r  being below a predetermined speed and sets the mode signal to the inactive state (disabling pulse skipping) in response to the output speed ω r  being above the predetermined speed. For example only, the predetermined speed may be approximately 8 Hz. 
     In this example, when starting the motor  400 , open-loop control of the motor  400  is used, pulse skipping is enabled in the PWM control module  528 , and center-based control is used by the PWM module  524 . At a predetermined speed, pulse skipping is disabled, and the PWM module  524  switches to using bus clamping. Subsequently, and in some implementations based on other criteria, control of the motor  400  is transitioned to closed-loop. However, enabling or disabling pulse skipping, and switching between center-based control and bus clamping may be performed at different times and based on different criteria. Likewise, enabling/disabling pulse skipping and/or switching clamping modes may be performed at the same time as, or subsequent to, the transition from open-loop control to closed-loop control. 
     In other implementations, the mode determination module  638  generates the mode signal based on whether the motor control module  260  is operating in open-loop mode or closed-loop mode, which may be indicated by the transition signal from the transition module  503 . For example only, while the motor control module  260  is operating in open-loop mode, the mode determination module  638  may set the mode signal to the active state (enabling pulse skipping), and while the motor control module  260  is operating in closed-loop mode, the mode determination module  638  may set the mode signal to the inactive state (disabling pulse skipping). Additionally or alternatively, the mode determination module  638  may also generate the mode signal according to the commanded speed ω r . The mode determination module  638  may also generate the mode signal according to other operating parameters, such as motor currents, load, and/or torque. 
     The pulse skip determination module  634  selectively generates a pulse skip signal, which controls the multiplexers  630  of the duty cycle adjustment module  626 . The pulse skip signal determines whether the multiplexers pass the commanded duty cycles through or select a predetermined value (such as 0%, as shown in  FIG. 6A ). Although shown as a single pulse skip signal, in other implementations each of the multiplexers  630  could be individually controlled with a respective pulse skip signal. 
     When the mode signal is in the inactive state (pulse skipping disabled), the pulse skip determination module  634  leaves the pulse skip signal in an inactive state, which causes the multiplexers  630  to pass the commanded duty cycles through unchanged. When the mode signal is in the active state (pulse skipping enabled), the pulse skip determination module  634  alternates the pulse skip signal between the active state and the inactive state according to one of the techniques described below. In the active state, the pulse skip signal causes the multiplexers  630  to pass through the predetermined value (e.g., 0%) instead of the commanded duty cycles. 
     The outputs of the multiplexers  630  are provided to first, second, and third pulse modules  642 ,  644 , and  648 , respectively. The pulse modules  642 ,  644 , and  648  output signals using pulse-width modulation (PWM) having duty cycles specified by the outputs of the multiplexers  630 . When the pulse skip signal is in the inactive state, the first, second, and third pulse modules  642 ,  644 , and  648  generate PWM signals according to the commanded duty cycles A, B, and C, respectively. 
     In various implementations, the first, second, and third pulse modules  642 ,  644 , and  648  are capable of varying the width of each pulse in response to the incoming duty cycle commands. In other words, for every period of the PWM waveform, the duty cycle of the PWM pulse will be based on the present duty cycle command, and the duty cycle commands can change once each period. When a 0% duty cycle is requested, no pulse is created during that period, and the pulse is considered to be “skipped.” By skipping the pulse, switching losses in the inverter power module  208  may be reduced. 
     The pulse skip determination module  634  may therefore vary the state of the pulse skip signal for each period of the PWM. PWM periods where the pulse skip signal is active are called skipped pulses, because a 0% duty cycle causes there to be no voltage change in the PWM signal. The pulse skip determination module  634  may determine which pulses to skip according to, for example, a predetermined pulse skip sequence and/or a pulse skip sequence generated on the fly. 
     For example, the predetermined pulse skip sequence can define which pulses to skip and, when the predetermined pulse skip sequence is a finite length, the predetermined pulse skip sequence can be repeated over and over again. The predetermined pulse skip sequence may specify that, for example only, every other pulse is skipped or every fourth pulse is skipped. The predetermined pulse skip sequence may be more complicated, and may include a binary sequence with each binary digit indicating whether the corresponding pulse should be skipped or not. Alternatively, the predetermined pulse skip sequence may include a series of integers, each integer specifying how many pulses to allow before skipping a pulse. 
     In various implementations, the pulse skip determination module  634  may implement a lookup table from which a pulse skip sequence is selected. The lookup table may store pulse skip sequences corresponding to different operating regimes of the motor  400  or of the motor control module  260 . For example, different pulse skip sequences may be selected from the lookup table based on a speed of the motor  400 . 
     Predetermined pulse skip sequences may be generated at design time using a pseudorandom number generator. If the predefined pulse skip sequence is long enough, it can be simply be repeated over and over again to achieve results insignificantly different from a truly random sequence. Alternatively, the pulse skip determination module  634  may implement a pseudorandom number generator to allow a randomized pulse skip sequence to be created on the fly. The pseudorandom number sequence may be, for example, uniformly distributed or normally distributed. In various implementations, the pseudorandom number sequence may be generated without replacement—that is, each value is used exactly once before the sequence repeats. 
     For example, the pulse skip determination module  634  may randomly select an integer from a set such as the inclusive set [0, 1, 2]. The integer determines how many pulses will be skipped—i.e., a value of 0 means that the next pulse will not be skipped. A value of 1 means that the next pulse will be skipped, and a value of 2 means that the next two pulses will be skipped. The set may include additional integers greater than 2 and may omit certain integers. For example only, the set of integers may consist of even values, including zero. In an alternative example, the set of integers may consist of zero as well as one or more odd values. 
     Because each phase of the inverter power module  208  includes complementary switches, complementary versions of the outputs of the pulse modules  642 ,  644 , and  648  are generated by inverters  652 ,  656 , and  660 , respectively. If the complementary switches in a given inverter phase were controlled with strictly complementary control signals, there may be some overlap between one switch turning off and the other switch turning on. When both switches are on, an undesirable short circuit current may flow. Therefore, a deadtime module  664  offsets the switching-on time of one signal from the switching-off time of the other control signal. 
     For example only, the deadtime module  664  may slightly advance an off-going (active to inactive) control signal and slightly delay an on-coming (inactive to active) control signal. In this way, any overlap between the conducting times of the complementary switches is avoided. Outputs of the deadtime module  664  are provided to the switches of the inverter power module  208 . 
     In various implementations, the order of the deadtime module  664 , the pulse modules  642 ,  644 , and  648 , and the duty cycle adjustment module  626  may be rearranged. For example only, the deadtime module  664  may be arranged after the pulse modules  642 ,  644 , and  648  but before the duty cycle adjustment module  626 . In such an implementation, the duty cycle adjustment module  626  would simply, in response to the pulse skip signal indicating that a pulse should be skipped, replace the six deadtime-adjusted pulses with an inactive signal (such as 0 in an active-high environment). 
     Referring now to  FIG. 6B , traces of four example pulse-width modulation (PWM) signals are shown, with a timescale in milliseconds along the x axis. A first PWM signal  670  has a switching frequency of 10 kHz and a duty cycle of 50%. Although 10 kHz is used in these examples, higher or lower switching frequencies may be used. In various implementations, motor currents are read while the first PWM signal  670  is low. These reading times are marked with vertical dashed lines. These motor currents may be used in closed-loop motor control. 
     To reduce switching losses, a PWM signal having a lower switching frequency can be used. For example only, a second PWM signal  674  is shown, with a 5 kHz switching frequency. The switching frequency is reduced by half, so the switching losses (caused by the low-to-high and high-to-low signal transitions) are reduced by approximately half. However, an audible signature of the 5 kHz switching may be less pleasing to the human ear than an audible signature of the 10 kHz switching. In addition, in some implementations, motor currents are not measurable while the PWM signal is high. In those implementations, the second PWM signal  674  only allows current readings to be performed half as often as for the first PWM signal  670 . This may decrease the responsiveness of closed-loop control. 
     A third PWM signal  678  is shown, which may allow for current readings at every reading time. The third PWM signal  678  can be thought of as a 5 kHz PWM signal operating at 25% duty cycle or equivalently as a 10 kHz PWM signal with every other pulse skipped. The switching losses are therefore reduced similarly to the second PWM signal  674 . However, there may still be an unpleasant audible signature to the third PWM signal  678  similar to that of the second PWM signal  674 . 
     A fourth PWM signal  682  operates at 10 kHz, as with the first PWM signal  670 , but skips individual PWM cycles. If these PWM cycles are skipped in an aperiodic manner, the resulting audible signature is decreased, as the energy of the PWM switching is no longer concentrated at 5 kHz. 
     In various implementations, after each pulse is generated, a random number is generated. That number of pulses is then skipped. In such implementations, the fourth PWM signal  682  would have resulted in response to a series of random numbers  686  having been generated as shown. Note that after a generation of the number 0, no pulses are skipped, and the pulses are consecutive. After generation of the number 1, a single pulse is skipped. Similarly, after generation of the number 2, two pulses are skipped. 
     Referring now to  FIG. 7 , a functional block diagram of an example implementation of the open-loop torque module  511  is shown. The open-loop torque module  511  generates an open-loop demanded torque, which may be limited according to the DC bus voltage and a device temperature. The open-loop torque module  511  includes a torque limit determination module  704 , an open-loop torque determination module  708 , and a torque limiting module  712 . 
     The open-loop torque determination module  708  determines a demanded torque suitable for starting the motor  400  in open-loop mode. The demanded torque may be a single predetermined value. In other implementations, the open-loop torque may be one of a plurality of values stored in a lookup table. 
     The torque limit determination module  704  determines an upper torque limit based on the DC bus voltage and a temperature of the switch block  402  of  FIGS. 3A-3C  (referred to as switch block temperature). For example only, the torque limit determination module  704  receives the switch block temperature from a temperature sensor (not shown) arranged to determine the temperature of the switch block  402 . In various implementations, the switch block temperature may be determined by combining temperature values from multiple temperature sensors. Each of the temperature sensors may be thermally coupled to a different circuit element. For example only, each temperature sensor may be thermally coupled to a respective switching module that includes two of the transistors and two of the diodes of the switch block  402 . The individual temperature values may be combined by averaging. Alternatively, a maximum value of the individual temperature values may be chosen. 
     For example only, the upper torque limit may be calculated using a function of the DC bus voltage and the switch block temperature. Additionally or alternatively, the upper torque limit may be determined from a lookup table indexed by the DC bus voltage and the switch block temperature. 
     The torque limiting module  712  generates a limited demanded torque by limiting the demanded torque according to the upper torque limit. In other words, the torque limiting module  712  outputs, as the limited demanded torque, the lesser of the demanded torque and the upper torque limit. 
     Referring now to  FIG. 8A , an example of PWM motor control is shown. At  804 , control determines whether a commanded motor speed is less than a predetermined threshold speed. If so, control continues at  806 ; otherwise, control continues at  808 . At  808 , control receives a commanded duty cycle. At  812 , control generates a pulse based on the commanded duty cycle and continues at  804 . 
     At  806 , control receives a commanded duty cycle. At  820 , control generates a pulse based on the commanded duty cycle. At  824 , control determines a number, N, of pulses to skip. At  828 , control determines whether N is greater than zero. If so, control continues at  832 ; otherwise, control returns to  804 . In various implementations, the commanded speed is checked again between  828  and  832 ; if the commanded speed is no longer less than the threshold, control transfers to  808  and otherwise, control continues at  832 . At  832 , control receives a commanded duty cycle. At  836 , control generates a pulse based on a zero duty cycle—in other words, skipping the pulse. At  840 , control decrements N. Control continues at  828 . 
     Referring now to  FIG. 8B , an alternative example of PWM motor control is shown. Reference numerals from  FIG. 8A  are used to indicate similar elements. After  820 , control continues at  850 . At  850 , control determines a random integer between zero and two, and sets N equal to the random integer. Control then continues at  828 . 
     Referring now to  FIG. 9 , control related to producing a limited demanded torque begins at  900 . At  904 , control determines whether the mode is open-loop or closed-loop. If open-loop, control continues at  906 . If closed-loop, control continues at  908 . At  908 , control controls the motor using the demanded torque from the speed loop and returns to  904 . 
     At  906 , control determines an open-loop torque. At  916 , control determines an upper torque limit based on a device temperature, such as a switch block temperature, and a bus voltage. At  920 , control determines whether the open-loop torque is greater than the upper torque limit. If so, control continues at  924 ; otherwise, control continues at  928 . At  924 , control reduces the open-loop torque to the upper torque limit. Control continues at  928 . At  928 , control controls the motor using the open-loop torque and returns to  904 . 
     The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. 
     In this application, including the definitions below, the term module may be replaced with the term circuit. The term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); a digital, analog, or mixed analog/digital discrete circuit; a digital, analog, or mixed analog/digital integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; memory (shared, dedicated, or group) that stores code executed by a processor; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. 
     The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, routines, functions, classes, and/or objects. The term shared processor encompasses a single processor that executes some or all code from multiple modules. The term group processor encompasses a processor that, in combination with additional processors, executes some or all code from one or more modules. The term shared memory encompasses a single memory that stores some or all code from multiple modules. The term group memory encompasses a memory that, in combination with additional memories, stores some or all code from one or more modules. The term memory may be a subset of the term computer-readable medium. The term computer-readable medium does not encompass transitory electrical and electromagnetic signals propagating through a medium, and may therefore be considered tangible and non-transitory. Non-limiting examples of a non-transitory tangible computer readable medium include nonvolatile memory, volatile memory, magnetic storage, and optical storage. 
     The apparatuses and methods described in this application may be partially or fully implemented by one or more computer programs executed by one or more processors. The computer programs include processor-executable instructions that are stored on at least one non-transitory tangible computer readable medium. The computer programs may also include and/or rely on stored data.