System and method for reducing delay in modulation of multi-phase output voltage of an inverter

Abstract

Systems and methods are provided that reduce delays in the modulation of a multiphase output voltage of an inverter. A power converter is configured to measure an output current and determine a multiphase voltage reference from the output current. Within the same switching cycle, the voltage reference is determined, a modulation routine determines modulation indices for each phase of the output voltage. In some cases, one or more phases must begin modulation during the switching cycle before new modulation indices are determined. The modulation routine stores values of the modulation indices generated from a previous switching cycle and uses the stored values when new values are not yet ready. Offset values for the phase voltages using the modulation indices from the previous switching cycle are determined in order to compensate for the phase voltages of the other phases and maintain a desired line-to-line voltage output from the power converter.

Description

Technical Field

[0001] The subject matter disclosed herein relates to systems and methods for reducing delay in modulation of a multiphase output voltage from an inverter. More specifically, an improved modulation routine determines switching signals for each phase of the multiphase output during the same switching cycle in which feedback current is sampled, and these switching signals are used to determine the output voltage of at least a portion of the phases of the multiphase output voltage during the same switching cycle. Background Art

[0002] Power converters are widely used to provide a voltage to a load at a varying amplitude and / or varying frequency. A power converter can receive an input voltage in one form, such as alternating current (AC) or direct current (DC), and output a voltage in a second form. A common application for power converters is in motor drives.

[0003] As known to those skilled in the art, a motor drive is used to control the operation of a motor. According to one general configuration, the motor drive includes a DC bus having a DC voltage of suitable amplitude, from which an AC voltage can be generated and provided to the AC motor. The DC voltage can be provided as an input to the motor drive, or alternatively, the motor drive can include a converter section that converts the AC voltage input into a DC voltage present on the DC bus. The converter section can be passive, including conventional diode rectification; or it can be active, including controlled power electronic switching devices, either of which can convert the AC voltage input into a DC voltage for the DC bus. The power electronic switching devices in the active rectifier can be selected from transistors, such as insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETs), thyristors, or silicon controlled rectifiers (SCRs). The power electronic switching devices can also include a reverse conducting power electronic device, such as a freewheeling diode, connected in parallel across the power electronic switching device. The reverse conducting power electronic device is configured to conduct during time intervals when the power electronic switching device is non-conducting. A controller in the motor drive generates switching signals to selectively turn on or off each switching device in the active front end to generate a desired DC voltage on the DC bus.

[0004] The motor driver receives a command signal indicating the desired operation of the motor. The command signal may be the desired torque, speed, or position at which the motor is to operate. The motor's torque, speed, or position is controlled by varying the amplitude and frequency of the AC voltage applied to the motor's stator. An inverter unit is provided between the DC bus and the output of the motor driver to generate the controlled AC voltage. The inverter unit includes power electronic switching devices such as IGBTs, MOSFETs, thyristors, or SCRs; and reverse-conducting power electronic devices connected in parallel across the power electronic switching devices. The motor is connected to the output terminals of the motor driver, and a controller generates switching signals that rapidly turn the switching devices in the inverter on and off at a predetermined switching frequency, alternately connecting and disconnecting the DC bus from the output terminals, and thus from the motor. Due to the nature of the switching, the output waveform is rectangular, connecting or disconnecting the output terminals of the motor driver from the DC bus results in the output being at the DC voltage present on the DC bus or zero volts. The amplitude of the output voltage is varied by varying the duration of each switching cycle during which the output terminals of the motor driver are connected to the DC voltage. The motor controller varies the amplitude of the output voltage over each cycle of a desired fundamental frequency using a modulation technique such as pulse width modulation (PWM) to synthesize a waveform having the desired amplitude and frequency.

[0005] Typically, a current regulator executed on a motor drive generates a reference signal for a modulation routine. The current regulator receives feedback signals from one or more current sensors that correspond to the current output from the motor drive to the motor controlled by the motor drive. The current feedback signal can be single-phase or multi-phase, but corresponds to the actual current provided to the motor on the corresponding phase. The current feedback signal is converted to a static or synchronous reference frame, where a current control loop including a proportional regulator, an integral regulator, a differential regulator, or a combination thereof outputs a voltage reference in the corresponding static or synchronous reference frame. These voltage references are converted back to voltage reference values ​​corresponding to the desired voltage on each phase of the motor. The modulation routine then generates switching signals for controlling the power electronic switching devices in the inverter section to output the desired voltage waveform to the motor.

[0006] Historically, it has been known to execute both the current regulator and the modulation routine at the same periodic update rate. This periodic update rate is also referred to as the switching frequency or switching period. However, processing the current feedback signal requires a finite amount of time. The analog signal first passes through an analog-to-digital converter to obtain a digital value of the signal, which is then sent to a processor. The processor must sample the digital value and store the sampled value in a memory for use by the current regulator. The current regulator executes based on the current reference signal and the stored current feedback signal to determine the desired voltage reference. Each of these steps requires time to execute and must be executed sequentially within the switching cycle. Within the same time interval and possibly in parallel with sampling the current and executing the current regulator, the modulation routine determines the modulation index of the power switching device to generate the desired output voltage.

[0007] Due to the delays involved in sampling and processing the current feedback signal and executing the current regulator, the modulation routine may have already switched the output voltage to the desired output voltage within a switching cycle before the current regulator's execution is complete. Therefore, the output of the current regulator is typically stored for use by the modulation routine during the next switching cycle. This introduces a one-cycle delay between determining the desired voltage reference and its use by the modulation routine. Although the switching frequency is typically one or more orders of magnitude greater than the fundamental frequency of the output voltage, and thus a one-cycle delay does not significantly affect the fundamental frequency component of the output voltage, it may introduce some undesirable effects on the motor driver's output. When there are rapid and / or significant changes in the current reference, the current regulator may generate some overshoot in response to such changes. This overshoot can cause torque ripple or a corresponding overshoot in the output current provided to the motor. In certain applications, such as during torque control of the motor, this overshoot and / or torque ripple may require detuning the motor driver's response to prevent the overshoot and / or torque ripple, or to accept the overshoot and / or torque ripple, either of which can degrade motor performance.

[0008] Therefore, it is desirable to provide an improved method for implementing a current regulator and modulation routine within an inverter.

[0009] Historically, two different approaches have addressed the issue of using the voltage reference in the modulation routine within a cycle after it has been determined. The first approach is to increase the switching frequency of the motor drive. Increasing the switching frequency reduces the delay between determining the voltage reference and utilizing it in the modulation routine. Consequently, overshoot and / or torque ripple can be reduced. However, increasing the switching frequency increases power losses in the inverter due to switching, reduces the period during which the output current can be sampled, and increases the complexity of the inverter control routine. The second approach to addressing the delay between determining the voltage reference and utilizing it in the modulation routine is to execute these routines consecutively within the same switching cycle. This second approach has its own drawbacks. Typically, expensive analog-to-digital converters and / or processors are required, so that the hardware can be configured to perform both analog-to-digital conversion and current regulator processing at a high execution rate. Alternatively, the second approach may impose a limit on the maximum modulation index or a limit on the switching frequency to provide a minimum amount of time during which analog-to-digital conversion and subsequent processing can occur at the beginning of each switching cycle.

[0010] It would therefore be desirable to provide systems and methods that reduce or eliminate delays in the modulation routine when using a voltage reference generated by a current regulator, but without incurring any of the disadvantages indicated above. Summary of the Invention

[0011] According to one embodiment of the present invention, a method for generating a multiphase output voltage includes: measuring a value of a current output from a power converter to a load during a first switching cycle; and determining a value of a multiphase reference voltage based on the value of the current measured during the first switching cycle. The output voltage of a first portion of a phase of the multiphase output voltage is generated based on the multiphase reference voltage determined in a switching cycle prior to the first switching cycle. An offset value of a second portion of a phase is determined based on the multiphase reference voltage and the output voltage of the first portion of the phase. The output voltage of the second portion of the phase of the multiphase output voltage is generated based on the multiphase reference voltage determined in a switching cycle prior to the first switching cycle and the offset value.

[0012] According to another embodiment of the present invention, a method for generating a three-phase output voltage from a power converter is disclosed, wherein the power converter executes a plurality of instructions at periodic intervals. During a first switching cycle, a first current feedback value of at least one phase output from the power converter is measured, and a first voltage reference for each phase of the three-phase output voltage is determined based on the first current feedback value. During a second switching cycle, a second current feedback value of at least one phase output from the power converter is measured, and a second voltage reference for each phase of the three-phase output voltage is determined based on the second current feedback value. A switching signal for a first phase of the three-phase output voltage is generated based on the first voltage reference of the corresponding phase. An offset value for a second phase and an offset value for a third phase of the three-phase output voltage are determined, and a switching signal for the second phase and a switching signal for the third phase are generated based on the second voltage reference and the offset value of the corresponding phase.

[0013] According to another embodiment of the present invention, a motor driver is configured to control the operation of a multi-phase motor connected to the motor driver. The motor driver includes at least one current sensor and a controller, wherein the at least one current sensor is configured to generate a current feedback signal corresponding to the value of the current output from the motor driver, and the controller is configured to execute at a periodic switching frequency. The controller is configured to receive the current feedback signal and determine a reference voltage output to each phase of the multi-phase motor based on the current feedback signal. During a first switching cycle, the controller is configured to generate switching signals for a first portion of the phases of the multi-phase motor based on a reference voltage determined during a switching cycle prior to the first switching cycle. During the first switching cycle, the controller is further configured to determine an offset value for a second portion of the phases of the multi-phase motor and generate switching signals for the second portion of the phases of the multi-phase motor based on the reference voltage and the offset value determined during the first switching cycle.

[0014] These and other advantages and features of the present invention will become apparent to those skilled in the art from the detailed description and accompanying drawings. However, it should be understood that although the detailed description and accompanying drawings indicate preferred embodiments of the present invention, the detailed description and accompanying drawings are given by way of illustration and are not limiting. Many changes and modifications may be made within the scope of the present invention without departing from the spirit of the present invention, and the present invention includes all such modifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings, wherein like reference numerals refer to like parts throughout, and in which:

[0016] Figure 1 is a block diagram of a motor driver including one embodiment of the present invention;

[0017] Figure 2Is from Figure 1 A block diagram representation of the rectifier section of a motor driver;

[0018] Figure 3 Is from Figure 1 A block diagram of the inverter section and gate driver module of the motor driver;

[0019] Figure 4 Is from Figure 1 A block diagram representation of one embodiment of a controller for a motor drive;

[0020] Figure 5 is a timing diagram of an exemplary sine-triangle comparison modulation technique;

[0021] Figure 6 is a flow chart illustrating exemplary steps for determining an output voltage according to one embodiment of the present invention;

[0022] Figure 7 is a timing diagram of an exemplary modulation routine for generating switching signals for a multi-phase output voltage in single update mode;

[0023] Figure 8 is a flow chart illustrating executing multiple modules in parallel within a switching cycle according to one embodiment of the present invention; and

[0024] Figure 9 is a timing diagram of an exemplary modulation routine for generating switching signals for a multi-phase output voltage in dual update mode.

[0025] In describing the various embodiments of the present invention shown in the accompanying drawings, specific terminology will be employed for the sake of clarity. However, it is not intended to limit the present invention to the specific terminology so selected, and it should be understood that each specific term includes all technical equivalents that operate in a similar manner to achieve a similar purpose. For example, the words "connect," "attach," or similar terms are often used. These are not limited to direct connections, but include connections made through other elements, where such connections are considered equivalent by those skilled in the art. DETAILED DESCRIPTION

[0026] Various features and advantageous details of the subject matter disclosed herein are more fully described with reference to the non-limiting embodiments described in detail in the following description.

[0027] The subject matter disclosed herein describes an improved method for executing a current regulator and modulation routine within an inverter that reduces or eliminates delays in the modulation routine when using a voltage reference generated from the current regulator. During each switching cycle, the output current is sampled and a current feedback signal is provided to the current regulator. The current regulator executes to determine the desired output voltage for the motor, and the voltage reference is passed to the modulation routine during the same switching cycle in which the voltage reference is calculated. However, rather than requiring an expensive converter with a fast conversion rate, a standard converter is provided for converting the analog value of the measured current into a digital value. In addition, the disclosed method does not require an increased switching frequency, does not limit the maximum modulation index, and does not limit the switching frequency for providing a minimum amount of time during which analog-to-digital conversion and subsequent processing can occur.

[0028] After the voltage reference is passed to the modulation routine, the new voltage reference determined by the current regulator is used by the modulation routine during the current switching cycle in which it is determined. According to one embodiment of the present invention, the voltage reference is used to determine the modulation index for each phase of the output voltage. Because the modulation routine does not impose restrictions on switching frequency, analog-to-digital conversion rate, maximum modulation index, etc., there is a possibility that switching signals for one or more phases must be generated before analog-to-digital conversion and current regulation are completed. The modulation routine stores the values ​​of the modulation index generated from the previous switching cycle and uses the stored values ​​when new values ​​are not yet ready. However, the modulation indices for at least two of the three phases are typically determined before switching signals need to be generated. While the first phase may need to use the modulation index from the previous switching cycle, the modulation indices for the other two phases are then utilized during the switching cycle in which they are calculated. An offset can be provided for the modulation index of each of the two additional phases to maintain the commanded differential voltage between the phases during that switching cycle.

[0029] Conventional voltage reference generation is configured to provide a balanced three-phase voltage to a motor. When a modulation index determined from a previous switching cycle is used together with a modulation index from a current switching cycle, the resulting multi-phase voltage supplied to the motor may no longer be balanced. To maintain a balanced three-phase voltage, an offset value is determined using the modulation index determined in the current switching cycle and the offset value is used to compensate for the phase voltage. An exemplary application may be a motor drive supplying power to a three-phase motor. Based on the voltage reference determined in the previous switching cycle, the modulation routine determines the first phase V before the modulation index for the current switching cycle has been determined. aSwitching is required. After determining the modulation index for the current switching cycle, the motor driver determines the difference between the actual phase output (using the voltage reference from the previous switching cycle) and the desired reference voltage for the current switching cycle. This difference is used to determine an offset value for correcting the other two phase voltages. Before the modulation routine begins switching either of the two phase voltages, the offset value is added to the desired reference voltages of the other two phase voltages. Therefore, even if at least one phase is switched based on the reference value determined from the previous switching cycle, the differential voltage between each phase is equal to the new reference value calculated during the current switching cycle.

[0030] First turn Figure 1 , a motor drive 20 for determining a modulation index according to one embodiment of the present invention is shown. An AC voltage 12 is provided to the motor drive 20 at an input 22. According to the embodiment shown, the AC voltage 12 is a three-phase AC input voltage. The motor drive provides an AC output voltage from an output 160 of the motor drive to a motor 10 operatively connected to the motor drive 20 via a cable 14. The output voltage is a three-phase AC output voltage, wherein a separate conductor is shown extending between the motor 10 and the drive 20 for each phase of the motor. It should be understood that the conductors shown can be combined within the cable 14, operated as separate conductors, or a combination thereof, depending on the application requirements.

[0031] The AC input voltage 12 is provided to the converter section 40 of the motor drive 20. One or more additional filters may be included between the input 22 of the motor drive and the converter section 40 as required by the application. The converter section 40 may include any electronics suitable for passive or active rectification as understood in the art. Figure 2The converter section 40 is shown as a passive converter and includes a set of diodes 44 forming a diode bridge. The converter section 40 receives the AC voltage 12 at an input 42, rectifies the three-phase AC voltage into a DC voltage, and provides the DC voltage to the DC bus 50 at the output of the converter section. Alternatively, the converter section can be an active converter that includes gate-controlled switching devices, including but not limited to thyristors, silicon-controlled rectifiers (SCRs), or silicon-based transistors such as insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETs). The converter section can alternatively include high-frequency switching devices, including but not limited to silicon carbide (SiC) or gallium nitride (GaN) wide-bandgap IGBTs or MOSFETs that can switch in the 20 kilohertz (20 kHz) to megahertz range to convert the voltage at the input 42 from an AC voltage to a DC voltage for the DC bus 50. A DC bus 50 is connected to the output terminals of the converter section 40 , and the DC voltage output by the converter exists between a positive rail 52 and a negative rail 54 of the DC bus 50 .

[0032] Refer again Figure 1 A DC bus capacitor 55 is connected between the positive rail 52 and the negative rail 54 to reduce the amplitude of the ripple voltage generated by converting the AC voltage into a DC voltage. It should be understood that the DC bus capacitor 55 can be a single capacitor or multiple capacitors connected in parallel, series, or a combination thereof. The amplitude of the DC voltage between the negative rail 54 and the positive rail 52 is generally equal to the peak amplitude of the AC input voltage. The DC bus 50 is connected in series between the converter section 40 and the inverter section 100. One or more additional filters may be included between the converter section 40 and the inverter section 100 depending on the application requirements.

[0033] The inverter section 100 includes gate controlled switching elements as described for the active converter 40, such as silicon and / or wide bandgap materials for IGBTs or MOSFETs, or silicon gate controlled thyristors (SCGTs) and gate turn-off thyristors (GTOs) for medium voltage, high power cell gap transistors as known in the art. Figure 3The inverter section 100 shown includes power metal oxide semiconductor field effect transistors (MOSFETs) 106 and reverse connection devices 108, which can be freewheeling diodes or the inherent body diodes of the MOSFETs, connected in pairs between the positive rail 52 and each phase of the output voltage (110U, 110V, 110W) and between the negative rail 54 and each phase of the output voltage. Each of the transistors 106 receives a switching signal 116 to selectively enable the transistor 106 and convert the DC voltage from the DC bus into a controlled three-phase output voltage for the motor 10. When enabled, each transistor 106 connects the corresponding rail 102, 104 of the DC bus to one output phase 110, which in turn is connected between the inverter section 100 and the output terminals 160. One or more additional filters may be included between the output of the inverter section 100 and the output terminals 160 of the motor drive 20, depending on the application requirements.

[0034] A current sensing module 150 is provided at the output of the motor drive. The current sensing module 150 includes a current sensor 152 on each phase of the AC output voltage. Each current sensor 152 generates a current feedback signal 154 corresponding to the current present at the output 160 of the motor drive for each phase of the AC output.

[0035] In operation, the motor driver 20 is configured to control the operation of the motor 10 connected at the output 160. Figure 1In the embodiment shown in FIG, the processor 112 and the driver circuit 114 may include modules for controlling the operation of the motor driver 20 and manage the execution of the above modules. The driver circuit 114 may be a dedicated modulation circuit, a separate core executed on the processor 112, or a module executed on the processor 112. For convenience, the driver circuit 114 and the processor 112 may be referred to herein as a controller 113 for the motor driver 20. The embodiment shown is not intended to be limiting, and it should be understood that the various features of each module may be performed by another module and / or various combinations of other modules may be included in the processor 112 or the driver circuit 114 without departing from the scope of the present invention. A module may be a stored program executed on one or more processors, logic circuits, or a combination thereof. The processor 112 may be implemented, for example, in a microprocessor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other such custom devices. The motor driver 20 also includes a memory device 115 in communication with the processor 112. The memory device 115 may include a transient memory, a non-volatile memory, or a combination thereof. The memory device 115 can be configured to store data and programs, including a series of instructions that can be executed by the processor 112. It is contemplated that the memory device 115 can be a single device, a plurality of devices, or can be incorporated, for example, as part of another device such as an application specific integrated circuit (ASIC). The processor 112 communicates with the memory 115 to read instructions and data as needed to control the operation of the motor drive 20.

[0036] According to one embodiment of the present invention, the processor 112 receives a reference signal that identifies the desired operation of the motor 10 connected to the motor drive 20. The reference signal can be, for example, a torque reference (T*), a speed reference (ω*), or a position reference (θ*). The processor 112 also receives a feedback signal indicative of the current operation of the motor drive 20. The motor drive 20 may include a voltage sensor and / or a current sensor that is operably connected to the DC bus 50 and generates a feedback signal corresponding to the magnitude of the voltage and / or current present on the DC bus. The motor drive 20 may also include: one or more current sensors 152 and one or more voltage sensors on each phase of the AC output voltage, which generate a feedback signal 154 corresponding to the magnitude of the output current and / or voltage present at the output terminal 160 of the motor drive 20.

[0037] The processor 112 uses the feedback signal and the reference signal to control the operation of the inverter section 100 to generate an output voltage having a desired amplitude and frequency for the motor 10. Figure 4, an exemplary controller 120 for the motor drive 20 is shown. The controller 120 can be implemented as a series of instructions stored in the memory 115 of the motor drive 20 and executed on the processor 112. The controller 120 shown receives a position reference signal (θ*) 47 as an input to a series of cascaded control loops. The controller 120 includes a position control loop, a velocity control loop, and a current control loop. The control loops are shown as cascaded control loops, where the output of one control loop is provided as an input to another control loop. It is contemplated that various other control topologies can be used within the motor drive 20.

[0038] In the position control loop, a position reference signal (θ*) 47 is compared to a position feedback signal (θ) 121 at a first summing junction 122. A position error signal is output from the first summing junction 122 and input to a position loop controller 124. According to the illustrated embodiment, the position loop controller 124 is a proportional-integral (PI) controller. Alternatively, the position loop controller 124 may be solely a proportional (P) controller or may also include a differential (D) component. Each of the proportional (P), integral (I), and / or differential (D) components of the position loop controller 124 includes a controller gain. The position loop controller gains are typically referred to as a position loop proportional gain (Kpp), a position loop integral gain (Kpi), and a position loop differential gain (Kpd). The output of the position loop controller 124 is a velocity reference signal (ω*).

[0039] In the speed control loop, the speed reference signal (ω*) is compared to the speed feedback signal (ω) at the second summing junction 126. The speed feedback signal (ω) is generated by differentiating the position feedback signal (θ), as shown in the differentiation block 123. The speed feedback signal (ω) may also be filtered by the speed filter block 125. A speed error signal is output from the second summing junction 126 and input to the speed loop controller 128. According to the illustrated embodiment, the speed loop controller 128 is a proportional-integral (PI) controller. Alternatively, the speed loop controller 128 may be solely a proportional (P) controller or may also include a differential (D) component. Each of the proportional (P), integral (I), and / or differential (D) components of the speed loop controller 128 includes a controller gain. The speed loop controller gains are generally referred to as the speed loop proportional gain (Kvp), the speed loop integral gain (Kvi), and the speed loop differential gain (Kvd). The output of the speed loop controller 128 is the acceleration reference signal.

[0040] The controller 120 may also include a feedforward branch. According to the illustrated embodiment, the controller 120 includes feedforward branches for both the velocity element and the acceleration element. The position reference signal (θ*) 47 passes through a first differential element 132 to obtain a velocity feedforward signal. The velocity feedforward signal is multiplied by a velocity feedforward gain (Kvff) 134 and combined with a velocity reference signal (ω*) and a velocity feedback signal (ω) at a second summing junction 126. The velocity feedforward signal passes through a second differential element 136 to obtain an acceleration feedforward signal. The acceleration feedforward signal is multiplied by an acceleration feedforward gain (Kaff) 138 and combined with the acceleration reference signal at a third summing junction 140 to generate a torque reference signal (T*). As is known in the art, the torque required for a motor to achieve a desired acceleration is related to the inertia of the motor. A gain block that includes the inertia of the motor may be included between the output of the velocity loop controller 128 and the acceleration feedforward gain and the output of the third summing junction 140 to convert the acceleration signal into a torque signal. Alternatively, the inertia of the motor can be incorporated into the gains of the velocity loop controller 128 and the acceleration feed-forward gains, so that the outputs of the velocity loop controller 128 and the acceleration feed-forward block are both torque signals. According to yet another option, the third summing junction 140 can combine the acceleration signals, and an inertia gain block can be included after the third summing junction 140 to generate a torque reference signal (T*).

[0041] The torque reference signal (T*) output from the third summing junction 140 is further processed before generating the switching signal 116 for the inverter section 100. The torque reference signal (T*) is provided as an input to a filter section 142, which may include one or more filters to remove unwanted components from the control system, including, for example, a low-pass filter to attenuate undesirable high-frequency components, or a notch filter to attenuate specific frequency components that have an undesirable effect on the controlled mechanical load. The output of the filter section 142 is passed through a torque gain block 144. The torque gain block 144 may include a torque constant (Kt) that defines the relationship between the current provided to the motor 10 and the torque output by the motor. The torque gain block 144 may include one or more additional gain elements, such as an inertia-related gain, which is combined with the torque constant (Kt) to produce a desired current reference (I*) to the current regulator 146. The current regulator 146 receives a current feedback signal (Ifdbk) from the current sensor 152 at the output of the motor driver 20 and utilizes a current controller, which may include proportional, integral, and / or derivative components, for regulating the current in the motor 10 .

[0042] The current feedback signal (Ifdbk) is received at the controller 120 as a signal corresponding to each phase of the motor. As shown in the figure, the current feedback signal (Ifdbk) includes multiple feedback signals, each of which corresponds to the current amplitude measured on one phase of the multi-phase motor 10. For a three-phase motor, it is expected that the current amplitude is measured in all three phases of the motor. Alternatively, the current amplitude is measured in two phases of the motor, and the amplitude of the third phase is determined based on the amplitudes of the two measured phases. The measured feedback signal is then provided to the reference frame converter 145 within the controller 120 for use by the current regulator 146.

[0043] Reference frame converter 145 is configured to convert the current feedback signal from the static, physical reference frame of the measured signal to a rotating reference frame. Reference frame converter 145 receives as input the phase current (Ifdbk) measured in the static reference frame and the position feedback signal (θ). The position feedback signal (θ) corresponds to the angular position of motor 10. The position feedback signal (θ) can be the mechanical angular position of the motor or the electrical angular position corresponding to the electrical angle of the current supplied to motor 10. However, the conversion between reference frames utilizes the electrical angular position of the current to convert the current feedback signal from the static reference frame to the rotating reference frame. Therefore, if the position feedback signal (θ) is the mechanical angular position of the motor, reference frame converter 145 first converts the mechanical angular position of the motor to the electrical angle of the current based on the number of poles in the motor. In the static reference frame, the current applied to motor 10 varies sinusoidally at the command frequency output by motor controller 20. If the current feedback signal is converted to a reference frame rotating at the same frequency as the command frequency, the rotating reference frame synchronizes with the output current, and the current feedback signal becomes a "DC" value, or constant value.

[0044] The reference frame converter 145 is used to convert the measured current feedback signal into a synchronous reference frame that rotates at the fundamental frequency of the current output to the motor 10. As shown in the following equations 1 and 2, the frame conversion uses the electrical angle of the motor and the current amplitude measured in the motor to generate a synchronous current feedback signal that rotates at the fundamental frequency of the motor. Equation 1 defines the current in the "d" axis, and Equation 2 defines the current in the "q" axis. When the feedback current is mentioned in the dq axis, both currents exist. The synchronous current feedback signal output from the reference frame converter 145 in the synchronous reference frame is also called a dq reference frame and includes both the d-axis component and the q-axis component of the current. The synchronous current feedback signal is provided as an input to the current regulator 146 to generate a voltage reference signal, wherein the voltage reference signal generates the desired current reference I* when applied to the motor 10, and the current reference I* is input to the current regulator 146.

[0045]

[0046]

[0047] in:

[0048] θ = electrical angle;

[0049] i a = current amplitude in phase "a" of the motor;

[0050] i b = the magnitude of the current in phase "b" of the motor; and

[0051] i c = current amplitude in phase "c" of the motor.

[0052] The output of the current regulator 146 is a voltage reference in the dq reference frame. The voltage reference can be converted to a static reference frame or expressed as an amplitude and phase angle. The electrical angle is used to convert the voltage reference back to a reference voltage for each phase voltage. The voltage reference is provided to the gate driver 114, which in turn generates a switching signal 116 to the inverter section 100. It is conceivable that the voltage reference can be converted back to a phase voltage before being provided to the gate driver 114. Optionally, the gate driver 114 can be configured to perform an inverse transformation and then generate a switching signal. The output of the gate driver 114 is shown as being provided to the plant 130 of the controlled system. In a motion control system, the plant 130 typically includes the inverter section 100 of the motor drive 20, the motor 10, a mechanical load, a position feedback device, and a mechanical coupling between the motor and the mechanical load or between the motor and the position feedback device. The position feedback device generates a position feedback signal (θ) used by the controller 120.

[0053] The gate driver module 114 converts the voltage reference output from the current regulator into a desired output voltage having a variable amplitude and frequency, wherein the amplitude and frequency are selected to produce the desired operation of the motor 10. The gate driver module 114 then generates a switching signal 116 used by pulse width modulation (PWM) or other modulation techniques to control the switching elements in the inverter section 100 to produce the desired output voltage. The switching signal 116 then enables / disables the transistor 106 to provide the desired output voltage to the motor 10, which in turn results in the desired operation of the mechanical load coupled to the motor 10.

[0054] Next refer to Figure 5FIG. 3 shows a segment of one phase of an AC voltage output according to an exemplary sinusoidal-triangular PWM modulation technique 161. In the sinusoidal-triangular PWM modulation technique 161, a triangular waveform 162 is compared to a voltage reference 164 to generate a switching signal 116. The switching signal 116 controls a switching element 106 that selectively connects or disconnects each phase 110 of the output terminal to either the positive voltage rail 52 or the negative voltage rail 54. One cycle of the triangular waveform 162 is defined by a switching period 166 of the PWM routine. During the switching period 166, the switching signal 116 is set to high if the voltage reference 164 is greater than the triangular waveform 162 and set to low if the voltage reference 164 is less than the triangular waveform 162. The resulting output voltage 168 can be represented by a step waveform, where the amplitude of the step waveform during each period 166 is the average value of the output voltage 168 during that period 166. The average value is determined by multiplying the amplitude of the DC voltage present on the DC bus 50 by the percentage of the switching period 166 that the switching signal 116 is set to high. As the switching period 166 of the PWM routine is decreased, the step output voltage 168 more accurately corresponds to the voltage reference 164. It is contemplated that other modulation techniques such as space vector or multi-level switching can also be used to generate the output voltage as known to those skilled in the art. Furthermore, the modulation technique can be implemented by comparing analog signals; digital signals such as incrementing registers up and down; or a combination thereof.

[0055] In practice, a direct comparison of the sinusoidal waveform to the triangular waveform is not typically performed to generate the switching signal 116. Rather, the processor 112 or gate drive circuit 114 uses the voltage reference signal output from the current regulator 146 to determine the modulation index required to produce the desired voltage for each phase of the motor 10. The modulation index defines the portion of the switching period that the output voltage should be on. The modulation index is typically defined as a value between 0 and 1, where 0 is zero percent of the switching period 166 and 1 is one hundred percent of the switching period 166. The present application will be discussed with respect to the determination of the modulation index for illustrative purposes. This embodiment is not intended to be limited to a particular modulation method, but is an exemplary embodiment of the present application.

[0056] According to an exemplary embodiment of the present invention, a voltage reference is used to generate a modulation index for each phase, determining the amount of time within the corresponding switching cycle 166 that the switching signal 116 of the corresponding phase will be output. Within the motor driver 20, many different processes are scheduled for execution. These processes may include sampling input signals, setting values ​​at output terminals, reading or writing communication buffers, executing different control modules, etc. The firmware within the motor driver 20 schedules the execution of each process based on timing requirements, processing power, communication bandwidth, etc. The processor 112 may include multiple cores that execute different processes in parallel. Similarly, dedicated processing circuits such as the gate driver 114 can execute in parallel with the processor 112. One of the scheduled processes within the motor driver 20 typically handles current and voltage regulation supplied to the motor 10. This process is executed at a frequency commonly referred to as the switching frequency. The reference value for each phase is updated once, and sometimes twice, within this periodic interval.

[0057] Next turn Figure 6 , the steps performed to determine the modulation index are shown. At the beginning of the flowchart shown, an interrupt or other trigger initiates the parallel operation of two tasks. The first task 190 provides the voltage reference value determined in the previous switching cycle to the modulation routine. Optionally, the voltage reference value can be provided as an initial step performed at the beginning of the switching cycle and in series with triggering the analog to digital conversion. However, as shown, the analog to digital conversion is performed in parallel with loading the reference to the modulation routine at step 180 and initiating the analog to digital conversion of the current feedback signal 154 from the current sensor 152. After the analog to digital conversion is completed as shown in step 182, the processor 112 executes the current regulator 146 as further indicated in step 184. The current regulator 146 determines a new voltage reference value for the current switching cycle to obtain the desired output voltage of the motor 10. According to Figure 1 In the embodiment shown, the motor 10 is a three-phase motor, and the voltage of each phase will be determined.

[0058] Under certain operating conditions, it is expected that the analog-to-digital conversion and the execution of the current regulator can all be completed before one phase needs to switch. Under these first set of operating conditions, the voltage reference value determined in the current switching cycle is then provided to the modulation routine to update the voltage reference value previously provided to the modulation routine at step 190. Under other operating conditions, it is expected that one phase of the output voltage must begin switching before the current regulator is completed. Under these second set of operating conditions, such as Figure 6 As shown, the modulation routine uses the voltage reference value from the previous switching cycle provided to the modulation routine at step 190 to begin generating switching signals before determining a new reference voltage for the current switching cycle, as shown in step 192. However, the new reference voltage is determined before the second or third phase needs to switch. Figure 6 As shown, at step 186, the processor 112 determines an offset value. The offset value is used to compensate the voltage reference values ​​of the second phase voltage and the third phase voltage. These compensated reference voltages are used by the modulation routine to generate the output voltage of the second phase and the output voltage of the third phase.

[0059] When one phase must begin switching before a new voltage reference is determined, it is desirable to maintain the desired line-to-line voltage of the motor during the switching cycle. The line-to-line voltage is determined as shown in Equations 3 to 5 below.

[0060] V ab =V a -V b (3)

[0061] V bc =V b -V c (4)

[0062] V ca =V c -V a (5)

[0063] in:

[0064] V a = the voltage present in phase A of the motor;

[0065] V b = the voltage present in phase B of the motor; and

[0066] V c = Voltage present in phase C of the motor.

[0067] In the exemplary application, it will be assumed that phase A of the motor must start switching before the new voltage reference is determined, while phases B and C of the motor do not start switching until the new voltage reference is determined. Referring to equations 3 and 5 above, the line voltage V ab and V ca will not correspond to the expected line-to-line voltage, since the phase V a Switching is started using the voltage reference value from the previous cycle. To obtain the desired line voltage, an offset value is determined, such as Figure 6 As shown in step 186 of the figure. This offset value is added to phases B and C of the motor. The offset value compensates for the difference in line voltage that occurs because phase A uses the voltage reference from the previous switching cycle. In addition, as shown in equation 7 below, when determining the line voltage V bcWhen the offset values ​​are equal, they cancel each other out. As shown in step 188, during the switching cycle in which the reference voltage is determined, the other two phases use the modified reference value that includes the offset value. Therefore, based on the sampled current values ​​during that switching cycle, all three line-to-line voltages become the desired line-to-line voltages during that switching cycle without incurring a full switching cycle delay. The modified line-to-line voltages are determined as shown in Equations 6 through 8 below.

[0068] V ab =V a -(V b +V offset ) (6)

[0069] V bc =(V b +V offset )-(V c +V offset ) (7)

[0070] V ca =(V c +V offset )-V a (8)

[0071] in:

[0072] V a = the voltage present in phase A of the motor;

[0073] V b = voltage present in phase B of the motor;

[0074] V c = the voltage present in phase C of the motor; and

[0075] V offset =Offset value added to Phase B and Phase C of the motor.

[0076] Next refer to Figure 7, shows sample plots of the switching signal 116 calculated for a three-phase motor over two different switching cycles. Within a switching cycle 166, three voltage reference values ​​164 are determined for the current switching cycle. Also shown are voltage reference values ​​165 for the previous switching cycle. A first voltage reference 164A corresponds to a desired voltage on the first phase of the motor 10, identified as phase A. A second voltage reference 164B corresponds to a desired voltage on the second phase of the motor 10, identified as phase B. A third voltage reference 164C corresponds to a desired voltage on the third phase of the motor 10, identified as phase C. When the voltage reference values ​​are determined in a per-unit system, the desired voltage reference can be a value between 0 and 1 for zero volts and rated volts. For values ​​between negative rated voltage and positive rated voltage, the desired voltage reference can be a value between -1 and 1. These voltage references, in per-unit values, directly convert to a modulation index, or the percentage of the switching cycle 166 to which the switching signal should be set. The modulation index corresponds to the period of time during which the voltage reference 164 is greater than the carrier signal 162. When the first voltage reference 164A is greater than the carrier signal 162, the first switching signal 116A transitions from low to high, and when the first voltage reference 164A falls back below the carrier signal 162, the first switching signal 116A transitions from high to low. When the second voltage reference 164B is greater than the carrier signal 162, the second switching signal 116B transitions from low to high, and when the second voltage reference 164B falls back below the carrier signal 162, the second switching signal 116B transitions from high to low. When the third voltage reference 164C is greater than the carrier signal 162, the third switching signal 116C transitions from low to high, and when the third voltage reference 164C falls back below the carrier signal 162, the third switching signal 116C transitions from low to high. The example shown is for a single update during each carrier cycle 166. The switching signals 116 for each phase are symmetrical about the center of the carrier cycle 166, with the center of the carrier cycle shown at time t2.

[0077] As in Figure 7 As seen in FIG1 , at least one phase of the multiphase voltage is typically turned on early in the carrier cycle 166. In the illustrated carrier cycle 166, phase B must be turned on early in the cycle. According to the illustrated embodiment, it is assumed that the duration between t0 (which is the start of the carrier cycle 166) and t1 corresponds to the amount of time required for the motor driver 20 to sample the output current and execute the current regulator 146 to generate the voltage reference 164. However, phase B has a voltage reference value 165B from the previous switching cycle, which requires that the switching signal 116B be generated before the current sampling and current regulator execution are completed. In this case, the modulation routine can use the voltage reference value 165B from the previous carrier cycle 166.

[0078] Also refer to Figure 8, shows a flow chart illustrating at least a portion of the execution performed by the motor driver 20 during a switching cycle 166. As previously discussed, multiple modules can execute in parallel. According to the illustrated embodiment, sampling the output current, as performed in step 252, and executing the current regulator, as shown in step 254, can be executed in parallel with the modulation routine 200. As shown in steps 202 and 204, the modulation routine 200 reads the voltage reference and any stored offset for each phase from the previous switching cycle and determines the modulation index value for each phase in the current switching cycle 166. In this manner, the modulation routine 200 has the modulation index and is ready to begin modulation as soon as any phase requires modulation within the switching cycle 166. However, at steps 206 and 208, the modulation routine can continuously monitor whether the current regulator 146 has completed execution and generated a new voltage reference value. At step 206, if switching is required for one or more phases, the modulation routine will begin generating the switching signal 116 for that phase, as shown in step 210. However, if the current regulator 146 completes execution and determines a new voltage reference before the phase needs to switch, the modulation routine 200 reads the new voltage reference value, as shown in step 212 .

[0079] At step 214, the modulation routine 200 determines any necessary offset value V offset .exist Figure 7 In the example shown, phase B begins switching before the current regulator 146 completes execution. Therefore, the voltage reference 164B for the current cycle remains unchanged. At time t1, a new voltage reference 165B is determined for the next switching cycle, but for the duration of the current switching cycle 166, modulation will use the voltage reference 164B for phase B from the previous cycle. At time t1, the current regulator 146 also determines a new voltage reference value 165A for phase A and a new voltage reference value 165C for phase C. The voltage offset V is determined. offset , the voltage offset V offset corresponds to the difference between the value of voltage reference 164B for phase B during the previous switching cycle and the value of voltage reference 165B during the current cycle. A voltage offset will be added to phases A and C to achieve the desired line-to-line voltage as shown in Equations 6 through 8 above. Due to the addition of the offset value to phases A and C, the compensated values ​​of voltage references 164A and 164C determined at time t1 are greater than the calculated voltage reference values ​​165A and 165C for that cycle. These new modulation indices are provided to the modulation routine, as shown in step 216.

[0080] When each phase that was not switched based on the voltage reference from the previous switching cycle 166 needs to switch, the modulation routine uses a new modulation index determined based on the sampled current, the voltage reference generated within the same switching cycle 166, and the determined offset. Thus, for at least a portion of the phases, a one-carrier-cycle delay in the phase voltage is eliminated. The one-carrier-cycle delay in the line-to-line voltage is completely eliminated by using the offset value. If the modulation index for each phase is sufficiently low, it is contemplated that the modulation routine can determine the modulation index for each phase based on the sampled current and the voltage reference generated within the same switching cycle 166. Alternatively, in situations where the motor drive 20 can operate in overmodulation, for example, and each phase has a large modulation index, the modulation routine can continue to operate by using the voltage reference from the previous switching cycle for each phase of the motor 10. Most commonly, it is contemplated that the motor drive 20 will need to utilize the voltage reference for one or two phases (considered to be the first portion of the motor's phases) from the previous switching cycle and will be able to utilize the voltage reference for another one or two phases (considered to be the second portion of the motor's phases) determined during the current switching cycle 166.

[0081] Refer again Figure 7 , it is contemplated that a voltage offset can be alternately added and subtracted from phase A and phase C. As previously indicated, modifying the modulation index of phase A and phase C allows the motor drive 20 to supply the desired line-to-line voltage to the motor 10. However, adding the offset voltage to two of the three phases creates an asymmetry in the three voltage waveforms and introduces a small zero-sequence voltage at the output of the motor drive. The zero-sequence voltage may introduce some undesirable common-mode current within the motor drive. However, by alternately adding and subtracting the offset, the zero-sequence voltage will alternate between small positive values ​​and small negative values. The average value of the zero-sequence voltage remains approximately zero, thereby reducing the effect of adding the offset to two phases of the output voltage.

[0082] Figure 7The motor driver 20 is shown performing in single update mode. During single update mode, the current is sampled once and the desired output voltage is determined once during each switching period 166. The motor driver 20 can optionally also be operated in a double update mode of operation. In double update mode of operation, the current is sampled at both the beginning and the middle of the switching period 166, as represented by times to and t2, respectively. The current regulator is executed to determine the desired output voltage in response to obtaining the two samples. During operation in double update mode, the sampling at the beginning of the switching period and execution of the current regulator ends at time ti. The sampling at the middle of the switching period and execution of the current regulator ends at time t3. Double update mode of operation allows the modulation routine 200 to modify the end time of the switching signal 116 for each phase based on a newly calculated reference voltage. This results in the switching signal 116 being asymmetric about the middle (t2) of each switching period 166. Double update mode improves the resolution of the modulation routine and reduces harmonic distortion in the output voltage waveform.

[0083] Next turning to Figure 9 , the operation of the motor driver 20 in double update mode is shown. At the beginning of the first switching period 166A, the modulation routine receives the voltage reference value 164 based on the previous switching period. At the beginning of the switching period 166A, the voltage reference value 164 is provided to the modulation routine 200 so that it can generate the switching signal 116 at any point within the switching period. The current sampling and execution of the current regulator occurs between the beginning of the period, to, and the first time, ti, indicated on the figure. Similar to the example discussed above Figure 7 with respect to phase B, the switching signal 116B for phase B needs to be switched before the first time, ti, based on the voltage reference value 164 from the previous period, and is generated based on the voltage reference from the previous period. After the current regulator determines the new voltage reference value, an offset value is determined and added to the voltage reference values for phase A and phase C, the offset value corresponding to the difference between the voltage reference value for the current period and the voltage reference value for the previous period. The new voltage reference value 164A for phase A and the new voltage reference value 164C for phase C are provided to the modulation routine at the first time, ti, and are used to generate the beginning switching signal 116A, 116C for the respective phases.

[0084] Because Figure 9A dual-update mode of operation is shown, so at the midpoint t2 of the switching cycle 166, the current is again sampled and the current regulator executes to determine a new voltage reference. The duration of this sampling and current regulator execution extends between the midpoint t2 and a third time t3 shown in the figure. The voltage reference determined at the midpoint can be used to modify the modulation index for each phase within the switching cycle, which changes the end point or transition from high to low for each switching signal 116, and the switching signal 116 becomes asymmetric about the midpoint of the switching cycle.

[0085] Similar to the start of a switching cycle, a phase may complete switching in the second half of the cycle before a new voltage reference value is determined. In the example shown, phase C completes its switching in the current switching cycle 166 before the updated voltage reference is determined. In this case, a new voltage offset value is determined using the difference between the new voltage reference determined in the second half of the switching cycle and the voltage reference determined in the first half of the switching cycle. The new voltage offset value is subtracted from the voltage reference values ​​for phase A and phase B. The modified voltage reference values ​​for phase A and phase B (164A and 164B, respectively) are provided to the modulation routine at a third time t3 and are used to transition switching signals 116A, 116B from high to low during the second half of the switching cycle. Even if phase C turns off switching signal 116C before the voltage reference is updated in the second half of the switching cycle, the offset is added to phases A and B to maintain the desired line-to-line voltage of motor 10 during the second half of the switching cycle.

[0086] In addition to compensating the phase voltages with an offset to maintain the desired line-to-line voltage, a second offset can be added to the voltage reference to compensate for the zero-sequence voltage introduced by the addition of the first offset value. The magnitude of the line-to-line voltage between each phase can remain the same as it is shifted as a whole. For example, the line-to-line values ​​between 200V and 100V are the same as the line-to-line values ​​between 150V and 50V. The amplitude of each phase is shifted by 50V, but the line-to-line values ​​remain unchanged. To compensate for the zero-sequence voltage introduced by the first offset, a second offset can be determined and added to each phase. When the motor drive is operating in dual update mode, as shown in FIG. Figure 9 As shown, it is contemplated that this second offset value may be added to the reference voltage at the beginning of each switching cycle and at the midpoint of each switching cycle. As can be observed at the midpoint t2 of the first switching cycle 166A, the voltage reference value 164 supplied to the modulation routine shifts downward for each of the three phases. Similarly, at the end of the first switching cycle 166A or the beginning of the second switching cycle 166B, the voltage reference value 164 shifts upward for each of the three phases. The addition of the second offset value compensates for the zero-sequence voltage introduced by the first offset value.

[0087] It should be understood that the present invention is not limited in its application to the construction details and arrangement of parts set forth herein. The present invention can have other embodiments and can be practiced or executed in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It should also be understood that the present invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or apparent from the text and / or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best known modes for practicing the present invention and will enable those skilled in the art to utilize the present invention.

[0088] In the foregoing description, various embodiments have been described with reference to the accompanying drawings. However, it will be apparent that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the present invention as set forth in the appended claims. Accordingly, the description and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A method for generating a multi-phase output voltage, comprising the following steps: measuring a value of a feedback current output from the power converter to the load during a first switching cycle; determining a value of a multi-phase reference voltage according to a value of the feedback current measured during the first switching period; generating an output voltage of a first portion of a phase of the multi-phase output voltage during the first switching period based on a multi-phase reference voltage determined in a switching period preceding the first switching period, wherein the first portion of the phase includes a first phase voltage; determining an offset value for a second portion of a phase in the first switching period based on the multi-phase reference voltage and the output voltage of the first portion of the phase; and An output voltage of a second portion of a phase of the multi-phase output voltage is generated by adding a multi-phase reference voltage determined in the first switching period and an offset value, wherein the second portion of the phase includes a second phase voltage and a third phase voltage.

2. The method according to claim 1, wherein The power converter is a motor drive, and the load is a motor operatively connected to the motor drive.

3. The method according to claim 1, wherein The step of generating an output voltage of the second portion of the phase further comprises the steps of: determining a difference between a phase voltage generated for the output voltage of the first portion of the phase and a value of the phase voltage of the first portion of the phase present in the multi-phase reference voltage during the first switching period, wherein the difference is the offset value; and The offset value is added to a value of a phase voltage of a second portion of the phases present in the multi-phase reference voltage during the first switching period to produce a compensated reference voltage for the second portion of the phases, and wherein the step of generating an output voltage for the second portion of the phases uses the compensated reference voltage for each phase in the second portion of the phases.

4. The method according to claim 1, wherein: The step of generating the output voltage is performed in parallel with the steps of measuring the value of the feedback current and determining the value of the multi-phase reference voltage, and The output voltage of the first portion of the phases requires that generation of the corresponding output voltage begin during the first switching period before the value of the multi-phase reference voltage is determined.

5. The method according to claim 1, wherein Each step is performed once during the first half of the switching cycle and once during the second half of the switching cycle.

6. The method according to claim 5, further comprising the steps of: determining a second offset value corresponding to a zero-sequence voltage caused by the offset value of a second portion of the phase; as well as The second offset value is added to the multi-phase reference voltage for each phase.

7. A method for generating a three-phase output voltage from a power converter, wherein: The power converter executes a plurality of instructions at periodic intervals, and the method comprises the following steps: During the first switching cycle: measuring a first current feedback value of at least one phase output from the power converter, and determining a first voltage reference for each phase of the three-phase output voltage according to the first current feedback value; During the second switching cycle: measuring a second current feedback value of at least one phase output from the power converter, determining a second voltage reference for each phase of the three-phase output voltage according to the second current feedback value, generating a switching signal for a first phase of the three-phase output voltage according to a first voltage reference of the corresponding phase, determining an offset value, which includes determining a difference between a first voltage reference for the first phase and a second voltage reference for the first phase, the offset value being applied to a second phase and a third phase of the three-phase output voltage, respectively, and The switching signal of the second phase and the switching signal of the third phase are generated by adding the second voltage reference of the corresponding phase and the offset value.

8. The method according to claim 7, wherein: The step of generating the second-phase switching signal and the third-phase switching signal further includes the following steps: adding the offset value to the second voltage reference of the second phase and the second voltage reference of the third phase to generate a compensated reference voltage of the second phase and a compensated reference voltage of the third phase, and The second-phase switching signal and the third-phase switching signal are generated according to the second-phase corresponding compensated reference voltage and the third-phase corresponding compensated reference voltage.

9. The method according to claim 7, wherein: The step of generating a switching signal is performed in parallel with the steps of measuring the second current feedback and determining the second voltage reference, and The switching signal of the first phase requires that the first phase begin switching during the second switching period before the second voltage reference is determined.

10. The method according to claim 7, further comprising the steps of: determining a second offset value corresponding to a zero-sequence voltage caused by the offset value of the second phase and the offset value of the third phase; and The second offset value is added to the second voltage reference of each phase.

11. The method according to claim 7, wherein: Measuring the second current feedback value and determining the second voltage reference for each phase are performed once during a first half of the second switching period and once during a second half of the second switching period.

12. The method according to claim 11, wherein During the first half of the second switching period, the following steps are performed: generating a switching signal for a first phase of the three-phase output voltage according to a first voltage reference of the corresponding phase, determining an offset value for a second phase and an offset value for a third phase of the three-phase output voltage, and generating a switching signal for the second phase and a switching signal for the third phase according to a second voltage reference of the corresponding phase and the offset value. The method further comprises the following steps: During the second half of the second switching cycle: generating a switching signal for a second phase or a switching signal for a third phase of the three-phase output voltage according to a second voltage reference of a corresponding phase determined during a first half of the second switching period, determining an offset value of the first phase and an offset value of the second phase or the third phase not selected for generating a switching signal, adding the offset value to the corresponding phase to obtain a compensated voltage reference value, and The switching signal of the first phase and the switching signal of the second phase or the third phase not selected for generating the switching signal are generated according to the compensated voltage reference value.

13. The method according to claim 7, wherein: The power converter is a motor drive, and a three-phase motor is operatively connected to receive the three-phase output voltage from the motor drive.

14. A motor drive configured to control operation of a multi-phase motor connected to the motor drive, the motor drive comprising: at least one current sensor configured to generate a current feedback signal corresponding to a value of a current output from the motor drive; as well as A controller configured to execute at a periodic switching frequency to: receiving the current feedback signal, determining a reference voltage output to each phase of the multi-phase motor according to the current feedback signal, generating, during a first switching period, switching signals for a first portion of the phases of the multiphase motor in dependence on a reference voltage determined during a switching period preceding the first switching period, determining an offset value for a second portion of a phase of the multiphase motor by determining a difference between a reference voltage for a first portion of the phase determined during the first switching period and a reference voltage for the first portion of the phase determined during a switching period prior to the first switching period; as well as During the first switching period, switching signals for a second portion of the phases of the multi-phase motor are generated by adding a reference voltage determined during the first switching period and the offset value, wherein the second portion of the phases includes a second phase voltage and a third phase voltage.

15. The motor driver according to claim 14, wherein: The controller is further configured to generate switching signals for the second portion of the phases by: adding the offset value to a reference voltage for a second portion of the phase determined during the first switching period to generate a compensated reference voltage for each phase in the second portion of the phase, and A switching signal for a second portion of the phase is generated based on the compensated reference voltage.

16. The motor driver according to claim 14, wherein: Generating the switching signal is performed in parallel with receiving the current feedback signal and determining the reference voltage, and The switching signals for the first portion of the phases require that the corresponding phases begin switching during the first switching period before the reference voltage is determined.

17. The motor driver according to claim 14, wherein: The controller is configured to: receive the current feedback signal and determine the reference voltage output by each phase once during the first half of each switching cycle; and receive the current feedback signal and determine the reference voltage output by each phase once during the second half of each switching cycle.

18. The motor driver according to claim 17, wherein: generating a switching signal for each phase during a first half of each switching cycle and modifying the switching signal for each phase during a second half of each switching cycle; as well as During a second half of each switching period, an offset value is determined for a third portion of the phases, the third portion including at least one phase from the first portion of the phases and at least one phase from the second portion of the phases.

19. The motor driver according to claim 14, wherein: The controller is further configured to: determining a second offset value corresponding to a zero sequence voltage caused by the offset value for a second portion of the phase; and The second offset value is added to the reference voltage of each phase.

20. The motor driver according to claim 14, wherein The offset value for the second portion of the phase is alternately a positive offset value and a negative offset value added to the second portion of the phase, wherein the positive offset value and the negative offset value alternate over consecutive switching cycles.

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