PULSE WIDTH MODULATION CONTROL OF A MATRIX CONVERTER

BACKGROUND

The present invention relates to power systems. More specifically, the present invention relates to a pulse width modulation control method for a matrix converter or direct frequency changer.

A matrix converter is an electronic device that converts AC voltage of one frequency at its input to AC voltage of a different frequency at its output. The matrix converter may also change the amplitude and the number of phases between the input signal and the output signal. The matrix converter includes a plurality of switching devices that are controlled by pulse width modulation (PWM) to provide voltages in a single phase or multiple phases at the output of the matrix converter. The number of switching devices in the matrix converter is a function of the number of phases in the input and output lines.

PWM changes the connections of the switches between the input and the output of the matrix converter such that the locally averaged output voltages follow reference voltages.

One application for a matrix converter is controlling the speed and torque of an AC motor. In this application, the matrix converter receives an AC input signal (for example, a three-phase signal from an electrical utility) and converts the input signal to a single phase or multi-phase output signal having a frequency and amplitude that is compatible with the AC motor. However, many control algorithms for converting the input signal to the appropriate output signal are very complicated and consume a large amount of processor resources. In addition, the incorporation of a matrix converter into a motor control system often necessitates complex control hardware, which increases the cost of the system.

SUMMARY

The present invention relates to control of a matrix converter including a plurality of switching elements. The matrix converter is adapted to receive a multi-phase alternating current (AC) input signal having an input frequency and to generate a multi- phase AC output signal having an output frequency. The phases of the input signal are sorted as a function of their instantaneous voltage amplitude. A reference signal is generated from output reference voltages that correspond to each phase of the output signal. Duty cycles are calculated for each phase of the output signal based on the sorted input

signal phases and the reference signal. Switching functions, which each control one of the switching elements, are then generated based on the duty cycles for each phase of the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a power system including a matrix converter with a plurality of switching elements and a controller for operating the switching elements.

FIG. 2A-2C are schematics of switching elements suitable for use in the matrix converter shown in FIG. 1. FIG. 3 is a block diagram of the controller for generating switching functions for the plurality of switching elements from the input signals.

FIG. 4 is a graph showing the input signals sorted according to their instantaneous values.

FIG. 5A is a graph of a triangle comparison signal for generating pulse width modulation functions.

FIG. 5B is a graph of intermediate switching functions generated by a triangle comparison method.

FIG. 5C is a graph of switching functions derived from the intermediate switching functions for controlling the switching elements of the matrix controller. FIG. 5D is a graph of an output voltage waveform for a phase of the matrix converter output.

DETAILED DESCRIPTION FIG. 1 is a schematic view of power system 10, which receives multi-phase alternating current (AC) power from power supply 12 at an input frequency and provides multi-phase AC power to a load at an output frequency. Power system 10 includes LC filter 20, matrix converter 22, and matrix converter (MxC) controller 24. In the embodiment shown, power supply 12 is a three-phase power supply (including input phases R, S, and T) that provides input voltages vi, v%, and V3 and supplies input currents ii, h., and i ₃ at the inputs of matrix converter 22. Also in the embodiment shown, matrix converter 22 provides three-phase power (including phases U, V, and W) to induction motor 14, including output voltages v,° , v£ , and V ₃ and output currents i" , i% , and Z ₃ ⁹ .

LC filter 20 includes inductors 26a, 26b, and 26c, and capacitors 28a, 28b, and 28c. Inductor 26a is connected in series with input phase R, inductor 26b is connected in series with input phase S, and inductor 26c is connected in series with input phase T. Capacitor 28a is connected across input phases R and S, capacitor 28b is connected across input phases S and T, and capacitor 28c is connected across input phases R and T. LC filter 20 controls the current levels and prevents voltage transients from power supply 12.

Matrix converter 22 includes switching elements sπ, S21, S31, S12, S22, S32, Sn, S2 ₃ , and S ₃₃ (collectively referred to as switches Sjk). Switching elements Sj _k are bidirectional switches that are connected to power supply 12 via LC filter 20 such that input voltage vi is received on the input node of switching elements Sn, s^, and S ₁₃ , input voltage V2 is received on the input node of switching elements S2i, S22, and S ₂ 3, and input voltage V3 is received on the input node of switching elements S _3l9 S ₃ 2, and S ₃₃ . The output node of switching elements Sπ, S21, and S31 is connected to provide output voltage v° to motor 14, the output node of switching elements S12, S ₂ 2 _> and S32 is connected to provide output voltage V _j to motor 14, and the output node of switching elements S ₁₃ , S ₂₃ , and S ₃₃ is connected to provide output voltage V ₃ to motor 14. While matrix converter 30 is shown receiving three-phase power at its input and providing three-phase power at its output, it will be appreciated that matrix converter 22 can be adapted to receive power from power supply 12 having any number of phases and to provide power to motor 14 with any number of phases. MxC controller 24 is connected to each of switching elements Sjk to provide switching functions Sjk that operate switches Sjk to provide output signals to motor 14 at an output frequency. In other words, MxC controller 24 operates switching elements Sj _k to convert the frequency of the input signal from power supply 12 to an output frequency that is compatible with motor 14. MxC controller 24 receives input voltages Vi, V2, and V ₃ as inputs and generates the switching function Sj _k based on these inputs. The algorithm employed to develop the switching functions will be described in more detail below.

FIG. 2A-2C are schematics of devices suitable for switching elements S _jk in matrix converter 22. Each of the devices receives input voltage Vj on its input node and provides output voltage v _k " on its output node. Each of the devices is controlled by a switching function Sjk and its complement Sjk , which are provided by MxC controller 24. FIG. 2A shows device 40 including transistors 42 connected in an anti-parallel configuration (emitter to collector), with reverse blocking capability. FIG. 2B shows device

44 including transistors 42 connected in a common collector configuration. Each transistor 42 in device 44 is connected to a diode 46 in an anti-parallel configuration to provide opposite conductivity between each transistor 42 and diode 46. FIG. 2C shows device 48 including transistors 42 connected in a common emitter configuration. Each transistor 42 in device 48 is connected to a diode 46 in an anti-parallel configuration to provide opposite conductivity between each transistor 42 and diode 46. In some embodiments, transistors 42 in FIGS. 2A, 2B, and 2C are insulated gate bipolar transistors (IGBTs). It should be noted that devices 40, 44, and 48 are merely exemplary, and any device capable of controllable to provide bi-directional switching between two nodes may be employed for switching elements Sj _k -

Transistors 42 in devices 40, 44, and 48 may be controlled by pulse width modulation (PWM) signals, which provide pulses to the gates of transistors 42 to control current flow through them. The gating pulses may be modeled by switching functions Sj _k , which assume values of "1" when switching element Sj _k is closed (i.e., conducting) and "0" when switching element S _jk is open. If an inductive load is provided at the output of matrix converter 22 (such as inductive motor 14), one of switching elements S _jk needs to be in a conduction state at any given time. In addition, to avoid a short circuit between input phases R, S, and T, no two switching elements Sj _k may be conducting at the same time. These constraints may be expressed as: ∑S _Jk = l;k = l,m . (1)

M

From Equation 1, it follows that for a given k, there are only n-1 independent switching functions Sj _k . Thus, the number of switching functions S _jk may be reduced from n x m to (n-1) x m switching functions.

As is shown in FIG. 1, the output signal for each output phase U, V:, and W is generated by controlling three switching elements si _k , S _2k , and S _3k , corresponding to the three-phase input power from power supply 12. Thus, the 3x3 matrix converter 22 shown in FIG. 1 can be viewed as three converters each including three input phases and a single output phase having a signal based on control of switching elements si, S ₂ , and S ₃ . The output voltage v° from a three-phase input and single phase output matrix converter is:

V-(O = [S ₁ (O S ₂ (O

Using local averaging over a short sampling interval T ₃ and assuming that input voltages vi, V ₂ , and V ₃ are constant over sampling interval T ₈ , Equation 2 can be written as: v° = Cl ₁ V ₁ +d ₂ v ₂ +d ₃ v ₃ , (3) where di, d ₂ , and d ₃ are duty cycle functions defined as d ₁₍ 2, ₃ T ₃ is the sum of time intervals Ti, T ₂ , and T ₃ , which correspond to the times that switching elements Si, S ₂ , and S ₃ , respectively, are conducting, and v° is the locally averaged output voltage. Thus, Equation 1 may expressed in terms of duty cycles as: d _{l +} d _{2 +} d ₃ = l, (4) where 0 ≤ d _ι ,d _z ,d ₃ ≤ l . Equation 4 shows that output voltage v° is a function of two of the duty cycle functions, since the third duty cycle function can be calculated from two known duty cycle functions.

Duty cycle functions di, d ₂ , and d ₃ may be employed not only to control output voltage v°, but also to provide additional criteria related to the distribution of output current i ⁰ over particular input phases in one sampling interval. In particular, input currents ii, i ₂ , and i ₃ are related to output current i ⁰ in that: d _λ i° = i _x ; d ₂ i" = i ₂ ; d ₃ i° = i ₃ . (5)

The ratio of two locally averaged contributions from output current i° to input currents ij, i ₂ , and 1 ₃ , may be selected to follow the desired ratio of phase shifted input voltages vi, V ₂ , and V ₃ , to control the displacement factor. This can be accomplished by introducing a current distribution factor a to the duty cycle functions, wherein current distribution factor a may be defined as:

« =£L ₌ ^- ₌ 4, (6) h ^d 3 V ₃ where voltages v ₂ ' and V ₃ " are phase angle reference voltages. Voltages v ₂ ^* and v] may be generated by a phase-locked loop (PLL) system such that they are in phase with input voltages V ₂ and V ₃ , respectively.

To reduce the number of unknown duty cycles from three to two, Equation 4 may be expressed as d _x = 1 — (d ₂ +d ₃ ) , and substituted into Equation 3:

v° - V ₁ ^ ₂ (V ₂ -V ₁ H d ₃ (V ₃ -V ₁ ) . (7)

Furthermore, Equation 6 may be expressed as d ₂ - ad ₃ and substituted into Equation 7 and rewritten to provide an expression for <fj:

^d> ~< (V ₃ - V ₁ T) ^T- OT(V ₂ - V ₁ ϊ) - ⁽⁸⁾ With di calculated to meet output voltage and input power factor demands, the remaining duty cycle functions dj and d ₂ may be calculated backward from Equations 4 and 7.

FIG. 3 is a block diagram of a portion of MxC controller 24 (referred to as MxC controller portion 24a) for generating switching functions S ₁ , S ₂ , and S3 for switching elements si, s∑, and S3. MxC controller portion 24a is an embodiment of a system that generates switching functions pursuant to the constraints outlined above. MxC controller portion 24a includes phase-locked loop (PLL) module 50, linearity extender module 52, signal polarity module 54, level shifter module 56, sorting module 60, duty cycle module 62, pulse width modulation (PWM) module 64, and de-multiplexing module 66. Each of the modules of MxC controller portion 24a may be implemented in hardware, software, firmware, or combinations thereof. In order to provide output signals for all three output phases U, V, and W of MxC controller 24, three MxC controller portions 24a may be connected in parallel to the input phases from power supply 12.

PLL module 50 receives input voltages vi, V ₂ , and V ₃ at its inputs and provides output reference voltages V ₁ ⁰ " , vf , and vf to linearity extender 52. Linearity extender module 52 provides signals 70 to signal polarity module 54 based on output reference voltages v° , V ₂ , and v ₃ and zero sequence signal v _∞ . Signal polarity module 54 provides signals 72 to level shifter module 56 based on the signal from linearity extender module 52 and polarity signal pol from duty cycle module 62. Level shifter module 56 generates modified output reference voltages Av ₁ ^0** , Av f, and δv ₃ ", and one of these modified output reference voltages are provided as an input to duty cycle module 62.

Sorting module 60 also receives input voltages vi, Vz, and V3 at its inputs and generates sorted voltage signals v _m ; _n , v _m ,-d, and v _max at its output and provides a decoding signal to de-multiplexing module 66. Duty cycle module 62 generates duty cycle signals dmin, dmid, ^an d <W from sorted voltage signals v _m in, v _m id, and v _max . PWM module 64 generates switching functions S _m in, S _m id, and S _max from duty cycle signals d _m i _n , d _m i _d , and

d _maxj and de-multiplexing module 66 provides output switching functions Si, S ₂ , and S ₃ based on switching functions S _m in, S _m id, and S _ma χ and the decoding signal from sorting module 60.

Sorting module 60 receives input voltages vι, v∑, and V ₃ and sorts them as a function of their instantaneous voltage amplitudes. Input voltages vi, v _∑ , and V ₃ are sorted such that v _max is the input phase with the highest amplitude, v _m j _n -is the phase with lowest amplitude, and v _m ; _d is the phase with an amplitude intermediate v _max and v _m ; _n . Signals v _max , Vmicb and Vmin are provided at the outputs of sorting module 60 and the inputs of duty cycle module 62. Sorting module 60 also provides a decoding signal to de-multiplexing module 66 that associates sorted input voltages v _m ; _n , v _m i _d» and v _max with their originating input voltages vi, V ₂ , and V ₃ .

PLL module 50 also receives input voltages vi, v ₂ , and V ₃ at its inputs and generates output reference voltages V ₁ ^0* , v| ^* , and vf at its outputs. Output reference voltages V ₁ ^0* , vζ , and vf are phase-locked with input voltages Vi, V2, and V ₃ , respectively. Output reference voltages V ₁ ^0* , v| ^* , and vf are provided to linearity extender module 52 which extends the linearity of output reference voltages v,° ^* , v| ^* , and v| ^* . The linearity of output reference voltages v,° ^* , vf , and vf may be extended by adding a zero sequence signal V ₂ S having a specific waveform and amplitude to reduce the peaks of output reference voltages v°~ , vζ* , and V ₃ ^5* . In some embodiments, the zero sequence signal V ₂₃ is the third harmonic of one of output reference voltages v° ^* , vf , and V ₃ ^3* . With proper selection of zero sequence signal V ₂₈ , the linearity of output reference voltages v,° ^* , v£ ^* , and vf can be extended by a factor of 2/ S , or up to 15.4%.

After zero sequence signal V ₂₃ has been added to output reference voltages v° ^* . vf , and vf , signals 70 are provided to polarity module 54. Polarity module 54 receives a polarity signal pol from duty cycle module 62 that has a value of "1" when sorted input voltage v _m id is zero or positive, and a value of "-1" when sorted input voltage v _m j _d is negative. The signals from linearity extender 52 are multiplied by polarity signal pol, which assures that the criterion 0 ≤ d _ι ,d ₂ ,d ₃ < 1 set forth above is satisfied.

FIG. 4 is a graph that illustrates the relationship between sorted input voltages v _min , v _m i _d , and v _max and polarity signal pol. Input voltages Vj, v ₂ , and V ₃ are plotted

versus time, and input voltages vj, V ₂ , and V3 are phase shifted by about 120" with respect to each other. Line pol shows that the value of polarity signal pol changes as the polarity of the input voltage with the intermediate amplitude changes. For example, at instantaneous time Ti _πst , sorting module 60 sorts input voltages vj, V2, and V ₃ such that {vi, V ₂ , v ₃ ) = {v _m i _n , v _m i _d , v _max } , and polarity signal pol has a value pol = 1 because v _m id ≥ 0.

Referring back to FIG. 3, polarity module 54 provides the polarity adjusted output reference voltages V ₁ ⁰ " , vζ , and vf (i.e., signals 72) to level shifter module 56.

Level shifter module 56 scales and shifts the signals to provide modified output voltages

δv° ^** , δv|", and Av f having an amplitude of up to about 86.6% of input voltages vi, V2, and V ₃ , respectively. One of modified output reference voltages Av ₁ ^0** , Av f , and δv" ^** is provided to duty cycle module 62 for use in calculating duty cycle functions di, d ₂ , and 6 ₃ .

Duty cycle module 62 receives sorted input voltages v _m , _n , v _m id, and v _raax and one of modified output reference voltages vf , v| ^** , and vf" and generates duty cycles dmin _> dmid, and diπax- The duty cycles for a sampling interval T _s are calculated pursuant to the following table. Signals V^ _jn , v _m ^* _ld , and v ^* ^ are phase-locked sorted input reference voltages

Vminj V _m id, and v _raa χ, respectively, a is the current distribution factor described above, and -dP° is the reference signal provided by level shifter module 56 to duty cycle module 62.

After duty cycle functions d _m j _n , d _m jd, and d _m2x . are calculated, PWM module 64 generates modulation functions u" and u™ that are a function of duty cycle functions d _m i _n , d _m i _d , and d _max . In some embodiments, uf - d _mld + Cl _103x = (l + ά)d _mwi and u" = d _max . PWM module 64 compares modulation functions w™ and w, ^m to a triangular carrier signal of known frequency to generate the switching functions for switching elements si, S2, and S3. FIG. 5A is a graph showing triangle carrier signal Vtή with duty cycle functions d _m in, d _m id, and d _max and modulation functions u" and «, ^m plotted on the graph.

The comparison of modulation functions u™ and u" to triangle carrier signal V _t ri generates intermediate switching functions S^ ¹ ₃₁₁ and Sζ _ω . FIG. 5B is a graph showing the waveforms for intermediate switching functions S^ _3x and S* _ω . Intermediate switching function S^ _3x has a logic "1" value when triangle carrier signal V _tπ is less than w™ and a logic "0" value at all other times. Intermediate switching function S^' has a logic "1" value when triangle carrier signal V _t ή is less than u" and a logic "0" value at all other times. FIG. 5C is a graph of switching functions S _m i _n - S _m id, and Smax. which are derived from the intermediate switching functions S^ _3x and S^' _u . Switching functions S _m i _n , Smid, and Smax are derived as follows:

0 ore

"max "max

S _πύα = XOR(S^' _{x >} S^' _α )

Logic gates may be connected to conventional triangle comparison hardware to generate switching functions S _m i _n , S _m id, and S _max from intermediate switching functions S^ _1x and s: ^ι _ld .

Switching functions S _m i _n , S _m id, and S _max are then provided to de-multiplexing module 66, which associates switching functions S _m i _n , S _m , _d , and S _max with switching elements si, S2, and S3 based on the decoding signal provided by sorting block 60. Thus, switching function Si is provided to switching element si, switching function S ₂ is provided to switching element S ₂ , and switching function S ₃ is provided to switching element S ₃ . FIG. 5D is a graph of the waveform generated switching functions Si, S ₂ , and S ₃ control

switching elements si, S ₂ , and S ₃ , respectively, when v _m id ≥ 0. The output voltage v° for

MxC controller portion 24a is the locally averaged contributions of v _m j _n , v _m id, and v _max .

In summary, the present invention relates to control of a matrix converter including a plurality of switching elements. The matrix converter is adapted to receive a multi-phase alternating current (AC) input signal having an input frequency and to generate a multi-phase AC output signal having an output frequency. The phases of the input signal are sorted as a function of their instantaneous voltage amplitude. A reference signal is generated from output reference voltages that correspond to each phase of the output signal.

Duty cycles are calculated for each phase of the output signal based on the sorted input signal phases and the reference signal. Switching functions, which each control one of the switching elements, are then generated based on the duty cycles for each phase of the output signal.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.