INTRODUCTION

As demands for energy savings have increased in recent years, inverters

are being used in a wider range of applications. Demands for lower cost, smaller size and higher

efficiency will continue to further expand the range of inverter applications. However, as a trend

towards eco-friendly products increases, some sort of measure is necessary to suppress the

harmonics contained in the inverter input current.

A matrix converter is capable of converting an input voltage directly into

an arbitrary AC voltage, instead of converting that voltage into a DC voltage as inverters. This

matrix converter has higher efficiency, smaller size, longer lifespan and fewer input current

harmonics than inverters and has high potential for realizing the above mentioned demands.

A three phase AC – DC – AC sparse matrix converter (SMC) having no

energy storage elements in the DC link and employing only 15 IGBTs as opposed to 18 IGBTs

of a functionally equivalent conventional AC-AC matrix converter (CMC) is proposed. The

realization effort could be further reduced to only 9 IGBTs (Ultra Sparse Matrix Converter,

USMC) in case the phase displacement of the fundamentals of voltage and current at the input

and at the output is limited to ±n/6. The dependency of the voltage and current transfer ratios of

the systems on the operating parameters is analyzed and a space vector modulation scheme is

described in combination with a zero current commutation procedure.

EVOLUTION OF MATRIX CONVERTER

The inverter is a well known device that converts an input AC voltage into

a DC voltage by a rectifier, and then controls the semiconductor switch of a PWM inverter to

convert the DC voltage into the desired AC voltage. A voltage smoothing capacitor is required

in the DC link circuit, and an electrolytic capacitor is typically used for this purpose.

The Sparse Matrix Converter is an AC/AC converter which offers a

reduced number of components, a low complexity modulation scheme, and low realization

effort. Invented in 2001 by Prof Johann W Kolar, sparse matrix converters avoid multi step

commutation procedure of the conventional matrix converter, improving system reliability in

industrial operations. Its principle applications are in highly compact integrated AC drives.

The matrix converter arranges semiconductor switches into a matrix

configuration and controls them to convert an input AC voltage directly into the desired AC

voltage. Since the input AC voltage is not converted to a DC voltage, there is no need for an

energy storage device such as an electrolytic capacitor. Bi directional switches are needed as the

semiconductor switches, since an AC voltage is impressed on it.

Fuji Electric is developing a matrix converter capable of converting an

input voltage directly into an arbitrary AC voltage, instead of converting that voltage into a DC

voltage as inverters. This matrix converter has higher efficiency, smaller size, longer lifespan

and fewer input current harmonics than inverters and has high potential for realizing the above

mentioned demands.

Fig. 1 Inverter and matrix converter

As can be seen in Fig. 1 (a), the inverter require charge-up circuit to

suppress the inrush current that flows to the electrolytic capacitor connected to the DC link

circuit. If a diode rectifier is used as the rectifier, a large amount of input current harmonics will

be generated and therefore, a DC reactor (DCL) is inserted to reduce the current harmonics in the

input current. In a conventional inverter, it is necessary to connect a braking unit to the DC link

circuit in order to dissipate the regenerated power from the motor. A PWM rectifier was often

used to reduce the input current harmonics and to realize motor regeneration. The matrix

converter, on the other hand, is able to realize motor regeneration with almost no input current

harmonics. In other words, a single converter unit is able to provide performance equivalent to

that of a PWM rectifier and an inverter. Additionally, the charge-up circuit is unnecessary since

the large electrolytic capacitor is not needed for the matrix converter. As a result, smaller size

and longer lifespan can be achieved. In Fig. 2, a matrix converter system is compared to a

conventional system that uses a PWM rectifier and an inverter. The conventional system needs a

filter capacitor, a filter reactor and a boost-up reactor in addition to a main unit. The matrix

converter system, however, only needs a main unit and a filter reactor. Therefore, the

configuration becomes simple and a panel size of the system can be reduced by half or more. In

addition, since the matrix converter uses one-stage AC-AC direct conversion, a low loss system

can be realized, achieving at least 1/3 lower loss than in the conventional system.

Fig. 2 Comparison of matrix converter with the conventional system

PRINCIPLE

The matrix converter consists of 9 bi-directional switches that allow any

output phase to b connected to any input phase. The circuit scheme is shown in Fig. 3.

Fig. 3 Circuit scheme of a three phase to three phase matrix converter

a, b, c are at the input terminals. A, B, C are at the output terminals.

The input terminals of the converter are connected to a three phase voltage

fed system, usually the grid, while the output terminal are connected to a three phase current fed

system, like an induction motor might be. The capacitive filter on the voltage fed side and the

inductive filter on the current fed side represented in the scheme of Fig. 3 are intrinsically

necessary. Their size is inversely proportional to the matrix converter switching frequency.

It is worth noting that due to its inherent bi-directionality and symmetry a

dual connection might be also feasible for the matrix converter, i.e. a current fed system at the

input and a voltage fed system at the output.

With nine bi-directional switches, the matrix converter can theoretically

assume 512 (29) different switching states combinations. But not all of them can be usefully

employed. Regardless to the control method used, the choice of the matrix converter switching

states combinations (from now on simply matrix converter configurations) to be used must

comply with two basic rules. Taking into account that the converter is supplied by a voltage

source and usually feeds an inductive load, the input phases should never be short circuited and

the output currents should not be interrupted. From a practical point of view these rules imply

that one and only one bi-directional switch per output phase must be switched on at any instant.

By this constraint, in a three phase to three phase matrix converter, 27 are the permitted

switching combinations.

The Output voltage

Since no energy storage components are present between the input and

output sides of the matrix converter, the output voltages have to be generated directly from the

input voltages. Each output voltage waveform is synthesized by sequential piecewise sampling

of the input voltage waveforms. The sampling rate has to be set much higher than both input and

output frequencies, and the duration of each sample is controlled in such a way that the average

value of the output waveform within each sample period tracks the desired output waveform. As

consequence of the input – output direct connection, at any instant, the output voltages have to fit

within the enveloping curve of the input voltage system. Under this constraint, the maximum

output voltage the matrix converter can generate without entering the over modulation range is

equal to v312 of the maximum input voltage. This is an intrinsic limit of matrix converter and it

holds for any control law.

Entering in the over modulation range, thus accepting a certain amount of

distortion in the output voltages and input currents, it is possible to reach higher voltage transfer

ratio.

In fig. 4 the output voltage waveform of a matrix converter is shown and

compared to the output waveform of a traditional voltage source inverter (VSI). The output

voltage of a VSI can assume only two discrete fixed potential values, those of the positive and

negative DC bus. In the case of the matrix converter, the output voltages can either input voltage

a, b or c and their value are not time invariant and the effect is a reduction of the switching

harmonics.

Fig. 4 Output voltage waveforms generated by a VSI and a matrix converter

The Input Current

Likewise to the output voltages, the input currents are directly generated

by the output currents, synthesized by sequential piecewise sampling of the output current

waveforms. If the switching frequency of the matrix converter is set to a value that is much

higher than the input and output frequency, the input currents drawn by the converter are

sinusoidal. Their harmonic spectrum consists only of the fundamental desired component plus a

harmonic content around the switching frequency.

In Fig. 5 the input current drawn by a matrix converter for a 2 kHz

switching frequency is shown. It can be noted that the amplitude of the switching harmonic

components is comparable to the fundamental amplitude. It is then obvious that an input filter is

needed in order to reduce the harmonic distortion of thee input line current to an acceptable

level. It follows that care should be taken in speaking about matrix converters as an “all silicon”

solution for direct AC/AC power conversion, since some reactive components are needed.

Fig. 5 Matrix converter input current and harmonic spectrum. Switching frequency 2 kHz.

The matrix converter performance in terms of input currents represent a

significant improvement with respect to the input currents drawn by a traditional VSI converters

with a diode bridge rectifier, whose harmonic spectrum shows a high content of low order

harmonics. By the light of the standards related to power quality and harmonic distortion of the

power supply this is a very attractive feature of matrix converter.

The Input Power Factor Control

The input power factor control capability is another attractive feature of

matrix converters, which holds for most of the control algorithms proposed in literature. Despite

of this common capability, it is worth noting that a basic difference exists with respect to the load

displacement angle dependency.

Fig. 6 matrix converter input line-to-neutral voltage, instantaneous input current and its average value.

Switching frequency 2 kHz

For instance, the algorithm proposed does not require the knowledge of

the load displacement angle in order to fully control the input power factor. On the contrary, the

algorithm does require the knowledge of thee load displacement angle whenever the reference

input power factor is different from unity. From an algorithm computational burden point of

view, this is a drawback, since it implies additional quite heavy calculations.

NEW TECHNOLOGIES FOR THE PRACTICAL APPLICATION OF

MATRIX CONVERTERS

The circuit configuration and operating principles of the matrix converter

have been known for some time, but there are many problems in achieving practical application.

The new technologies that solved these problems are introduced below.

Technology for realizing a reverse blocking IGBT

Table 1 shows the bi-directional switches that are used in matrix

converter. An AC voltage is impressed on the bi-directional switches. Because conventional

semiconductor switch such as IGBTs do not have reverse blocking capability, diodes for reverse

blocking are needed as shown in Table 1 (a). The problem with this diode, however, was that it

increased on-state loss and decreased efficiency.

Table 1 Bi-directional switches

In order to solve this problem, Fuji Electric is developing a new IGBT

having reverse blocking capability (RB-IGBT). Under a reverse bias, the conventional IGBT

generates a large leakage current because its depletion region extends to the dicing surface at the

chip side, where severe strain exists after the mechanical dicing process. In the newly developed

RB-IGBT, a deep isolation region is formed in the dicing area to prevent expansion of the side

surface of the depletion region and to ensure the reverse blocking capability. Recent advance in

IGBT manufacturing technology have enabled the realization of the device. The RB-IGBT has

the same basic structure as the conventional IGBT, and thus their characteristics are also similar.

Moreover, the reverse recovery characteristics are also similar. .Moreover, the reverse recovery

characteristic of the RB-IGBT is approximately the same as that of the conventional diode.

Fig. 7 Comparison of the matrix converter losses

Fig. 7 compares the loss of matrix converters with each of bi-directional

switches shown in table 1(a) and 1 (b). By using the RB-IGBT, the on state loss of a series

connected diode is eliminated and although the switching loss remains nearly the same, on state

loss can be reduced by approximately 30%.

Protection Technology

Fig. 8 shows the commutation and protection circuit of the matrix

converter. Commutation is the process wherein the current flowing to a switch Sa for example, is

transferred by turning on a switch Sb and turning off a switch Sa so as to transfer that current to

switch Sb. The switch must be controlled, so that there is no short circuit and the load current is

not interrupted. If the load current is interrupted, a large surge voltage is impressed upon the

semiconductor switch and the switch is damaged.

Therefore, similar to conventional PWM inverter, dead time is provided to

prevent a short circuit condition and surge voltage generated during this dead time interval is

absorbed by a protection circuit. As a result, loss increases and the protection circuit grow in

size, as it requires a large electrolytic capacitor to absorb energy. This reduced advantage of the

matrix converter.

Fig. 8 Commutation and protection circuit

The commutation problem is solved by controlling the two RB-IGBTs that

compose a bi-directional switch independently. In other words, by keeping a reverse biased

switch constantly in its on state, the device is made to behave the same as the freewheeling diode

in the conventional PWM inverter, and the load current is not interrupted. The forward biased

switch is turned on and off with dead time and controlled similar to a conventional PWM

inverter to prevent a short circuit condition. For example, in Fig. 8, if VRS > 0, San and Sbp are

reverse biased and therefore are always turned on, while Sap and Sbn are turned on and off with

dead time. As a result, while short circuit conditions are being prevented, interruption of the

load current is also prevented and the current is commutated safely. In addition, a protection

circuit is necessary to protect the device from over current and/or overvoltage. An electrolytic

capacitor is generally used in the protection circuit to absorb energy stored in the load. However,

using the electrolytic capacitor for the protection circuit reduces the advantage of the matrix

converter. To overcome the problem, a new protection circuit is developed. The new protection

circuit dissipates the load energy quickly without absorbing the energy to the capacitor. As a

result, the electrolytic capacitor is not necessary.

Control Technology

With the matrix converter, simultaneous control of the output voltage and

input current is possible, but simultaneous and independent control is not easy to implement.

The control method becomes complicated because switching one bi-directional switch in order to

output a certain voltage causes the change of the input current condition. The higher speed,

higher performance and lower cost of control devices in recent years, however, have made it

possible to realize even complicated control with ease. In the conventional control method for a

matrix converter, the pulse pattern for each bi-directional switch is calculated directly from the

condition for obtaining the desired AC output voltage and the condition in which the input

current becomes a sinusoidal wave. The control method is unique to the matrix converter and is

capable of outputting various pulse patterns. However, since the pulse pattern is calculated

directly, it is difficult to control the input current and the output voltage independently.

Fig. 9 Control method for the matrix converter

Then, a new control method was developed, and is shown in Fig. 9. This

method is based on the virtual indirect control of a virtual PWM rectifier and a virtual PWM

inverter. The matrix converter pulse pattern is obtained by synthesizing the pulse patterns of the

virtual PWM inverter and the virtual PWM rectifier. This method enables the input current and

output voltage to be controlled independently. In addition, since this control method can be

implemented as a direct extension of the control of the conventional PWM inverter, techniques

developed in the past can be applied largely without change. The virtual indirect method

controls the input current and output voltage, and as shown in Fig. 10, assumes a virtual

converter comprised of a virtual PWM rectifier and a virtual PM inverter.

Fig. 10 Principle of virtual indirect control method

The virtual indirect control method is based upon the principle that states,

“In a three phase power converter, if the final input and output connections relations are made

equal, then the input and output waveforms will not depend on circuit topologies.” In Fig. 10,

for example, if there exist intervals during which the virtual rectifier turns on S rp and Stu, and the

virtual PWM inverter turns on switches Sup, Svp and Swn, then the input and output connection

relations will be such that R-phase is connected to U-phase and V-phase, and T-phase is

connected to W-phase. Consequently, the matrix converter similarly turns on switches S ru, Srv

and Stw'. As a result, R-phase is connected to U-phase and V-phase, and T-phase is

connected to W-phase, and the operation of the matrix converter becomes same as that

of the conventional PWM system.

Fig. 11 Input and output waveforms

Fig. 11 shows waveforms of the matrix converter with the virtual

indirect control method. The load is an induction motor. Unity power factor of the

input is observed, and good sinusoidal waveforms were obtained for both the input and

output currents.

Fig. 12 Input power factor and THD vs. load torque

Fig. 12 shows the input power factor and total harmonic distortion

(THD) of the input current versus load torque. The input power factor is more than

99% at 50% load torque or higher. THD of the input current is also less than 10% at

50% load torque or higher.

Fig. 13 Acceleration and deceleration characteristics

(100 r/min → 1,200 r/min → 100 r/min)

Fig. 14 Impact load torque characteristic

(0% → 100% → 0%)

Figs 13 and 14 show waveform of the acceleration deceleration

characteristic and impact load torque characteristic, respectively, in the case of using

the vector control method for the induction motor control. The magnetizing current

remains constant even when the torque current changes and it can be verified that

vector control achieves good results, similar to those of the conventional motor

control. Moreover, during deceleration it can b seen that input current increases and

power is regenerated.

BLOCK DIAGRAM

BLOCK DIAGRAM DESCRIPTION

The decade counter is one of the most important components in the matrix converter configuration. The decade counter used here is CD4017. It has sixteen pins. The switching signals are provided by the decade counter. The signals to the decade counter are provided by the 555 IC used as an astable multivibrator.

The optical isolator is used to isolate the DC from the AC stage. The switches used are TIP127 transistors. A 12V, 50Hz AC supply is converted to a higher frequency by the switches.

CIRCUIT DIAGRAM

DESCRIPTION

The circuit diagram of the matrix converter has 3 main functional sections. The three functional sections are:

Switching frequency source

Decade Counter

Switches

Switching Frequency Source

The switching frequency source is the 555 timer IC. It provides the switching frequency. The frequency range of the timer IC is from 50Hz to 3.5 kHz. The output from pin 3 of the 555 timer IC is fed to the counter IC.

Decade Counter

CD4017 is used as the decade counter. It is a 16 pin IC. It has 10 outputs and a carry. The output from the decade counter is configured to a 3 pin out system and to the fourth output pin RESET is applied. Any three of the output pins can be used for the output configuration required for the matrix converter. The pins 3, 2 and 4 have been used as the output in the circuit diagram. The pin number 7 is set as the RESET.

Each of the three pins provides an output frequency with the time period of fraction of a millisecond. This output frequency is fed to the switching transistors via an optical isolator. IS4N35 is the optical isolator used. The optical isolator is used only to provide isolation between the input and the output sides.

Switches

TIP 127 is used as switches. TIP 127 is a pnp configuration transistor. The AC from the supply is fed directly to the switches. The switches turn ON and OFF depending upon the output from the decade counter. Depending on the ON and OFF condition of the switches, the AC current from the supply splits. This split in the AC gives a high frequency characteristic to the output wave.

A WAVEFORM FILE

PCB LAYOUT

COMPONENTS

details

ADVANTAGES

Sinusoidal i/p and o/p waves with min harmonics.

Input power can be fully controlled.

Loss is one third of that in conventional converters.

Minimum energy storage requirements, no more bulky, limited energy storage capacitors hence there is possibility of a more compact design

Adjustable (including unity) power factor

Bidirectional power flow

High quality waveform

DISADVANTAGES

Higher complexity in modulation and analysis effort

Requires more semiconductor devices

Sensitive to disturbances of the i/p voltage system

APPLICATIONS

Implementation as power supplies

Realization of highly compact AC drives

High potential in industrial, military, marine and avionics

Employed in Wind Energy Conversion Systems(WECS)

FUTURE SCOPE

Matrix converters can be seen as a future replacement concept for variable speed drives technology

Future applications in fields that now use PWM rectifiers and inverters.

CONCLUSION

As proposed, the full functionality of a 3 phase AC-AC converter can be achieved by using a few number of switches, and the absence of a dc link with high efficiency.

REFERENCES

[1]. J. W. Kolar, M. Baumann, F. Stogerer, F. Schafmeister, H. Ertl, “Novel Three-Phase AC-DC-AC Sparse Matrix Converter”.

[2]. L. Wei, T. A. Lipo, H. Chan, “Matrix Converter Topologies with Reduced Number of Switches”.

[3]. F. Schafmeister, “Sparse and Indirect Matrix Converter”.

[4]. J. W. Kolar, F. Schafmeister, S. D. Round, and H. Ertl, “Novel Three-Phase AC-AC Sparse Matrix Converters”.

[5]. Wikipedia, National semiconductors, Toshiba power supplies.