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THE MATRIX CONVERTER – A NEW CONCEPT IN AC VARIABLE SPEED DRIVE
TECHNOLOGY.
by W Law, Applications Engineer, Varispeed, a Division of Hudaco Transmissions (Pty) Ltd,
P O Box 4202, Halfway House, 1685, South Africa; e-mail: [email protected]
Abstract
This paper describes the implementation of matrix converter technology in a commercial AC variable speed drive. The
benefits in an AC variable drive application are compared to that of a conventional DC link pulse width modulated
(PWM) inverter. The content of this paper is intended to be descriptive in nature so as to provide an insight into this
new technology.
Introduction
The theory and design of AC variable speed drives using a matrix converter has been the subject of discussion and
research for over thirty years. From a mathematical view point the theory has proved to be feasible but a satisfactory
practical implementation has been the drawback due to difficulty in finding a power device that can switch an
alternating current in both positive and negative directions at the required speed. Early attempts at implementing such
an inverter used such devices as inverse parallel thyristors that required cumbersome commutating circuitry. This
approach proved to be impractical because of the complicated and bulky circuitry and limited switching times. The
limitation of the power switching device has now been overcome by the availability of a bi-directional AC switch. The
advantages that a matrix converter has over a conventional inverter are:

Direct AC to AC conversion without bulky DC capacitors.

Low input current harmonics, unity power factor.

Four quadrant operation, i.e. inherent regenerative braking and energy saving.

Continuous zero speed, full torque operation.

Reduced switching spikes at the motor terminals due to cable capacitance.
The power circuit of a matrix converter
The matrix converter allows a direct AC to AC conversion of an alternating current. It consists of nine bi-directional
power switches. There is no DC link and therefore no DC link capacitors.
The bidirectional switches consist of two sets IGBT’s with series blocking diodes connected in an inverse parallel
configuration as shown in Fig 1 below:
Fig 1.
The Bidirectional AC Switch
Nine bidirectional switches are used in the matrix converter. The name ‘matrix converter’ arises from the physical
appearance of the power circuit looking like a matrix and also and possibly more correctly the mathematical derivation
of the output voltage is a mathematical matrix. The main parts of the power circuits of a conventional PWM inverter
and a matrix converter are shown in Fig. 2a and 2b.
+
Q1
Q3
Q5
+
L1
L2
L3
U
V
W
Q2
Q4
M
Q6
_
RECTIFIER
DC LINK
CAPACITOR
INVERTER
Fig 2a
The Power Circuit of a Conventional PWM Inverter
L1
=
L2
L3
M
Fig 2b
The Power Circuit of a Matrix Converter
Performance of a matrix converter
Input Waveforms.
The matrix converter offers a considerable advantage over a conventional PWM inverter in terms of the input waveform
due to the low harmonic distortion present. This results in a unity power factor as seen by the supply, and hence
inherently energy saving occurs. Also the need for expensive and bulky input reactors to the drive falls away. An EMI
filter is however needed but it will not be as bulky as harmonic suppression devices. Fig. 3 shows a comparison
between the input voltage and current waveforms of a PWM inverter and a matrix converter. From the diagram it is
seen that phase current is virtually sinusoidal with no significant harmonic content, this is how unity power factor is
achieved.
Matrix Converter
Phase current
Phase Voltage
PWM Inverter
Fig. 3
The Input Waveforms of a PWM Inverter and a Matrix Converter
Output Waveforms.
The output waveform from a matrix converter is very similar to that from a PWM inverter, with only a slight
improvement. There is a reduction in the switching spikes at the motor which occur due to cable capacitance.
Fig. 4 shows a comparison between the output waveform of a PWM inverter and that of a matrix converter.
Phase current
Line – Line
Voltage
PWM Inverter
Matrix Converter
Fig. 4
The Output Waveforms of a PWM Inverter and a Matrix Converter.
A Comparison with Other Types of Inverters.
The table shown in Fig. 5 overleaf shows a comparison of various parameters that are of importance when considering
the overall performance of a variable speed drive system.
Diode Rectifier
+ PWM Inverter
6 stepped Converter
+ PWM Inverter
PWM Converter
+ PWM Inverter
Matrix Converter
Efficiency
Good
Moderate
Moderate
Good
Power Factor
Input Distortion
Poor
Moderate
Excellent
Excellent
Regen. Capability
Impossible
Good
Excellent
Excellent
Potential Level of
Line Voltage
3 Level
( +VPN, 0, -VPN )
3 Level
( +VPN, 0, -VPN )
3 Level
( +VPN, 0, -VPN )
Reliability
(Life Time)
Moderate
Moderate
Moderate
5Level (except VMAX≠ VMID)
( +VMAX, +VMID. 0, -VMID, -VMAX)
High
(No Electrolytic Capacitors)
Leakage Current
Moderate
Moderate
High
Low
Surge Voltage
High
High
High
Moderate
Size
Small
(No Input Filter)
Moderate
(Small Input Filter)
Large
(Large Input Filter)
Moderate
(Small Input Filter)
Topologies
Circuit Diagram
Fig. 5:
Comparison of Features of Different Types of Inverter.
Control principle
The output wave form is generated by the switching devices performing a chopping or sampling sequence on all three
input phases in sequence. A sample from each of the three input phases in any one time space are added together to
make one combined waveform of an output phase. Notice that the combined output wave is a multilevel appearance
similar to a three level inverter. The number of levels in practice is more than three and is dependant on the switching
frequency and sequencing of the power devices. This principle is explained graphically in Fig 6.
Input Phase Voltage
VR
0ﾟ
8
9 10 11 0
VT
VS
90ﾟ
1
2
270ﾟ
180ﾟ
3
4
5
6
7
8
360ﾟ
9 10 11 0
VR
VS
VT
VR
VS
VT
I
II
III
IV
V
Fig 6
Output Waveform Generation in a Matrix Converter.
The converter is capable of power flow from the supply to the motor and vice versa. It is fully regenerative and
therefore capable of four quadrant operation. The response time to load changes is also inherently very fast.
Output voltage quality
Motor Terminal Voltage.
Conventional inverters have the disadvantage that high voltage spikes appear at the motor terminals when long cables
are used. These spikes are generated by the rapid charging and discharging of the cable’s natural capacitance. Spikes
with a voltage of twice that of the DC link voltage in the inverter are generated and can lead to premature failure of
motor insulation.
The matrix converter also generates some spikes by the same means but their voltage is limited to less than 1,6 times
the DC link voltage that would have been present. This reduction in the magnitude of the spikes is a considerable
improvement. A comparison of the motor terminal voltage generated by a PWM inverter and a matrix converter is
shown in Fig. 7 below.
PWM Inverter
Matrix Converter
Fig 7.
Comparison of Motor Terminal Voltage in a PWM Inverter and a Matrix Converter.
Motor Shaft Voltage.
The fast switching nature of the output of both PWM inverters and matrix converters induce a voltage at the motor
shaft. This voltage causes leakage currents in the motor bearings which can cause electrolytic corrosion. Because the
magnitude of the steps in the shaft voltage of a motor drive by a matrix converter is half of that experienced with a
PWM inverter, the leakage current is reduced accordingly. Fig 8. shows a comparison between the typical shaft voltage
generated by a PWM inverter and a matrix converter.
[250V/DI
V]
[250V/DI
V]
+Vpn/
2
+Vpn/2
Vpn/2
[250V/DI
V]
PWM Inverter
Vpn/2
[250V/DI
V]
Matrix Converter
Fig 8.
Comparison of Motor Shaft Voltages in a PWM Inverter and a Matrix Converter.
Harmonics
All forms of converters cause harmonic distortion of the input current to some degree. A conventional PWM inverter
with no harmonic suppression can cause as total harmonic distortion (THD) of as much as 88%. With the addition of s
line reactor the THD will be reduced to about 39%. A matrix converter produces a THD of only 6% without the use of a
line reactor. A cost and space saving is therefore achieved. This is not to be confused with the EMC filter which is
necessary and is normally internal to the drive. The harmonic currents that can be expected from a matrix converter are
shown in Fig 9.
7
Harmonics [%]
6
5
4
3
2
1
0
5
7
11
13
17
19
23
25
29
31
35
37 total
Compornents
Fig 9.
Input Current Harmonics of a Matrix Converter.
Conclusion
The matrix converter, while initially may appear to be relatively expensive it will prove to be very economical when all
of its advantages are considered. The main advantages being inherent four quadrant operation, unity power factor and
low total harmonic distortion.
Further developments in switching devices may reduce the cost and improve the performance even more.
References
Information from Yaskawa Electric Corporation.