Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2000-04-06
2001-11-06
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C363S045000
Reexamination Certificate
active
06313602
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The technical field of this invention is electric motor control.
BACKGROUND OF THE INVENTION
Power converters of the AC-DC-AC type are widely used in motor drives and power supply applications. These AC-DC-AC converters consist of a rectifier-inverter system along with a DC voltage link. The DC link is normally equipped with a large electrolytic capacitor which provides the stiff ripple free DC bus voltage required for proper inverter operation. However, this DC link capacitor is a large, heavy, and expensive component which continues to be a matter of concern in the industrial environment. Moreover, the DC bus capacitor is the prime factor in degradation of system reliability. This has been the driving motivation for much work aimed at decreasing the required size of this DC link capacitor.
A number of techniques have been reported which have been aimed at the complete elimination of the link capacitor, or reduction in its size. The implementation of these methods have required significant changes in the well known, extremely simple, and reliable rectifier-inverter power circuit configuration, and have involved complex control circuitry with added concerns of system stability. Some implementations have even involved the use of expensive floating point digital signal processing elements. In general, these solutions are not applicable to low cost, high volume applications such as fans, blowers and pumps.
Pulse Width Modulation (PWM) techniques have long been used to improve the performance and reliability of power conversion devices. Within the past decade such devices have improved steadily, yet the capacitor size and expense issue, as well as the reliability issue have remained paramount. This invention provides another level of improvement in power conversion devices by unique modifications to the PWM technique.
PWM in its basic form as applied to power inverters is illustrated in
FIG. 1
, the schematic of a conventional three-phase voltage source inverter. The diode bridge rectifier circuit (components
151
,
152
,
153
, and
154
) receives its power input from the AC power source
140
, and its output is partially filtered by means of the DC link capacitor
150
. The phase A, phase B, and phase C inverter outputs are derived from the inverter configuration comprised of N-channel MOSFET transistors
100
to
102
and
110
to
112
. These output signals appear at the three nodes
190
,
191
and
192
. Providing appropriate drive signals for the gates of transistors
100
to
102
and
110
to
112
is the key to generating proper inverter output voltage. Diodes
160
to
162
and
170
to
172
act to clip any occurrences of reverse voltage at the drain to source of the inverter transistors
100
to
102
and
110
to
112
.
The basic PWM equations which relate to the inverter switching functions, essentially the gating signals at the gate of transistors
100
to
102
and
110
to
112
will now be described. Let SW
1
, SW
2
, and SW
3
be the inverter switching functions. The Fourier series expansions of the switching functions can be written as:
SW
_
=
[
SW1
SW2
SW3
]
=
[
∑
n
=
1
A
n
sin
(
n
ω
i
t
)
∑
n
=
1
A
n
sin
n
(
ω
i
t
-
120
°
)
∑
n
=
1
A
n
sin
n
(
ω
i
t
+
120
°
)
]
[
1
]
where &ohgr;
i
is the inverter operating frequency. The switching function for any PWM scheme consists of the fundamental frequency component, (n=1), and higher order harmonics. For a ripple free dc bus voltage V
i
=V
DC
, the inverter line to neutral output voltages are given by:
V
n
=
[
V
an
V
bn
V
an
]
=
V
dc
×
SW
_
[
2
]
The line to line voltage at the inverter output is given by:
V
L
_
=
[
V
an
-
V
bn
V
bn
-
V
cn
V
cn
-
V
an
]
=
3
V
DC
[
∑
n
=
1
A
n
sin
n
(
ω
i
t
+
π
/
6
)
∑
n
=
1
A
n
sin
n
(
ω
i
t
-
π
/
2
)
∑
n
=
1
A
n
sin
n
(
ω
i
t
+
5
π
/
6
)
]
[
3
]
From equation 3 it is clear that the output voltage contains only the fundamental and the higher order harmonics present in the switching function. However, ripple components in the DC bus voltage of the inverter will have an effect on the AC output voltage.
To understand the effect of the ripple component, consider a case in which the DC bus voltage is not ripple free, as is the case when a smaller capacitor is used in the DC link. Assume that the DC voltage contains sinusoidally varying component of frequency &ohgr;
t
, and of magnitude kV
DC
. Then the inverter input voltage can be represented as:
V
i
=V
DC
(1+k sin &ohgr;
t
t) [4]
The line to neutral inverter output voltage can be computed from equation 2 and is given by:
{overscore (V
i
+L )}=V
DC
(1+k sin &ohgr;
t
t) {overscore (SW)} [5]
The inverter line to line voltage can be obtained from equations 3 and 5, and is given by:
V
ab
=
3
V
DC
∑
n
=
1
A
n
sin
n
(
ω
i
t
+
π
/
6
)
+
3
V
DC
∑
n
=
1
[
A
n
(
k
/
2
)
sin
(
(
ω
t
-
n
ω
i
)
t
-
n
π
/
6
)
]
-
3
V
DC
∑
n
=
1
[
A
n
(
k
/
2
)
sin
(
(
ω
t
-
n
ω
i
)
t
+
n
π
/
6
)
]
[
6
]
From equation 6 it is evident that the DC bus voltage variation has a significant affect on the output voltage, due to the appearance of lower order harmonics (&ohgr;
i
−n&ohgr;
i
) and (&ohgr;
i
−n&ohgr;
i
) not present in the switching function SW.
In order to counteract the DC bus voltage fluctuation, the PWM switching function needs to be altered so that a counter modulation is introduced in the inverter control. The modified switching function for the above described illustration would be:
SW
_
new
=
1
(
1
+
k
sin
ω
r
t
)
SW
_
[
7
]
Using equation 5, the inverter line to neutral voltage then becomes:
{overscore (V)}n=V
DC
(1+k sin &ohgr;
t
t) {overscore (SW)}
new
[8]
Substituting {overscore (SW)}
new
from equation 7 into equation 8, gives:
{overscore (V)}
n
=V
DC
×{overscore (SW)} [9]
Equation 9 represents the inverter line to line voltage with a ripple component in the DC bus voltage. However, equation 9 is identical to equation 2, in which the DC bus voltage was assumed to be ripple free. Therefore, by employing the proposed technique, i.e. by suitably altering the inverter switching function (SW), immunity to the DC bus voltage ripple component can be achieved. In this invention the conventional space vector PWM technique is modified to meet that goal.
PRIOR ART:
Space Vector Pulse Width Modulation (PWM)
One of the most common methods of PWM is based on the ‘space vectors’ of the inverter voltages. Space vectors of the line to neutral voltages are shown in FIG.
2
. These eight vectors may be understood with reference to the eight inverter states described in Table 1. Each inverter state represents a single combination of the states of inverter switching transistors
100
to
102
. Any one, two or all three of these transistors may be ‘on’ in the eight possible combinations given in Table 1. The state of each of the complementary transistors
110
to
112
is the opposite. For example if transistor
100
is ‘on’, transistor
110
is ‘off’, and so on for transistor complementary pairs
101
, the complement of
111
, and transistor
102
, the complement of transistor
112
. The eight vectors of
FIG. 2
relate to the instantaneous inverter states of Table 1.
TABLE 1
Eight Switching States of Voltage Source Inverter
Transistor
Transistor
Transistor
State
100
101
102
UPPER INVER
Arefeen Mohammed
Shireen Wajiha
Brady III W. James
Donels Jeffrey
Marshall, Jr. Robert D.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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