Method for controlling pwm pulse

Details Method for controlling pwm pulse Method for controlling pwm pulse

2002-11-18

2004-06-15

Nguyen, Matthew V. (Department: 2838)

Electric power conversion systems

Current conversion

With means to introduce or eliminate frequency components

C318S801000

Reexamination Certificate

active

06751105

ABSTRACT:

TECHNICAL FILED
The present invention relates to a pulse width modulation (hereinbelow abbreviated “PWM”) pulse control method in a power converter such as an inverter that implements system interconnection or variable-speed drive of a motor that is load, and more particularly to a PWM pulse control method for realizing lower noise of an inverter that performs three-phase drive.
BACKGROUND ART
We refer first to
FIG. 1
, which is a circuit diagram showing the configuration of a three-phase two-level PWM inverter. As shown in
FIG. 1
, a three-phase two-level PWM inverter is composed of dc power supply
101
, capacitor
102
, U-phase, V-phase, and W-phase output terminals
117
-
119
of a motor, semiconductor switching elements (for example, IGBT and GTO)
103
-
108
, and diodes
109
-
114
.
When semiconductor switching elements
103
,
105
, and
107
are turned ON, output terminals
117
-
119
for each phase are connected to positive bus
115
, which leads from the positive electrode of the dc power supply, and the output phase voltage of each phase becomes a high level (hereinbelow abbreviated “H”). When semiconductor switching elements
104
,
106
, and
108
are turned ON, output terminals
117
-
119
for each phase are connected to negative bus
116
, which leads from the negative electrode of the dc power supply, and the output phase voltage of each phase becomes a low level (hereinbelow abbreviated “L”).
Modulation methods for such a three-phase two-level PWM inverter include a three-phase modulation method and a two-phase modulation method. The three-phase modulation method realizes modulation by varying the voltage levels of all three phases. The two-phase modulation method realizes modulation by fixing the output phase voltage of one of the three phases to the high level (H) or low level (L) and then modulating the remaining two phases. In the two-phase modulation method, the fixed phase is switched by the phase of the output phase voltage.
In such a three-phase two-level PWM inverter, the two-phase modulation method is generally used when the modulation percentage and output frequency of the inverter are both high, and the three-phase modulation method is generally used when the modulation percentage and output frequency are low.
Since pulses of three phases are output at the same time in the three-phase modulation method, the width of PWM pulses can be made longer than in the two-phase modulation method. When the modulation percentage and the output frequency of the inverter are both low, the time interval in which the output voltage vector is a zero vector increases and the width of the PWM pulses of each phase therefore decreases.
Since the switching of the semiconductor switching elements cannot be followed when the width of the PWM pulses becomes too small, the three-phase modulation method, in which the PWM pulse width is greater, is employed when the output frequency of the inverter is low.
As for the prior-art PWM pulse control method of a PWM inverter in which the three-phase modulation method is employed, the phase state when output terminals
117
-
119
for each phase are connected to positive bus
115
is referred to as the first state (hereinbelow abbreviated “P”), and the phase state when output terminals
117
-
119
are connected to negative bus
116
is referred to as the second state (hereinbelow abbreviated “N”). Further, when the output states of the three phases in the order of U-phase, V-phase, and W-phase is (P, P, P), the output voltage vector is the Op-vector, and when the output states of the three phases is (N, N, N), the output voltage vector is the On-vector. The output voltage vector is the a-vector when the output states are (P, N, N), (N, P, N), and (N, N, P); and the output voltage vector is the b-vector when the output states are (P, P, N), (N, P, P), and (P, N, P). The a-vector is the output voltage vector when any one phase of the three phases is P, and the b-vector is the output voltage vector when any one phase of the three phases is N.
We now refer to
FIG. 2
, which is a timing chart showing the PWM pulse control method of the prior art. Triangular wave voltage
4
is the PWM carrier signal of the PWM inverter. Voltage commands
5
-
7
indicate voltage commands of the W-phase, the V-phase, and the U-phase, respectively. PWM pulse
1
of the U-phase, PWM pulse
2
of the V-phase, and PWM pulse
3
of the W-phase are shown beneath the signal and commands. The output terminals of each phase are connected to positive bus
115
and the output state of each phase is P when PWM pulses
1
-
3
are H; and the output terminals of each phase are connected to negative bus
116
and the output states of each phase is N when PWM pulses
1
-
3
are L. Since the cycle of voltage commands
5
-
7
is extremely long, the values of voltage commands
5
-
7
show virtually no fluctuation within a single cycle of triangular-wave voltage
4
.
In the PWM pulse control method of the prior art, PWM pulses
1
-
3
are each L when the value of triangular wave voltage
4
exceeds the value of each of the respective voltage commands
5
-
7
, and PWM pulses
1
-
3
are each H when the value of triangular wave voltage
4
falls below each of the respective values of voltage commands
5
-
7
. In this case, the output voltage vectors undergo transitions within one cycle of triangular wave voltage
4
in the order: Op-vector-b-vector-a-vector-On-vector-a-vector-b-vector-Op-vector.
We next refer to
FIG. 3
, which is a circuit diagram showing the constitution of a three-phase three-level PWM inverter. As shown in
FIG. 3
, a three-phase three-level PWM inverter is made up by: dc power supply
201
; capacitors
202
and
203
; U-phase, V-phase, and W-phase output terminals
117
-
119
of a motor; neutral point
252
, semiconductor switching elements
230
-
241
, and diodes
204
-
221
.
When semiconductor switching elements
230
and
231
,
234
and
235
, and
238
and
239
are turned ON, output terminals
117
-
119
of each phase are connected to positive bus
250
and the output phase voltage of each phase becomes H. When semiconductor switching elements
231
and
232
,
235
and
236
, and
239
and
240
are turned ON, output terminals
117
-
119
of each phase are connected to neutral point
252
and the output phase voltage of each phase becomes an intermediate level between H and L (hereinbelow abbreviated “M”). When semiconductor switching elements
232
and
233
,
236
and
237
, and
240
and
241
are turned ON, output terminals
117
-
119
of each phase are connected to negative bus
251
and the output phase voltage of each phase becomes L.
The three-phase three-level PWM inverter modulation method described in the foregoing explanation includes unipolar modulation and dipolar modulation. Unipolar modulation is a mode in which PWM pulses are output in which the output level of the output phase voltage is repeatedly H and M when the voltage command value is a positive value, and PWM pulses are output in which the output level of the output phase voltage is repeatedly M and L when the voltage command value is a negative value. Dipolar modulation is a mode in which a PWM pulse is output in which the output level of the output phase voltage repeatedly alternates between H and L on either side of M within one cycle of the PWM carrier signal regardless of whether the voltage command value is negative or positive.
In such a three-phase three-level PWM inverter, unipolar modulation is generally used when the output frequency and modulation percentage are both high, and dipolar modulation is generally employed when output frequency and modulation percentage are both low. This is because one side of the semiconductor switching elements repeatedly switch between ON and OFF over a long time period if unipolar modulation is used when the output frequency is low, raising the danger of breakdown of these semiconductor switching elements.
Explanation next regards the prior-art PWM pulse control method for a PWM inverter in which dipolar modulation is employed. Th

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