Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2001-09-20
2003-09-09
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S811000
Reexamination Certificate
active
06617821
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to motor controllers and more particularly to a method and an apparatus for compensating for switching device dynamics in inverter systems.
One type of commonly designed motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source providing time varying voltages across the stator windings. Often, an adjustable speed drive (ASD) will be positioned between the voltage source and the motor to control motor speed by controlling the stator voltages and frequency.
Many ASD configurations include a pulse width modulated (PWM) inverter consisting of a plurality of switching devices and a PWM controller. Referring to
FIG. 1
, an exemplary PWM inverter leg
10
corresponding to one of three motor phases includes two series connected switches
12
,
13
between positive and negative DC rails
18
,
19
and two diodes
16
,
17
, a separate diode in inverse parallel relationship with each switch
12
,
13
. By turning the switches
12
,
13
ON and OFF in a repetitive sequence, leg
10
receives DC voltage via rails
18
and
19
and provides high frequency voltage pulses to a motor terminal
22
connected to a stator winding
24
. By firing the switching devices in a regulated sequence the PWM inverter can be used to control both the amplitude and frequency of voltage that are eventually provided across windings
24
.
Referring to
FIG. 2
, an exemplary sequence of high frequency voltage pulses
26
that an inverter might provide to a motor terminal can be observed along with an exemplary low frequency alternating fundamental voltage or terminal voltage
28
and related alternating current
30
. By varying the widths of the positive portions
32
of each high frequency pulse relative to the widths of the negative portions
34
over a series of high frequency voltage pulses
26
, a changing average voltage which alternates sinusoidally can be generated. The changing average voltage defines the terminal voltage
28
that drives the motor. The terminal voltage
28
in turn produces a low frequency alternating current
30
that lags the voltage by a phase angle &phgr;.
The hardware that provides the firing pulses to the PWM inverter is typically referred to as a PWM controller. A typical controller includes, amount other things, a PWM generator. Various controller components receive a command operating frequency and convert the operating frequency into three phase modulating waveforms that are provided to the PWM generator. Referring to FIG.
3
(
a
), illustrative waveforms used by a signal generator to generate the firing pulses for leg
10
may be observed. As well known in the art, a carrier waveform
36
is perfectly periodic and operates at what is known as the carrier frequency. A modulating voltage waveform
38
is typically sinusoidal, having a much greater period than the carrier waveform
36
.
Referring also to FIGS.
3
(
b
) and
3
(
c
), an upper signal
40
and a lower signal
42
that control the upper and lower switches
12
,
13
respectively can be observed. The turn-on tu
1
, tu
2
and turn-off to 1, to 2 times of the upper and lower signals
40
,
42
come from the intersections of the modulating waveform
38
and the carrier waveform
36
.
When the modulating waveform
38
intersects the carrier waveform
36
while the carrier waveform has a positive slope, the upper signal
40
goes OFF and lower signal
42
goes ON. On the other hand, when the modulating waveform
38
intersects the carrier waveform
36
while the carrier waveform has a negative slope, the upper signal
40
goes ON and the lower signal
42
goes OFF. Thus, by comparing the carrier waveform
36
to the modulating waveform
38
, the state of the upper and lower signals
40
,
42
can be determined.
Referring also to FIGS.
2
and
3
(
d
), an ideal high frequency voltage pulse
26
resulting from the ideal upper and lower signals
40
,
42
in FIGS.
3
(
b
) and
3
(
c
) that might be provided at terminal
22
can be observed. When the upper signal
40
is ON and the lower signal
42
is OFF, switch
12
allows current to flow from the high voltage rail
18
to motor terminal
22
thus producing the positive phase
44
of pulse
26
at motor terminal
22
. Ideally, when the upper signal
40
goes OFF and the lower signal
42
goes ON, switch
12
immediately turns OFF and switch
13
immediately turns ON connecting motor terminal
22
and the low voltage rail
19
producing the negative phase
46
of pulse
26
at terminal
22
. Thus, the ideal high frequency voltage pulse
26
is positive when the upper signal
40
is ON and is negative when the lower signal
42
is ON. Also, ideally, the low frequency terminal voltage and corresponding current (see
FIG. 2
) should be completely sinusoidal and mirror the operating waveforms.
As well known in the motor controls art, when an inverter terminal is linked to an inductive load, the load current cannot reverse directions immediately upon switching of inverter switches and therefore the currents caused by the high frequency voltage pulses are at least partially smoothed by the inductive load. Early technology used to configure inverter switches was relatively rudimentary and therefore, not surprisingly, switching speed was relatively slow. In fact, despite the waveform smoothing load inductance, early inverters often caused appreciable amounts of terminal voltage ripple that, in some applications, was unacceptable. For this reason many early inverter configurations included complex and bulky filter configurations to smooth out the terminal voltages.
Recently much faster switches have been developed and adopted by the controls industry that reduce terminal voltage ripple and therefore, at least in some inverter configurations, substantially minimize the need for complex filter configurations. For instance, high speed IGBTs are capable of turning on or off in as little as several tens of nanoseconds (i.e., 50 nsec).
While fast switching IGBTs are now routinely used to configure inverters, in many cases the higher switching speeds have resulted in other adverse operating characteristics and operating phenomenon. One adverse characteristic is that some motors, due to their construction, resonate and generate ringing noise at frequencies corresponding to specific carrier frequencies. To address this problem several prior references teach that the carrier signal frequency can be reduced as the operating frequency is increased. To this end, see U.S. Pat. No. 4,691,269 and U.S. Pat. No. 5,068,777.
Other adverse operating phenomenon, including reflected waves, bearing currents, conducted interference, turn-on delays and radiated interference, have been effectively dealt with by modifying operating waveforms and in other ways calculated to compensate for associated distortions. These phenomenon that are already compensated for will be referred to generally as “other phenomenon”.
Unfortunately, while efforts to reduce the effects of these other phenomenon have reduced terminal voltage distortions, experience has shown that even after these efforts, under certain circumstances, appreciable terminal voltage distortion still occurs. While not well understood, some in the industry have generally recognized that these circumstantial distortions are due to complex capacitive and inductive interaction between inverter devices and other system components (e.g. supply lines, motor windings, filter devices, etc.). The causes of these circumstantial distortions are generally referred to herein as “parasitics” and the distortions as parasitic distortions.
FIGS. 4-6
demonstrate the effects of parasitics on drive performance.
FIG. 4
shows the phase current for a 10 hp industrial drive with full compensation for the other phenomenon with a load cable of approximate
Kerkman Russel J.
Leggate David
Schlegel David W.
Quarles & Brady LLC
Ro Bentsu
Rockwell Automation Technologies Inc.
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