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Claims  |
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We claim:
1. A method for controlling a pulse width modulated inverter, the method
for eliminating unintended pulses in a PWM output signal, the method
comprising the steps of:
(a) a controller receiving a carrier signal and producing the PWM output
signal;
(b) determining whether the slope of the carrier signal is positive or
negative;
(c) permitting the output signal to change only from high to low when the
slope of the carrier signal is positive; and
(d) permitting the output signal to change only from low to high when the
slope of the carrier signal is negative.
2. The method as recited in claim 1 wherein the controller provides a PWM
input signal and the method further including the steps of determining if
the PWM output signal is high or low, determining if the PWM input signal
is high or low, and, the step of permitting the PWM output signal to
change from high to low includes the step of, if the PWM output signal is
high, and the PWM input signal is low, forcing the PWM output signal to go
low, and the step of permitting the PWM output signal to change from low
to high includes the step of, if the PWM output signal is low, and the PWM
input signal is high, forcing the PWM output signal to go high.
3. The method as recited in claim 2 wherein the controller also receives a
command signal and further including the step of producing the PWM input
signal prior to determining if the PWM input signal is high or low, the
PWM input signal being high when the command signal is greater than the
carrier signal and low when the command signal is less than the carrier
signal.
4. The method as recited in claim 3 further including the step of filtering
the carrier signal prior to producing the PWM input signal to remove high
frequency signals.
5. The method as recited in claim 1 wherein a carrier signal generator
provides a slop signal that is high when the carrier signal has a positive
slope and is low when the carrier signal has a negative slope and the step
of determining the slope of the carrier signal includes the step of
receiving the slope signal.
6. The method as recited in claim 5 further including the steps of
filtering the slope signal to produce a synchronized slope signal and
replacing the slope signal with the synchronized slope signal.
7. The method as recited in claim 1 wherein the controller also receives a
PWM input signal and the steps of permitting the PWM output signal to
change include the steps of:
(a) determining if the PWM output signal is high or low;
(b) if the carrier signal slope is positive and the PWM output signal is
low or if the carrier signal slope is negative and the PWM output signal
is high, maintaining the present PWM output signal; and
(c) if the PWM output signal and carrier signal are other than specified in
step (b), equating the PWM output signal to the PWM input signal.
8. A method for controlling a pulse width modulated inverter, the method
for eliminating unintended pulses in a PWM output signal, the controller
receiving a command signal and a carrier signal and producing a PWM input
signal that is high when the command signal is greater than the carrier
signal and is low when the command signal is less than the carrier signal,
the controller also producing the PWM output signal, the method comprising
the steps of:
(a) determining if the slope of the carrier signal is positive or negative;
(b) determining if the PWM output signal is high or low;
(c) determining if the PWM input signal is high or low;
(d) if the carrier signal slope is positive, the PWM output signal is high,
and the PWM input signal is low, allowing the PWM output signal to go low;
(e) if the carrier signal slope is negative, the PWM output signal is low,
and the PWM input signal is high, allowing the PWM output signal to go
high; and
(f) if the PWM output signal and carrier signal are other than specified in
steps (d) or (e), maintaining the present PWM output signal.
9. The method as recited in claim 8 wherein a carrier signal generator
provides a slope signal that is high when the carrier signal has a
positive slope and is low when the carrier signal has a negative slope and
the step of determining the slope of the carrier signal includes the step
of receiving the slope signal.
10. An apparatus for controlling a pulse width modulated inverter, the
apparatus used to eliminate excess PWM pulses in a PWM output signal, the
apparatus receives a carrier signal and produces a PWM output signal, the
apparatus comprises:
(a) a comparator to determine whether the slope of the carrier signal is
positive or negative; and
(b) a lockout circuit that permits the output signal to change only from
high to low when the slope of the carrier signal is positive and, permits
the output signal to change only from low to high when the slope of the
carrier signal is negative.
11. The apparatus as recited in claim 10 wherein the controller also
receives a command signal and produces a PWM input signal that is high
when the command signal is greater than the carrier signal and is low when
the command signal is less than the carrier signal, the apparatus further
comprising, an output module to determine if the PWM output signal is high
or low, an input module to determine if the PWM input signal is high or
low, and, the lockout circuit includes a command corrector that, if the
PWM output signal is high, and the PWM input signal is low, forces the PWM
output signal to go low, and, if the PWM output signal is low, and the PWM
input signal is high, forces the PWM output signal to go high.
12. The apparatus as recited in claim 10 wherein the lockout circuit also
receives a PWM input signal and the lockout circuit includes:
(a) an output module to determine if the PWM output signal is high or low;
and
(c) a lockout module that, if the carrier signal slope is positive and the
PWM output signal is low or if the carrier signal slope is negative and
the PWM output signal is high, maintains the present PWM output signal,
and, under any other circumstances, equates the PWM output signal to the
PWM input signal.
13. An apparatus for controlling a pulse width modulated inverter, the
apparatus used to eliminate excess pulses in a PWM output signal, the
controller receives a command signal and a carrier signal and produces a
PWM input signal that is high when the command signal is greater than the
carrier signal and is low when the command signal is less than the carrier
signal, the inverter also provides a slope signal that is high when the
slope of the carrier signal is positive and is low when the slope of the
carrier signal is negative, the apparatus receives both the PWM input
signal and the slope signal and produces the PWM output signal, the
apparatus comprises:
(a) an inverter module that receives the PWM input signal, PWM output
signal, and slope signal and produces an inverted input signal, an
inverted output signal and an inverted slope signal;
(b) a first summing module that receives the inverted input signal, the
slope signal and, the PWM output signal and produces a first sum signal
that is high when all of the signals received by the first summing module
are high and is low under any other condition;
(c) a second summing module that receives the PWM input signal, the
inverted slope signal, and the inverted output signal and produces a
second sum signal that is high when all of the signals received by the
first summing module are high and is low under any other condition;
(d) a tally module that receives both the first and second sum signals and
produces a tally signal that is low if both of the sum signals is low and
is high under any other circumstances; and
(e) a switching mechanism that receives the PWM input signal and the tally
signal and produces the PWM output signal, the switching mechanism being
enabled when the tally signal is high and disabled when the tally signal
is low, the PWM output signal imitating the PWM input signal when the
switching mechanism is enabled and remaining unchanged when the switching
mechanism is disabled.
14. The apparatus as recited in claim 13 wherein the summing modules are
AND gates, the tally module is an OR gate, the switching mechanism is a
flip-flop, and the inverter module includes three inverters that provide
the inverted PWM input, inverted PWM output, and inverted slope signals. |
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Claims |
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Description |
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BACKGROUND
1. Field of the Invention
The present invention relates to AC motors. More particularly, the present
invention relates to an apparatus to be used with a pulse width modulated
(PWM) inverter driving an AC motor to eliminate unintended voltage pulses
caused by discontinuities in a modulating command signal.
2. Description of the Art
Many motor applications require that the motor be driven at various speeds.
Motor speed can be adjusted with an adjustable speed drive (ASD) and which
is placed between a voltage source and an associated motor that can excite
the motor at various frequencies.
One commonly used type of ASD uses a pulse width modulated (PWM) inverter
and associated PWM controller which can control both voltage and frequency
of signals that eventually reach the stator windings of a motor. Referring
to FIGS. 2(a) and 2(b), a PWM controller receives both a modulating
command signal 16 and a carrier signal 18, compares the command and
carrier signals 16, 18 and produces a firing signal 20. When the command
signal 16 is greater than the carrier signal 18, the firing signal 20 is
high. When the command signal 16 is less than the carrier signal 18, the
firing signal 20 is low.
The firing signal 20 is used to control an associated PWM inverter. The
inverter produces a series of high frequency voltage pulses that excite
the stator windings of a motor. Referring also to FIG. 2(c), an exemplary
sequence of high frequency pulses 40 that an inverter might provide to a
motor can be observed along with an exemplary low frequency alternating
voltage 42 and related alternating current 44. The high frequency pulses
40 are positive when the firing signal 20 is high and negative when the
firing signal 20 is low.
By varying the widths of the positive portions 43 of each high frequency
pulse relative to the widths of the negative portions 45 over a series of
high frequency pulses 40, a changing average voltage 42 can be generated.
To produce a sinusoidal average voltage 42, a simple sinusoidal modulating
command signal 16 can be used.
The changing average voltage 42 defines the low frequency alternating
voltage that drives the motor. The low frequency alternating voltage 42 in
turn produces a low frequency alternating current 44 that lags the voltage
by a phase angle .phi.. As well known in the art, the motor operates at
the frequency of the alternating current 44.
By changing the frequency of the sinusoidal command signal 16, the
frequency of the alternating current 44, and thus the motor speed, can be
altered. For example, by increasing the frequency of the command signal
16, the frequency of the alternating current can be increased and motor
speed can in turn be increased. Motor speed can be decreased by decreasing
the sinusoidal command signal 16 frequency. In addition, by changing the
peak-to-peak amplitude variation of the command signal 16 while
maintaining a constant frequency, the amplitude of the stator winding
current can be altered.
In theory, a PWM inverter can be used to drive a motor accurately at
various motor speeds. In reality, however, due to controller-inverter
system noise and unforseen command signal discontinuities, often the
alternating voltage commnad produces excessive switching of the power
devices.
To correct for errors in motor current, many PWM controllers include a
feedback loop which compares actual motor current to desired motor current
and increases or decreases the command signal 16 in order to compensate
for current errors. While a properly designed feedback loop can correct
for noise and signal discontinuities, often an abruptly corrected command
signal can introduce greater error than it eliminates.
Referring to FIGS. 3(a) and 3(b), a corrected command signal 16' and a
carrier signal 18' along with a resulting firing signal 20' can be
observed. Prior to t.sub.1, the command signal 16' is greater than the
carrier signal 18' and the firing signal 20' is appropriately high. At
t.sub.1, the command signal 16' crosses the carrier signal 18' and the
firing signal 20' goes appropriately low.
At t.sub.2, the amplitude of the command signal 16' is increased 46 to
correct for a deviation between the desired and actual stator winding
currents. When corrected, the command signal 16' becomes greater than the
carrier signal 18' and the firing pulse 20' goes high. The firing pulse
20' remains high until the command signal 16' again crosses the carrier
signal 18' at t.sub.3. At t.sub.4 the command signal 16' again goes above
the carrier signal and the firing signal 20' goes high.
The command signal correction 46 at t.sub.2 results in a command signal
discontinuity at t.sub.2 and a "double crossing" between the carrier and
command signals 18', 16' first at t.sub.1 and then at t.sub.3. The double
crossing in turn produces an additional and unintended firing pulse 22.
Referring also to FIG. 3(c), the unintended firing pulse creates an
unintended additional high frequency pulse 47 which ultimately increases
the switching losses
Beside feedback, double crossings can be caused by command signal
discontinuities and generally by system noise. Together, noise, signal
discontinuities, and feedback corrections can produce enough double
crossings to produce imprecise motor operation to a degree which is
intolerable for many motor applications.
One way to eliminate the effects of double crossings is to employ a low
pass filter just prior to the comparator circuit. The low pass filter
allows the feed back loop to correct the command signal but slopes the
correction so that it takes place more gradually (i.e. the correction is
sloped so as to be less steep than the carrier signal).
Unfortunately, low pass filter operation is sensitive to motor application,
motor size, and the carrier frequency. Motor application and size affect
the amount of noise in a system and thus can instantaneously affect the
difference between carrier signal slope and the slope of a corrected
command signal. Similarly, carrier frequency clearly affects the
difference between carrier signal slope and the slope of a corrected
command signal. Thus, a low pass filter designed to operate under one set
of conditions cannot operate effectively in all applications to eliminate
the effect of double crossing signals.
Thus, it would be advantageous to have a method or apparatus which could
eliminate the effects of double crossing. Ideally, the method or apparatus
should be independent of motor application and size and independent of
carrier signal frequency.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus used with a motor
controller having a PWM inverter for eliminating discontinuities in a
stator current that are caused by noise, command signal discontinuities,
and feedback corrections. While noise, discontinuities, and corrections
can cause unintended double crossings between a carrier signal and a
command signal during any half cycle of a carrier signal, the present
invention only allows a single crossing during any carrier signal half
cycle to alter the applied voltage.
The method of the present invention is to be used with a motor controller
for controlling a pulse width modulated inverter. The method is used for
eliminating unintended pulses in a PWM output signal. The controller
receives a carrier signal and produces a PWM output signal. The method
comprises the steps of determining if the slope of the carrier signal is
positive or negative, if the slope of the carrier signal is positive,
permitting the output signal to change only from high to low, and if the
slope of the carrier signal is negative permitting the output signal to
change only from low to high.
Thus, one object of the present invention is to eliminate the possibility
of more than one change in the state of the output signal during each half
cycle of the carrier signal. If the output signal is only permitted to
change from high to low when the slope of the carrier signal is positive,
once the output signal goes low during a positive half cycle, it will
remain low during the remainder of the existing positive half cycle.
Similarly, if the output signal is only permitted to change from low to
high when the slope of the carrier signal is negative, once the output
signal goes high during a negative half cycle, it will remain high during
the remainder of the existing negative half cycle.
Importantly, the output signal controls the high frequency voltage pulses
provided to the motor. As only one high frequency voltage pulse is desired
during each carrier signal cycle, by limiting the state changes of the
output signal to once per half cycle, the intended number of high
frequency voltage pulses can be produced.
In a preferred method the controller provides a PWM input signal and the
method further includes the steps of determining if the PWM output signal
is high or low, determining if the PWM input signal is high or low and the
step of permitting the PWM output signal to change from high to low
includes the step of, if the PWM output signal is high, and the PWM input
signal is low, forcing the PWM output signal to go low, and the step of
permitting the PWM output signal to change from low to high includes the
step of, if the PWM output signal is low, and the PWM input signal is
high, forcing the PWM output signal to go high.
Also, preferably, the controller also receives a command signal and the
method further includes the step of producing the PWM input signal prior
to determining if the PWM input signal is high or low, the PWM input
signal being high when the command signal is greater than the carrier
signal and low when the command signal is less than the carrier signal.
The method may also include the step filtering the command signal prior to
producing the PWM input signal to filter out high frequency signal
components.
Yet another object of the present invention is to add a level of redundancy
to the correction method. A low pass filter can dampen any instantaneous
large changes in the slope of the command signal so that multiple
crossings occur less often.
Preferably, a carrier signal generator provides a slope signal that is high
when the carrier signal has a positive slope and is low when the carrier
signal has a negative slope and the step of determining the slope of the
carrier signal includes the step of receiving the slope signal.
The method may further include the steps of filtering the slope signal to
produce a synchronized slope signal and replacing the slope signal with
the synchronized slope signal.
Thus, another object of the invention is to make an accurate carrier signal
slope determination which is noise independent. Without a filter, the
slope signal might be instantaneously negative at some time during a
positive half cycle and instantaneously positive at some time during a
negative cycle. Clearly such deviations would limit the effectiveness of
the present invention as a double crossing that occurs during an
instantaneous slope signal deviation could produce unintended voltage
pulses at the stator windings. The filter eliminates high frequency
disturbances from the slope signal and thus is a true representation of
which half cycle, positive or negative, presently exits.
In another preferred method the controller also receives a PWM input signal
and the steps of permitting the PWM output signal to change include the
steps of determining if the PWM output signal is high or low, if the
carrier signal slope is positive and the PWM output signal is low or if
the carrier signal slope is negative and the PWM output signal is high,
maintaining the present PWM output signal and under any other
circumstances, equating the PWM output signal to the PWM input signal.
The present invention includes a method to be used with a motor controller
to control a pulse width modulated inverter, the method for eliminating
unintended pulses in a PWM output signal, the controller receiving a
command signal and a carrier signal and producing a PWM input signal that
is high when the command signal is greater than the carrier signal and is
low when the command signal is less than the carrier signal, the
controller also producing a PWM output signal, the method comprising the
steps of determining if the slope of the carrier signal is positive or
negative, determining if the PWM output signal is high or low, determining
if the PWM input signal is high or low, if the carrier signal slope is
positive, the PWM output signal is high, and the PWM input signal is low,
allowing the PWM output signal to go low, if the carrier signal slope is
negative, the PWM output signal is low, and the PWM input signal is high,
allowing the PWM output signal to go high, and under any other
circumstances, maintaining the present PWM output signal.
The present invention also includes an apparatus to be used with a motor
controller to control a pulse width modulated inverter for eliminating
unintended pulses in a PWM output signal. The apparatus receives a carrier
signal and produces a PWM output signal and comprises a comparator to
determine if the slope of the carrier signal is positive or negative, and
a lockout circuit that, if the slope of the carrier signal is positive,
permits the output signal to change only from high to low and, if the
slope of the carrier signal is negative, permits the output signal to
change only from low to high.
Preferably, the controller also receives a command signal and produces a
PWM input signal that is high when the command signal is greater than the
carrier signal and is low when the command signal is less than the carrier
signal, the apparatus further comprising an output module to determine if
the PWM output signal is high or low, an input module to determine if the
PWM input signal is high or low and, the lockout circuit includes a
command corrector that, if the PWM output signal is high, and the PWM
input signal is low, forces the PWM output signal to go low, and, if the
PWM output signal is low, and the PWM input signal is high, forces the PWM
output signal to go high.
The invention further includes an apparatus to be used with a motor
controller to control a pulse width modulated inverter for eliminating
excess PWM pulses in a PWM output signal. The controller receives a
command signal and a carrier signal and produces a PWM input signal that
is high when the command signal is greater than the carrier signal and is
low when the command signal is less than the carrier signal. The
controller also provides a slope signal that is high when the slope of the
carrier signal is positive and is low when the slope of the carrier signal
is negative. The apparatus receives both the PWM input signal and the
slope signal and produces a PWM output signal. The apparatus comprises an
inverter module that receives the PWM input signal, PWM output signal, and
slope signal and produces an inverted input signal, an inverted output
signal and an inverted slope signal, a first summing module that receives
the inverted input signal, the slope signal and, the PWM output signal and
produces a first sum signal that is high when all of the signals received
by the first summing module are high and is low under any other condition,
a second summing module that receives the PWM input signal, the inverted
slope signal, and the inverted output signal and produces a second sum
signal that is high when all of the signals received by the first summing
module are high and is low under any other condition, a tally module that
receives both the first and second sum signals and produces a tally signal
that is low if both of the sum signals is low and is high under any other
circumstances, and a switching mechanism that receives the PWM input
signal and the tally signal and produces the PWM output signal, the
switching mechanism being enabled when the tally signal is high and
disabled when the tally signal is low, the PWM output signal imitating the
PWM input signal when the switching mechanism is enabled and remaining
unchanged when the switching mechanism is disabled.
Other and further objects and aspects of the present invention will become
apparent during the course of the following description and by reference
by the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a motor control system including the
lockout circuit of the present invention;
FIG. 2(a) is a graph illustrating carrier and command signals, FIG. 2(b) is
a graph illustrating resultant firing pulses, and FIG. 2(c) is a graph
illustrating the high frequency pulses generated by a PWM inverter, the
resulting low frequency alternating voltage applied to, and an associated
alternating current provided to, a stator winding by a PWM inverter;
FIG. 3(a) is a graph illustrating a carrier signal and a corrected command
signal, FIG. 3(b) is a graph illustrating a resulting firing signal, and
FIG. 3(c) is a graph illustrating resulting high frequency pulses produced
by an inverter;
FIG. 4 is a circuit diagram illustrating switching devices which form an
inverter;
FIG. 5 is a diagram illustrating a filter circuit;
FIG. 6 is a schematic illustrating the lockout circuitry of the present
invention;
FIG. 7(a) is a graph illustrating a command signal and a carrier signal,
FIG. 7(b) is a graph illustrating a PWM input signal, FIG. 7(c) is a graph
illustrating an enable signal, and FIG. 7(d) is a graph illustrating a PWM
output signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will be described in the context of the exemplary
motor control system 10 and motor 12 shown in FIG. 1. To facilitate the
understanding of the present invention, the present invention is described
in relation to only a single phase of a three phase motor. It should be
understood that all three phases of a three phase motor are individually
controlled as described herein and that the present invention contemplates
control of all three phases.
The control system 10 consists of a comparator 27, a lock out circuit 31, a
PWM inverter 32, a feedback loop 17, and other components which will be
discussed below. The control system 10 receives an initial command signal
on line 14 indicating both the amplitude and frequency of a desired stator
winding current and produces an actual current at stator winding terminal
15 which excites the motor stator winding The actual current delivered at
node 15 often differs from the current prescribed by the initial command
signal on line 14.
To compensate for any discrepancies between the actual and desired stator
winding currents, the feedback loop 17 is provided which detects the
actual current at terminal 15 and provides an actual current signal on
loop 17 for comparison with the desired current signal on line 14. The
actual current is then corrected using a PI controller 24 in a mann | | |
