NZ761486B2 - Manufacturing-sensitive control of high rotor pole switched reluctance motors - Google Patents
Manufacturing-sensitive control of high rotor pole switched reluctance motors Download PDFInfo
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Abstract
method for controlling switched reluctance machine (SRM) utilizing a SRM control system. The method allows for adaptive pulse positioning over a wide range of speeds and loads. An initial rotor position is provided for the SRM utilizing an initialization mechanism. A pinned point on a phase current waveform is defined during an initial current rise phase of the current waveform. A slope of the current rise is determined as the current waveform reaches the pinned point. The slope is then fed to the commutation module of the SRM control system. An error signal from calculated inductance or current slope is used as an input to a control loop in the SRM control system. The time determining module determines an optimum time signal to fire a next pulse. The optimum time signal is fed to the SRM for turning the plurality of SRM switches to on and off states. t waveform is defined during an initial current rise phase of the current waveform. A slope of the current rise is determined as the current waveform reaches the pinned point. The slope is then fed to the commutation module of the SRM control system. An error signal from calculated inductance or current slope is used as an input to a control loop in the SRM control system. The time determining module determines an optimum time signal to fire a next pulse. The optimum time signal is fed to the SRM for turning the plurality of SRM switches to on and off states.
Description
MANUFACTURING-SENSITIVE CONTROL OF HIGH ROTOR POLE
SWITCHED RELUCTANCE MOTORS
Inventors: Trevor Creary, Tim Knodel and Mahesh Krishnamurthy
Applicant: Software Motor Company
BACKGROUND OF THE DISCLOSURE
TECHNICAL FIELD OF THE DISCLOSURE
[0001] The present disclosure relates generally to methods for controlling switched
reluctance machines (SRMs), and more particularly to a method and system for
controlling SRM to enable adaptive pulse positioning over a wide range of speeds and
loads.
DESCRIPTION OF THE RELATED ART
A switched reluctance machine (SRM) is a simple type of electric motor that
operates by reluctance torque. SRM includes salient rotor and stator poles. There are
concentrated windings on the stator but no windings or permanent magnets on the rotor.
These features enable the SRM to achieve very high-speed relative to conventional non-
SRM motors. Since there are no windings in the rotor, power is only delivered to the
windings in the stator rather than the rotor, and due to this simple mechanical construction
SRMs offer lower maintenance costs relative to conventional electric motors. When
current is passed through the stator windings, torque is generated by the tendency of the
rotor poles to align with the excited stator pole. Continuous torque can be generated by
synchronizing each phase’s excitation with the rotor position. Accurate rotor position
information is essential for controlling the motor torque.
Several techniques have been proposed for position estimation using
inductance of the active or inactive phase. In most methods, a controlled signal is utilized
and may be applied to the phase winding to estimate inductance and thus determine rotor
position without the use of a position encoder. Certain other methods describe auto-
calibration of a motor. One such method describes a sensor-less rotor position
measurement system having a digital processor which receives signals from current and
flux sensors of the current and flux associated with a phase winding of the machine. The
measurement of the current and flux is enabled at a predicted reference rotor position.
Current and flux are sampled only once per energization cycle. This method is based on
position estimation methodology, which fails to provide absolute rotor position
information.
Another method describes a circuit for controlling a switched reluctance motor
through indirect sensing of rotor position within the switched reluctance motor. This
method measures time for the current to rise between two predetermined levels. The
measured current rise time can be compared to a desired current rise time to determine
whether conduction intervals in the motor phases are in-phase with the position of the
rotor or are lagging or leading the position of the rotor. However, this method utilizes
complex algorithms for calculating the current rise time.
Yet another method for controlling a switched reluctance electric machine
includes a switched reluctance electric machine having a sensor generating and
transmitting a sensor signal indicative of an operating characteristic, a controller
operatively coupled to the switched reluctance motor and the sensor and the controller
executing a method. Here, the sensor-less control of SRM is done by injecting a pulse of
voltage and measuring resultant current in the phase. However, this method injects
additional voltage pulses for controlling the switched reluctance electric machine.
There is thus a need for a method for controlling a switched reluctance machine
to achieve adaptive pulse positioning. Such a method would reduce manufacturing
imperfections and aging effects in the machine. Further, such a method would adjust
control parameters for each individual machine instead of the entire batch of manufactured
machines. Moreover, such a method would provide accurate rotor position information.
Such a method would utilize simple algorithms for calculating the current rise time.
Further, such a method would not inject additional voltage pulses for controlling the
switched reluctance electric machine. These and other objectives are accomplished by the
present embodiment.
SUMMARY OF THE INVENTION
To minimize the limitations found in the prior art, and to minimize other
limitations that will be apparent upon the reading of the specification, the preferred
embodiment of the present invention provides a switched reluctance machine (SRM)
control system that controls an SRM and enables adaptive pulse positioning over a wide
range of speeds and loads. The SRM control system includes an initialization module to
provide an initial rotor position for the SRM utilizing an initialization mechanism. A point
defining module in the SRM control system defines a pinned point on a phase current
waveform during an initial current rise phase of the current waveform. The defined pinned
point is static with respect to an underlying inductance value of the SRM.
Preferably, there are two options to determine a new pinned point in order to
handle a change in operating conditions and load torque profile. The first option depends
on the knowledge of inductance value for this current for the new operating condition or
can be calculated. And the second option is that, if simplifications in the control
methodology allow, only the slope of the current profile (desired current rise) over a fixed
time period based on this inductance is needed. The slope of the current (rise) is measured
as the waveform reaches the pinned current level.
A slope determining module in the SRM control system determines the slope
of the current rise as the current waveform reaches the pinned point. A commutation
module in the system is designed to receive the slope of the current rise from the slope
determining module and a frequency input signal. The SRM control system further
includes an error calculating module to calculate an error signal. The SRM control system
is designed to utilize the underlying inductance or the measured current rise to calculate
the error signal. In one configuration, the slope of the current rise is utilized to calculate
the underlying inductance that is used to calculate the error signal from the desired
inductance. In another configuration, the SRM control system is designed to utilize the
measured current rise over a fixed time period to calculate the error signal from a desired
current rise. The error signal from the calculated inductance or current slope is used as an
input to a control loop in the SRM control system. A time determining module determines
an optimum time to fire a next pulse.
The preferred method describes an overall control architecture of the SRM
control system. According to this control architecture, a reference speed or torque is
provided as an input to the system. The slope of the current rise is calculated as the current
waveform reaches the pinned point and fed to the commutation module. The underlying
inductance value is calculated utilizing the slope of the current rise. Frequency input signal
is the other input provided to the commutation module that gives a digital estimate for
shaft speed. A current speed is calculated utilizing the slope of the current rise and
frequency input signal. The error generator between the reference speed and the current
speed is processed through a regulator unit which generates a commanded current. The
regulator unit may be a proportional-integral (PI) regulator. The commanded current is
compared with a measured current by an inner current loop in the SRM control system.
Thereafter, pulse width modulation (PWM) signals are generated to create a plurality of
commutation angles for turning a plurality of switches of the SRM to on and off states
utilizing the time signals T , T .
on off
[0011] The preferred embodiment includes a method for controlling the SRM utilizing
the SRM control system. The method commences by providing the SRM control system.
Next, the initial rotor position is provided to the SRM utilizing the initialization
mechanism. Then, the pinned point on the phase current waveform is defined during the
initial current rise phase of the current waveform. Thereafter, the slope of the current rise
is determined as the current waveform reaches the pinned point. The slope is then fed to
the commutation module. Thereafter, the error signal from the calculated inductance or
current slope is used as an input to a control loop in the SRM control system. Finally, the
time determining module determines an optimum time signal to fire the next pulse. The
optimum time signal is fed to SRM for turning the plurality of SRM switches to on and
off states.
Optimum efficiency and greatest load capacity of the SRM is obtained when
the pinned point of the current waveform is near the top of the initial rise of the current
waveform and the point on the induction profile it is pinned to is near the start of the
induction rise for that phase of the machine.
It is a first objective of the present invention to provide an SRM control system
that enables accurate pulse positioning in a sensor-less environment.
A second objective of the present invention is to provide an SRM control
system for controlling an SRM that reduces manufacturing imperfections and aging
effects in the machine.
A third objective of the present invention is to provide an SRM control system
adaptable to adjust control parameters for each individual machine instead of the entire
batch of manufactured machines.
A further objective of the present invention is to provide an SRM control
system that utilizes simple algorithms for calculating the current rise time.
A still further objective of the present invention is to provide an SRM control
system that does not inject additional voltage pulses for controlling the switched
reluctance electric machine.
These and other advantages and features of the present invention are described
with specificity so as to make the present invention understandable to one of ordinary skill
in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to enhance their clarity and improve the understanding of the various
elements and embodiments of the invention, elements in the figures have not necessarily
been drawn. Furthermore, elements that are known to be common and well understood to
those in the industry are not depicted in order to provide a clear view of the various
embodiments of the invention. Thus, the drawings are generalized in form in the interest
of clarity and conciseness.
illustrates a block diagram of a switched reluctance machine (SRM)
control system in accordance with the preferred embodiment of the present invention;
illustrates a graphical representation showing change in the inductance
profile with respect to change in the electric angle in accordance with the preferred
embodiment of the present invention;
illustrates an overall control architecture of the SRM control system
with speed and current loops in accordance with the preferred embodiment of the present
invention;
illustrates a flowchart of a method for controlling the SRM utilizing
the SRM control system in accordance with the preferred embodiment of the present
invention;
[0024] illustrates an asymmetric bridge configuration for controlling phase
current of the SRM in accordance with the preferred embodiment of the present invention;
illustrates a pinned point at a current waveform of a three-phase SRM
in accordance with the preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
In the following discussion that addresses a number of embodiments and
applications of the present invention, reference is made to the accompanying drawings
that form a part hereof, and in which is shown by way of illustration specific embodiments
in which the invention may be practiced. It is to be understood that other embodiments
may be utilized, and changes may be made without departing from the scope of the present
invention.
Various inventive features are described below that can each be used
independently of one another or in combination with other features. However, any single
inventive feature may not address any of the problems discussed above or only address
one of the problems discussed above. Further, one or more of the problems discussed
above may not be fully addressed by any of the features described below.
As used herein, the singular forms "a", "an" and "the" include plural referents
unless the context clearly dictates otherwise. “And” as used herein is interchangeably used
with “or” unless expressly stated otherwise. As used herein, the term ‘about” means +/-
% of the recited parameter. All embodiments of any aspect of the invention can be used
in combination, unless the context clearly dictates otherwise.
Unless the context clearly requires otherwise, throughout the description and
the claims, the words ‘comprise’, ‘comprising’, and the like are to be construed in an
inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense
of “including, but not limited to”. Words using the singular or plural number also include
the plural and singular number, respectively. Additionally, the words “herein,” “wherein”,
“whereas”, “above,” and “below” and words of similar import, when used in this
application, shall refer to this application as a whole and not to any particular portions of
the application.
The description of embodiments of the disclosure is not intended to be
exhaustive or to limit the disclosure to the precise form disclosed. While the specific
embodiments of, and examples for, the disclosure are described herein for illustrative
purposes, various equivalent modifications are possible within the scope of the disclosure,
as those skilled in the relevant art will recognize.
illustrates a block diagram of a switched reluctance machine (SRM)
control system 10 for controlling an SRM 26. The SRM control system 10 enables
adaptive pulse positioning over a wide range of speeds and loads. The SRM control system
includes an initialization module 12 to provide an initial rotor position for the SRM 26
utilizing an initialization mechanism. In the preferred embodiment, the initialization
mechanism is adaptable to implement several approaches like hard alignment or any other
mathematical approach. A point defining module 14 in the SRM control system 10 defines
a pinned point on a phase current waveform during an initial current rise phase of the
current waveform. The defined pinned point is static with respect to an underlying
inductance value of the SRM 26 is a function of desired operating point. The PI controller
for the speed loop controls the amount of time from x (pinned point) to the turn on of the
next phase. If demand for speed changes then the demand for current also changes. This
means that the slopes are different and requires change in the pinned point. The pinned
point is defined at a specifically chosen magnitude of current between 50% and 100% of
the steady state current on the initial current rise, particularly when there is a sudden
change in the operating condition (load torque). This can also be useful in improving
accuracy as we enter single pulse mode and the current waveform begins to plateau as the
waveform gets closer to the aligned position. The goal is to have the pinned point low
enough or far enough from the curved profile of current.
Preferably, there are two options to determine a new pinned point in order to
handle a change in operating conditions and load torque profile. The first option depends
on the knowledge of inductance value for this current for the new operating condition or
can be calculated. And the second option is that, if simplifications in the control
methodology allow, only the slope of the current profile (desired current rise) over a fixed
time period based on this inductance is needed.
A slope determining module 16 determines a slope 42 (see of the
current rise as the current waveform reaches the pinned point. The slope of the current
(rise) is measured as the waveform reaches the pinned current level. As shown in the
graphical representation illustrated in if we change the current value, the
inductance profile also changes with it. This means the angle corresponding to the pinned
position must be changed until the same slope is arrived as was arrived at the previous
case.
[0034] A commutation module 18 is designed to receive the slope 42 of the current
rise from the slope determining module 16. The SRM control system 10 further comprises
an error calculating module 20 to calculate an error signal. The SRM control system 10 is
designed to utilize the underlying inductance or the measured current rise to calculate the
error signal. In one configuration, the slope 42 of the current rise is utilized to calculate
the underlying inductance which is used to calculate the error signal from the desired
inductance. In another configuration, the SRM control system 10 is designed to utilize the
measured current rise over a fixed time period to calculate the error signal from a desired
current rise. The error signal from the calculated inductance or current slope is used as an
input to a control loop 22 in the SRM control system 10. Finally, a time determining
module 24 determines an optimum time Ton, Toff 40 (see to fire a next pulse. The
optimum time Ton, Toff 40 turns a plurality of switches of the SRM 26 to on and off states.
In one configuration of the preferred embodiment, position is determined to fire a next
pulse.
shows an overall control architecture of the SRM control system 10
with speed and current loops. Here, a reference speed (Ref Speed) 32 or torque is provided
as an input to the system 10. Preferably, the proposed method for controlling SRM 26
utilizes a current feedback. The slope 42 of the current rise is calculated as the current
waveform reaches the pinned point and is fed to the commutation module 18. The
underlying inductance value is calculated utilizing the slope 42 of the current rise.
Frequency input signal Tp 44 is the other input provided to the commutation module 18
that gives a digital estimate for shaft speed. A current speed 36 is calculated utilizing the
slope 42 of the current rise and frequency input signal T 44. The error generator between
the reference speed 32 and the current speed 36 is processed through a regulator unit 30
that generates a commanded current (Iced) 34. The regulator unit 30 may be a proportional-
integral (PI) regulator. The commanded current 34 is compared with a measured current
(Iphase) 38 by an inner current loop in the SRM control system 10 to generate pulse width
modulation (PWM) signals. The PWM signals create a plurality of commutation angles
for turning the plurality of switches of the SRM 26 to on and off states utilizing the time
signals T , T 40.
on off
[0036] shows a flowchart of a method for controlling the SRM 26 utilizing the
SRM control system 10. As shown in block 50, the SRM control system having the
commutation module is provided. Next, the initial rotor position is provided to the SRM
utilizing the initialization mechanism as shown in block 52. Then, the pinned point on the
phase current waveform is defined during the initial current rise phase of the current
waveform as indicated at block 54. Thereafter, the slope of the current rise is determined
as the current waveform reaches the pinned point as shown in block 56. The slope is then
fed to the commutation module. Thereafter, the error signal from the calculated inductance
or current slope is used as an input to a control loop in the SRM control system as shown
in block 58. Finally, the time determining module determines an optimum time signal to
fire the next pulse as indicated at block 60. The optimum time signal is fed to SRM for
turning the plurality of SRM switches to on and off states.
shows an asymmetric bridge configuration typically used for
controlling the SRM 26. This configuration has each phase connected between two
switches T1, T2 which allows independent control and ensures that the inverter does not
have a shoot-through failure. The turn-on and turn-off signals 40 in are used to
control switches T1 and T2.
shows three current waveforms for a three-phase machine. In this
example, “x” is the pinned point of the current waveform in phase A of the machine. Here,
the pinned point is roughly at 80% of steady state current for the operating condition.
Optimum efficiency and greatest load capacity of the SRM is obtained when the pinned
point of the current waveform is near the top of the initial rise of the current waveform
and the point on the induction profile it is pinned to is near the start of the induction rise
for that phase of the machine.
In the current embodiment, the feedback from one commutation pulse is used
for the positioning of the next pulse. Instead, the feedback from this pulse could be used
to adjust the position of the next pulse in the same phase, or the next time that specific
stator rotor pole combination is reached, or anything in between.
Using each pulse to modify only pulses of the same phase has the benefit of
allowing phases to be adjusted independently due to non-uniform inductance on each
phase; however, the position feedback is slower by a multiple of the number of phases in
the machine. This could be overcome by using the error of the current pulse to input to
two control loops. Among the two control loops, one adjusts the current phase and the
other adjusts all of the phases allowing for both minor adjustments between phases while
still achieving rapid feedback to the main control methodology.
[0041] Using each pulse to modify only the same stator rotor poll combination has the
benefit that it allows adjustments to non-uniform pole positions, air-gap and inductance;
however, this position feedback is slower by a multiple of the number of phases times the
number of rotor poles. A similar methodology to the previous could be used to introduce
the extra degree of freedom while still maintaining rapid feedback.
[0042] In the current embodiment, an event base control loop was utilized. Any form
of control loop operating from the error between the desired inductance (or desired current
rise) and the measured inductance (or measured current rise) meets the intent of the
preferred embodiment.
In the current embodiment, the current was pinned on the initial rising edge of
the pulse; however, any point along an arbitrary waveform can be used as the pinned point.
In the current embodiment, the current rise was used at the specified point on
the current rise; however, at the desired waveform position, the phase could be switched
off or freewheeled and the current drop/decay at that point could be used in the same
manner to control position.
[0045] In the current embodiment, the output of the control loop is the desired time
between pulses and when the time from the last pulse is reached, the next pulse is fired.
The output of the control loop could also be tuned such that it is the desired position on a
software encoder which is being updated continuously based on the speed estimations.
This methodology induces further error because the software encoder is prone to drift due
to error in the speed measurements but would achieve the same effect. Similarly, a
hardware encoder could be used and this methodology could position the pulses relative
to the hardware encoder.
This methodology could be extended further to allow for adjustments in the
desired inductance (or desired current rise) based on speed, load, or desired optimization.
These adjustments could be applied from a lookup table based on current operating point
or calculated real time based on an adjustment formula.
The foregoing description of the preferred embodiment of the present
invention has been presented for the purpose of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise form disclosed. Many
modifications and variations are possible in light of the above teachings. It is intended that
the scope of the present invention not be limited by this detailed description, but by the
claims and the equivalents to the claims appended hereto.
Claims (9)
- What is claimed is: 5 1. A method for controlling a switched reluctance machine (SRM), the method comprising: a) providing an SRM control system having a commutation module; b) providing an initial rotor position for the SRM utilizing an initialization mechanism; 10 c) defining a pinned point on a phase current waveform during an initial current rise phase of the current waveform, the current waveform starting to rise in correspondence of an inductance rise of a phase of the SRM, wherein the pinned point is defined at a point corresponding to a magnitude between 50% and 100% of the steady state current on the initial current rise; 15 d) determining a slope of the current rise as the current waveform reaches the pinned point, the slope being fed to the commutation module; e) calculating an error signal and providing the error signal as an input to a control loop in the SRM control system; and f) determining an optimum time to fire a next pulse; 20 whereby the SRM control system enables adaptive pulse positioning over a wide range of speeds and loads.
- 2. The method of claim 1 wherein the defined pinned point is static with respect to an underlying inductance value of the SRM.
- 3. The method of claim 1 wherein the SRM control system is designed to utilize the measured current rise over a fixed time period to calculate the error signal from a desired current rise.
- 4. A method for controlling a switched reluctance machine (SRM), the method comprising: a) providing an SRM control system having a commutation module; b) providing an initial rotor position for the SRM utilizing an initialization 5 mechanism; c) defining a pinned point on a phase current waveform during an initial current rise phase of the current waveform, the current waveform starting to rise in correspondence of an inductance rise of a phase of the SRM, wherein the pinned point is defined at a point corresponding to a magnitude between 50% and 100% of the steady 10 state current on the initial current rise, the pinned point being static with respect to an underlying inductance value of the SRM; d) determining a slope of the current rise as the current waveform reaches the pinned point, the slope being fed to the commutation module; e) providing a frequency input signal to the commutation module to obtain a 15 digital estimate for shaft speed; f) calculating a current speed and an optimum time signal to fire a next pulse, utilizing the slope of the current rise and frequency input signal; g) generating a commanded current by a regulator unit utilizing a reference speed and the current speed; 20 h) comparing the commanded current with a measured current by an inner current loop in the SRM control system; and i) utilizing the optimum time signal to fire a next pulse, generating pulse width modulation (PWM) signals to create a plurality of commutation angles for turning a plurality of switches of the SRM to on and off states; 25 whereby the SRM control system enables adaptive pulse positioning over a wide range of speeds and loads.
- 5. The method of claim 4 wherein the regulator unit is a proportional-integral unit.
- 6. A switched reluctance machine (SRM) control system for controlling an SRM, the system comprising: an initialization module to provide an initial rotor position for the SRM utilizing an initialization mechanism; 5 a point defining module to define a pinned point on a phase current waveform during an initial current rise phase of the current waveform, the current waveform starting to rise in correspondence of an inductance rise of a phase of the SRM, wherein the pinned point is defined at a point corresponding to a magnitude between 50% and 100% of the steady state current on the initial current rise; 10 a slope determining module to determine a slope of the current rise as the current waveform reaches the pinned point; a commutation module to receive the slope of the current rise from the slope determining module and a frequency input signal; an error calculating module to calculate an error signal that being fed to a control 15 loop in the SRM control system; and a time determining module to determine an optimum time to fire a next pulse; whereby the SRM control system enables adaptive pulse positioning over a wide range of speeds and loads. 20
- 7. The SRM control system of claim 6 wherein the defined pinned point is static with respect to an underlying inductance value of the SRM.
- 8. The SRM control system of claim 6, wherein the system is designed to utilize the measured current rise over a fixed time period to calculate the error signal from a desired 25 current rise.
- 9. The SRM control system of claim 6 wherein the optimum time determined from the time determining module is utilized to turn a plurality of switches of the SRM to on and off states.
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
PCT/US2018/025609 WO2019190569A1 (en) | 2018-03-31 | 2018-03-31 | Manufacturing-sensitive control of high rotor pole switched reluctance motors |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ761486A NZ761486A (en) | 2021-11-26 |
NZ761486B2 true NZ761486B2 (en) | 2022-03-01 |
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