 |
Claims  |
|
|
What is claimed is:
1. A method of controlling operation of a synchronous motor, said
synchronous motor making polyphase alternating currents flow through
polyphase windings and rotating a rotor by an interaction of a magnetic
field formed by said windings with a magnetic field formed by a permanent
magnet, said method comprising the steps of:
detecting an electrical angle of said rotor according to a first detection
process immediately after a start of operation of said synchronous motor,
and regulating the polyphase currents flowing through said polyphase
windings based on the detected electrical angle, said first detection
process having a practical accuracy when a revolving speed of said rotor
is not less than a predetermined level;
calculating the revolving speed of said rotor based on a variation in
detected electrical angle; and
detecting the electrical angle of said rotor according to a second
detection process in case that the calculated revolving speed is less than
the predetermined level, and regulating the polyphase currents flowing
through said polyphase windings based on the detected electrical angle,
said second detection process having a practical accuracy when the
revolving speed of said rotor is less than the predetermined level.
2. A method of controlling operation of a synchronous motor, said
synchronous motor making polyphase alternating currents flow through
polyphase windings and rotating a rotor by an interaction of a magnetic
field formed by said windings with a magnetic field formed by a permanent
magnet, said method comprising the steps of:
determining whether or not said rotor rotates at a revolving speed of not
less than a predetermined level, prior to a start of operation of said
synchronous motor;
detecting an electrical angle of said rotor according to a first detection
process in case that said rotor rotates at the revolving speed of not less
than the predetermined level, and regulating the polyphase currents
flowing through said polyphase windings based on the detected electrical
angle, said first detection process having a practical accuracy when the
revolving speed of said rotor is not less than the predetermined level;
and
detecting the electrical angle of said rotor according to a second
detection process in case that said rotor rotates at the revolving speed
of less than the predetermined level, and regulating the polyphase
currents flowing through said polyphase windings based on the detected
electrical angle, said second detection process having a practical
accuracy when the revolving speed of said rotor is less than the
predetermined level.
3. A method in accordance with either one of claim 1, wherein said first
detection process detects the electrical angle of said rotor based on a
motor model, and said second detection process detects the electrical
angle based on an inductance of said polyphase windings.
4. A method in accordance with either one of claim 2, wherein said first
detection process detects the electrical angle of said rotor based on a
motor model, and said second detection process detects the electrical
angle based on an inductance of said polyphase windings.
5. A motor control apparatus that controls operation of a synchronous
motor, said synchronous motor making polyphase alternating currents flow
through polyphase windings and rotating a rotor by an interaction of a
magnetic field formed by said windings with a magnetic field formed by a
permanent magnet, said motor control apparatus comprising:
a first electrical angle detection unit that detects an electrical angle of
said rotor according to a first detection process, said first detection
process having a practical accuracy when a revolving speed of said rotor
is not less than a predetermined level;
a second electrical angle detection unit that detects the electrical angle
of said rotor according to a second detection process, said second
detection process having a practical accuracy when the revolving speed of
said rotor is less than the predetermined level;
a revolving speed computation unit that calculates the revolving speed of
said rotor at least based on a variation in electrical angle detected by
said first electrical angle detection unit;
a switching unit that activates said first electrical angle detection unit
to detect the electrical angle of said rotor immediately after a start of
operation of said synchronous motor, and activates said revolving speed
computation unit to calculate the revolving speed of said rotor based on
the electrical angle detected by said first electrical angle detection
unit, when the revolving speed calculated by said revolving speed
computation unit being less than the predetermined level, said switching
unit activating said second electrical angle detection unit to detect the
electrical angle of said rotor; and
a control unit that regulates the polyphase currents flowing through said
polyphase windings based on the electrical angle detected by either one of
said first electrical angle detection unit and said second electrical
angle detection unit.
6. A motor control apparatus in accordance with claim 5, wherein said first
detection process adopted in said first electrical angle detection unit
detects the electrical angle of said rotor based on a motor model, and
said second detection process adopted in said second electrical angle
detection unit detects the electrical angle based on an inductance of said
polyphase windings.
7. A motor control apparatus in accordance with claim 5, wherein said
second electrical angle detection unit comprises:
a voltage application unit that applies a predetermined voltage to a
certain combination selected among said polyphase windings;
a current behavior detection unit that detects behaviors of the polyphase
currents flowing through said polyphase windings in response to the
voltage applied by said voltage application unit;
a storage unit that stores a relationship between the electrical angle of
said rotor and the behaviors of the polyphase currents flowing through
said polyphase windings in response to the predetermined voltage applied
to said certain combination, said relationship being determined in
advance; and
an electrical angle computation unit that refers to said relationship
stored in said storage unit and specifies the electrical angle of said
rotor in a range of 0 to 2.pi. corresponding to the behaviors of the
polyphase currents detected by said current behavior detection unit.
8. A motor control apparatus in accordance with claim 5, wherein said
second electrical angle detection unit comprises:
a voltage application unit that applies a predetermined voltage to said
polyphase windings;
a current behavior detection unit that detects behaviors of the polyphase
currents flowing through said polyphase windings in response to the
voltage applied by said voltage application unit;
a driving current detection unit that detects a driving current supplied to
said polyphase windings at a time of application of said predetermined
voltage;
a storage unit that stores a relationship between the electrical angle of
said rotor and the behaviors of the polyphase currents flowing through
said polyphase windings in response to application of the predetermined
voltage while the driving current flows through said synchronous motor,
said relationship being determined in advance: and
an electrical angle computation unit that refers to said relationship
stored in said storage unit and specifies the electrical angle of said
rotor in a range of 0 to 2.pi. corresponding to the behaviors of the
polyphase currents detected by said current behavior detection unit and
the driving current detected by said driving current detection unit.
9. A motor control apparatus that controls operation of a synchronous
motor, said synchronous motor making polyphase alternating currents flow
through polyphase windings and rotating a rotor by an interaction of a
magnetic field formed by said windings with a magnetic field formed by a
permanent magnet, said motor control apparatus comprising:
a revolving speed determination unit that determines whether or not said
rotor rotates at a revolving speed of not less than a predetermined level,
prior to a start of operation of said synchronous motor;
a first control unit that detects an electrical angle of said rotor
according to a first detection process in case that said rotor rotates at
the revolving speed of not less than the predetermined level, and
regulates the polyphase currents flowing through said polyphase windings
based on the detected electrical angle, said first detection process
having a practical accuracy when the revolving speed of said rotor is not
less than the predetermined level; and
a second control unit that detects the electrical angle of said rotor
according to a second detection process in case that said rotor rotates at
the revolving speed of less than the predetermined level, and regulates
the polyphase currents flowing through said polyphase windings based on
the detected electrical angle, said second detection process having a
practical accuracy when the revolving speed of said rotor is less than the
predetermined level.
10. A motor control apparatus in accordance with claim 9, wherein said
revolving speed determination unit comprises:
a short-circuit current detection unit that short-circuits said polyphase
windings, through which the polyphase currents flow, for a predetermined
time period and detects a short-circuit current flowing through said
short-circuited polyphase windings; and
a revolving speed computation unit that calculates the revolving speed of
said rotor from the detected short-circuit current,
wherein said revolving speed determination unit carries out said
determination based on the revolving speed calculated by said revolving
speed computation unit.
11. A motor control apparatus in accordance with claim 9, wherein said
first detection process adopted in said first control unit detects the
electrical angle of said rotor based on a motor model, and said second
detection process adopted in said second control unit detects the
electrical angle based on an inductance of said polyphase windings. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of detecting an electrical angle
in a synchronous motor in a sensor-free manner and regulating electric
currents flowing through polyphase windings of the synchronous motor, so
as to control operation of the synchronous motor. The present invention
also pertains to an apparatus for the same.
2. Description of the Related Art
A motor control apparatus for controlling operation of a synchronous motor
generally has a unit that detects an electrical angle of a rotor, and
regulates the electric currents flowing through polyphase windings
according to the observed electrical angle of the rotor. Resolvers,
encoders, and other sensors may be applicable for the electrical angle
detection unit. Sensor-free structures that detect the electrical angle
based on the electric currents flowing through the windings have, however,
proposed recently, in order to enhance the reliability of detection.
The applicants of the present invention have proposed a device that detects
an electrical angle of a synchronous motor at a high accuracy even when a
rotor is at a stop or rotates at a low revolving speed (JAPANESE PATENT
LAID-OPEN GAZETTE No. 7-177788). The proposed device measures behaviors of
electric currents flowing through the windings and detects the inductance
of the windings, which is affected by the position of the rotor, and
thereby the electrical angle according to the observed behaviors of the
electric currents. This device is based on the finding that detection of
the behaviors of electric currents flowing through any two phases leads to
unequivocal determination of the electrical angle in the case of a
three-phase synchronous motor. This technique is specifically effective
when the rotor rotates at a low revolving speed. The applicants of the
present invention accordingly have proposed a structure that detects the
electrical angle according to the behaviors of electric currents flowing
through the respective phase coils while the rotor is at a stop or rotates
at a revolving speed of less than a predetermined level. The structure
adopts the conventional method of detection utilizing the
counterelectromotive forces when the revolving speed of the rotor is not
less than the predetermined level. This structure thus enables the
electrical angle to be detected at a high accuracy, irrespective of the
rotation of the rotor, that is, whether the rotor is at a stop, rotates at
a low revolving speed, or rotates at a high revolving speed.
The proposed electrical angle detection device, which detects the
electrical angle based on the behaviors of electric currents, does not
require a sensor, such as a resolver, but enables the electrical angle to
be detected at a high accuracy even when the rotor rotates at a low
revolving speed. When the electrical angle detection device is actually
incorporated in a motor control apparatus that controls operation of a
synchronous motor, however, problems discussed below arise in some
applications of the synchronous motor. In the normal application of the
synchronous motor, operation of the synchronous motor starts from the
state in which the rotor is at a stop. It can thus be assumed that
measurement of the electrical angle starts from the state in which no
electromotive forces are generated in the respective windings of the
synchronous motor. In case that the synchronous motor is attached to, for
example, a drive shaft of a vehicle, the rotor may rotate with a rotation
of the drive shaft, although the synchronous motor is not specifically
used either as a motor or a generator. In such cases, the electromotive
forces due to the rotation of the rotor cause electric currents flow
through the respective phase coils. Under these conditions, a voltage is
applied between the phase coils and the behaviors of electric currents are
measured. The electrical angle determined from the observed behaviors of
electric currents may accordingly not represent the actual electrical
angle of the rotor accurately.
SUMMARY OF THE INVENTION
The object of the present invention is thus to accurately detect an
electrical angle and adequately regulate phase currents flowing through
polyphase windings of a synchronous motor, irrespective of application of
the synchronous motor or more specifically irrespective of the rotation of
the synchronous motor.
At least part of the above and the other related objects is realized by a
first method of controlling operation of a synchronous motor, the
synchronous motor making polyphase alternating currents flow through
polyphase windings and rotating a rotor by an interaction of a magnetic
field formed by the windings with a magnetic field formed by a permanent
magnet. The first method of the present invention enables accurate
detection of the electrical angle of the rotor and adequate regulation of
electric currents flowing through the polyphase windings, even when the
rotor has already been rotated at the time of starting operation of the
synchronous motor.
Part of the objects is also realized by a second method of controlling
operation of a synchronous motor, the synchronous motor making polyphase
alternating currents flow through polyphase windings and rotating a rotor
by an interaction of a magnetic field formed by the windings with a
magnetic field formed by a permanent magnet. The second method of the
present invention determines whether or not the rotor rotates at the
revolving speed of not less than a predetermined level prior to a start of
operation of the synchronous motor, and adopts the appropriate electrical
angle detection process according to the revolving speed of the rotor.
This method thus enables the electric currents flowing through the
polyphase windings to be regulated adequately.
In accordance with one preferable application of the method, the first
detection process detects the electrical angle of the rotor based on a
motor model, and the second detection process detects the electrical angle
based on an inductance of the polyphase windings. These techniques may not
be adopted in pair, but the first detection process of the second
detection process may be combined with another available technique.
The present invention is also directed to a first motor control apparatus
that controls operation of a synchronous motor, the synchronous motor
making polyphase alternating currents flow through polyphase windings and
rotating a rotor by an interaction of a magnetic field formed by the
windings with a magnetic field formed by a permanent magnet. In the first
motor control apparatus of the present invention, immediately after a
start of operation of the synchronous motor, the first electrical angle
detection unit detects the electrical angle according to the first
detection process, and the revolving speed computation unit calculates the
revolving speed of the rotor from the detected electrical angle. In case
that the calculated revolving speed is less than a predetermined level,
the switching unit activates the second electrical angle detection unit to
detect the electrical angle according to the second detection process. The
second detection process has a practical accuracy when the revolving speed
of the rotor is less than the predetermined level. The control unit then
regulates the electric currents flowing through the polyphase windings,
based on the electrical angle detected by the second electrical angle
detection unit, thereby enabling adequate operation of the synchronous
motor. In case that the calculated revolving speed is not less than the
predetermined level, on the other hand, the first electrical angle
detection unit continues detecting the electrical angle. The control unit
then regulates the electric currents flowing through the polyphase
windings, based on the electrical angle detected by the first electrical
angle detection unit. The structure of the first motor control apparatus
thus enables operation of the synchronous motor to be controlled
adequately, irrespective of the revolving speed of the rotor.
In accordance with one preferable application of the first motor control
apparatus, the first detection process adopted in the first electrical
angle detection unit detects the electrical angle of the rotor based on a
motor model, and the second detection process adopted in the second
electrical angle detection unit detects the electrical angle based on an
inductance of the polyphase windings.
In accordance with another preferable application of the first motor
control apparatus, the second electrical angle detection unit includes: a
voltage application unit that applies a predetermined voltage to a certain
combination selected among the polyphase windings; a current behavior
detection unit that detects behaviors of the polyphase currents flowing
through the polyphase windings in response to the voltage applied by the
voltage application unit; a storage unit that stores a relationship
between the electrical angle of the rotor and the behaviors of the
polyphase currents flowing through the polyphase windings in response to
the predetermined voltage applied to the certain combination, the
relationship being determined in advance; and an electrical angle
computation unit that refers to the relationship stored in the storage
unit and specifies the electrical angle of the rotor in a range of 0 to
2.pi. corresponding to the behaviors of the polyphase currents detected by
the current behavior detection unit. This structure enables detection of
the electrical angle based on the inductance of the circuit, which depends
upon the position of the rotor.
In accordance with still another preferable application, the second
electrical angle detection unit may detect the electrical angle while a
driving current flows through the polyphase windings. In one concrete
structure, for example, the second electrical angle detection unit
includes: a voltage application unit that applies a predetermined voltage
to the polyphase windings; a current behavior detection unit that detects
behaviors of the polyphase currents flowing through the polyphase windings
in response to the voltage applied by the voltage application unit; a
driving current detection unit that detects a driving current supplied to
the polyphase windings at a time of application of the predetermined
voltage; a storage unit that stores a relationship between the electrical
angle of the rotor and the behaviors of the polyphase currents flowing
through the polyphase windings in response to application of the
predetermined voltage while the driving current flows through the
synchronous motor, the relationship being determined in advance; and an
electrical angle computation unit that refers to the relationship stored
in the storage unit and specifies the electrical angle of the rotor in a
range of 0 to 2.pi. corresponding to the behaviors of the polyphase
currents detected by the current behavior detection unit and the driving
current detected by the driving current detection unit. This structure
eliminates the effects of the driving current and enables the electrical
angle to be determined according to the behaviors of the electric currents
flowing through the polyphase windings.
The present invention is further directed to a second motor control
apparatus that controls operation of a synchronous motor, the synchronous
motor making polyphase alternating currents flow through polyphase
windings and rotating a rotor by an interaction of a magnetic field formed
by the windings with a magnetic field formed by a permanent magnet. In the
second motor control apparatus of the present invention, it is determined
whether or not the rotor rotates at the revolving speed of not less than a
predetermined level, prior to a start of operation of the synchronous
motor. The appropriate detection process is then adopted to detect the
electrical angle according to the revolving speed of the rotor. The first
control unit and the second control unit thus adequately regulate the
electric currents flowing through the polyphase windings.
In accordance with one preferable application of the second motor control
apparatus, the revolving speed determination unit includes: a
short-circuit current detection unit that short-circuits the polyphase
windings, through which the polyphase currents flow, for a predetermined
time period and detects a short-circuit current flowing through the
short-circuited polyphase windings; and a revolving speed computation unit
that calculates the revolving speed of the rotor from the detected
short-circuit current, wherein the revolving speed determination unit
carries out the determination based on the revolving speed calculated by
the revolving speed computation unit. While the rotor rotates, a
short-circuit current flows through the windings. The revolving speed of
the rotor can be readily obtained from the magnitude of the detected
short-circuit current.
In accordance with another preferable application of the second motor
control apparatus, the first detection process adopted in the first
control unit detects the electrical angle of the rotor based on a motor
model, and the second detection process adopted in the second control unit
detects the electrical angle based on an inductance of the polyphase
windings.
These and other objects, features, aspects, and advantages of the present
invention will become more apparent from the following detailed
description of the preferred embodiments with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram schematically illustrating structure of a motor
control apparatus 10 as a first embodiment according to the present
invention;
FIG. 2 schematically illustrates structure of a three-phase synchronous
motor 40, which is to be controlled in the first embodiment;
FIG. 3 is an end view illustrating a stator 30 and a rotor 50 of the
three-phase synchronous motor 40;
FIG. 4 is a circuit diagram illustrating internal structure of the inverter
110 of the first embodiment;
FIG. 5 is a flowchart showing a synchronous motor control routine executed
in the first embodiment;
FIG. 6 is a flowchart showing a first electrical angle detection routine
executed in the first embodiment;
FIG. 7 is a circuit diagram illustrating an equivalent circuit of the
three-phase synchronous motor 40 of the first embodiment;
FIG. 8 is a graph showing a transient response of U-phase current Iu(t)
when a voltage E1 is applied between the U phase and the VW phase;
FIG. 9 is a graph showing the phase currents Iu, Iv, and Iw plotted against
the electrical angle .theta.;
FIG. 10 is a map showing the relationship between each combination of phase
currents and the electrical angle .theta.;
FIG. 11 is a flowchart showing a second electrical angle detection routine
executed in the first embodiment;
FIG. 12 is a graph showing phase currents in response to application of a
voltage E1:
FIG. 13 schematically illustrates a power transmission system of a hybrid
vehicle with the motor control apparatus of the first embodiment
incorporated therein;
FIG. 14 is a flowchart showing a synchronous motor control routine executed
in a second embodiment according to the present invention;
FIG. 15 is a flowchart showing a motor revolving speed detection routine;
FIG. 16 is a graph showing the relationship between the amplitude
.vertline.I.vertline. of the short-circuit current and the revolving speed
Nh of the rotor 50 used in the motor revolving speed detection routine of
FIG. 15;
FIG. 17 is a circuit diagram schematically illustrating a motor current
control circuit 400 with a circuit for realizing the first electrical
angle detection process based on a motor model;
FIG. 18 is a flowchart showing an electrical angle detection routine as
another available technique to detect the electrical angle in two separate
steps;
FIG. 19 is a graph showing the phase currents when the technique of FIG. 18
is applied; and
FIG. 20 is a graph showing the normalized state of the respective phase
currents.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some modes of carrying out the present invention are described below as
preferred embodiments. FIG. 1 is a block diagram schematically
illustrating structure of a motor control apparatus 10 as a first
embodiment according to the present invention. FIG. 2 schematically
illustrates structure of a three-phase synchronous motor 40, which is the
object to be controlled. FIG. 3 is an end view illustrating a stator 30
and a rotor 50 of the three-phase synchronous motor 40.
The general structure of the three-phase synchronous motor 40 is described
first with the drawing of FIG. 2. The three-phase synchronous motor 40
includes the stator 30, the rotor 50, and a casing 60 for accommodating
the stator 30 and the rotor 50 therein. The rotor 50 has permanent magnets
51 through 54 attached to the outer circumference thereof and is provided
with a rotating shaft 55. The rotating shaft 55 passes through the axial
center of the rotor 50 and is rotatably supported by bearings 61 and 62
disposed in the casing 60.
The rotor 50 is prepared by laying a plurality of rotor elements 57, which
are punched out of a non-directional electromagnetic steel plate, one upon
another. Each rotor 57 has four salient poles 71 through 74 arranged in a
cross configuration as shown in FIG. 3. The rotating shaft 55 is pressed
into the laminate of the rotor elements 57, so as to temporarily fix the
laminate of the rotor elements 57. Each rotor 57 composed of the
electromagnetic steel plate has an insulating layer and an adhesive layer
formed on the surface thereof. The laminate of the rotor elements 57 is
heated to a predetermined temperature, which fuses the adhesive layers and
thereby fixes the laminate of the rotor elements 57.
After the assembly of the rotor 50, the permanent magnets 51 through 54 are
attached to the outer circumferential surface of the rotor 50 along the
axis of the rotor 50 at the positions between the salient poles 71 through
74. The permanent magnets 51 through 54 are magnetized in the direction of
their thickness. In the state that the rotor 50 is combined with the
stator 30, a pair of the permanent magnets 51 and 52 forms a magnetic path
Md that passes through the rotor elements 57 and stator elements 20
(described below) as shown by the one-dot chain line in FIG. 3.
The stator 30 is prepared by laying a plurality of stator elements 20,
which are punched out of a non-directional electromagnetic steel plate
like the rotor elements 57, one upon another. Each stator element 20 has
twelve teeth 22 as shown in FIG. 3. Coils 32 for causing the stator 30 to
generate a revolving magnetic field are wound on slots 24 formed between
the teeth 22. Bolt holes, each of which receives a fixation bolt 34, are
formed in the outer circumference of the stator element 20, although being
omitted from the illustration of FIG. 3.
The stator 30 is temporarily fixed by heating the laminate of the stator
elements 20 under pressure and fusing the adhesive layers thereof. In this
state, the coils 32 are wound on the teeth 22 to complete the stator 30.
The stator 30 is then placed in the casing 60 and fixed to the casing 60
by fitting the fixation bolts 34 in the bolt holes. The rotor 50 is then
rotatably attached to the casing 60 by means of the bearings 61 and 62.
This completes the assembly of the three-phase synchronous motor 40.
When an excitation current is flown to generate a revolving magnetic field
on the stator coils 32 of the stator 30, a magnetic path Mq is formed to
pass through the adjoining salient poles as well as the rotor elements 57
and the stator elements 20 as shown by the two-dot chain line in FIG. 3.
The axis `d` represents the axis through which the magnetic flux formed by
the permanent magnet 52 passes in the radial direction of the rotor 50,
whereas the axis `q` represents the axis through which the magnetic flux
formed by the stator coils 32 of the stator 30 passes in the radial
direction of the rotor 50. In this embodiment (where the number of
poles=4), the axes `d` and `q` are electrically arranged at right angles.
Referring to FIG. 1, structure of the motor control apparatus 10 is
described. The motor control apparatus 10 mainly includes a control ECU
100 that regulates three-phase (U, V, and W phases) motor currents of the
three-phase synchronous motor 40 in response to a torque command value T*
input from outside, an inverter 110 that regulates electric currents
flowing through the respective phases of the stator coils 32 in the
three-phase synchronous motor 40 in response to an instruction from the
control ECU 100, electric current meters 122, 124, and 126 that
respectively measure the electric currents flowing through the respective
phases of the stator coils 32 in the three-phase synchronous motor 40, and
A/D converters (ADC) 132, 134, and 136 that respectively convert the
observed values of these meters 122 through 126 to digital signals.
As illustrated in FIG. 1, the control ECU 100 includes a microprocessor
(CPU) 101 that carries out arithmetic and logic operations, a ROM 102 in
which data required for the operations of the CPU 101 are stored in
advance, a RAM 103, which data required for the operations of the CPU 101
are temporarily written in and read from, and a timer 104 that counts the
time. These elements 101 through 104 are mutually connected by a bus 105.
An input port 106 and an output port 107 are also connected to the bus
105. The CPU 101 reads electric currents Iu, Iv, and Iw flowing through
the respective phases U, V, and W of the three-phase synchronous motor 40
and controls the driving state of the inverter 110 via these ports 106 and
107.
FIG. 4 is a circuit diagram illustrating internal structure of the inverter
110. The inverter 110 includes an interface unit 112 functioning as an
interface with the control ECU 100, a main drive circuit 114 that consists
of six high-power switching transistors and directly regulates supply of
electricity to the respective phases of the three-phase synchronous motor
40, and pre-drive circuits 116 and 118 that respectively drive the main
drive circuit 114 on the side of the source and on the side of the drain.
The symbols `+` and `-` in FIG. 4 are connected to a main power source for
driving the three-phase synchronous motor 40. The symbols `+V` and `-V`
represent stabilized positive and negative power sources for control in
the inverter 110, which are respectively connected to power circuits (not
shown). Outputs of the main drive circuit 114 are connected to the
respective phase coils of the three-phase synchronous motor 40. The
electric current meters 122, 124, and 126 that measure the electric
currents flowing through the U phase, the V phase, and the W phase are
disposed in the lines connecting the main drive circuit 114 with the
three-phase synchronous motor 40.
The interface unit 112 is a circuit that receives signals from the control
ECU 100 and outputs required signals to the pre-drive circuits 116 and
118. The interface unit 112 specifically includes a dead-time generating
circuit that generates a dead time to prevent any pair of the transistors
in the main drive circuit 114 from being activated simultaneously. The
interface unit 112 also includes a gate that blocks transmission of all
the signals to the pre-drive circuits 116 and 118 in response to an SD
signal output from the control ECU 100 and thereby cuts off the electric
currents in case of emergency.
The pre-drive circuits 116 and 118 switch on and off the high-power
switching transistors of the main drive circuit 114 at a high speed. In
this embodiment, insulated-gate bipolar transistors (IGBT) are applied for
the high-power switching transistors.
In the motor control apparatus 10 thus constructed, the control ECU 100
carries out the following control procedures in the normal driving state:
(1) In case that the three-phase synchronous motor 40 rotates at or above a
predetermined speed (100 rpm in this embodiment), a first process of
detecting electrical angle (described later) is applied to detect an
electrical angle .theta. of the rotor 50 and control the inverter 110, in
order to regulate the electric currents of the respective phases according
to the observed electrical angle .theta..
(2) In case that the revolving speed of the three-phase synchronous motor
40 is less than the predetermined speed, a second process of detecting
electrical angle (described later) is applied to detect the electrical
angle .theta. of the rotor 50 and control the inverter 110, in order to
regulate the electric currents of the respective phases according to the
observed electrical angle .theta..
The following describes the first electrical angle detection process, the
second electrical angle detection process, and a method of controlling
operation of the three-phase synchronous motor 40, together with a
start-time control procedure of the three-phase synchronous motor 40. FIG.
5 is a flowchart showing a synchronous motor control routine executed in
the first embodiment. When the method enters the routine of FIG. 5, it is
first determined whether or not the three-phase synchronous motor 40 is to
be activated at step S100. The determination of the activation or
non-activation of the three-phase synchronous motor 40 depends upon
whether or not a torque command value T* is given externally. In case that
the torque command value T* for activating the synchronous motor 40 is not
given from outside, the CPU 101 remains on standby. In case that the
torque command value T* is externally given to activate the synchronous
motor 40, on the other hand, the method proceeds to step S110. The method
controls the inverter 110 for the flow of predetermined electric currents
at step S110, and subsequently detects the electrical angle .theta.
according to the first process at step S120.
The first electrical angle detection process is realized by a routine shown
in the flowchart of FIG. 6. The first electrical angle detection process
determines the electrical angle .theta. based on counterelectromotive
forces of the three-phase synchronous motor 40. As the magnetic field
formed by the permanent magnets 51 through 54 shifts or rotates with a
rotation of the rotor 50, voltages in the direction reverse to the
direction of externally applied voltages are generated in the stator coils
32, which are conductors placed in the magnetic field. These voltages
continuously rotate with the rotation of the rotor 50 while keeping the
balance with the externally applied voltages. Measurement of these
voltages accordingly leads to detection of the electrical angle .theta..
The counterelectromotive forces follow the Fleming's right-hand rule. It
is accordingly difficult to measure the counterelectromotive forces when
the magnetic field shifts at a low speed, that is, when the rotor 50
rotates at a low speed. The condition of measurement is accordingly that
the rotor 50 rotates at or above the predetermined speed.
The first electrical angle detection process first assumes that the rotor
50 rotates substantially at the predetermined speed, and controls the
inverter 110 at step S121 in the flowchart of FIG. 6. The process then
reads the signals from the electric current meters 122 through 126 to
measure the respective phase currents Iu, Iv, and Iw at step S122. The
process subsequently calculates counterelectromotive forces from the
observed phase currents at step S123. The counterelectromotive forces are
calculated in the following manner. The counterelectromotive forces
represent the voltages on the d axis and the q axis and are expressed as
the counterelectromotive forces [Ed,Eq]. In order to calculate the
counterelectromotive forces [Ed,Eq], the observed electric currents Iu and
Iv of the U phase and the V phase are subjected to three phase-to-two
phase conversion (three phase-to-dq conversion), which is carried out
according to Equation (1) given below. The electrical angle .theta. used
in the calculation of Equation (1) is arithmetically estimated and, for
example, applied for the control of the inverter 110 at step S121 in the
flowchart of FIG. 6.
##EQU1##
where Iu+Iv+Iw=0 and (n) denotes a sampled point at a time point `n`.
The counterelectromotive forces [Ed,Eq] are then calculated according to
Equation (2) given below. The counterelectromotive forces [Ed,Eq]
calculated here are counterelectromotive forces on the d and q axes based
on the arithmetically estimated electrical angle .theta..
##EQU2##
where T denotes a sampling period, R a resistance of the motor, L an
inductance of the three-phase coils, and .omega. an angular velocity of
the motor.
Equation (2) is rewritten as Equations (3) given blow:
##EQU3##
In this manner, counterelectromotive forces Eu, Ev, and Ew are obtained
from the observed electric currents Iu and Iv of the U phase and the V
phase.
The counterelectromotive forces thus obtained rotate with a rotation of the
rotor 50 and thereby vary with an elapse of time. The counterelectromotive
forces plotted on the time axis accordingly form sine waves. In this
embodiment, the relationship between the electrical angle and the
counterelectromotive forces is determined in advance and stored as a table
in the ROM 102. The process refers to this table at step S124 and reads
the electrical angle .theta. of the rotor 50 corresponding to the
counterelectromotive forces calculated at step S123 from the table at step
S125.
As described above, the electrical angle .theta. can be detected when the
revolving speed of the rotor 50 is not less than the predetermined level.
When the revolving speed of the rotor 50 is less than the predetermined
level, however, the electrical angle .theta. can not be detected.
Referring back to the flowchart of FIG. 5, it is then determined whether
or not the electrical angle .theta. has been detected successfully at step
S130. In case that the electrical angle .theta. has been detected, the
method proceeds to step S135. In case that the electrical angle .theta.
has not been detected, on the other hand, the method proceeds to step S160
to detect the electrical angle .theta. according to the second electrical
angle detection process. When the electrical angle .theta. has been
detected successfully, the method calculates a revolving speed Nh of the
rotor 50 at step S135. The revolving speed Nh of the rotor 50 is obtained
as a rate of change in electrical angle .theta. per unit time. The
calculated revolving speed Nh is compared with a predetermined revolving
speed Nref at step S140. When the calculated revolving speed Nh is,
greater than the predetermined revolving speed Nref, the method determines
that the detection of the electrical angle .theta. can be continued with
the counterelectromotive forces. The method accordingly repeats the
detection of the electrical angle .theta. according to the first
electrical angle detection process at step S150 and regulates the
respective phase currents of the three-phase synchronous motor 40 based on
the detected electrical angle .theta. at step S170. The first electrical
angle detection process applied at step S150 is identical with the first
electrical angle detection process applied at step S120 and described with
the flowchart of FIG. 6.
When the electrical angle .theta. has not been detected successfully with
the counterelectromotive forces immediately after the activation of the
three-phase synchronous motor 40 at step S130 or when the calculated
revolving speed Nh of the rotor 50 is not greater than the predetermined
revolving speed Nref at step S140, the method detects the electrical angle
.theta. according to the second electrical angle detection process at step
S160. The second electrical angle detection process is based on the
inductance of the circuit, which depends upon the position of the rotor
50. The following describes the details of the second electrical angle
detection process.
FIG. 7 is a circuit diagram illustrating an equivalent circuit of the
three-phase synchronous motor 40. When a predetermined voltage E1 is
applied between the U phase and the VW phase of the three-phase
synchronous motor 40 as a step function, an electric current Iu(t) flowing
therethrough shows a transient response, which depends upon an inductance
component L of the circuit. Namely a value Im of the electric current at a
predetermined time point after application of the voltage is affected by
the magnitude of the inductance L of the circuit. FIG. 8 is a graph
showing the transient response of the electric current Iu(t). In the
actual three-phase synchronous motor 40, the inductance L is a function of
the electrical angle .theta. of the rotor 50 at the moment. When a
predetermined voltage is applied between specific windings for a
predetermined time, each phase current flowing therethrough has an
intrinsic value according to the electrical angle.
The electric current Iu(t) flowing through the equivalent circuit shown in
FIG. 7 (hereinafter referred to as the U-phase current) shows a response
defined by Equation (4) given below:
Iu(t)={1-exp(-Rt/L)}E1/R (4)
where exp() denotes an exponential function, R an impedance of the circuit,
and t a time. The U-phase current Iu(t) increases slowly in the case of
the large inductance L. Measurement of the electric current at a
predetermined time point after application of the voltage leads to
determination of the electrical angle .theta. as the function of the
inductance L as mentioned above. The respective phase currents were
measured against the electrical angle .theta.. The results of measurement
are shown in the graph of FIG. 9. FIG. 9 shows curves of the respective
phase currents when a voltage is applied between the U phase and the VW
phase as shown in FIG. 7 and large electric currents are flown to
magnetically saturate the coils. The electric currents Iu,
.vertline.Iv.vertline., and .vertline.Iw.vertline. were measured by the
electric current meters 122 through 126. These measurement values Iu,
.vertline.Iv.vertline., and .vertline.Iw.vertline. cause magnetic
saturation and are thereby unsymmetrical. The electrical angle .theta. can
thus be clearly determined from these measurement values, except in the
range where the U-phase current Iu is less than a predetermined value ie.
FIG. 10 is a map showing the relationship between each combination of | | |
