Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRICAL MACHINE AND METHOD OF CONTROLLING THE SAME
RELEATED APPLICATIONS
This application claims the benefit of nonprovisional U.S. Patent Application
No.
______________ , filed on April 21, 2006, which claims the benefit of
provisional U.S. Patent
Application No. 60/734,855 filed on November 9, 2005.
FIELD OF INVENTION
The invention relates to an electrical machine and specifically a brushless
electrical
machine.
BACKGROUND AND SUMMARY OF THE INVENTION
Brushless direct current (BLDC) motors are becoming more prevalent in
industries
that typically did not use BLDC motors. For example, the need for increased
efficiency in the heating
and air conditioning market has led to the use of BLDC motors for powering the
blower. BLDC
motors include a rotor having a plurality of magnetic poles (e.g., a plurality
of poles produced with
permanent magnets) of alternating polarity disposed on a surface of a rotor
core, and a stator that
receives electrical power and produces a magnetic field in response thereto.
The magnetic field of the
stator interacts with a magnetic field of the rotor to cause movement of the
rotor.
BLDC motors require a means for determining the position of the rotor in order
to
commutate the motor. One method of commutating the motor is referred to as
"sensorless" motor
commutation. Sensorless motor commutation is often performed by sensing the
back electromotive
force (BEMF) produced by the motor. Typically, the BEMF signal produced in the
stator windings is
not large enough for sensorless motor commutation until the speed of the rotor
reaches about ten
percent of the rated motor speed. As a result, a means of starting the motor
without using the BEMF
signal may be necessary.
For a three-phase motor, one method of starting the motor is to align the
rotor by
providing current to one phase of the motor and wait until the rotor has
stopped oscillating, then step
through the other phases of the motor (with each subsequent phase getting
shorter, thus ramping the
speed up without any position feedback) until the rotor reaches 10% of rated
speed. This method
traditionally has two drawbacks. First, the time required during the align
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phase can be long where the inertia of the attached load is large and the
friction is low (e.g., if the load
is a large blower). Second, information about the load (e.g., inertia and
torque) is typically required in
order to step the motor.
The purpose of aligning the rotor as described earlier is to start the motor
from a
known rotor position. One way to avoid this aligning process is by knowing the
rotor position by
some other method. The second drawback described earlier can be overcome by
not stepping blindly
(without rotor position information) but by knowing the rotor position at
almost zero speed.
In one embodiment, the invention provides a method of controlling an
electrical
machine having a stator and a rotor. The stator includes a core and a
plurality of windings disposed on
the core in a three-phase arrangement. The three-phase arrangement includes a
first phase, a second
phase, and a third phase having a first terminal, a second terminal, and a
third terminal, respectively.
The rotor is disposed adjacent to the stator to interact with the stator. The
method includes the steps of
applying a pulsed voltage differential to the first and second terminals
resulting in movement of the
rotor; monitoring the back electromotive force (BEMF) of the third phase to
sense rotor movement;
after the applying and monitoring steps, monitoring the BEMF of each of the
first, second, and third
phases to determine the direction of rotation of the rotor; determining
whether the rotor is rotating in a
desired direction, and electrically commutating the motor when the rotor is
rotating in the desired
direction and zero or more other conditions exist.
In another embodiment, the invention provides a method of controlling an
electrical
machine having a stator and a rotor. The stator includes a core and a
plurality of windings disposed on
the core in a multiphase arrangement. The rotor is disposed adjacent to the
stator to interact with the
stator. The method includes, prior to purposely causing movement of the rotor,
sensing a BEMF of at
least one of the phases, determining whether the rotor is moving based on the
sensed BEMF, defining
a state of the motor (e.g., a no moving state, a slow moving state, and a fast
moving state), and
stopping movement of the rotor if the motor falls under a slow moving state.
The method can further
include starting movement of the rotor as discussed above.
In yet another embodiment, the invention provides a method for controlling an
electrical machine having a stator and a rotor. The stator includes a core
having a plurality of
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phase windings disposed on a core. The rotor is disposed adjacent to the
stator and includes a plurality
of magnetic poles. The method includes initiating an aligning process of the
stator and the rotor by
generating a moving force to cause rotation of the rotor with respect to the
stator and generating a
braking force to at least slow rotation of the rotor with respect to the
stator. The generating of a
moving force to cause rotation of the rotor can include exciting at least one
of the phase windings to
generate an attracting magnetic force between the excited at least one phase
winding and at least one
of the magnetic poles, and the generating of a braking force to at least slow
rotation of the rotor can
include exciting at least one of the phase windings to generate a force
opposite to the rotational
direction of the rotor with respect to the stator. The method may also include
alternating between
generating the moving force and generating the braking force. The method may
further include
defining a specific amount of time to align the stator and the rotor, where
the specific amount of time
may include a plurality of cycles such as an exciting cycle, a braking cycle,
and a coast cycle.
In a further embodiment, the invention provides a method for controlling an
electrical
machine with a stator and a rotor. The stator includes a core with a plurality
of phase windings
disposed on the core. The rotor is disposed adjacent to the stator and
includes a plurality of rnagnetic
poles. The method includes generating a moving force to cause rotation of the
rotor with respect to the
stator, and generating a braking force to at least slow rotation of the rotor
with respect to the stator.
The method also includes alternating the generating a moving force and the
generating a braking force
for a period of time, and stopping rotation of the rotor at one of one or more
known rotor positions.
In another embodiment, the invention provides a method of controlling an
electrical
machine with a stator having a core and a plurality of windings disposed on
the core in a multiple
phase arrangement, and a rotor disposed adjacent to the stator to interact
with the stator. The method
includes applying a first pulsed voltage to a first terminal of a first phase
of the multiphase
arrangement, monitoring back electromotive force (BEMF) of at least one phase
of the multiphase
arrangement, and determining a peak value of BEMF. The method also includes
obtaining a first
monitored value of BEMF, comparing the peak value of BEMF against the first
monitored value of
BEMF, and determining whether the rotor is rotating based of the comparison.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is partial exploded view of the stator and rotor of a brushless
permanent magnet
electrical machine;
Fig. 2 is an isometric view showing the geometry used to define an arc of
magnetization skew ( ) on the rotor;
Fig. 3 is a longitudinal view of one construction of the rotor of Fig. 1;
Fig. 4 is a cross-sectional view of the stator and rotor of Fig. 1;
Fig. 5 is a block diagram of an electrical drive circuit for powering the
electrical
machine of Fig. 1;
Fig. 6 is a stator-winding pattern in a double-layer arrangement with compact
coils for
an 18-slot, 12-pole, 3-phase machine;
Fig. 7 is a stator-winding pattern in a single-layer arrangement with compact
coils for
an 18-slot, 12-pole, 3-phase machine;
Fig. 8 shows schematic diagrams representing three pulses being applied to a
three-
phase motor;
Fig. 9 represents a comparison of BEMFs for a three phase machine;
Fig. 10 shows one start routine of a BLDC motor; and
Fig. 11 shows another start routine of a BLDC motor.
DETAILED DESCRIPTION
Before any embodiments of the invention are explained in detail, it is to be
understood
that the invention is not limited in its application to the details of
construction and the arrangement of
components set forth in the following description or illustrated in the
following drawings. The
invention is capable of other embodiments and of being practiced or of being
carried out in various
ways. Also, it is to be understood that the phraseology and terminology used
herein is for the purpose
of description and should not be regarded as limiting. The use of "including,"
"comprising," or
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"having" and variations thereof herein is meant to encompass the items listed
thereafter and
equivalents thereof as well as additional items. The terms "connected,"
"coupled," "supported," and
"mounted" and variations thereof herein are used broadly and, unless otherwise
stated, encompass both
direct and indirect connections, couplings, supports, and mountings. In
addition, the terms connected
5 and coupled and variations thereof herein are not restricted to physical
and mechanical connections or
couplings.
Portions of an exemplary brushless direct current (BLDC) machine incorporating
the
invention are shown in Figs. 1-4. However, the invention is not limited to the
machine disclosed in
Figs. 1-4; other BLDC machines or electrically commutated machines (ECMs) can
incorporate the
invention.
Fig. 1 is a partial exploded view of the stator and rotor of one construction
of an
electrical machine (e.g., motor, generator, etc.). For Fig. 1, the electrical
machine is a motor 10 having
a rotor 15 and a stator 20. The rotor 15 is coupled to a shaft 17. In general,
the stator 20 receives
electrical power, and produces a magnetic field in response thereto. The
magnetic field of the stator 20
interacts with a magnetic field of the rotor 15 to produce mechanical power on
the shaft 17.
The rotor 15 includes a plurality of magnetic poles 25 of alternating polarity
exhibited
on a surface of a rotor core 30. The rotor core 30 includes laminations (e.g.,
magnetic steel
laminations), and/or solid material (e.g., a solid magnetic steel core),
and/or compressed powdered
material (e.g., compressed powder of magnetic steel). One construction of the
rotor 15 includes a
sheet of permanent magnet (e.g., hard magnetic) material disposed on the rotor
core 30. Another
construction of the rotor 15 can include a plurality of strips of permanent
magnet material attached
(e.g., with adhesive) around the core 30. The permanent magnet material can be
magnetized by a
magnetizer to provide a plurality of alternating magnetic poles. Additionally,
the number of magnetic
strips can be different than the number of rotor magnetic poles. Yet another
construction of the rotor
15 contains blocks of permanent magnet material placed inside the rotor core
30.
The description of the invention is not limited to a particular mechanical
construction,
geometry, or position of the rotor 15. For example, Fig. 1 shows the rotor 15
located inside and
separated by a radial air gap from the stator 20. In another construction, the
rotor 15 can
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be positioned radially exterior to the stator 20 (i.e., the machine is an
external- or outer- rotor
machine.)
One method to reduce cogging and ripple torque, which may arise in some BLDC
motors, is skewing the magnetization of the magnetic poles 25 with respect to
the stator 20.
Alternatively, stator teeth of the stator 20 can be skewed with respect to the
rotor magnetization. As
shown in Figs. 1 and 2, the "magnetization" of the rotor 15 refers to the line
pattern 31 along the
length of the rotor 15 delineating alternating magnetic poles 25 on the rotor
core 30.
Fig. 2 illustrates the geometrical concepts involved in defining the
magnetization skew
of the rotor. The arc of magnetization skew can be defined as the arc (i ),
measured in radians in
between the longitudinal lines 32 and 33 on the rotor surface facing the air-
gap, which separates the
stator 20 and the rotor 15.
Fig. 3 is a schematic representation of the rotor 15 divided into a plurality
of axial
sections 55 (e.g., 70, 71, and 72) along a rotational axis 50 of the rotor 15.
The number of axial
sections 55 can vary and is not limiting on the invention. An axial section 55
refers to a portion of the
rotor 15 differentiated by imaginary lines 60. Imaginary lines 60 refer to
locations on the rotor 15
where the direction of skew of the magnetization pattern 31 changes. One
construction of the rotor 15
includes alternating magnetic poles with substantially the same arc of
magnetization skew C along
each axial section 55, resulting in a herringbone pattern of magnetization.
The length of each axial
section 55 can vary.
Fig. 3 shows one construction of the rotor 15 including three axial sections
70, 71, and
72. The stator 20 interacts with one or more of the three axial sections 70,
71, and 72. The first axial
section 70 includes magnetic poles aligned with a first skew direction, the
second axial section 71
includes magnetic poles aligned with a second skew direction, and the third
axial section 72 includes
magnetic poles aligned with the first skew direction. The total number of
axial sections and the total
number of ratings for a given motor profile are not limiting on the invention.
Various designs of stator
20 can be used to interact with each construction of the rotor 15 described
above and shown in Figs. 1-
3.
With reference to Figs. 1 and 4, the stator 20 includes a stator core 105
having a
plurality of stator teeth 110, slots 120, and a back iron portion 115. A slot
120 is defined by the space
between adjacent stator teeth 110 and receives stator windings 112. In one
construction, the stator
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core 105 includes a stack of magnetic steel laminations or sheets. In other
constructions, the stator
core 105 is formed from a solid block of magnetic material, such as compacted
powder of magnetic
steel. The stator windings 112 can include electrical conductors placed in
slots 120 and around the
plurality of teeth 110. Other constructions and types of the stator core 105
and stator windings 112
known to those skilled in the art can be used and are not limiting on the
invention.
Electrical current flowing through the stator windings 112 produces a magnetic
field
that interacts with the magnetization of the rotor 15 to provide torque to the
rotor 15 and shaft 17. The
electrical current can be an (m) phase alternating current (AC), where (m) is
an integer greater than or
equal to two. The electrical current can have various types of waveforms
(e.g., square wave, quasi-
sine wave, etc). The stator windings 112 receive electrical current from an
electrical drive circuit.
The number (t) of stator teeth 110 equals the number of slots 120, where (t)
is an
integer. In the construction shown in Fig. 4, the rotor 15 is produced by
fixing three arc shaped
magnets 26 on the rotor core 30. Other rotor designs and constructions are
also possible. A
magnetizer is used to produce on the rotor 15 a number (p) of alternating
magnetic poles that interact
with the stator 20.
Fig. 5 shows a drive circuit 125 that receives AC power from a power source
130 and
drives the motor 10 in response to an input 135. The AC power is provided to a
filter 140 and a
rectifier 145 that filter and rectify the AC power, resulting in a bus voltage
VDC. The bus voltage
VDC is provided to an inverter 150 and to a voltage divider 155. The voltage
divider 155 reduces the
bus voltage VDC to a value capable of being acquired by a controller 160 (at a
terminal 162). The
controller 160 includes a processor 165 and a memory 170. Generally speaking,
the processor 165
reads, interprets, and executes instructions stored in the memory 170 to
control the drive circuit 125.
The controller 160, which may be in the form of a microcontroller, can include
other components such
as a power supply, an analog-to-digital converter, filters, etc. The
controller 160 issues drive signals at
terminals 175 and 180 to control the inverter 150. The inverter 150 includes
power electronic switches
(e.g., MOSFETs, IGBTs) to vary the flow of current to the motor 10. For
example, the inverter 150
can be in the form of a bridge circuit. A sense resistor 185 is used to
generate a voltage having a
relation to a bus current of the inverter 150. The voltage of the sensor
resistor 185 is provided to the
controller 160 at a terminal 187. Other methods of sensing current can be
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used to sense the bus current. The controller 160 can receive values
associated with phase currents
and phase voltages provided by the inverter 150. The drive circuit 125 also
includes a BEMF voltage
divider 190 and variable gain amplifiers I95A, 195B, and 195C. The BEMF
voltage divider 190 and
variable gain amplifiers 195A, 195B, and 195C provide voltage values to the
controller 160 at
terminals 200A, 200B, and 200C, respectively. The voltage values provided to
the controller 160 by
the variable gain amplifiers 195A, 195B, and 195C have a relation to the BEMF
of each phase voltage.
With reference to Figs. 6 and 7, the stator core 105 having the above-
described
construction can be used to design and manufacture motors 10 with various (m)
electric phases,
windings 112 composed of compact coils, and rotors 15 having poles (p). One
construction of the
stator windings 112 includes a double layer arrangement of compact coils (Fig.
6), which are placed
around each tooth 110 (i.e. the coils have a pitch of 1-slot). In this double
layer arrangement, each slot
120 is shared by two coil sides, each of the coil sides belonging to a
different coil and phase. The two
coil sides sharing a slot 120 can be placed side by side or one on top of the
other. Fig. 6 shows the
double-layer winding pattern for an example 18-slot, 12-pole, 3-phase winding.
Another construction of the windings 112 includes a single layer arrangement
of
compact coils (Fig. 7), which are placed around every other tooth 110 (i.e.
the coils have a pitch of 1-
slot and are only placed around half the number of teeth 110). In this single
layer arrangement, each
slot 120 contains only one coil side. Fig. 7 shows the single layer winding
pattern for an example 18-
slot, 12-pole, 3-phase winding. A typical manufacturing technique to provide a
double layer stator
winding with compact coils includes use of a needle or gun winder. A typical
manufacturing
technique to provide a single layer stator winding with compact coils includes
use of an insertion
winder. Other types and techniques known to those in the art to provide the
stator windings 112 of the
stator 20 can be used.
With reference to Fig. 5, the drive circuit 125 can estimate the rotor 15
position
through sensorless control. Sensorless motor commutation is often performed by
sensing the back
electromotive force (BEMF) produced by the motor 10. Typically, the BEMF
signal produced in the
stator windings 112 is not large enough for sensorless motor commutation until
the speed of the rotor
15 reaches about ten percent of the rated motor 10 speed. Described below are
various starting
procedures for starting a BLDC motor 10 utilizing sensorless control.
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The starting procedure is described below in three sections. The first section
is a rotor
position detection section. The second section is an initial pulsing section.
The last section is a low-
speed BEMF detection section. The starting procedure is stored as software
instructions in the
memory 170. The processor 165 reads the instructions from the memory 170,
interprets the
instructions, and executes the interpreted instruction resulting in the
operation of the motor 10 as
described below. Other circuit components (e.g., an AS1C) can be used in place
of the processor 165
and the memory 170 to control the motor 10.
A. Initial Rotor Position Detection
The initial position detection of the rotor 15 is based on a more simplified
version of
an algorithm disclosed in U.S. Patent No. 5,001,405 (the '405 Patent), which
is fully incorporated
herein by reference. The '405 Patent describes a method of exciting one phase
of a three phase motor
with one polarity, and then, exciting the same phase with the opposite
polarity. Through a comparison
of the peak current, the rotor position is known within 60 degrees.
Some of the starting algorithms described in this application do not excite
the winding
with the opposite current. This reduces the initial position resolution to 120
degrees (for a three-phase
motor). Using this more simplified method of determining the position of the
rotor 15 with a
resolution of 120 degrees provides enough information to get the motor 10
started in the correct
direction.
With reference to Fig. 8, the controller 160 uses the following pulse
sequence:
Pulse [0]=Aon, Bdc, Coff (current goes in phase B and returns in phase A);
Pulse [1]=Ade, Boff, Con (current goes in phase A and returns in phase C); and
Pulse [2]=Aoff, Bon, Cdc (current goes in phase C and returns in phase B);
where dc represents a pulsed bus voltage, on represents the phase being
grounded, and offrepresents
no current in the winding. The current is measured at the end of each pulse.
The sequence with the
greatest current determines the rotor position and which phase to apply the
first pulse movement.
In an alternate construction, the controller 160 uses the following pulse
sequence:
PulseParallel [0]=Aon, Bdc, Cdc (current goes in phase B and returns in phases
A and C);
PulseParallel [1]=Adc, Bdc, Con (current goes in phase A and returns in phase
C and B); and
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PulseParallel [2]=Adc, Bon, Cdc (current goes in phase C and returns in phase
B and A);
where dc represents a pulsed bus voltage and on represents the phase being
grounded. The current is
measured at the end of each pulse. The sequence with the greatest current
determines the rotor
5 position and which phase to apply the first pulse movement.
The winding sequence with the highest current is the winding that has the
magnet
most aligned with the field created by the winding. It is assumed that the
direction of the current is
also the direction of the north pole created by the winding current. For the
example shown in Figure 8,
phase B has the magnet most aligned (see arrangement shown as "Pulse[2]" in
Fig. 8). Therefore, the
'10 next sequence to turn on is Aon, Bdc, Coff or an intermediate sequence
of Aon, Bof, Cdc. Preferably,
the durations of the initial rotor pulses are fast enough and the current
level is small enough to not
cause the rotor 15 to move.
B. Initial Pulsing
An initial pulse, long enough to cause movement in the rotor 15, is applied to
the
appropriate phase from the information gathered from the previous section. The
duty cycle or voltage
applied to the winding 112 is set during the initial pulse such that the
voltage for the phase that :is open
can be amplified to a level that movement is detected by monitoring a change
in the voltage. If the
initial pulsed voltage is too large, then the motor accelerates too fast
causing a torque transient that
results in an undesirable audible noise at start. If the initial pulsed
voltage is too small then there
might not be enough torque to cause movement in the rotor 15. The initial
movement of the rotor 15
depends on where the rotor 15 is positioned within the 120 degree window.
Sampling BEMF at the
start of the pulse gets a baseline voltage before movement has occurred. The
BEMF is then monitored
for a change in voltage, which is related to rotor movement. During the
initial pulse sequence, the
rotor 15 can actually move backwards before it moves forward. If this occurs,
the controller 160
applies a braking pulse to stop or slow the rotor movement, and the controller
160 returns to the
previous section.
C. Coast; sense BEMF crossings (low speed BEMF detection method)
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Once movement is detected and all phases are turned off, the BEMF is monitored
for
phase crossings. The negative half of the BEMF is clamped by diodes in the
inverter 150. A
commutation point occurs when the BEMF phases intersect, as shown in Fig. 9.
More specifically, the software monitors three parameters:
1) Aphase>Bphase
2) Bphase>Cphase
3) Cphase>Aphase
These parameters are used to decode the rotor commutation position as follows:
Aphase>Bphase Bphase>Cphase Cphase>Aphase
TRUE FALSE FALSE Commutation[0]
TRUE TRUE FALSE Commutation[1]
FALSE TRUE FALSE Commutation[2]
FALSE TRUE TRUE Commutation[3]
FALSE FALSE TRUE Commutation[4]
TRUE FALSE TRUE Commutation[5]
At the first change in any of the three conditions, the software in memory 170
starts a
timer, and then, subsequently looks for the next "proper" transition. This is
to make sure the motor 10
is running in the proper direction. Upon the second change in BEMF condition,
the software stops the
timer and measures the period. The controller 160 then commutates the motor
with the appropriate
commutation phase sequence (assuming the rotor 15 is rotating in the proper
direction). The software
keeps the phase on as specified by the previous period, while looking for a
conventional BEMF zero-
cross event. The motor 10 can then commutate as is conventionally known in the
art. For example,
the controller 160 can use a six-step control technique for driving the motor
10. An example six step
phase sequence to commutate the motor is
Commutation [0]=Adc, Bon, Coff (current goes in phase A and returns in phase
B);
Commutation [1] =Adc, Boff, Con (current goes in phase A and returns in phase
C);
Commutation [2]=Aoff, Bdc, Con (current goes in phase B and returns in phase
C);
Commutation [3] =Aon, Bdc, Coff (current goes in phase B and returns in phase
A);
Commutation [4]=Aon, Boff, Cdc (current goes in phase C and returns in phase
A);
Commutation [5] =Aoff, Bon, Cdc (current goes in phase C and returns in phase
B);
where de represents a pulsed bus voltage and on represents the phase being
grounded.
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Fig. 10 illustrates a flow chart describing one possible method for detecting
the
position of the rotor 15 and starting rotational movement of the rotor 15
utilizing the electrical drive
circuit 125. The process illustrated by the flow chart can be started
automatically or manually (at
block 300). A hardware initialization procedure takes place at block 305. The
hardware initialization
procedure can include charging energy storing devises (e.g., capacitors) to
help control the flow of
current to the stator windings.
Occasionally, the rotor 15 is in motion when the method for starting the motor
10 is
initiated. The controller 160 measures the BEMF to detect movement of the
rotor 15 (at block 310).
The variable gain amplifiers 195 are switched to a high gain mode to detect
possible low BEMF
signals produced by the motor 10. Low BEMF signals are generally indicative of
significantly slow
motion of the rotor 15. The controller 160 usually determines the rotational
speed of the rotor 15 by
measuring the time between BEMF crossings, such as the ones illustrated in
Fig. 9. For example, if
the time between BEMF crossings increases, it is determined that the rotor 15
is slowing down. The
speed of the rotor 15 may be classified as one of various states. For example,
states determined by the
speed of the rotor 15 can include a no moving state, a slow moving state, or a
fast moving state. If the
speed of the rotor 15 falls under the slow moving state, the rotor 15 is
stopped by shorting phases A, B,
and/or C (at block 315).
The controller 160 classifies the speed of the rotor 15 under the no moving
state when
there is relatively no rotation of the rotor 15. In such case, the controller
160 determines the position
of the rotor 15 with respect to the stator 20 (at block 320) as previously
discussed. Based on the
determined position, the controller 160 applies a relatively longer duration
pulse (at block 325) in
comparison to the pulsed bus voltage used to determine the position of the
rotor 15 at block 320. The
longer duration pulse intends to cause rotational movement of the rotor 15 in
a desired direction (for
purposes of description, the desired rotational direction is identified as the
"forward" direction).
Because the pulsed bus voltage does not provide information to the controller
160
regarding the load to be manipulated by the motor 10, it is possible that the
longer duration pulse does
not cause significant movement of the rotor 15. The controller 160 determines
that there is no
movement of the rotor 15 when it does not detect zero-cross events (at block
327). The controller 160
then implements alternative methods for detecting variations of BEMF signals
indicative of rotor 15
movement. For example, the controller 160 searches for peaks of BEMF signals
(at block 330). When
no peaks are detected, it is determined that the longer duration pulse had
insufficient strength to cause
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rotation of the rotor 15, and the controller 160 returns to the hardware
initialization procedure (at block
305). If the controller 160 detects a BEMF peak, the controller searches for a
dropping BEMF (at
block 335). The controller 160 returns to the hardware initialization
procedure (at block 305) when
there is no significant change in the BEMF signals.
The controller 160 proceeds to a coast state (at block 340) when the
controller 160
detects a dropping BEMF (at block 335), or when it is determined (at block
327) that the applied
longer duration pulse produces rotation of the rotor 15. The controller 160
turns off the inverter 150
and monitors the BEMF as the rotor 15 is allowed to coast (at block 340).
Monitoring the BEMF
allows the controller 160 to determine a period in relation to the rotational
speed of the rotor 15 (at
block 345), and-the rotational direction of the rotor 15 (at block 347).
With reference to block 310, the controller 160 proceeds directly to determine
the
period (at block 345) when the rotational speed of the rotor 15 is classified
under a fast moving state
(at block 310). Because the controller 160 detects the position of the rotor
15 with accuracy up to 120
degrees (at block 320), it is possible for the rotor 15 to rotate in the
direction opposite to the one
desired (also referred as the "reverse" direction) after initiating motion (at
block 325). In some cases
when the controller 160 determines that the rotational speed of the rotor 15
is classified under the fast
moving state (at block 310), the rotor 15 may also be rotating in the reverse
direction. In the cases
when the controller 160 determines that the rotor 15 is moving in the reverse
direction (at block 347),
the controller 160 shorts the phases A, B, and/or C to stop rotational
movement of the rotor 15 (at
block 315).
The controller 160 allows the rotor 15 to rotate in a forward direction after
turning on
the inverter 150, and monitors the BEMF for a predetermined amount of time (at
block 350). The
controller 160 determines if the rotational speed of the rotor 15 is above a
threshold value after the
predetermined amount of time. It is assumed that noise and crosstalk, usually
generated by the motor
10 or other electric components, are mistaken for BEMF signals if the speed of
the rotor 15 is above
the threshold value. The threshold value in relation to rotational speed of
the rotor 15 may vary based
on factors such as the size of the motor 10 or the load coupled to the motor
10. When the speed of the
rotor 15 is above the threshold value, the controller 160 returns to the
hardware initialization procedure
(at block 305). Alternatively, when the speed of the rotor 15 is below the
threshold value, the
controller proceeds to a run mode (at block 355).
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Fig. 11 illustrates a flow chart describing another method for starting
rotational
movement of the rotor 15 utilizing the electrical drive circuit 125. More
particularly, the method
includes alinging the rotor 15 with the stator 20 to position the rotor 15 at
one of one or more known
starting positons. The process illustrated by the flow chart can be started
automatically or manually (at
block 400). A hardware initialization procedure takes place at block 405. The
hardware initialization
procedure can include charging energy storing devises (e.g., capacitors) to
help control the flow of
current to the stator windings.
Occasionally, the rotor 15 is in motion when the method for starting the motor
10 is
initiated. The controller 160 measures the BEMF to detect movement of the
rotor 15 (at block 410).
The variable gain amplifiers 195 are switched to a high gain mode to detect
possible low BEMF
signals produced by the motor 10. Low BEMF signals are generally indicative of
very slow motion of
the rotor 15. The controller 160 usually determines the rotational speed of
the rotor 15 by measuring
the time between BEMF crossings, such as the ones illustrated in Fig. 9. The
speed determined from
the BEMF signals may be classified under various states. For example, some
states may include a no
moving state, a slow moving state, or a fast moving state.
If the speed of the rotor 15 falls under the slow moving state, the rotor 15
is stopped
by shorting phases A, B, and/or C (at block 415). In some embodiments, the
slow moving state is
indicative of a speed below 7-10% (e.g., 8%) of the full rotational speed of
the rotor 15. In such cases,
the fast moving state is indicative of speeds equal or above 7-10% (e.g., 8%)
of the full rotational
speed of the rotor 15. The controller 160 accounts for the transition between
determining the speed of
the rotor 15 under the slow moving state (at block 410) and stopping the rotor
15 (at block 415) by
taking a numeric count (at block 412). After stopping the rotor 15 (at block
415), the controller 160
compares the numeric count to a predetermined value X (at block 416). The
predetermined value X is
indicative to the maximum number of times the rotor 15 is stopped (at block
415) in a continuous
matter after the controller 160 determines the speed of the rotor 15 falls
under the slow moving state.
This control sequence defined by blocks 412 and 416 is generally applicable in
cases when outside
influences cause the rotor 15 to rotate after shorting the phases A, B, and/or
C (at block 415). As
indicated in block 416, once the numeric count becomes larger than the
predetermined value X, the
controller 160 resets the numeric count and proceeds to align the rotor 15
with the stator 20 as
subsequently explained.
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The controller 160 classifies the speed of the rotor 15 under the no moving
state when
there is relatively no rotation of the rotor 15 with respect to the stator 20.
In such cases, the control
160 initiates a procedure to align the rotor 15 with the stator 20 to position
the rotor 15 at one of one or
more known starting positions (at block 420). The alignment procedure includes
applying a pulse to
5 one or more of the phase windings A, B, and C to generate an
electromotive force (EMF). The
generated EMF causes a section of the rotor 15 to attract to the section of
the stator 20 being excited,
thereby causing the rotor 15 rotate. As a result, the position of the rotor 15
at the end of the aligning
procedure (at block 420) is set to one of one or more known positions.
In some constructions, the rotor 15 is connected to a relatively larger mass
resulting in
10 oscillation of the rotor 15 for an extended period of time until the
rotor 15 and stator 20 are aligned.
The procedure represented by block 420 may also include shorting one or more
phase windings A, B,
and/or C to generate an opposing braking force. The braking force is described
as "opposing" because
it opposes rotation of the rotor 15 regardless of the rotational direction.
Because the braking force is
proportional to the BEMF generated by rotation of the rotor 15, the braking
force is generally
15 proportional to the rotational speed of the rotor 15. Thus, shorting the
phase windings as the rotor 15
rotates at a relatively higher speed will generate a greater braking force, as
opposed to the force
generated when the rotor 15 is rotating at a relatively lower speed. It is
possible to damp the rotor 15
oscillations by alternating powering one phase winding (to rotate the rotor
15) and shorting the phase
windings A, B, and C (to stop the rotor 15). Alternating powering and shorting
phases helps reduce
the rotor 15 oscillating time over an aligning cycle.
Before proceeding further, it should be understood that when referring to
generating a
breaking force, the controller 160 controls the power or current to or in the
windings to result in a
force opposing the rotation of the rotor 15. For example, the generating a
braking force can be
accomplished by shorting two or more phases of the windings together. As
another example, the
controller 160 can supply and switch current into the windings in such a way
as to oppose motion of
the rotor 15. Other variations are possible.
In some constructions, the controller 160 establishes specific periods of time
for
alternating the powering of one or more phase windings and the shorting of one
or more phase
windings to align the rotor 15 and stator 20. For example, the time assigned
to the excitation of one
phase winding may be about 350 ps and the time assigned to the braking of the
rotor 15 (by shorting at
least one of the phase windings) may vary between 150 is to 2.86 ms.
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In one construction, the braking time progressively increases from 150 jts to
2.86 ms
through an aligning cycle of 900 ms. The braking time is intended to increase
through the aligning
cycle to allow for relatively little braking force at the beginning of the
aligning cycle and large braking
force at the end of the aligning cycle. Varying the braking time can also
prevent the motor 10 from
generating resonant noise, otherwise created by implementing a constant
excitation time and a constant
braking time.
One variation to the procedure represented by block 420 can be accomplished by
changing the number of phase windings shorted during each braking cycle. For
example, the
controller 160 can generate the opposing braking force by shorting two phase
windings. It is also
within the scope of the invention to short different phase windings in each
braking cycle through the
aligning cycle. Another variation includes adjusting the excitation and
braking times. Yet another
variation includes introducing a coast cycle to the aligning cycle. The coast
cycle allows the rotor 15
to rotate without excitation or shorting of the phase windings. In some
occasions, introducing the
coast cycle is found to allow the rotor 15 to rotate back to the position
where it started before any
excitation when the rotor 15 starts 180 from aligned.
After the rotor 15 is aligned with the stator 20, the controller 160 applies a
set of
relatively longer duration pulses to two phases and monitors the BEMF of the
third phase (at block
425). The longer duration pulses intend to cause rotational movement of the
rotor 15 in the forward
direction. In comparison to the start routine illustrated in Fig. 10, the
pulses applied at block 425 are
generally longer than the pulse applied at block 325. The starting algorithm
in Fig. 10 illustrates
finding the position of the rotor 15 with accuracy of 120 , thus applying the
longer duration pulse (at
block 325) may cause the rotor 15 to rotate in the reverse direction. In
comparison, the starting
algorithm in Fig. 11 illustrates aligning the rotor 15 with the stator 20 to
one of one or more known
positions, thus there is higher certainty of the location of the rotor 15 with
respect to the stator 20
allowing for the application of stronger pulses.
In one construction, block 425 illustrates applying a defined number of pulses
(e.g., 6
pulses) sequentially to two phase windings while concurrently monitoring the
BEMF from the third
phase winding as illustrated in Fig. 8. The BEMF generated by the third phase
winding is indicative of
the relative velocity and location of the rotor 15 with respect to the phase
windings A, B, and/or C of
stator 20. Each pulse is applied or active (at block 425), while the BEMF
generated by the third phase
winding is above a predetermined amount of decrease. That is, the actual value
of BEMF is not
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necessarily required. Rather, the controller can monitor the BEMF for an
amount of decrease (or delta
BEMF) from a previously sensed BEMF (e.g., a sensed BEMF when first applying a
pulse). Once the
delta BEMF has been met, the pulse is no longer generated, a subsequent pulse
is applied, and the
process of monitoring for a delta BEMF is repeated. Applying pulses
sequentially allows for the rotor
15 to gain speed in comparison to applying the longer duration pulse (at block
325). Additionally, as
the rotor 15 gains speed, the pulses become shorter in response to the BEMF
raising and dropping at a
faster rate. Alternative constructions can include applying pulses (at block
425) in sequences which
skip one phase winding when applying the subsequence pulse. Other
constructions may also include
varying the number of pulses applied to the rotor 15 based on the current
speed of the rotor 15.
After the controller 160 applies the longer duration pulses (at block 425),
the
controller 160 proceeds to a coast state (at block 430), turning off the
inverter 150 and monitoring the
BEMF. Monitoring the BEMF allows the controller 160 to determine a period in
relation to the
rotational speed of the rotor 15 (at block 435), and the rotational direction
of the rotor 15 (at block
437).
With reference to block 410, the controller 160 proceeds directly to determine
the
period (at block 435) when the speed of rotor 15 is classified under the fast
moving state (at block
410). It is possible for the rotor 15 to rotate in the reverse direction after
initiating motion (at block
425). It is also possible that the longer duration pulse causes no significant
movement of the rotor 15,
thus disabling the controller 160 from calculating a period at block 435. In
the case the controller 160
determines that the rotor 15 is not moving or the rotor 15 is moving in the
reverse direction (at block
437), the controller 160 shorts the phases A, B, and/or C to stop rotational
movement of the rotor 15
(at block 415). Alternatively, if the controller 160 determines that the rotor
15 is moving in the
forward direction, the controller 160 proceeds to a run mode (at block 440).
Various features and advantages of the invention are set forth in the
following claims.
