Note: Descriptions are shown in the official language in which they were submitted.
i~7190'1
21-DSH-2531
APPARATUS AND METHOD FOR HIGH SLIP
OPERATION OF AN AC ELECTRIC MOTOR AT SUBSTAN-
TIALLY ZERO ROTATION AND SUBSTANTIALLY ZERO TORQUE
BACKG~OUND OF T~E INVENTION
.
Field Of The Invention
The present invention relates generally to AC drive
power conversion systems and, more particularly, to an
apparatus and method for high slip operation of an AC
electric motor at substantially zero rotation and
substantiaLly zero torque, which electric motor being
supplied an outgoing signal, such as a drive current, of
variable magnitude and frequency by an AC electric motor
drive system.
Prior Art
D'rect current (DC) motors have traditionally been
used in electric drive systems to produce a mechanical
rotation over a variable rotation range at substantial
torque levels. However, DC motors exhibit several major
deficiencies, including high maintenance costs and radio
frequency interference problems caused by arcing and
concomitant mechanical deterioration of the brushes used
in such motors.
The trend in recent years has been to use AC motors
in electric drive systems which produce variab'e mechan-
ical rotation of substantial tor~ue. AC motors are
attractive technically and commercially because of their
lack of brushes and inherent ruggedness of design.
11719Q1 21-DSH-2531
--2--
An excellent analysis of the theory and operation
as tlell as the attributes and deficiencies of DC and AC
motor types is found in Electrical Machinery, the
Processes, Devices and Systems of Electromechanical
5 Energy Conversion, 3d Ed., by A. E. Fitzgerald et al,
McGraw-Hill Book Company, New York, 1971.
One type of AC motor is the AC induction motor.
I'he AC induction motor has been used in AC drive power
systems for producing a variable mechanicai rotation of
substantial torque.
In such drive sy~tems, the AC induction motor
produces a variable mechanical ro1:ation of variable
torqu~ in response to an outgoing signal, such as a
drive current, of variable magnitude and frequency.
15 This d rive current typically is supplied from a variable
frequency inverter. The inverter converts a DC current
of cortrollable magnitude into the drive current of
variabLe magnitude and frequency; in the case of the
thyristor inverter, the drive current is generated as a
2~ result of controlled gating of the thyristors. The
inverter typically has commutating capacitors used to
commutate automatically the thyristors. This automatic
commutation produced by the commutating capacitors
requires, however, a charge of appropriate magnitude and
polarity on each commutating capacitor.
The DC current of controllable magnitude provided
to the inverter can be supplied fxom any DC current
source, but typically is provided by a DC converter via
a DC link having an inductor.
A conventional drive system utilizing an AC induc-
ti~n motor typically can provide substantially zero
rotation at substantial torque. One way this can be
accomplished is by operating the induction motor in a
"constant slip" mode. The concept of slip is explained
in detail below, but it is sufficient for present
purposes to state that per-unit slip s is expressed as
9~l
21-DS~-2531
--3--
s - 1 , where n is the rotation produced by the rotor
nb
of the motor in revolutions per minutes (rpm), nl is th~
synchronous rotation of the stator field of the motor in
rpm, and nb is the synchronous speed of th~ stator field
at motor rated rpm. In this regard, reference is made
to pages 188-89 of the FitzgeraLd, et al reference
presented above.
When the drive system produces zero rotation at
substantial torque by operating the induction motor in
a conetant slip mode, the amount of generated torque is
contrclled by varying the magnitude of the drive current.
To prc~uce the substantial torque, the per-unit slip
must have a very low value, for example, 0.02, in order
to operate the induction motor in the required region of
its torque-slip curve.
E~apid acceleration of the motor rotor out of the
zero ~otation, substantial torque mode is possible for
three reasons. First, a sufficient charge of proper
polarity i8 maintained on each commutating capacitor
becauce the frequency of the drive current is very low,
for example~, 1 to 2 Hertz (Hz), but is not O Hz, and the
magnitude of the drive current is high. Secondly, the
flux level in the motor needed to generate torque is
high due to the high magnitude of the drive current.
Lastly, the magnitude of the current flowing through
inductor of the DC link is already high, thus not
requixing a substantial rate of change of current.
Conventionally, when it is desired to operate the
AC induction motor in the substantially zero rotation
and substantially zero torque mode, the magnitude and
frequency of the drive current ar~ reduced substantially
to zeYo values. This results in 3everal problems in
system performance. In high perf~rmance drive systems,
it i9 essential that the AC induction motor be able to
accelerate rapidly on command fro~ the substantially
9~l
- 4 - 21-DSH-2531
zero rotation and substantially zero torque condition.
However, this rapid acceleration is not presently
possible in existing AC electric motor drive systems
because of two inherent problems.
First, in order to produce substantially zero
rotation and substantially zero torque, the frequency
of the drive current must be at a very low value,
typically zero Hz. This low frequency value,
however, causes the requisite charge on the
commutating capacitors to bleed off because the
inverter is not being commutated. The insufficient
charge on the commutating capacitors results
in unsatisfactory commutation when the drive
system is rapidly taken out of the substantially
zero rotation and substantially zero torque
condition.
The second problem is that the required
reduction in the magnitude of the drive current causes
the level of the DC current in the DC link
connecting the inverter with the DC current source
to be at a low level. As stated above, the DC link
typically includes an inductor connected in series
between the DC current source and the inverter. As
is well known, the current through an inductor
cannot be changed instantaneously; instead, a
finite amount of time is required to raise
substantially the level of the current flowing
through the inductor. Thus, a time delay is also
introduced in a conventional system when the system
is taken out of the substantially zero rotation
and substantially zero torque condition due to the
inductor in the DC link~
OBJECTS OF THE INVENTION
.
It is an object of the present invention to provide
an apparatus and method for operating an induction motor
,~
~i719Ql
21-DS~-2531
_5_
at high sli.p to produce substantially zero rotation and
substa~tial.ly zero torque.
It is a further object of the present invention to
provide an apparatus and method for furnishing an idle
control signal when the AC electric motor drive system
is in a substantially zero rotation and substantially
zero torque mode, where the magnitude and freguency of
the drive current are each substantially zero, and for
changing in response to the idle control signal the
magnitude and frequency of the outgoing signal, such as
a drive current, supplied to the induction motor by the
AC electric motor drive system.
It is another object of the present invention to
provide an apparatus and method for substantially simul-
taneously rapidly changing the magnitude of the drive
current to a predetermined magnitude level and ~orraising rapidly the frequency of the drive current to a
higher frequency value when the idle control ~ignal i8
furnished in order to produce a per-unit slip s greater
than 0.1.
It is a further object of the present invention to
provide an apparatus and method for substantially simul-
taneously changing rapidly the magnitude of the drive
current and rapidly raising the frequency of the drive
current when the idle control signal is present so that
the motor is operating in the high slip region of its
torque-rotation cur~e and the rotation and torque
produced by the motor are substantially zero.
These and other objects have been achieved by the
apparatus and method of the present invention.
SUMMARY OF T~E I~ENTION
An apparatus and method for high slip operation of
an AC electric motor at substanti.ally zero rotation and
substantially zero torque is disclosed. The motor is
71 9 ~ 21-DSH-2531
--6--
supplied an outgoing signal, such as a drive current,
of variable magnitude and frequency from an inverter
circuit. The drive system generates an actual rota-
tional signal proportional to the rotation produced by
the motor or in accordance with the frequency of the
drive current. A rotation reference signal i5 estab-
lished proportional to a desired level of rotation. A
rotaticn di~erence signal is generated as a function of
any difference between the rotation reference signal and
the actual rotation signal. A torque command signal is
provided in accordance with the rotation difference
signal. Alternately, the drive system can establish a
torque reference signal proportional to a desired level
of torque, and can generate the torque command signal as
a function of the torque reference signal. In either
case, the torque command signal is used as a function to
produce a feequency control signal and a current control
signal. An inverter supplie~ to the motor the outgoing
signal, such as the drive current, at a frequency
controlled in response to the frequency control signal
and at a magnitude varied in accordance with the current
control signal.
A substantially zero rotation and aubstantially
zero torque mode where the magnitude and frequency of
the drive current are each substantially zero is indi-
cated when an idle control signal is furnished. In the
version of the drive system employing a desired level of
rotation, the idle control signal is furnished when the
torque command signal and at least two of the rotation
reference signal, the actual rotation signal, and the
rotation difference signal are less than respective
predetermined values. In the version of the drive
system employing a desired level of torque, the idle
control signal is supplied when the actual rotation
signal and the torque command signal are 'ess than
respective predetermined values, or, alternately, when
~l ~'7~
21-DSH-2531
--7--
the actual rotation signal and the torque reference
signal are less than respective pr3determined values.
The apparatus and method of the present invention
substantially simultaneously rapidly changes the magni-
tude of the drive current to a predetermined magnitudelevel and rapidly raises the frequency of the drive
current to a higher frequency value when the idle
control signal is present. The higher requency value
is selected to produce a per-unit slip s greater than
0.1, and the predetermined magnitude level is chosen to
maintain the current in the motor at a desired level.
The rapid change in the magnitude of the drive current
and the rapid increase in the frequency of the drive
current causes the motor to produce sub~tantially zero
li rotation and substantially zero torque because it is
being operated in the high slip region of its torque-
slip curve.
BRIEF DESCRIPTION OF ~HE DRAWINGS
Figure 1 is a schematic diagram of an exemplary
motor driv~ system comprising a three-phase AC power
source, corverter, DC link, and autosequentially commu-
tated cont~olled current inverter coupled to an AC
elect~ic mc,tor suitable for use with the apparatus and
methoc of lhe present invention.
Figure 2 plots on graphs 2A, 2B and 2C, respec-
tively, the magnitude of the source current Is, the
motor current IM, and the motor voltage (L-N) on the
respective vertical axes with respect to time on the
horizontal axes for the converter, DC link, and inverter
of Fi~ure 1.
Figure 3 plots 1ux on the vertical axis versus
torque on the horizontal axis of the motor for the
constant slip mode of operation.
li719~1
21-DSH-2531
--8--
Figure 4 plots on the vertical axis slip in Hertz
(Hz) with respect to torque on ~he horizontal axis
produced by a motor operating in the constant slip mode.
Figure 5 plots torque in per unit (P.U.) on the
vertical axis with respect to per-unit slip on the hori-
zontal axis for torque-rotation curves produced by a
conventional induction motor opera~ed at fixed frequency,
with trace A being the torq.ue-rotation curve for rated
drive voltage, with trace B being the torque-rotation
curve for a constant drive current of I - 1.0 P.U., and
with trace C being the torque-rotation curve for a
constant drive current of I = 0.5 P.U.
Figure 6 is a ~chematic block diagram of a preferred
embodiment of the apparatus and method for high slip
operation of an AC electric motor at.substantially zero
rotation and substantially zero torque of the pr~s~nt
invention utilized in an AC electric motor drive system
employing a desired level of rotation.
Figure 7 plots torque on the vertical axis with
respect to rotation on the horizontal axis of the torque-
rotation curves of an induction motor, with trace A
being the torque-rotation curve of the motor driven by a
constlnt drive current I = 1.0 P.U. to produce a high
torque, substantially zero rotation condition, and with
~s trace B being the torque-rotation curve of the motor
driven by a constant current of I = 0.2 P.U. and after a
sudden jump in frequency of the motor drive current to
produce high slip operation at substantially zero rota-
tion and substantially zero torque in accordance with
^o the apparatus and method of the present invention.
Figure 8 includes five separate graphs plotting
id~ntlcal time periods on horizontal axes, and plotting
on vertical axes, respectively, the levels of the actual
rotation signal, the torque command signal, the torque,
the motor stator current, and the idle control signal as
the drive system enters, stops, and then exits the high
,
~7~.9 ~ ~
21-DSH-2531
_g_
slip subfitantially zero rotation and substantially zero
torque mode in accordance with the apparatus and method
of the present invention.
Figure 9 i~ a schematic block diagram of a preferred
embo~1iment of the apparatus and method for high slip
o~eratlon of an AC electric motor at sub~tantially zero
rotation and substantially zexo torque utilized in an AC
electric motor drive system employing a desired level of
torque.
1~ DETAII.ED DESCRIPTION OF THE INVENTION
Figure 1 is a schematic diagram of a typical three-
phase AC power source converter 22, DC link 24, and
autosequentially commutated controlled current inverter
14 in an AC electric motor drive system employing an AC
elect~ic motor 18. Figure 2 plots on graphs 2A, 2B
and 2C, respectively, on vertical axes the magnitude of
the source current Is, the motor current IM, and the
motor voltage (L-N) with respect to time on the hori-
zontal axes for the converter 22 and inverter 14 of
Figure 1.
As graphs 2A and 2B show, the frequency of the
source current IS supplied by the AC power source 28
to converter 22 is substantially constant, for example,
60 Hz, whereas the motor or drive current IM of variable
magnitude and frequency supplied by the autosequentially
commutated controlled current inverter 14 can be varied
from, for example, 0 to S0 Hz.
The rotation n produced by the rotor of the AC
electric motor is varied in accordance with the frequency
of the drive current IM and the amount of torque required
to be produced.
The concept of slip of an AC induction motor is now
defined. Rotation of the rotor is defined as being
equal to n revolutions per minute (rpm), and the rotation
~ ~ 7~
21-DSH-2531
--10--
of the stator field of the motor is defined as being
equal ~o nl rpm; the rotation of the rotor and the
stator field are assumed to be in the same dir~ct~on.
Tle slip of the ro~or is defined as being equal to
nl - n rpm. Another way to look at it is that the slip
is the difference between the value of rotation of the
stator field and the value of rotation produced by the
rotor. This relationship often is expressed as a per-
unit slip s expressed as s = 1 , where n is the
nb
1~ rotation produced by the rotor of the motor in rpmt n
is the synchronous speed of the stator field of the
motor in rpm, and nb is the synchronous speed of the
stato~ field at motor rated rpm. Thus, it i8 apparent
that as the value of the per-unit slip s increases from
1~ o to 1, the value of the rotation n produced by the
rotor decreases with respect to ~he xotation value nl of
the stator field.
Figure 5 plots the value of torque on the vertical
axis with respect to per-unit slip s on the horizontal
axis for an AC induction motor excited at constant
frequency for the condition where the motor i3 driven by
a drive signal at rated voltage ~trace A), by a drive
current of a constant value at I - 1.0 per unit (P.U.)
(trace B), and by a drive current of a constant value at
I = 0 5 P.U. (trace C).
~ rrace A shows that in the case of a drive signal at
rated voltage the torque produced by the motor first
incre~ses between per-unit slip values of 0 to .275 and
then decreases at a substantially constant rate between
per-unit slip values of .3 to 1Ø
Trace B shows that when the motor is provided with
a drive current having a constant value of I - 1.0 P.U.,
torque increases rapidly between the per-unit slip values
lt7196~1
21-DSH-2531
of O to 0.04, then decreases very rapidly between the
per-unit slip values of .05 ~o .3, and then decrea~e~ at
a much more gradual and constant rate between the per-
unit slip values of .3 to 1.0
Trace C plots the torque produced by the electric
motor supplied with a drive current having a constant
value of I = 0.5 P.U. Trace C shows that the tor~ue
produced by the motor with respect to per-unit slip is
substantially similar to the tor~ue produced by the
motor plotted by trace B, with the exception that the
torque level for each corresponding per-unit slip value
is reduced approximately as the square of the value of
the drive current. As is well known, there is a whole
family of curves for drive current3 from 1.0 P.U. to
0 P.U. between trace B and the horizontal axis of
Figure 5.
Conventional electric motor drive systems typi-
cally provide the desired amount of torque and rotation
by maintaining a low value for the per-unit slip, for
example, 0.01 to 0.03, and by varying the magnitude
and/or frequency of the drive current.
Trace A of Figure 7 plots torque on the vertical
axis versus rotation on the horizontal axis for the
constant slip mode used in variable frequency electric
motor drive systems to generate high torque at substan-
tiall~ zero rotation from the motor using a drive
current of a high constant current value of I = 1.0
P.U., as was described above. As trace A shows, the
rotat~on pxoduced by the motor is substantially zero
when e~slip value of approximately 2 Hz is used. ~he
use o~ constant slip for producing high torque at
substantially zero rotation is also shown by the plot of
Figure 4.
,g~e~
21-DSH-2531
-12-
As was discussed above, prior art variable frequency
electric motor drive systems utilizing a controlled
curren~ inverter cannot produce a substantially zero
rotation and substantially zero torque without detri-
mentally affecting the ability of the system to exit andaccelerate rapidly out of this condition because the
magnitude of the drive current in the constant slip mode
must be reduced substantially to zero or the frequency
of the inverter must be reduced to zero, causing the
charge on the commutating capacitors to bleed off.
The apparatus and method of the present invention
overcomes these deficiencies by operating the AC elec-
tric motor at high slip to produce substantially zero
rotation and substantially zero torqu~. High slip
operation allows the frequency of the drive current to
be high enough to maintain desired charge on the commu-
tating capacitors. In addition, it allows a higher
magnitude for the drive current and still obtain the
desired substantially zero rotation and substantially
2~J zero torque. Because the drive current can have a
higher magnitude, the current flowing through the
inductor of the DC link is also at a higher value.
Thus, the apparatus and method of the present invention
overcomes the two deficiencies discussed above which are
present in conventional AC electric motor drive systems
operating at substantially zero rotation and substan-
tially zero tor~ue.
The apparatus and method of the present invention
is now described in summary form; preferred embodiments
for implementing the present invention are pre~ented in
detail below.
The conventional electric motor drive system is
brought into the conventional substantially zero rota-
tion and substantially zero torque condition in response
~5 to a desired level of rotation or a desired level of
torque. At this point, the drive system is providing
21 DSH 2531
- 13 -
drive current havin~ substantially zero frequency.
Immediately after this condition is achieved, however,
the apparatus and method of the present invention causes
substantially simultaneously the magnitude of the drive
current to be changed to a predetermined magnitude level
and the frequency to be rapidly raised to a higher
frequency value so that the motor is operating at a
per-unit slip greater than 0.1, as shown by trace B of
Fi~ure 7. Because -the motor is ~eing supplied drive
current having the current magnitude and per-unit slip
shown by trace B of Figure 7, a substantially zero
rotation and substantially zero torque is produced.
Thus, the apparatus and method of the present invention
overcomes the deficiencies present in conventional systems
by utilizing effectively the low torque produced by an
AC lnduction motor provided with a dri.ve current of
substan-tial magnitude when the motor is operating in
high per-unit slip.
Turning to Figure 6, a schematic block diagram of a
preferred embodiment of the apparatus for high slip
operation of an AC electric motor at substantially zero
rotation and substantially zero torque of the present
invention utilized in an AC electric motor drive system
employing a desired level of rotation is shown. The
apparatus of the present invention is shown in a preferred
embodiment basically within a dashed-line box 10; the
circuitry outside box 10 is essentially a conventional AC
electric motor drive system employing a desired level o~
rotation command.
The apparatus and method of the present invention
can be utilized in other types of electric drive systems.
The system shown in Figure 6 is only for purposes of
illustration, and is similar to the system disclosed and
claimed in Canadian Patent Application Serial No. 331,769
entitled "Controlled Current Inverter and Motor Control
System," to P.M. Espelage et al, filed July 13, 1~79 r
'.~
~ .~ 7 1 9~1 21-D~-2531
-14-
assigned to the assignee of the present invention.
Another suitable electric drive motor system is shown
in A. B. Plunkett, D'Atre, J. D., Lipo, T. A.,
entitled "Synchronous Control of a Static
5 AC Induction Motor Drive," IEEE/IAS Annual Meeting
Con~erence Record, 1977, pp. 609-15.
Referring to Figure 6, a variable frequency
inverter 14 provides an outgoing signal, such as a drive
current, of variable magnitude and frequency via a
0 line 16 to a load, such as AC motor 18. AC motor 18 can
be of any suitable type, but prefera~ly is an AC induc-
tion motor.
~ nverter 14 can be of any suitable type for
converting a DC input signal to a drive current of
variable frequency under control of a variable frequency
gating signal, also referred to as a frequency control
signaL, on an input line 20. One preferable form for
inverter 14 is an autose~uentially commutated controlled
current inverter having a 6-thyristor bridge, such as
inverter 14 of Figure 1, which generates the drive
current of variable magnitude and frequency in accor-
dance with the gating of the thyristors.
The DC input current to inverter 14 can be provided
by any suitable variable DC current source. One
preferred embodiment for the variable DC current source
is a converter 22, which supplies variable magnitude DC
current via a DC link 24 to the input of inverter 14.
Converter 22 converts AC power supplied through termi-
nals 28 under control of phase controlled gating signals
on lines 26 to a DC current of variable magnitude.
The phase controlled gating signals are also referred to
herein as the current control signal. Converter 22 can
be of any suitable type but, most typically, would be a
6-thy~istor phase controlled converter whose thyristors
are p~ovided with gating pulses by the current control
signal on line 26, as shown by converter 22 of Figure 1.
7~ 9 ~ 21-DSH-2531
-15-
The DC current of variable magnitude (I~c) i~
provided to inverter 14 via DC link 24. DC link 24 can
take any suitable form, but preferably includes an
inductor 30 connected in series between converter 22 and
S inverter 14. Inductor 30 acts as a filter.
Thus, the magnitude of the drive current supplied
by inverter 14 to line 16 i~ controlled by the current
control signal supplied to converter 22, and the
requency of the drive current is varied in accordance
with the frequency control signal furnished on line 20
to inverter 14.
The electric motor drive system shown in Figure 6
is a closed loop system having the following feedback
paths. The actual rotation produced by motor 18 is
sensed and used to generate an actual rotation signal
on a line 34 proportional to the mechanical rotation.
One c;uitable form for generating the actual rotatian
signal is,a DC tachometer 32. Another approach for
generating the actual rotation signal i~ by ~ensing
the ~requency of the drive current. Furthermore,
othe~ approaches for generating the actual rotation
signal are contemplated by this invention.
A desired le~el of rotation is used to establish a
rota:ion reference signal proportional thereto. The
desired level of rotation can be furnished from either a
system or user command; and most typically is in the
form of a rotation user command from an operator settable
rheostat 38 having a wiper arm 40 connected to a user or
operator rotation control level (not shown).
The rotation reference signal from wiper arm 40 is
provided to a first input of a summing junction 42.
The actual rotation signal is negatively fed back and
~l-DS~2531
-16-
provided to a second input of summing junction 42. The
output of summing junction 4~ is a rotation difference
signal, which is representative of any diference between
the rotation reference si~nal and ~he actual rotation
signal and is provided to the input of a rotation regu~
lator 44. ~otation regulator 44 can ~e o~ any ~uitabl~
type to genera~e on a line 46 a torque cosnmand ~ignal as
a function of the ro~ation diference signal. One suit-
able f~rm for rotation regulator 44 is an operational
amplifier configured ~o operate as a gain amplifier
havinc, for example, a transfer ~unction of k 1 ~ st ,
where s is a LaPlace operator, t is a time constant, and
k is ~ gain constant.
Lina 46 is connected to the input 165 of an elec-
tronic switch 16~. As is discussed below, el~ctronic
switch 164 is part of the present invention. Electronic
switch 164 is adopted to connect its output 166 effec--
tively to elec~rical ground in response to a ~wi~ching
signal or i~le control signal applied to a switching
input 167 so as to cause the torque command signal
effectively to assume a substantially ~ero valu~. ~lec~
tronic switch 164 can be of any suitable type, such as a
bipolar transistor or field e~ect transistor switch or
an electro~echanical relay.
The torque command signal is applied via llne 46 to
the input of an absolute value stage 156 of conve~tional
design. The absolute value version of the torque com~and
signal at the outpu~ of the absolute value stage 156 is
applied to the input of an electronic switch lS8. As is
discussed below, electronic switch 158 ia part of the
present invention. Electronic switch 158 normally
connects its input to a first output 159, but is adapted
to connect its first output 159 to a second input 160
in response to a switch signal or idle control signal
5 applied to a switching input.I61. Electronic switch 158
can be of any suitable type, such as a bipolar transistor
19~1
21 DS~I 2531
- 17 -
or field effect transistor switch or an electromechan-
ical relay. When the first output 159 of electronic
switch 158 is caused to be connected to the second input
160, the level of the torque command at the first output
159 is caused to be forced to a level corresponding to
a predetermined current level, whereby the magnitude
of the drive current is forced to a predetermined
level.
The first output 159 is applied to the first input
10 of a summer 162 of conventional design. A shunt 168 is
mounted to sense the magnitude of the DC current (IDC) at
the side of inductor 30 connected to inverter 14. Shunt
168 provides on a line 169 a signal indicative of this
magnitude level. The signal on line 169 is negatively
fed back and provided to a second input of summer 162.
The output of summer 162 provides to an input 48 of a
current control stage 50 a signal representative of the
difference between the absolute value version of the torque
command signal and the signal indicative of the magnitude
of the DC current (IDC).
Current control stage 50 can be of any suitable type
for generating the current control signal on line 26 in
accordance with the signal at input ~8. One suitable form
for current control stage 50 is that of a ramp and pedestal
gating control of conventional design.
The torque command signal on line 166 is also applied
to the first input of a summer 170 of conventional design.
The actual rotation signal is positively fed back and
provided to a second input of summer 170. The output of
summer 170, which is a signal proportional to the sum
of the torque command signal and the actual rotation signal,
is provided to the input of an electronic switch 172.
Electronic switch 172 is part of the present invention.
Switch 172 normally connects the output of summer 170
to its output, but is adapted to connect the output
to a high frequency reference signal source when
.~
:1 ~ t7~ 9~
21 DSH 2531
- 18 -
a frequency command signal or an idle control signal is
provided by a line 173 to its switching input 174.
Electronic switch 172 can be of any suitable type,
such as a bi-polar or field effect transistor switch or
an electromechanical relay. As is discussed in detail
below, when switch 172 is caused to connect its output
to the high frequency reference signal source, the
signal at its output is effective to cause a f.equency
control stage 54 to force inverter 14 to raise the
frequency value in order to produce the desired high
slip.
The output of electronic switch 172 is connected
to an input 52 of frequency control stage 54. Frequency
control stage 54 can be of any suitable type for
generating the frequency control signal as a function of
its input, which for the normal operation discussed above
is the signal proportional to the sum of the torque command
signal and the actual rotation signal. The frequency
control signal is provided to inverter 14 via line 20.
One suitable form for frequency control stage 54 is
that of a voltage controlled oscillator and a non- -
recirculating shift register disclosed and claimed in
United States Patent No. 4,258,414, entitled "Inverter
Power Conversion System Having Improved Control Scheme",
25 to Loren H. Walker et al, issued March 24, 1981 and
assigned to the assignee of the present application.
Another suitable form for frequency control stage 54
is that of a voltage controlled oscillator and a ring
counter.
The drive system shown in Figure 6 allows the
mechanical rotation and torque generated by AC
induction motor 18 to be controlled in accordance with
the desired level of rotation. Figure 8 plots on five
separate graphs important drive system parameters as
the drive current causes motor 18 to enter, stop r
~*~
9g~
21-DSH-2531
--19--
and exit the substantially zero rotation and substan-
tially zero torque mode at high 51ip.
The horizontal axis of each of the graphs 8A-8E
repre~ents an identical time period in the drive system
operation, where the time period to the left of symbol Tl
represents the drive system supplying drive current to
motor 18 causing it to enter the conventional substan-
tially zero rotation and substantially zero torque mode
where the frequency of the drive current is substantially
0 zero. The time period between symbols Tl and T2 repre-
sents the time period in which the drive system supplies
drive current causing motor 18 to remain in the substan-
tially zero rotation and substantially zero torque mode
at moderate current magnitude and high ~lip; the time
period to the right of symbol T2 represents the time
period. where the drive system supplies drive current
causing motor 18 to exit and accelerate out of the
subst~ntially zero rotation and substantially zero
torque high slip mode.
6raph 8A plots on the vertical axis the level of
the actual rotation signal. It i5 seen that the actual
rotation signal is approximately zero when the drive
system causes motor 18 to stop in the substantially zero
rotation and substantially zero torque high 51ip mode.
Graph 8B plots on the vertical axis the level of
the torque command signal, which is approximately zero
when the drive system causes motor 18 to stop in the
subst3ntially zero rotation and substantially zero
torque high slip mode.
Graph 8C plots on the vertical axis the level of
the t)rque generated by motor 18, which is approximately
zero when the drive system causes motor 18 to ~top in
the substantially zero rotation and substantially zero
torque high slip mode.
Graph 8D plots on the vertical axis the level of
the motor stator current for one winding of polyphase
719~1
21DSH 2531
- 20 -
motor 18. Graph 8D shows that this stator current is
at high frequency, moderate magnitude level when motor
18 is in the substantially zero rotation and
substantially zero torque high slip mode.
An idle control signal is furnished in the
electric motor drive system of Figure 6 when the system
is in the conventional substantially zero rotation and
substantially zero torque mode where the frequency of
the drive current is substantially zero. An apparatus
and method for furnishing the idle control signal is
disclosed in Canadian Patent Application Serial
Number 350,526, entitled "Zero Rotation and Zero
Torque Detector and Method for an AC Electric Motor
Drive," to Loren H. Walker and John H. Cutler, filed
April 24, 1980, and assigned to the assignee of the
present invention.
In the case of a drive system utilizing a
desired level of rotation, the idle control signal is
furnished when the torque control signal is furnished
when the torque command signal and at least two of the
rotation reference signal, the actual rotation~signal,
and the rotation difference signal are less than
respective predetermined values. Alternately, in the
case of a drive system utilizing a desired level of
torque as shown in Figure 9, the idle control signal is
furnished when the actual rotation signal and the
torque reference signal are less than respective
predetermined values.
Graph 8E plots the vertical axis the presence
of the idle control signal. The idle control signal is
in the low state when the drive system is not in the
substantially zero rotation and substantially zero
torque high slip mode, and goes to the high state when
the system enters the conventional substantially zero
rotation and substantially zero torque condition.
9~
21-D5H-2531
-21-
Referring again to Figure 6, a preferred embodiment
of t~e apparatus of the present invention is shown for a
drive system employing a desired level of rotation. An
absolute magnitude circuit 100 has an input connected
via line 102 to the torque command signal on line 46.
Absolu~:e magnitude circuit 100 can be of any suitable
type for providing on an output line 104 an ab~olute
magnitude version of the torque command signal.
The absolute magnitude version of the torque
command signal is provided to a first input of a voltage
comparator 106, whose second input is connected to a
source of reerence voltage 108. The level of the
reference voltage correspon~s to the respective prede-
termined value below which the ab~olute magnitude
version of the torque oommand signal must have in order
for th~ system to be in the substantially zero rotation
and substantially zero torque mode. Reference voltage
source 108 can be of any suitable type for generating a
reference voltage at the predetermined value.
0 Voltage comparator 106 can be of any suitable form
for furnishing a first output signal on line 110 when
the absolute magnitude version of the torque command
signal is less than the level of reference voltage
source 108. One suitable form for voltage comparator
'5 106 is that of an operational amplifier connection in
the voltage comparison mode.
The input of a ~econd absolute magnitude circuit
112 is connected via a line 114 to the actual rotation
signal on line 34 for providing an absolute magnitude
~0 version of the actual rotation signal on an output
line 116. Absolute magnitude ~tage 112 can take any
suitable form. The absolute magnitude version of the
actual rotation signal on line 116 is provided to a
first input of a voltage comparator 118. The second
1,~'719~1
21-DS~-2531
-22-
input of comparator 118 is connected to reference
volta~ge source 108, and provides at an output 120 a
second ou~put signal when the magnitude of the absolute
magni.tude version of the actual rotation signal is less
than the level of reference voltage source 108.
The input of a third absolute magnitude circult 122
is connected via an input line 124 to the rotation
reference s:.gnal at wiper arm 40. Absolute magnitude
circuit 122 provides at an output line 126 an absolute
magnitude version of the rotation reference signal. The
absolute ma~nitude version of the rotation reference
signal on oltput line 126 is supplied to a fir~t input
of a voltag 3 comparator 128, whosé second input i~
connected to reference voltage source 108. Comparator
128 provide 3 on an output line 130 a third output signal
when the ab301ute magnitude version of the rotation
reference signal is less than the level of the reference
voltage signal.
T.`le input of a fourth absolute magnitude circuit 180
is connected via a line 182 to the rotation difference
signal at the output of summer 42 for providing an abso-
lute magnitude version of the rotation difference signal
on an output line 184. Absolute magnitude stage 180 can
take any suitable form. The absolute magnitude version
Of the rotation difference signal on line 184 is provided
to a first input of a voltage comparator 186. The second
input of comparator 186 is connected to reference voltage
source 108, and provides at an output 190 a fourth output
signal when t.he magnitude of the absolute magnitude
~ version of the rotation difference signal i~ less than
the level of reference voltage source 108.
It should be understood that voltage comparators
106, 118, 128 and 186 each could be connected to a
different reference signal source providing reference
signals of different levels. The use of different
reference sources is one way to provide for different
9 ~ 1 21-DS~-2531
-23-
predetermined levels below which the torque command
signal and at least two of the rotation referen~e
signal, the actual rotation signal, and the rotation
diferl~nce 3ignal must be in order for the idle control
signal to be furnished.
A; sho~n, the output signal~ from voltage compa-
rators 106, 118, 128 and 186 on lines 110, 120, 130
and 190, re~pectively, are applied to a logic circuit
140, wnich furnishes the idle control signal only when
1~ the first control signal and two of the second to fourth
control signals are present. It should be understood
that the present invention can al~o be configured to
provide th~ idle control signal when each of the fir~t
to fourth control signals are present. Xormal,ly, only
l'j two of the second to fourth control signals are used,
however, because the information in the unused control
signal is present in the two control signal~ that are
used. Logic circuit 140 can be of any 3uitable type,
such aq an AND gate or a NAND gate. The output from
logic circuit 140 on line 142 is the idle control
signal, and indicates that the drive system is in the
substantially zero rotation and sub~tantially zero
torque mode of operation.
A delay of prede~ermined time amoun~ can be intro-
duced before furni3hing of the idle control signal to
prevent the idle control signal from being generated
transiently when the drive system momentarily pa~es
through the conven~ional ~ubstantially zero rotation and
substantially zero torque mode. This delay of a prede-
termined time amount aan be produced by applying the
idle control signal on line 142 to a delay stage 144
which can be of any suitable decign~ e.g., a one ~hot
and a gate. Delay stage 144 ha~ a delay on ri~e and no
delay on fall, for example, 0.1 second on rise.
The idle control signal on output line 150 of delay
stage 144 causes three functions as represented by block
~'J~ 9 ~ 1 21-DSH-~531
-24-
148. Block 148 represents the three functions produced
by the signal on line 150. In structure, block i48 may
be nothing more than three lines to conduct the signal
on line 150 to the three switches as shown.
The first function represented by block 148 is to
provide a witching signal, i.e., the idle control
signal on line 173 to electronic switch 172 effectively
to cause the frequency of the drive current to be rapidly
raised to a higher frequency value (in accordance with
l~ the signal provided by the high frequency reference
source) to produce the desired high per-unit slip. The
idle control signal on line 173 can cause frequency
control sta-~e 54 effectively to raise the frequency
of the driv3 current to a preselected higher frequency
value ~n ac~ordance with the signal provided by the high
frequency r~ference source, or can cause the frequency
control stage to generate a higher frequency value to
produc~ the per-unit slip greater than 0.1. In either
case, the i~le control signal on line 173 to electronic
switch 172 causes switch 172 to provide the signal from
the high frequency reference source to input 52 of
control stage 54, effectively causing frequency control
stage 54 tc raise the frequency of the drive current to
produ(e th~ desired per-unit slip greater than 0.1. A
suitable v~lue for the higher frequency value is 12 Hz,
when the maximum frequency supplied by converter 14 is
60 Hz.
$he second function represented by block 148 ig
to prc,vide a switching signal, i.e., the idle control
signal, on a line 176 to electronic switch 158 effec-
tively to cause the magnitude of the drive current to be
rapidly changed to a predetermined magnitude level deter-
mined by the predetermined current level. The idle
control signal on line 176 ~or rapidly changing the
magnitude of the current can cause current control
stage 50 arbitrarily to change the current to a prese-
~ 1.9~
21-DSH-2531
-25-
lected magnitude level in accordance with ~he predeter-
mined current level. The predetermined magnitude levql,
as stated above, causes the level of the voltage o~ the
commutating capacitors in the inverter to be maintaine~
at a desired level.
In another aspect of the apparatus and method of
the present invention, a third function represented by
block 148 can be included which provides a switching
signal, i~e., an idle control signal, on a line 177 to
electronic ~witch 164, cau~ing switch 164 to clo~e so as
to reduce rapidly the torque command signal to sub~tan-
tially zero. ~5 iS seen in Figure 6, this reduction to
substantially zero is due to electronic switch 164
grounding line 166. The reduction of the torque command
signal to zero when the drive system is in the sub~tan-
tially zero rotation and substantially zero torque high
slip mode prevents sudden transients in drive system
performance from occurring when the drive system exits
this mode.
~g is apparent, the present invention can produce
the desired high slip condition of the motor in th-
substantially zero rotation and substantially zero
torque mode using other forms of control signalling
and using different types of AC electric motor drive
systems.
Referring to Figure 9, a preferred embodiment of
the apparatus and method of the present invention for
use in an AC electric motor drive system employing a
desired level of torque command is shown within a dashed-
line box 200. The conventional AC electric motor drive
system employing a desired level of torque commaDd shown
in Fiyure 9 is outside of dashed-line box 200, and is
similar to the drive sy~tem disclosed in the Espelage
et al patent application, Canadian Patent Application
Serial No. 331,769, discussed above Like numbers in
Figures 6 and 9 corresponds to identical components;
only different components are discussed herein.
:1 1', ~9~
21-DSH-2531
-26-
A torque reference signal proportional to a de~ired
level of torque is provided on a line 300. This torque
reference signal can be provided by the drive sy~tem, or
can be furnished by an operator settable rheostat 302
having a wiper arm 304. The position of wiper arm 304
corresponds to the desired level of torque indicated by
the position of a user torque lever (not shown).
The torque reference signal is provided a~ an input
of a torque regulator 306, which generates the torque
command signal on line 46 as a function of the torque
reference signal. Torque regulator 306 can be of any
suitable type to generate the torque command signal in
accordance with the torque reference signal. One ~uit-
able form for torque regulator 306 is an operational
amplifier of conventional design configured to operate
as an amplifier exhibiting a suitable gain.
In the drive system employing a desired level of
torque, the idle control signal i~ furnished when the
actual rotation signal and the torque reference signal
are less than respective prsdetermined values. The
respective predetermined values can be different for the
torque co~mand signal and the actual rotation signal,
but the values indicate when the drive system is in the
conventional s~bstantially zero rotation and substan-
tially zero torque mode when the frequency of the drive
current is of a very low value so as to produce the
constant slip mode of operation.
Referring again to Figure 9, the actual rotation
signal is applied via line 322 to an absolute magnitude
circuit 320. Absolute magnitude circuit 320 can be any
suitable type or providing on an output line 324 an
absolute magnitude version of the actual rotation
signal.
The absolute magnitude version of the actual rota-
tion signal is applied to the first input of a voltage
comparator 326. A second input of voltage comparator
~ ~'719~1
21-DSH-2531
-27-
326 is connected to a reference voltage source 328,
which provides a reference signal at a value equal to
the respective predetermlned value. Voltage comparator
326 provides a second output signal when the absolute
magnitude version of the actual rotation signal is less
than the predetermined value. Voltage comparator 326
can be of any suitable type, for example, an operational
amplifier connected in the voltage comparison mode. An
alternative shown on Figure 9 use the signal at line 171
rather than the actual rotation at line 34 via dot-dash
line 308 as the input to 322. The signal at line 171
is normally proportional to actual frequency rather than
actual rotation.
An input line 330 of an absolute magnitude circuit
332 is connected to the torque reference signal on line
300. Absolute magnitude circuit 332 provides an abso-
lute magnitude version of the torque command signal on a
line 334 con~ected to the first input of a comparator
336. ~he second input of voltage comparator 336 i3
'0 conne~,ted to reference voltage source 328. Voltage
comparator 336 generates a first output signal when the
va}ue of the absolute magnitude Version of the torque
command signal is less than the reference voltage signal
equal to the respective predetermined value furnished by
reference voltage ~ource 328. Voltage comparator 336
can be of any suitable type, for example, an operational
amplifier connected in the voltage compari~on mode.
The first output signal from comparator 336 and the
second output signal from voltage comparator 326 are
applied to a logic circuit 340, which provides the idle
control signal as an output on line 342 when both of the
two output signals are in the high state. Logic stage
1 1'7~ ~Q~
21-DS~-2531
-28-
340 can be of any suitable type for providing the idle
centI-ol signal when each of the two output signals are
in the high state, for example, an AND gate or a NAND
gate.
As in :he case of the embodiment sh~wn in Figure 6,
the idle control signal at output 346 of delay stage 344
causes three functions to be produced by block 348. It
should be noted that the idle control signal without
delay can be provided by line 342 directly to block 34B
which simil~rly to block 148 in Figure 1 may be only a
function re?resentation block.
The ficst function produced by block 348 i5 to
provid3 a s~itching signal, i.e., the idle control
signal on line 173 to electronic switch 172 effectively
to cause the freguency of the drive current to be
raised rapidly to a higher frequency value in ac.cor-
dance with the signal provided by the high frequency
reference cource to produce the desired high per-unit
slip. The idle control signal on line 173 can cause
frequency control stage 54 effectively to raise the
frequency c,f the drive current to a preselected higher
frequency value in accordance with the signal provided
by the high frequency reference ~ource, or can cause the
frequency control ~tage to generate a higher fre~uency
;'5 value to produce the per-unit 51ip greater tha~ 0.1. In
eithe case, the idle control ~ignal supplied on line 173
to electronic switch 172 causes switch 172 to provide
the s:.gnal from the high frequency reference ~ouxce to
input 52 of control stage 54,'effectively causing
frequ3ncy control stage 54 to raise the frequency o the
drive current to produce the desired per-unit slip
greater thln 0.1. A suitable value for the higher
frequency ~alue is 12 Hz, when the maximum frequency
SUppl ied by converter 14 is 60 Hz.
.
~ ~'719~'1
21-DSH-2531
-29-
The second function produced by block 348 i~ to
provide a switching signal, i . e ., the idle ~ontrol
signal, on a line 176 to electronic switch 158 effec-
tively to cause the magnitude of the drive current to
be rapidly changed to a predetermined magnitude level
determined by the predetermined current level. The
idle control signal on line 176 for rapidly changing the
magnitude of the current can cause current control
stage 50 arbitrarily to change the current to a prese-
lected magnitude level in accordance with the predeter-
mined level. The predetermined magnitude level, as
stated above, causes the level of the voltage on the
commutating capacitors in the inverter to be maintained
at a desired level.
In another aspect of the apparatus and method of
the present invention, a third function can be produced
by block 348 when it provides a switching signal, i.e.,
an idle control signal on a line 177 to electronic
switch 164, causing ~witch 164 to close so as to reduce
rapidly the torque command signal to substantially zero.
As is seen in Figure 6, this reduction to substantially
zero is due to electronic switch 164 grounding line 166.
The reduction of the torque command signal to zero when
the drive system is in the substantially zero rotation
and substantially zero torque high slip mode prevents
sudden transients in drive system performance from
occurring when the drive system exits this mode.
~ hile there have been shown and describQd what i8
at pre.sent considered to be the preferred embodiments
~0 of the present invention, modifications thereto will
readi:Ly occur to those skilled in the art. It is not
desired, therefore, that the invention be limited to
the specific arrangements shown and described, and it
is intended to cover in the appended claims all such
:5 modifications as fall within the true spirit and scope
of the invention.
