Note: Descriptions are shown in the official language in which they were submitted.
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- 1 - Case 2806
CONTROLLED PULSING OF AIR CLUTCHES
= FOR STOPPING A GRINDING MILL
The present invention relates to a system having
one or more dynamoelectric machines driving a load. In
particular, it relates to a control means for stopping
a drive system having a large load driven by one or
more synchronous motors.
It is common to drive large loads, such as
grinding mills, by having a large diameter ring gear
attached to the mill driven by pirlions which are in
turn mechanically coupled through clutches to one or
two synchronous motors. Usually, driving power to the
grinding mill is discontinued by open circuiting
circuit breakers to cut power to t:he motors and by
releasing the clutches. As a result, separately
rotating pieces of equipment, i.e., motors and mill
; 15 are left coasting. A synchronous motor may take as
long as twenty minutes to run down to standstill. The
mill normally begins to oscillat~ and may be in motion
for three or four minutes. It is not possible to work
on the mill until the mill has stopped oscillating;
however, work normally does not begin on the mill
until the motors have come also to a standstill so as
to ensure the safety of the workers.
It is very desirable to have all moving units
come to rest as quickly as possible to reduce the mill
shutdown time as well as to reduce the risk of the
load freezing in the mill.
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One system that overcomes the above disadvantages
is disclosed in U.S. Patent No. 4,298,113 issued
November 3, 1981 to Shaver et al. In this patent, a
wet clutch is utilized to mechanically couple the
synchronous motors with the mill. Further, a brake is
provided on the output side of the wet clutches. A
distinction should be noted between wet cutches and
dry clutches. Wet clutches transmit torque by fluid
shear between the mating friction surfaces. The
fluid, usually oil, is circulated in a very thin film
between the clutch plates which are not in actual
sliding contact. The heat is generated in the oil
film between the plates and may be removed from the
oil by external cooling means. Wet clutches have
inherently high thermal capability, and as a result
may be allowed to slip continuously without
significant wear. Wet clutches are expensive, and
tend to be mechanically critical. Conversely, dry
clutches, where friction shoes actually rub against
the mating clutch surfaces, have limited thermal
capacity, cannot stand a significant amount of
continuous slipping, are not mecharlically critical,
are a fraction of the cost of the wet clutch, and as
such are widely used in industry. The aforementioned
U.S. Patent discloses a method of stopping the mill
comprising slowing the mill down until it stops
rotating and thereafter slowly lowering the mill so
that the load in the mill comes to rest at the bottom
dead center position of the mill. To accomplish this,
pressure in the wet clutches is reduced wikh the
motors running at synchronous speed to permit the mill
to slow until it stops rotating. Thereafter, the wet
clutches provide sufficient torque to just balance the
force created by the mill load which is held up at an
angle. The angle is normally of a considerable size
in the order of 55 to 60 degrees. This requires a
large amount of heat dissipation because the motors
.,
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are running at full speed and the torque required by
the clutches to balance the load is large, perhaps in
the order of 75 percent of full load torque. Clutch
pressure is then gradually reduced to its minimum to
permit the mill to approach bottom dead center, at
which time the brakes are applied to hold the mill.
The U.S. Patent further discloses that, if desired,
the synchronous motors may then be stopped by
disconnecting the power supply to the motors and
engaging the clutches periodically to bring the
synchronous motors to a stop. The problem with this
method is that a significant amount of heat must be
dissipated through the clutches. This amount being
beyond the capability of dry clutches. Therefore, the
type of clutch required to stop the mill quickly is
limited to clutches that are able to dissipate large
amounts of heat quickly such as wet clutches with
external oil coolers. Wet clutches are considerably
more expensive and add significant costs to the mill
possibly making the purchase of the mill by a customer
economically undesirable.
It is therefore an object of the present
invention to provide a control means for quickly
stopping the motion of a mill and its synchronous
motors which does not have to dissipate large
quantities of heat in a concentrated manner through
the clutches.
It is a further object of the invention to
provide a control means for stopping the motion of a
mill regardless of the speed of the mill and motors
when the stop is initiated.
Briefly, the present invention provides a control
means for stopping the motion of a drive system having
a large load driven by one or more dynamoelectric
machines coupled to the load through respective
clutches. The control means comprises a clutch
; actuating means and an enabling means. The clutch
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actuating means applies a predetermined amount of
pressure of predetermined duration to the clutches
sufficient to at least partially close the clutches
but insufficient to fully close and lock-up the
S clutches. The enabling means periodically enables the
clutch actuating means to apply the predetermined
amount of pressure until the control means determines
that the motion of at least one of the load or
dynamoelectric machines ceases. The advantage with
the present invention is found in the manner in which
the enabling means and clutch actuating means act to
dissipate heat through the clutches in a controlled
manner so that the heat dissipated is not of such an
intensity that would be likely to damage the clutches.
Because heat dissipated through the clutches is
conteolled, the present invention permits for the use
of dry clutches in the preferred application of the
present invention; namely, drive systems having a
grinding mills driven by two synchronous motors. As
can be appreciated, mills utilizing fluid actuated dry
clutches are less expensive than mills employing wet
clutches.
Another advan~age of the present invention is the
ability of the control means to slow down the
synchronous motors and the mill simultaneously.
Because the momentum of the mill and the momentum of
the synchronous motors are approximately equal to one
another, once one of the mill and synchronous motors
has come to a halt, the other should be rotating or
oscillating at a considerably reduced speed.
Additionally, the control means may function in
response to electric power to the dynamoelectric
machines being discontinued.
Preferrably, the control means continues to
periodically enable the clutch actuating means for a
predetermined time duration sufficient to stop motion
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of both the load and the dynamoelectric machines.
In an alternate embodiment, in the event the
motion of the snychronous motors stops prior to the
motion of the mill, direct current may be supplied to
the stator and rotor windings of the synchronous
motors to lock the motors. After this, the periodic
enabling the of clutch actuating means will bring the
mill to a halt. Alternatively, in the event that the
mill has come to rest prior to the synchronous motors
coming to rest, direct current is applied to the
stator windings of the synchronous motors to bring the
synchronous motors to a halt by dynamic braking.
While the present invention contemplates the use
of a control means, the invention may be also
applicable to a method. Such a method would be for
stopping motion of a drive system having a large load
driven by one or more dynamoelectric machines coupled
to the load through respective clutches. The method
would comprise the steps of:
A) removing the source of electrical driving
power to the motors;
B) applying a predetermined amount of pressure
of predetermined duration to the clutches suficient
to at least partially close the clutches but
insufficient to fully close and lock-up the clutches;
and,
C) periodically continuing the step of applying
pressure to the clutches until such time as the motion
of at least one of the load and the machines ceases.
Throughout the specification and claims, there is
reference to a predetermined amount of pressure of
predetermined duration. It should be understood that
the predetermined amount of pressure is of sufficient
magnitude to cause the clutches to close but
insufficient to fully close and lock-up the clutches.
The expression l'predetermined duration" relates to the
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length of time during which the clutches will be
partially closed. The time duration chosen should not
be of sufficient length as to cause the clutches to
overheat. Also, reference is made throughout the
disclosure and claims to the enabling means. The
period of the enabling means should be of sufficient
length to allow dissipation of heat from the
clutches. Further, the period is preferably a random
period, that is the period is not necessarily related
to the mill oscillation period, however it can be
chosen to either occur on both the normal rotation and
backward rotation of the mill, or solely on the
backward rotation of the millO
Normal and backward rotation of the mill occur
when the mill starts to oscillate like a pendulum.
When the mill is partially coupled to the synchronous
motors and the mill is rotating in its normal
direction, there may be a slight reduction in the
speed of the mill and the synchronous motors because
the clutches will not be locked. The major reduction
in the speed of the mill and the synchronous motors `
occurs when the mill is rotatinq in its backward
direction counter to the direction of movement of the
dynamoelectric machines.
In this event, considerable amounts of heat will be
dissipated through the clutches. It is for this
latter event, involving large heat generation, that
the predetermined duration of the partial closing of
the clutches is determined~
For a better understanding of the nature and
objects of the present invention reference ma~ be had
by way of example to the accompanying drawing,
designated as Figure 1, which is a simplified
schematic drawing showing the invention.
Referring to the drawing, the present invention
is described. There is illustrated diagrammatically a
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large ring gear 10 which is attached to a grinding
mill (not shown). The ring gear 10 is driven by two
pinions 12 and 14 whose teeth respectively engage the
teeth of the ring gear in continuous meshing
relation. Two shafts 16 and 18 respectively connect
pinions 12 and 14 through air fluid actuated dry
clutches 20 and 22 to shafts 24 and 25 of synchronous
motors 28 and 30. Operation of clutches 20 and 22 is
controlled by air pressure respectively supplied along
piping 32 and 34 through pressure valves 36 and 38
from air supply 40. A.C. electrical power is provided
to the stators of synchronous motors 28 and 30 from
power supply 42. The electrical connection between
power supply 42 and synchronous motors 28 and 30 is
diagrammatically illustrated by respective power lines
44 and 46. Each power line 44, 46 is provided with a
respective circuit breaker 48 and 50, each shown in
its closed position. DC power is provided for the
fields of motors 28 and 30 ~rom DC supply ~7.
In accordance with the present invention there is
provided a control means generally illustrated at 52
~or stopping the motion of the grinding mill and the
synchronous motors 28 and 30 after the driving
electrical power is switched off or disscontinued.
The control means 52 comprises a clutch actuator means
or control 54, an periodic enable means or control 56
and a direct current supply 58 including commutating
contactors. As illustrated, circuit breakers 49 on
power lines 72, 74 provide DC power from DC suply 58
to stators of motors 28, 30 when breakers 48, 50 are
open.
The clutch actuator control is provided with an
output line 6Q which sends a signal to pressure valves
36 and 38 so as to provide to respective clutches 20
and 22 a predetermined amount of pressure of
predetermined duration. Upon the application of the
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predetermined amount of pressure to clutches 20 and
22, the clutches will at least partially close but
will not lock-up. ThuS clutches 20 and 22 provide a
percentage of full load ~orque which is employed to
slow the mill and the synchronous motorsO The clutch
actuator control 54 provides a control signal along
line 60 is response to an enable signal on line 62
from enable control 56. The enable control 56
generates enable signals in response to a signal on
input line 64 indicating interruption of power to the
synchronous motors. As illustrated, the enable
control is also provided with two other inputs lines
66 and 68 which respectively indicate that the motion
of the mill and the motion of the synchronous motors
is stopped. The sensing of of either the mill or the
synchronous motors stopping is illustrated in the
drawings by a sensor which would sense the rotaton of
a motor shaft or a pinion shaft. While this sensing
is part of the illustrated embodiments, it should be
understood that the enable control 56 continues to
enable the aGtuator controller 56 for a predetermined
time duration sufficient to stop motion of the
grinding mill and synchronous motors ~8, 30. This is
the simplest manner to stop the mill and motors as it
does not involves the use of sensors.
As illustrated, the enable control 56 is further
provided with an output line 70 which sends a signal
to the direct current power supply 58. In response to
receiving a signal, direct current power supply 58
provides direct current to energize the stator
windings of synchronous motors 28 and 30. Upon the
application of direct-current along lines 72 and 74
respectively to the stator windings of synchronous
motors 28 and 30, the motors brake dynamically and
then lock-up electrically when the rotor filed
excitation is applied. This will be useful for
,
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example, in the event that the mill stops prior to the
synchronous motors stopping. The application of
direct current to the stator windings of synchronous
motors 28 and 30 will dynamically brake the motors and
bring the synchronous motors to rest. In the event
that the synchronous motors 28, 30 are the first to
stop, the direct current power supply 58 in response
to a signal from the enable control 56 supplies direct
current power to the synchronous motor stators so as
to lock up the motors, and at the same time a signal
will be sent along line 62 from the enable control to
enable the clutch actuator control to continue to
operate the clutches, i.e. pulse the clutches. In
this latter event, the clutches in combination with
the stopped and locked motor act as a brake to stop
the mill. By this means the synchronous motors are
only required by to dynamically brake thermselves to
rest. The rotational energy of the mill and charge is
dissipated as heat in the clutches.
The present invention may also be used to lower
the charge Erom the cascade angle to the position of
rest when low speed inching is being used. When the
load is at the cascade angle during inching, the motor
field windings are fully excited with direct current
and the stator is being fed from power supply 58. The
motors are now at rest and capable of at least 125%
torque. Clutches 20 and 22 may now be pulsed by
control 54 through valves 36 and 38 to provide
controlled rotational lowering of the mill charge from
the cascade angle of perhaps 60 degrees from the
vertical to its position of rest at zero degrees.
The foregoing has been a description of the
preferred embodiment of the present invention and it
should be understood that alternate embodiments may be
readily apparent to a man skilled in the art.
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Accordingly, the scope of the present invention should
be limited only to that which is claimed in the
accompanying claims.
