Note: Descriptions are shown in the official language in which they were submitted.
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CENTRIFUGAL STARTER SWITCH APPARATUS
Background Of l'he Present Invention
The present invention relates to a
centrifugal starter switch apparatus and particularly
to a centrifugal actuator for switching high starting
current motors and the like.
Integral horsepower motors often are
constructed with a special start winding which is
connec$ed in circuit during the initial starting of the
motor to bring the motor up to speed. The start
winding is preferably disconnected from the circuit as
the motor approaches or reaches running speed. A
conventional switch mechanism or disconnecting of the
start winding includes a switch unit mounted within the
motor and a centrifugal actuator mounted on the shaft
and rotating therewith to actuate the switch unit. The
actuator holds the switch unit closed to provide
current to the start winding. As the motor speed
increases and approaches or reaches running speed, the
centrifugal actuator moves a switch operator to open
the switch unit and thereby disconnect the start
winding from circuit.
The centrifugal actuator generally includes a
spring loaded lever mechanism coupled to a pivoting
bracket to establish a snap action movement of switch
operator. A sliding hub or collar is coupled to and
moved by the pivoting bracket between a stop position
established by the spring and an axially offset
position established by the centrifugal forces. The
hub includes the switch operator for opening and
closing the start switch unit and thereby the start
winding circuit connection. Motors using such switch
mechanisms are normally started and stopped many times
over the life of the motor. The centrifugal switch
actuator should have a corresponding life. If the
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switch unit is not reset upon stopping, for example,
the start winding is connected and at the next stop and
start cycle the motor will not start and high current
flows through the run winding. Destruction of the
motor windings will occur unless personnel is present
to immediately disconnect power from the motor. In any
event, costly replacement of the centrifugal actuator
would be required.
Larger induction motors are currently in
demand in which starting currents of 110 amperes are
required. The switch for switching such currents
require special consideration to maintain a reliable
life due to the increased forces and higher current
ratings found in the larger motor applications. The
actuator must include a positive snap action which
insures movement from the start position to the run
position without hesitation. This movement allows the
contacts of the stationary switch to open cleanly with
no restrikes, which particularly is important in the
larger motor applications where more severe starting
conditions and longer acceleration times are common
place. Actuators without the positive snap action
eature will have contact restrike due to the
hesitation at switching speed during long acceleration
times. Contact restrikes can result in capactior
failure and contact welding. Further, a major problem
encountered in the actuator for large motors is the
increased wear due to higher centrifugal forces and
impact forces. The higher forces are inherently
necessary to provide the snap action, and are
particularly severe due to the larger geometry.
A particularly satisfactory construction of
an actuator is shown in U. S. Patent 4,386,290 which
issued on May 31, 1983 to Monette. A hub slides on a
shaft and and is coupled to actuate the shaft switch
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unit. The centrifugal actuator includes a U-shaped
mounting bracket fixed to the shaft with a pair of
oppositely located and similarly mounted pivot levers
coupled to position the hub on the shaft between the
switch closing position and switch opening position.
The pivoted levers generally include pivot arms pivoted
in suitable pivot bearing supports in the plastic
hub. After a great number of cycles but well within
the normal opera~ing life of the motor, the plastic
bearing supports wear, changing the pivot movement of
the levers. In moving to the switch open position, the
levers may move beyond a normal tolerance range,
resulting in the spring members moving over-center and
positively holding the switch in the actuat.ed
position. This can of course result in malfunction of
the switching circuit, with possible destruction of the
motor.
There is a demand and need for a modified
centrifugal actuator which prevents such locking of the
switch actuator and thereby the switch unit in the
actuated position~
Summary Of Th_ Present Invention
The present invention is particularly
directed to a centrifugal switch actuator for a motor
starter unit particularly adapted to actuators for
large induction motors, and includes a simple
mechanical structure which positively prevents the
movement of the centrifugal weight member from a
predetermined tolerance position and thereby
significantly extends the effective operating life of
the centrifugal actuator. Generally, in accordance
with the teaching of the present invention, the switch
actuator includes typical pivoted and spring-loaded
actuating arm members which are moved between a spring
loaded standby position and a pivoted actuated
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position. The switch actuator in accordance with this
invention includes a mechanical stop unit located
between the pivot arms a relatively fixed member of the
switch mounting bracket. The stop unit is located with
a space between the fixed member and the pivot arms
under normal operation such that there is no effect
under the normal switch operation. Wear within the
pivot support unit of the switch actuator permits the
switch pivoting bracket or arms to move toward the
fixed member, and close the gap between the members.
After a predetermined wear, the stop unit will engage
between ~he arms and the fixed member, preventing any
further excessive movement of the switch operator to an
over-center position of the spring members and
positively holding the switch operator in position for
subsequent operation. The simple mechanical stop thus
maintains continued subsequent operation of the
centrifugal actuator and significantly extends the
useful life of the centrifugal actuator.
In a particularly practical construction~ the
actuator includes a pair of U-shaped brackets or
arms. The opposite ends of the U-shaped arms end in
curved pivot arms~ The U-shaped arms project over the
hub of the actuator, with the ends located in pivot
slots or recesses on the opposite sides of the hub.
The curved end of the arm particularly bears on the
opposed curved surface of the opposed arm. The hub is
formed with relatively flat surfaces in the pivot slot
area to provide a lateral bearing support for the pivot
arms. The base of the arms include a pair of
rearwardly extended pivot tabs which are pivotally
mounted in the pivot slot which in turn are connected
to the mounting hub. The arms include small laterally
spaced projecting ~abs aligned with the mounting
bracket and limiting outward pivoting of the arms, and
~hereby functioning to prevent the switch arms from
moving sufficiently to move the bias spring over
cent~r. Alternatively, stop tabs may be secured to ~he
mounting bracket and extended toward the arms. Wear
wi~hin the actuator may permit the arms to move into
engagement with the limit tabs, into a locked actuated
state.
The simple mechanical stop thus positively
prevents excessive movement within the centrifugal
actuator as will cause the switch mechanism to lock in
the actuated position with possible start of the motor
with only the run winding connected to the power
supp ly .
The present invention thus provides a simple
reliable and effective means to maintain and extend the
operative life of a centrifugal switch actuator.
Brief Descrip~ion Of Drawin~s
The drawings furnished herewith generally
illustrate the best mode presently contemplated for the
invention and are described hereinafter.
In the drawings:
Fig. 1 is a side elevational view of a
centrifugal actutor constructed in accordance with the
teaching of the present invention applied to a motor;
Fig. 2 is a view similar to Fig. 1
illustrating the running position of the centrifugal
actuator with an associated centrifugal switch opening;
Fig. 3 is a pictorial view of the weight
shown in Figs. 1 and 2;
Fig. 4 is a fragmentary sectional view taken
generally on line 4-4 of FigO 2;
Fig. 4a is a simplified line or free body
drawing of the pivot points and movement of the
actuator shown in Figs. 1-4;
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Fi~. 5 is a view similar to Fig. 2
illustrating an alternate embodiment of the invention;
and
Fig. 6 is a view taken generally on line 6-6
of Fig. 5.
Description Of The Illustrated Embodiment
Referring to the drawings and particularly to
Figs. 1 and ~, a fragmentary end portion of an
induction motor 1 is illustrated including an annular
stator 2 mounted within an outer tubular frame, not
shown. The s~a~or 2 includes a magne~ic stator core 4
secured to the frame, not shown~ A field winding 5
generally includes a run winding and an offset separate
start winding which are interconnected to each other
and to power supply as hereinafter described. Because
such detail is well known, it is not specifically
separately shown herein. In accordance with
conventional practice, a rotor 6 having a rotor shaft 7
is rotatably mounted within the ~tator 3. The
illustrated rotor 6, as widely used in an A.C.
induction motor, includes a cast squirrel-cage winding
embedded within the rotor and thus located within the
magnetic field of the field winding 5. The motor shaft
7 is supported within a cup~shaped end enclosure bell,
not shown. The end bell defines an end chamber 10
within which a centrifugally actuated start switch
assembly 11 is located. The start switch assembly 11
is electrically connected into circuit with the start
winding of the field winding 5 for selected connection
of the start winding into the circuit only during the
starting of the motor and until such time as the rotor
reaches or closely approaches normal operating speed.
The start winding is primarily connected into the
circuit during the initial period ~o increase the
starting torque for accelerating the motor to operating
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speed~ The centrifugally actuated start switch
assembly 11 includes a start switch unit for apparatus
12 which is mounted on a suitable motor bracket 13
within the end chamber 10 immediately adjacent to the
axial end of field winding 5. A centrifugal actuator
14 is located on the motor shaft 7 and in particular
includes a hub 15 which is slidably mounted on the
shaft 7. A pair of oppositely located centrifugal
weights 16 having similar pivot arms 17 are pivotally
coupled to the hub 15 and spring loaded~ as by coil
springs 18 located to the opposite sides of the hub to
a first position as shown in Fig. 1. The centrifugal
weights 16 are pivotally mounted to a securement shaft
bracket 19 which is firmly affixed to the rotor shaft
7. The bracke~ 19 is mounted to locate an annular
operator of the hub 15 in engagement with the switch
unit at rest (Fig. 1) and serves to hold the switch
unit closed. The start winding is thereby connected
into circuit during the initial starting period. At
the desired or switching speed, the centrifugal weights
16 move outwardly, as shown in Fig. 2, with a snap
action under the force of the rotational centrifugal
forces and provide a corresponding retract movement vf
the hub lS which snaps outwardly from the start-switch
unit 12 to the position of Fig. 2. As a result, the
start-switch unit 12 moves to an open circuit position,
thereby disconnecting of the start winding from
circuit.
The details of the stator, rotor and the
like, as well as the centrifugal switch unit 12 may be
of any known or other desired construction~ No further
description of the detail of such components is
therefore given other than as necessary to fully
describe the present invention which is particularly
directed to a unique centrifugally actuated switch unit
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apparatus, a preferred embodiment o~ which is shown in
the drawings and its mounting within the motor. The
centrifugal switch unit is preferably constructed as
disclosed in U.S. patent 4,419,550 which issued on
December 6, 1983 to G. S. Monette or in the co-pending
Canadian application Serial No. 570,913 entitled
"Centrifugal Actuator For A.C. Induction Motor" which is
filed herewith and which is assigned to the same assignee
as this application.
The actuator 14 of the present invention
includes limit or stop elements 21 located between the
pivoting weights 16 and the hase 22 of bracket 19. The
stop elements 21 positively limits the position of the
pivot location of the weights 16, and thereby movement
past over center position. This positively insures the
reset of the weights 16 for proper subsequent start
cycles.
More particularly, the hub 15 is a tubular
member slidably mounted on shaft 7 and having a switch
operator 24 secured to one end of hub 15. The hub 15
has pivot slots 23 on opposite sides of the hub within
which the pivot ends 24 of the pivot arms 17 pivot.
As more fully shown in U.S. patent 4,419,550,
the internal weights 16, which position switch hub 15
under centrifugal action, are pivotally mounted by a
linkage to the bracket 19. The weights 16 are
pivotally connected to hinges 25 by weight projections
26 which extend through slots in hinges 25. The hinges
25 in turn are also pivotally ~onnected to side members
27 of base bracket 19 by ears 28 which are lodged in
slots 29 in side members 27. The pair of coil springs
18 are hooked at opposite sides to the hinges 25 and
are hooked at the opposite end to side members 27 of
the bracket 19 as at 27a. The springs 1~ operate to
displace the actuator weights -16 to the rest position
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of Fig. 1 when rotation of the actuator ceases and also
to preload the assembly.
The pivot slots 23 in hub 15 are formed by
spaced abutment walls 31 and 32 which are preferably
integrally molded with hub 15 adjacent the operator
24. The hub 15 is formed with a flat sur~ace between
the walls 31 and 32. The walls 31 and 32 are generally
curved walls formed by a plurality of flat, chordal
surfaces.
The movement of the metal weights 16 from
start to run creates sliding or rolling friction
between the pivot ends 24 of weight arms 17 and the hub
15. T~e friction of members 24 causes erosion or wear
to occur on the two abutment walls 31 and 32 of the hub
15. The progressive wear ultimately changes the
weight/hub pivot location 35 of the weights 16 to the
hub 15, as shown in Fig. 2. A hinge linkage pivot
location 36 is established by the location of the
weight projections 26 and the slots 26a in the hinges
25. This location 36 is normally to the motor side of
the pivot location 35 and shown to the right thereof in
Fig. 2. The reference to above and below herein refers
to a vertically oriented motor with the switch
apparatus at the upper end of the motor. The pivot
location 35 is established to maintain a positive
return force on the hinges and weights 16 so the -
centrifugal actuator can return to the start position
when the motor co~es to a stop. The positive return
force is maintained if the weight/hub pivot location 35
is above, or to the left of, the linkage pivot location
36, as shown in Fig. 2 and in the simplified linkage
drawing of Fig. 4a for the actuated switch position.
Fig. 4a illustrates the lower actuator portion of Fig.
2. The switch actuator 24, spring 17a and bracket 19
are shown in phantom. Worn abutments 31 and 32 on the
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hub 15 would allow the weight/hub pivot location 35 to
move in line with or below the linkage pivot location
36, which results in a zero or negative return force.
Thus, with the pivot locations in alignment, the system
i5 in equilibrium with a zero return force. With the
pivot location 35 moved past pivot location 36, the
system is locked in the actuated state. The
centrifugal actuator 14 wou~d then be pinned in the run
position of Fig. 2 and could not close the switch unit
11. Once this failure occurs, the motor will not start
on the naxt start cycle and the run winding may
potentially burn out.
In a practical construction, the pivot
locations 35 and 36 have a small separation in the
original structures. The spacing is however larger
than the space 37, and is shown substantially enlarged
in Fig~ 4a for clarity of description. The stop
elements 21 of the present invention, with the smaller
spacing, prevent the weight/hub pivot location 35 from
moving in line with/or below the linkage pivot location
36 when wear occurs to the abutments walls 31 and 32 of
the switch hub 15. Therefore a positive return force
is maintained.
More particularly in the embodiment of Figs.
1-3, the stop elements 21 are integral projection tabs
on the back edge of the arms 17 of the U-shaped weight
16. In the collapsed position of the weights, the arms
17 are angled from the base 22 and the s~op tabs 21 are
spaced from the bracket base 22. ~hen the weight 16
pivots outwardly, the arms 17 with the stop tabs 21
pivot toward the bracket base 22. The tabs 21 move to
a position essentially parallel to the base 22. In the
original structure, the tab 21 is spaced from the
bracket base 22 as at 37. The weights 16 then move in
the designed sequence.
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In starting the motor, the windings are
energized with the switch 12 closed to supply current
to the windings 5 including the start winding, not
separately shown. The motor accelerates, and at the
designed speed,the weights 16 snap outwardly, carrying
the hub 15 toward the base 22 of bracket 19. The hub
15 and operator 24 moves axially away from the switch
unit 12, which opens and disconnect the start
winding.
As the wear continues and prior to movement
to or below the pivot location 36, the tabs 21 move
into engagement with the base 22 of bracket lg. The
bracket base 22 thus forms a positive stop which
prevents the arms 17, and particularly the weight/hub
location 35 pivot of the arms, from moving to or past
the linkage pivot location 36. The stop members thus
serve to insure continued proper actuator functioning,
with reset of the operator.
An alternate embodiment is shown in Figs. 5
and 6. The embodiments have the same components and
operation. In Figs. 5 and 6 however, the stop limit
includes block members 40 secured as by welding to the
base 22 in alignment with the side arms 17 of the U-
shaped weights 16. The block members 40 extend from
the base toward the actuator arms 17.
The centrifugal actuator 14 of Figs. 5 and 6
functions in the sequence as the first embodiment. As
the plastic hub 17 wears, the pivotal weight move
inwardly. The arms 17 moves into engagement with the
stop limit blocks 40 before the pivot point moves in
line with or below the linkage location 36 to insure
continued proper functioning of the actuator.
The use of the stop limit elements on the
mounting bracket provides a convenient and effective
structure for retrofitting existing centrifugal
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actuators such as disclosed in the previously
identified Monette patent.
The stop limit may of course use the
combination of tabs on the arms and projections on the
base, or any other similar position limit means ~o
maintain the collapsing force of the springs 18.
The present invention thus provides a simple,
but effective system for extending reliable operation
of centrifugal actuators.
