Electricity: circuit makers and breakers – Electric switch details – Contact
Reexamination Certificate
1999-11-05
2001-02-06
Friedhofer, Michael (Department: 2832)
Electricity: circuit makers and breakers
Electric switch details
Contact
C200S275000
Reexamination Certificate
active
06184484
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention pertains to a switch for an electric motor. More specifically, the present invention pertains to a switch having a resilient terminal arm that is slotted to reduce its biasing force exerted on an actuator of the switch.
(2) Description of the Related Art
Common capacitor start and split phase induction motors have a run winding and a start winding wrapped around poles of a stator of the motor. An example of this type of motor is disclosed in the U.S. Patent of Hildebrandt et al. U.S. Pat. No. 4,296,366. The start winding of the motor stator is energized during start up of the motor, or when the operating speed of the motor falls below a specified operating speed. Energizing the start winding of the stator creates a rotating magnetic field in the stator that applies a sufficient torque to the rotor of the motor to begin rotation of the rotor. However, once the rotor has begun its rotation and has reached a desired operating speed, it is able to follow the alternations of the magnetic field created by the run windings of the stator and energizing the start windings is no longer needed. Commonly, in motors of this type, the start winding is not intended for continuous use and may fail if not de-energized during normal run operation of the motor. Therefore, motors of this type are typically operated by a two position switch having an actuator that is moveable between first and second positions. In the first position of the actuator it closes a first set of electrical contacts that establishes a circuit through the start windings of the motor, and in the second position of the actuator it closes a second set of electrical contacts that establishes a circuit through the run windings of the motor while opening the first circuit of the start windings.
Two position switches of this type are typically moved between their two positions by a centrifugal actuator assembly mounted on the rotor shaft of the motor.
FIGS. 1 and 2
show a two position switch
12
of the prior art and a centrifugal actuator assembly
14
mounted on the rotor shaft
16
of a motor (not shown). The centrifugal actuator assembly
14
rotates with the motor shaft and is responsive to the speed of rotation of the shaft for moving the switch actuator
18
from its first or start position to its second or run position in response to the rotation of the motor shaft attaining a predetermined operating speed. Some centrifugal actuator assemblies
14
include an annular collar
22
that is mounted on the rotor shaft for axially shifting movement between two positions, a start position of the collar on the shaft shown in
FIG. 1
, and a run position of the collar on the shaft shown in FIG.
2
.
The co-assigned U.S. Patents of Hildebrandt et al. U.S. Pat. No. 4,296,366 and Lewis et al. U.S. Pat. No. 5,744,883 each disclose a two position motor switch that is acted on by a centrifugal actuator assembly. These patents are incorporated herein by reference. The same type of switch
12
is shown in FIG.
3
and includes a switch actuator
18
that resembles a bell crank. The switch actuator
18
is mounted by a pivot connection
24
to the housing
26
of the switch. The switch actuator has an exterior arm
28
that extends from the pivot connection
24
to the exterior of the switch housing and an interior arm
32
that extends from the pivot connection
24
to the interior of the switch housing. The exterior arm
28
is provided with a follower surface
34
on a distal end of the arm that engages with the annular collar
22
of the centrifugal actuator assembly mounted on the motor shaft. The interior arm
32
engages with two resilient terminal arms in the interior of the switch housing. The first or start terminal arm
36
is fixed to the switch housing at its proximal end and has an electrical contact
38
at its distal end. The start terminal arm contact
38
engages a first or start winding electrical contact
42
in the switch housing to close the circuit through the start winding of the motor. The second or run terminal arm
44
also is fixed to the switch housing
26
at its proximal end and has an electrical contact
46
at its distal end. The run terminal arm contact
46
engages a second or run winding electrical contact
48
in the switch housing to close the circuit through the run winding of the stator. Thus, the switch actuator
18
, with its exterior arm
28
in sliding engagement with the cam surface of the centrifugal actuator collar
22
, moves between two positions in response to the axial movement of the collar between its two positions on the rotor shaft
16
. In the first position of the collar shown in
FIG. 1
, it positions the switch actuator
18
in its start position relative to the switch housing
26
. This closes the circuit through the first, start winding terminal arm
36
of the switch, energizing the start winding of the motor. The start position of the actuator
18
is shown in solid lines in FIG.
3
. When the collar moves to its second, run position on the rotor shaft shown in
FIG. 2
, the exterior arm
22
of the switch actuator slides over the exterior cam surface of the collar
22
allowing the exterior arm to move radially inwardly relative to the rotor shaft
16
. This movement of the exterior arm is caused by the resiliency of both the first and second terminal arms. The first
13
and second
14
terminal arms exert a biasing force on the interior arm
32
of the switch actuator. The biasing force causes the interior arm to pivot about the pivot connection
24
in the switch housing. As the interior arm is moved, the resiliency of the first terminal arm moves its electrical contact
38
out of engagement with the start winding electrical contact
42
of the start circuit, opening the start circuit. Also as the interior arm is moved, the resiliency of the second terminal arm moves its electrical contact
46
into engagement with the electrical contact
48
of the run winding, establishing a circuit through the run winding of the stator. The run position of the actuator
18
is shown in dashed lines in FIG.
3
.
As stated above, movement of the switch actuator
18
that causes the exterior arm
28
to move radially inwardly toward the rotor shaft
16
of the motor is caused by a biasing force exerted on the interior arm
32
of the switch actuator by both the first, start terminal arm
36
and the second, run terminal arm
44
. The resiliency of the two terminal arms results in the arms functioning as leaf springs that each exert a biasing force on the interior arm of the switch actuator. As shown in
FIG. 3
, first
52
an second
54
abutments on the interior arm
32
of the switch actuator engage the respective first
36
and second
44
terminal arms when the actuator is in the start position, causing the terminal arms to bow upwardly between their opposite ends. The biasing force exerted by the terminal arms holds the exterior arm follower surface
34
in sliding engagement with the collar
22
of the centrifugal actuator. The biasing force also causes the exterior arm of the switch actuator to move radially inward toward the rotor shaft in response to the axial movement of the centrifugal actuator assembly to its run position on the shaft.
Although the two position switch functions well for its intended purpose, it has been observed that the biasing force exerted by the first
36
and second
44
terminal arms on the interior arm
32
of the switch actuator causes the follower surface
34
on the exterior arm of the switch actuator to engage in sliding contact with the collar
22
of the centrifugal actuator assembly with a force that increases the wear rate of the follower surface. In addition, the wear rate problem of the follower surface cannot be overcome by simply reducing the biasing force of the terminal arms because the biasing force of the start terminal arm
36
must be sufficient to break a weld that often forms between the contact
36
of the start terminal arm and the start winding electri
Emerson Electric Co.
Friedhofer Michael
Howell & Haferkamp LC
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