Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-05-07
2002-11-12
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C200S08000A, C200S08000A
Reexamination Certificate
active
06479913
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electric motors incorporating a centrifugal governor mounted to a rotor shaft of the motor, and a starting switch which is allowed to open or close depending on the rotational speed of the rotor shaft.
BACKGROUND OF THE INVENTION
Many electric motors include governor assemblies which react to changes in the rotational speed of a rotor shaft to open and close an electric switch. Centrifugal governors generally utilize the centrifugal force generated by rotation of the shaft to engage and disengage an electric starting switch. Illustrated in
FIG. 9
is a known starting switch assembly
10
and governor assembly
14
. The starting switch assembly
10
includes a bracket
18
mounted to the motor end frame
22
with a mounting screw
26
, a switch plate
30
mounted to the bracket
18
, and a pair of elongated arms or terminal blades
34
and
38
which are supported by switch plate
30
and each of which includes an associated electric contact
42
and
46
. An actuator button
50
is attached to arm
34
. The governor assembly
14
includes a spring
52
and a pivot plate
54
, and the governor assembly
14
is mounted to the rotor shaft
58
. During the power off mode, and during start-up and until the shaft
58
reaches a predetermined rotational speed, the spring
52
causes the pivot plate
54
to contact the actuator button
50
to force the arms
34
and
38
and, therefore, the contacts
42
and
46
, together to close the electric switch. After the shaft
58
reaches the predetermined rotational speed, centrifugal forces acting on the pivot plate
54
overcome the force of the spring
52
and cause the pivot plate
54
to pivot in a direction (i.e., a counterclockwise direction as shown in
FIG. 9
) which allows the actuator button
50
to move in a direction that enables the arms
34
and
38
and, therefore, the contacts
42
and
46
, to separate to open the electric switch.
SUMMARY OF THE INVENTION
With continued reference to
FIG. 9
, during start-up and until the shaft
58
reaches the predetermined rotational speed, the spring
52
causes the pivot plate
54
to push the actuator button
50
away from the end
62
of the shaft
58
, such that the bottom surface
66
of the actuator button
50
is not perpendicular to the centerline
70
of the shaft
58
. As can be appreciated, during start-up and operation of the motor, the governor assembly
14
spins with the rotor shaft
58
. The spinning action of the governor assembly
14
and the nonperpendicular engagement between the pivot plate
54
and the bottom surface
66
of the actuator button
50
can cause the actuator button
50
to wobble. Wobbling of the actuator button
50
can result in intermittent arcing between the electric contacts
42
and
46
. In order to offset the wobbling action of the actuator button
50
, it is known to utilize a significant portion of the available stroke or pivotal movement of the pivot plate
54
of the governor assembly
14
. In this way, the engagement between the pivot plate
54
and the actuator button
50
prevents the electric contacts
42
and
46
from separating as the actuator button
50
wobbles. It has been observed that a problem with using a significant portion of the available pivotal movement of the pivot plate
54
to offset the wobbling action of the actuator button
50
is that there may not be a sufficient amount of pivotal movement left for the pivot plate
54
to absorb the stack-up tolerances associated with assembly of the electric motor. In other words, during operation of the electric motor, after the shaft
58
has reached a predetermined rotational speed, there may not be a sufficient amount of stroke left in the pivot plate
54
so as to allow the electric starting switch to open, thereby resulting in possible damage to the starting switch or even the electric motor itself. For example, if a significant portion of the available stroke of the pivot plate
54
is reserved to offset the wobbling action of the actuator button
50
, and the length of the shaft
58
is near the high end of the allowed tolerance range, after the shaft
58
reaches the predetermined rotational speed, the pivot plate
54
may not move a sufficient amount to allow the electric contacts
42
and
46
to separate. Consequently, careful adjustment is required to obtain the proper relationship between the switch assembly
10
and the governor assembly
14
.
To address the foregoing problems and other problems, the present invention provides an electric motor which includes a “no-adjust” starting switch. The starting switch according to the present invention does not require any reservation of the available stroke of a pivot plate of a governor assembly to offset wobbling of an actuator button because, for one reason, the actuator button is prevented from wobbling. One advantage of such an arrangement is that the entire available stroke of the pivot plate of the governor assembly can be used to offset the stack-up tolerances of the motor assembly.
According to one aspect of the present invention, a switch actuator button remains substantially parallel to the centerline of a rotor shaft at all times. As a result, there is minimal wobbling of the actuator button during motor start-up and motor operation. In one embodiment of the present invention, the actuator button is not secured to either terminal blade of the starting switch, so that the actuator button is better able to stay substantially parallel to the centerline of the rotor shaft. In another embodiment of the present invention, the actuator button is constrained for movement parallel to or along the centerline of the rotor shaft to further ensure that the actuator button will not wobble during motor start-up and motor operation. Preferably, a switch housing is provided to simplify the placement of the starting switch into the electric motor, and the switch housing includes an aperture through which the actuator button is slidably positioned, so that during rotation of the rotor shaft, the actuator button is constrained for movement along the centerline of the shaft.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features.
REFERENCES:
patent: 1604497 (1926-10-01), Stocking
patent: 2493140 (1950-01-01), Heath
patent: 2991340 (1961-07-01), Bickham
patent: 3660741 (1972-05-01), Walter
patent: 4315118 (1982-02-01), Kramer et al.
patent: 4689452 (1987-08-01), Quick
patent: 4885440 (1989-12-01), Kachuk
patent: 4894496 (1990-01-01), Palumbo
patent: 4958096 (1990-09-01), Kachuk
patent: 5220226 (1993-06-01), Long et al.
patent: 5291086 (1994-03-01), Shekalim
A. O. Smith Corporation
Michael & Best & Friedrich LLP
Mullins Burton S.
LandOfFree
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