Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Reexamination Certificate
1999-10-20
2002-08-13
Sircus, Brian (Department: 2839)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
Reexamination Certificate
active
06433975
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a motor protector which is to be used in a compressor or the like to be employed, for instance, in air conditioners, and more particularly to a motor protector of the type to be used inside of the motor that is to be protected.
BACKGROUND OF THE INVENTION
Examples of prior art motor protectors of this type are shown in FIGS.
9
(
a
) through
9
(
c
). As shown in FIGS.
9
(
a
) and
9
(
b
), a typical motor protector
101
has a support
103
having a heater
103
a
disposed in the main body
102
a
of a casing
102
made of steel. A terminal pin
104
which has been fixed to the support member
103
extends out of a header
105
that has been provided at the opening of casing
102
, with the gap between this terminal pin
104
and header
105
being sealed by a glass seal
106
and an epoxy pellet
107
. A bimetal disc
108
is fixed to support member
103
by means of a slug
109
, with its movable contact
110
engageable with a stationary contact
111
mounted inside of casing
102
.
In applications in which a motor protector as described is to be installed in a motor compressor of the sealed type (hereafter referred to as the electromotive compressor
100
), first and second electric wires
112
are connected respectively to terminal pin
104
and casing
102
. Motor protector
101
is disposed inside of an insulating sleeve
113
and, as shown in FIG.
9
(
c
), is connected to windings
114
(main winding
114
a
and auxiliary winding
114
b
). Thus, motor protector
101
is serially connected with driving circuit
130
of electromotive compressor
100
connectable to an alternating current source
115
.
Bimetal disc
108
snaps from one dished configuration to an opposite dished configuration due to the generation of heat by the bimetal disc stemming from overload current or, otherwise, by an elevation of the ambient temperature within the protector including the generation of heat by heater
103
a
, with a consequence that the driving circuit is opened thereby preventing any possible damage from being inflicted on electromotive compressor
100
.
Nevertheless, such conventional motor protectors
101
have the following problem:
In the event that an abnormal state, as described above, develops in electromotive compressor
100
, electromotive compressor
100
is protected from heat generation or possible burning as the motor protector
101
repeatedly conducts and interrupts the electric current. In the case where the situation is not remedied, motor protector
101
continues its protection of electromotive compressor
100
above and beyond the designed number of cycles of its life expectancy. Under such circumstances, when motor protector
101
eventually exceeds its expected product life, melting and welding of movable contact
110
and stationary contact
111
of the bimetal disc
108
occur, thereby bringing about a state of continuous current flow. If such a state continues, winding
114
in electromotive compressor
100
will become over-heated, with a result that the electrical insulating resin of winding
114
will be melted by the heat, thereby bringing about a short-circuited state. This results in a marked increase in electric current and concomitant lowering of the resistance value of the driving circuit
130
, with a resultant abnormally heated state due to a large current flowing in the inner circuit of electromotive compressor
100
. As a consequence of such abnormal heating, winding
114
of electromotive compressor
100
can be burned producing carbide soot which adheres to the surface of glass
106
of the fusite pin
104
, thereby bringing about a loss of electrical insulation between the fusite pin
104
and header
105
.
In the worst case, tracking develops on the surface of glass
106
causing the glass to soften and melt due to the heat generated by the passage of electric current, culminating in the blow-out of fusite pin
104
which is no longer able to withstand the inner pressure of electromotive compressor
100
in some cases. In order to cope with such a problem, it is conceivable to provide a fuse, for example, for the purpose of de-energizing driving circuit
130
prior to the possible blow-out of the fusite pin. In such case, there is also a concern that the arc that is generated in connection with the melting of the fuse could ignite the gas in the electromotive compressor
100
, thereby causing a possible explosion.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a motor protector which solves the problem of the prior art described above. Another object of the invention is the provision of a motor protector which is capable of ultimately avoiding a dangerous situation even in the case where the contacts of a motor protector weld. Yet another object of the invention is the provision of a motor protector which is capable of preventing any possible adverse affect of an arc, generated when the driving circuit has been interrupted, on the various environments of the electromotive compressor.
Briefly stated, a motor protector made in accordance with the invention comprises a casing that can be tightly sealed, a first switch provided inside the casing that carries out the switching of a movable contact in an electric path as a snap-acting disc snaps from one dished configuration to an opposite dished configuration in response to a selected level of electric current and the ambient temperature, and a second switch which is connected in series with the first switch inside of the casing and which opens the current path when it is melted by a predetermined overflow of electric current.
In cases where the contacts of the first switch melt and weld in the conductive state, a large electric current is generated. According to the invention, however, the second switch will melt, thereby de-energizing the motor. Therefore, it becomes possible to prevent the possible burning of the electromotive compressor of the sealed type and any damage from being inflicted on the sealing terminal of the protector, thereby making it possible to prevent the possible adhesive tracking of carbide to the vicinity of the sealing terminal. As a consequence of this, it becomes possible by means of the invention to prevent the possible melting of the glass portion as induced from the tracking phenomenon caused by the loss of electrical insulating characteristics of the glass seal and prevent any possible blow-out of the fusite pin caused by the loss of holding characteristic of the glass. Further, by means of the invention, there is no adverse affect on the various environments of the electromotive compressor of the sealed type by the arc generated upon melting of the second switch since the second switch is disposed in an air-tight casing.
According to a feature of the invention, a heater generates heat in dependence on the level of electric current that flows in the electric current path so that it is possible to elevate the atmospheric temperature inside the casing by the heat generated by the heater and to adjust the characteristics of the motor protector by suitably selecting the material of the heater and the temperature responsive characteristics. According to another feature of the invention, accurate adjustment of the characteristics of the protector is enhanced when the second switch also serves as a heater and when the first and second switches are connected via a connective member extending through the support member which serves as the heater. According to yet another feature of the invention, the heater can be used to satisfactorily generate heat by arranging the first and second switches to sandwich the support member, for example, thereby making it possible to easily adjust the characteristics as desired. The first switch can be supported on the support member and the second switch can be arranged on the opposite side of the support member relative to the first switch. According to an alternative feature of the invention, the second switch can be arranged on the same side of th
Masuda Takashi
Satoh Tatsuhiko
Unno Mitsuru
Baumann Russell E.
Duverne J. F.
Sircus Brian
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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