Background of the Invention
[0001] The present invention relates to a bimetallic relay, with normally closed contacts,
particularly suitable for use as a starting relay for single-phase induction motors,
of the kind used in compressors for refrigerators and the like. More particularly
the invention concerns a fast release relay, with double bimetal, relatively insensitive
to the variations of ambient temperature.
[0002] As is known, a single-phase induction or asynchronous motor requires special devices
for overcoming the obstacle of starting, especially under load, in order then to go
into normal running operation. That problem of starting has been resolved with various
devices, one of which provides the use of a supplementary winding, more particularly
called a starting winding, which is deenergized or excluded from use in the motor
after the motor is started. To obtain this result, one uses a relay connected in series
with the starting winding with normally closed contacts which open after a certain
delay. At present, many different relays are used for this application, all comprising
a certain delay function, for which reason they are called delayed relays or delay
relays, generally coming under two basic types:
- electromechanical or gravity relays; and
- thermal relays with PTC (positive temperature coefficient) resistive heaters.
Brief Summary of the Invention
[0003] The general object of the present invention is to make a relay which has the advantages
of both above-mentioned types but without having their respective limitations. Besides
the fact of being able to have the normally closed contacts, so as to make possible
their use with motors of the PSC (Permanent Start Capacitor) type, the other characteristics
to be realized in the relay of the present invention are the following:
- low energy consumption;
- maximum independence of the operating characteristics from variations of the ambient
temperature;
- reasonably short reset times;
- high reliability, in the range of millions of cycles;
- resistance to vibrations;
- small number of parts, all of fully established technologies, to hold down the manufacturing
costs; and
- guarantee of reliable operation in any position independent of gravity.
[0004] These objects set forth above and the advantageous structural and functional characteristics
mentioned before are realized in the present invention which, in its preferred form
of realization, consists of a delayed action thermal relay comprising a housing molded
of plastic material with a first, a second, and a third electrical terminal already
inserted and preferably molded into the plastic housing material, a first U-shaped
bimetallic strip extending between the first and third electric terminal and carrying
in an intermediate position at the base of the U-shape a contact element, and another
contact element which is carried by a second bimetallic strip connected to the second
electrical terminal. There is provided on said bimetallic strips associated permanent
magnet means and an armature respectively which tend to keep said bimetallic strips
together and hence the contact elements provided on them electrically engaged in a
closed circuit position.
[0005] Although the invention is described as consisting of a relay for the above indicated
application, it must of course be understood that the staring of single-phase induction
motors, of which the motors of refrigerator compressors and the like are examples,
do not exhaust the possibilities of application of such relays, which in fact can
be used whenever two electric users must be fed, one of which must subsequently be
excluded from the power supply with a certain delay.
[0006] The two bimetallic strips are disposed so that their temperature increase brings
about deformation of the strips in equal directions. This makes operation of the device
independent of the ambient temperature.
[0007] From the structural point of view, the bimetallic strips are of a simple type, not
preformed and a low strength, so that, also assisted by the presence of the magnetic
means, they are able to bring about a snap opening of the contacts so as to obtain
a distance between them of the order of several millimeters.
Description of the Drawings
[0008] Other objects and advantages of the present invention will become evident to those
expert in the field from the following detailed description and from the attached
drawings, in which the preferred form of realization is shown in an illustrative and
non-restrictive sense.
[0009] In the drawings:
Figure l shows a top view, with the housing partially in section, of the device of
the present invention;
Figure 2 shows a section view along line 2-2 of Fig. l;
Figure 3 shows a section view along line 3-3 of Fig. l; and
Figure 4 represents schematically the circuit in which the device of the present invention
is inserted and of which it forms a part.
Description of Preferred Embodiments
[0010] Referring now to the drawings, it is seen that the relay comprises a housing ll one
end of which is closed with a housing cover part l2 molded of plastic material in
which three electric terminals l3, l4 and l5 are already mounted, preferably by being
molded into the plastic material of the housing cover part, for connection with an
external circuit. The details of this connection will be clarified below in reference
to Figure 4.
[0011] At the terminal l3 is connected a bimetallic strip 20 by welding or the like, and
between the terminals l4 and l5 another bimetallic strip 2l is similarly connected.
The first strip 20 is a simple linear elongated form, for example rectangular, while
the second bimetallic strip 2l is U-shaped and has its legs 2l
a, 2l
b secured to terminals l4 and l5 respectively.
[0012] The bimetallic strip 20 carries at its free end, that is, the distal end with respect
to the terminal l3, a small permanent magnet l6 which cooperates functionally with
an armature l9 integral with the other bimetallic strip 2l, fastened to the bow or
base zone of the U-shaped strip 2l, which is the distal zone with respect to the two
terminals l4 and l5. The two bimetallic strips 20 and 2l also carry respectively two
contact elements l7 and l8 which are urged to remain electrically engaged in a closed
circuit position by the force of attraction exerted by the functional cooperation
between the permanent magnet l6 and the armature l9.
[0013] With a construction thus realized and with reference to the wiring diagram of Figure
4, the operation of the device is as follows:
[0014] It is seen in Figure 4 that the terminal l5 is connected to one or two power source
or network terminals while the terminals l3 and l4 are connected to the other network
terminal across two respective motor windings - a start winding of the motor which
must be cut off after the start up of the motor, and the main winding of the motor
used for a normal running operation of the motor. Upon start up of the motor, the
starting current I
S, owing to the normally closed state of the contacts l7, l8, flows through the terminal
l3 and the bimetallic strip 20, which is dimensioned and provided with selected electrical
resistance properties so that the temperature increase in the strip 20 due to the
Joule effect of the start winding current circulating therein is relatively small,
so that strip 20 tends to remain substantially in its initial position relative to
strip 2l even when traversed by the current. The current I
M of the main or normal run winding of the motor passes through the terminal l4 and
into the first arm 2l
a of the U-shaped bimetallic strip 2l. In the second arm 2l
b of the strip 2l, therefor, a current I
S + I
M flows. The bimetallic strip 2l is dimensioned and provided with selected electrical
resistance properties so that the heating of strip 2l caused by said currents urges
it to deflect in a direction opposed to the attraction of the permanent magnet l6
to a relatively greater extent than the strip 20 so that the magnetic attraction force
is overcome. At that point the bimetallic strips 20 and 2l move apart with a snap
action, abruptly opening the contacts l7 and l8. The starting winding is thus cut
off from the power supply.
[0015] When running, the current I
M of the motor is such as to maintain the bimetallic strip 2l at a temperature, and
hence at a deformation, sufficient for the contacts l7 and l8 to remain open and thereby
the starting winding remains disconnected.
[0016] When the power supply to the motor is interrupted, current no longer circulates in
the bimetallic strip 2l, so that it cools off and again engages against the bimetallic
strip 20 until the force of attraction of the permanent magnet l6 becomes predominant
again, bringing the contact l7 and l8 into (normally) closed or reset position. Thereby
the auxiliary or starting winding is again connected.
[0017] The self-heating due to the Joule effect of the currents flowing in the bimetals
is such that this relay has a relatively low consumption and typically absorbs a power
less than l/l0 of a Watt in a refrigerator compressor motor application for example.
[0018] As already mentioned, the snap opening of the contacts is ensured by the presence
of the permanent magnet l6, so that no particular forming of the bimetallic strips
needs to be provided, which can therefore work under conditions of extremely low stress,
thus guaranteeing a required long fatigue life. The bimetallic strips have thermal
response characteristics such that, when the contacts are engaged in close circuit
position, the strips tend to be deformed in a similar manner in response to ambient
temperature changes to retain the contacts in closed circuit position independent
of such ambient temperature changes.
[0019] The above explained principle of opening of the contacts is such that, with appropriate
dimensioning and provision of electrical resistance properties in the parts, the distance
between the open contacts can be of the order of several millimeters, thus giving
assurance against the priming of electric arcs and against accidental closing of the
contacts because of vibrations. This is very important from the viewpoint of meeting
the national and international standard requirements.
[0020] In the foregoing the preferred form of realization of the present invention has been
described, but it is understood that the experts in the field can make changes and
variants in the structural details without thereby going outside the scope of the
present invention.
1. Delayed action thermal relay comprising a housing molded of plastic material with
a first, a second, and a third electrical terminal mounted thereon, characterized
in that the relay comprises a first bimetallic strip inserted between the first and
the third electrical terminal and carries in position intermediate the first and third
electrical terminals a contact element cooperating in normally closed condition with
a corresponding contact element carried by the second bimetallic strip which is connected
to the second electrical terminal, there being provided on said bimetallic strips
respective associated permanent magnet means and armature means which tend to maintain
said bimetallic strips together with the contact elements provided thereon electrically
engaged in a closed circuit position, the bimetal strips being provided with selected
proportions and electrical resistance properties relative to each other so that directing
a selected current between the first electrical terminal and the second and third
electrical terminals is adapted to separate the contacts against the bias of said
permanent magnet means after a selected delayed period.
2. Delayed action thermal relay according to claim l, further characterized in that
said first bimetallic strip is U-shaped and is connected to the first and third electrical
terminal at the ends of two legs of the U-shape.
3. Delayed action thermal relay according to claim l, further characterized in that
the second bimetallic strip is of elongated form and is connected by one of its ends
to said second terminal.
4. Delayed action thermal relay according to claim l, further characterized in that
said contact elements and said permanent magnet means and armature means are provided
on said bimetallic strips at distal ends thereof relative to the electrical terminals.
5. Delayed action thermal relay according to claim l, further characterized in that
said bimetallic strips are free of preforming and of relatively low strength.