Abstrict
A self-regulating electric heater particularly useful for heating
compressor lubricant and the like is disclosed in which a cylindrical
body of electrically insulative ceramic is formed with an axially
extending generally parallelepiped shaped slot in communication
with an end of the body. A positive temperature coefficient of resistivity
(PTC) resistor, configured slightly smaller and complementary with
the slot, is disposed therein. Axially extending grooves are formed
in two opposed parallel walls of the body which, with two other
cooperating walls, define the slot. The grooves are located on opposite
sides of an axial plane perpendicular to the opposed walls. A platform
is formed in each groove to serve as a stop surface to limit the
insertion of spring biased terminals which are inserted in the grooves
to provide electrical connection with the PTC resistor. The grooves
are coextensive in length with the slot to permit reception of an
injection nozzle to facilitate injection of thermal transfer material
from the inside of the device to the outside to obviate trapping
of air pockets. Finally, sealant material is disposed in the open
end of the body with the terminal leads extending therethrough.
Claims
We claim:
1. A self regulating electrical heater comprising a rigid body
of thermally conductive, electrically insulative material having
first and second end portions, the first end portion having an outer
surface, a slot having an open end and a closed end formed in the
body with the open end in communication with the outer surface of
the first end portion and extending toward the second end portion
with the closed end of the slot at the second end portion of the
body, the slot defined by first two opposed surfaces joined by second
two opposed surfaces, a groove having a bottom wall formed in each
of said first two opposed surfaces in communication with the first
end portion and extending toward the second end portion, a resistor
element composed of a ceramic material with spaced, flat, electrical
contact layers provided thereon disposed in the slot, the resistor
element configured to slide into the slot and occupy essentially
all the available space in the slot, electrically insulative, thermal
transfer material disposed between the resistor element and the
surfaces defining the slot filling in any remaining space, a terminal
received in each groove, the terminals having electrically conductive
spring means which are compressed between the bottom wall of each
respective groove and a respective flat contact layer on the resistor
element to provide good electrical connection with the resistor
element and means sealing the slot at the first end portion with
the lead attachments received therethrough.
2. A self regulating electrical heater according to claim 1 in
which the grooves are coextensive in length with the slot whereby
injection of thermal transfer material is facilitated.
3. A self regulating electrical heater according to claim 1 in
which a plateau is formed in the grooves intermediate their ends
which serves as a stop surface to limit insertion of the lead attachments.
4. A self regulating electric heater according to claim 1 in which
the body is cylindrical in configuration and the first two opposed
surfaces are parallel and the grooves are located on opposite sides
of a plane in which the longitudinal axis of the cylinder lies and
which is perpendicular to the first two surfaces.
5. A self regulating electric heater according to claim 4 further
including orienting means formed in the body to facilitate automated
manufacturing.
Description BACKGROUND AND SUMMARY OF INVENTION
This invention relates in general to self-regulating heaters and
more specifically to PTC ceramic heater devices particularly suitable
for heating compressor oil.
In conventional refrigeration compressors, a refrigerant, such
as one sold under the trademark "Freon" by E. I. Du Pont
de Nemours, & Co., under certain temperature conditions tends
to migrate from the condenser into the compressor lubricant. Such
migration is undesirable since it causes deleterious effects including
the reduction in lubricating properties of the lubricant. In order
to avoid this problem it is conventional to employ a crankcase heater
to maintain the compressor crankcase at a temperature above that
of the rest of the refrigeration system which has the effect of
boiling out any Freon already in the lubricant and of preventing
migration of the refrigerant into the crankcase lubricant. Recently,
improvements have been effected in these heaters making them self-regulating,
thus improving their reliability while doing away with the costs
involved in associated regulation controls. By way of example: U.S.
Pat. Nos. 3,564,199; 3,720,807; 3,748,439; 3,824,328; 3,940,591;
3,996,447; 4,086,467; and 4,091,267 all disclose self-regulating
heaters useful in many applications including the heating of compressor
crankcases. These devices employ a heater made of ceramic material
having a positive temperature coefficient (PTC) of resistivity.
Such heaters have a relatively low resistance at normal ambient
temperatures, but following energization by a source of electric
power will self heat and increase in temperature and resistance.
Once a threshold or anomaly temperature is reached the resistance
increases rapidly by several orders of magnitude and will stabilize
when the heat generated balances the heat dissipated. At this point
the resistance level is many times the initial room temperature
value.
While the heaters of the above mentioned patents are effective
for many applications, it is an object of the present invention
to provide a self-regulating heater and a method for making such
a heater, which is more conducive to mass production assembly techniques
than prior art devices. Another object is the provision of a self-regulating
heating device which uses a minimal number of components and thus
can be produced at a low cost while still producing such heaters
which are reliable and efficient.
The self-regulating heater of this invention preferably comprises
a cylindrical body of steatite or other electrically insulative
ceramic in which a slot of parallelepiped configuration is formed
extending in an axial direction from an open end toward a closed
end of the cylindrical body. A single PTC resistor, formed of ceramic
material such as a doped barium titanate, is configured slightly
smaller than and complementary with the slot and is received therein.
In two of the walls defining the slot, an axially extending groove
is formed coextensive in length with the slot. Intermediate the
ends of the groove a platform is formed in the body to serve as
a stop surface to limit the extent that a spring biased terminal
can be inserted. Insertion of the terminals in the grooves place
them in electrical connection with spaced portions of the PTC resistor.
The grooves are disposed on opposite sides of a plane in which the
longitudinal axis of the cylinder lies and which is perpendicular
to the walls in which the grooves are formed in order to optimize
spacing between the leads. Automated assembly of the device includes
the steps of sliding or inserting the resistor into the slot, inserting
injection nozzles into the grooves until they are adjacent the closed
end of the cylinder and injecting thermally conductive grease like
material into the space between the resistor and the cylinder at
the same time the nozzles are removed from the grooves, sliding
a spring biased terminal into each groove until it bottoms against
a respective platform and then dispensing a first sealing layer
of RTV silicon in the open end of the cylinder around the two leads
passing therethrough and a second layer of epoxy on top of the first
layer to provide effective pull strength for the leads. If the device
is assembled by hand the above procedure is modified by coating
the grease like material on the resistor before sliding it into
the slot. The remainder of the procedure remains the same.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front elevation of a heater device made in accordance
with the invention;
FIG. 2 is a cross sectional view taken on lines 2--2 of FIG. 1;
FIG. 3 is a cross sectional view similar to FIG. 2 but rotated
on the axis of the cylindrical device 90.degree. therefrom; and
FIG. 4 is a top plan view of cylindrical body of the heater without
the heater assembly.
Corresponding reference characters indicate corresponding parts
through the several views of the drawings.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to the drawings, numeral 10 is used to generally identify
a heater device made in accordance with a preferred embodiment of
the invention. Heater device 10 comprises a generally cylindrical
body 12 of ceramic or ceramic like material such as a molded impervious
steatite in which a generally rectangular slot 14 is formed which
extends from a first open end 16 of body 12 along the axis of the
cylindrical body toward a second closed end 18 of body 12 terminating
at surface 19. Open end 16 is formed preferably by providing a cylindrical
bore 20 which extends to the surface 22 and communicates with parallelepiped
slot 14. Also in communication with slot 14 are two grooves 24,
26 which extend axially from surface 22 along the length of slot
14. Although as is apparent in FIG. 4, slots 24, 26 are rectangular
in cross section, they could be formed in any convenient configuration.
Grooves 24, 26 have a first width and depth which extend to platforms
28, 30 respectively located intermediate the open end 16 and bottom
surface 19 of body 12. Extending from platforms 28, 30 the grooves
have a second configuration includig a semi-cylindrical portion
32, 34 respectively connecting with walls 36, 38 spaced from slot
14.
Resistor 40, preferably formed generally in the configuration of
a parallelepiped is formed slightly smaller than and complementary
with slot 14. Resistor 40 is composed of ceramic material having
a positive temperature coefficient of resistivity such as barium
titanate doped with a rare earth such as lanthanum and is provided
with contact layers 42, 44 on opposite sides thereof. Layers 42,
44 of electrically conductive material such as electroless nickel
or an inner layer of aluminum and an outer layer of copper or any
other suitable material may be applied to resistor 40 in any conventional
manner. Resistor 40 is disposed in slot 12 and terminals 46, 48
are provided to electrically connect resistor 40 with a power supply.
As seen in FIG. 2, terminal 46 comprises a resilient electrically
conductive member 50, formed of material having good electrical
and spring characteristics, such as tin plated beryllium copper,
clinched at 56 onto the wire lead and has an elongated strip which
at 52 is bent back upon itself and with a dimple 54 formed in its
distal free end which serves as the electrical contact surface biased
against layer 44. In its unrestrained state, the distal end of member
50 extends further away from its base than is shown in FIG. 2 and
is formed so that it will take a preselected minimum force to cause
the distal end of member 50 to close. During assembly, with resistor
40 disposed in slot 12, terminal 46 is pushed into groove 24 forcing
contact surface 54 to move toward the base of member 50 thereby
providing sufficient contact force between contact surface dimple
56 and layer 44. The amount of insertion of terminal 46 is limited
by platform 28 which reacts against portion 52. Terminal 48 is constructed
in the same manner (not shown) and is received in groove 26. Terminals
46, 48 are provided with suitable electrically insulating sleeves
58, 60 respectively, such as a cross linked polyethylene. It will
be noted that grooves 26, 28 are disposed on opposite sides of a
plane 62 in which the axis of cylindrical body 12 lies and which
is perpendicular to surfaces 62, 64 so that the outside diameter
of body 12 can be kept to a minimum while still providing desired
heat sink characteristics and sufficient space between sleeves 58,
60 to avoid any interference therebetween.
Heat transfer material is placed between resistor 40 and the walls
of body 12 defining slot 14 to optimize heat transfer from resistor
40 to body 12. In order to avoid contamination of the side walls
of bore 20 which would deleteriously affect any seal thereafter
placed in the open end of the body it has been found that the heat
transfer material should be curable to preclude any outgassing.
By way of example, a suitable material is alcohol cured RTV 738,
sold by Dow Corning Corporation, mixed with particles of aluminum
oxide of varying size. The heat transfer material, which is of grease
like consistency prior to curing, is either coated on resistor 40
before it is inserted in slot 14 or injected in situ as will be
explained below. Once in place, the thermal transfer material is
cured for up to twelve hours.
A first vapor barrier seal 66 of RTV silicone or other suitable
material which is compatible with resistor 40, that is, will not
deleteriously effect the PTC characteristics of the resistor is
disposed in bore 20 and an epoxy seal 68 to provide required pull
strength for leads from terminal 46, 48 is placed thereover. A self
leveling, acetic acid cured RTV 112 sold by General Electric Company
has been found to be suitable for seal 66. This is cured for approximately
one hour. For seal 68 epoxy 925-13 sold by Amicon Corporation has
been found to be suitable and will provide pull strength of well
over twenty pounds per lead which is required in this type of device.
This epoxy, after curing for approximately two hours, has the characteristic
of being flexible and matches the thermal coefficient of the ceramic
body. The materials used for seals 66 and 68 form both mechanical
and chemical bonds with each other and with body 12.
The device and its components are configured in such a way as to
facilitate automated manufacture. Groove 70 is provided in the outer
surface portion of body 12 to serve as a means for orienting the
body at a work station. A resistor 40 of selected base resistivity
is dropped into slot 14 and injection nozzles are inserted into
grooves 26, 28 and semi-cylindrical sections 32, 34 with the outlet
of the nozzles in close proximity to bottom surface 19. Heat conductive
but electrically insulative, curable thermal transfer grease is
injected into the body in order to fill all voids between resistor
40 and body 12 to optimize heat transfer therebetween. The nozzles
are withdrawn during the injection procedure so that the grease
is inserted from the closed to the open end thereby avoiding trapped
air pockets. Once the nozzles are completely withdrawn, terminals
46, 48 are inserted into their respective grooves and due to the
spring bias of the terminals the contact surfaces wipe the grease
away from the conductive layers of resistor 40 thereby making good
electrical connection therewith. After allowing time for the thermal
grease to cure, the silicone seal is then poured into place, allowed
to cure and finally is followed by the epoxy seal which in turn
is allowed to cure. Once the sealant materials have cured, the heater
is ready for use.
By way of example, heaters made in accordance with the invention
employed a parallelepiped PTC resistor 40 of approximately 23.8
mm .times. 15.0 mm .times. 2.5 mm with an anomaly temperature of
120.degree. C. and a base resistivity of between 4000 - 12,700 .OMEGA.
-CM @25.degree. C. @ 240 VAC (1/4 cycle). Body 12 was 32.0 mm in
length and had a diameter of 19.0 mm. Slot 14 was approximately
23.8 mm .times.15.25 mm .times. 3.0 mm. Leads 58 and 60 were 18
gauge with a crosslinked polyethylene sleeve. The combined thickness
of sealing layers 66 and 68 was approximately 5.7 mm. This size
heater is particularly useful with relatively small horsepower compressors
such as 1.5 to 4.5 H.P.
Thus it will be seen that the heater is easily assembled with minimal
labor thereby minimizing manufacturing costs. The only difference
between different voltage ratings such as 240 VRMS and 480 VRMS
is in the composition of the PTC resistor material, i.e. various
applied voltage levels are accomodated merely by using PTC resistors
having different base resistivity levels. Thus an economy is realized
both in manufacturing and in maintaining inventory since fewer different
parts are required compared to prior art devices in which the design
of the device is modified to accommodate different voltage levels,
for instance by using PTC elements of varying thicknesses. The cylindrical
shape of heater 10 not only is very efficient as a heat source enabling
higher wattage per unit volume compared to heating devices with
flat surfaces, it also facilitates handling and is easily receivable
in a well in a crankcase. Additionally, the heater of the present
invention offers an advantage in the construction of the well itself.
Since the well is subjected to significant operating pressures,
a cylindrical configuration is more efficient, less expensive and
easier to construct than other configurations.
In view of the above, it will be seen that the several objects
of the invention are achieved and other advantageous objects attained.
As various changes could be made in the above construction without
departing from the scope of the invention, it is intended that all
matter contained in the above description or shown in the accompanying
drawings shall be interpreted as illustrative and not in a limiting
sense. |