US3622930A

US3622930A - Motor protector apparatus and method

Info

Publication number: US3622930A
Application number: US866993A
Authority: US
Inventors: John R D Entremont
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1969-10-16
Filing date: 1969-10-16
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

A motor protector particularly suited for use with appliance motors having a rate of rise up to 30* C. per second and higher, comprising a heater/thermostatic disc assembly in which the major heat transfer from the heater to the disc is by conduction. The disc is supported solely by the heater in such a way that the first ''''on'''' time is short and the first ''''off'''' time is long. The structure is such that the standard deviation of the first cycle ''''on'''' time is within a 3 second range necessary to protect such motors. The assembly is placed in a heat conductive can with the heater cantilever mounted on the bottom wall thereof. The thermostatic disc is in turn cantilever mounted on the heater, a contact is attached to the distal free end of the disc and is adapted to engage and disengage a stationary contact mounted on a cover which is clampingly attached to the can but electrically insulated therefrom. The device is calibrated by applying a force to the can in the vicinity of the heater mount to change the prestress of the contacts.

Description

United States Patent [72] inventor John R. DEntremont Foxboro, Mass. [21] Appl. No. 866,993 [22] Filed Oct. 16, 1969 [45] Patented Nov. 23, 1971 [73] Assignee Texas Instruments Incorporated Dallas, Tex.

[ 541 MOTOR PROTECTOR APPARATUS AND METHOD 9 Claims, 6 Drawing Figs;

[52] U.S.Cl 337/107, 337/89 [51] Int. Cl HOlh 37/04,

H01h 37/52, H0lh6l/0l3 [50] Field ofSearch 337/89, 102, 103, 107,112, 139,365, 380

[56] References Cited UNITED STATES PATENTS 3,538,478 11/1970 DEntremont 337/89 3,453,577 7/1969 DEntremont.. 337/112 X 3,443,259 5/1969 Wielil etal H 337/89 3,431,526 3/l969 Ambler et al. 337/89 3,430,177 2/1969 Audette 337/365 3,218,4l5 11/1965 Voormah,.lr

Primary Examiner-Bernard A. Gilheany Assistant Examiner-Dewitt M. Morgan AlmrneysHarold Levine, Edward J. Connors, Jr., John A.

Haug, James P. McAndrews and Gerald B. Epstein ABSTRACT: A motor protector particularly suited for use with appliance motors having a rate of rise up to 30 C. per second and higher, comprising a heater/thermostatic disc as sembly in which the major heat transfer from the heater to the disc is by conduction. The disc is supported solely by the heater in such a way that the first on time is short and the first off time is long. The structure is such that the standard deviation of the first cycle on"' time is within a 3 second range necessary to protect such motors. The assembly is placed in a heat conductive can with the heater cantilever mounted on the bottom wall thereof. The thermostatic disc is in turn cantilever mounted on the heater, a contact is attached to the distal free end of the disc and is adapted to engage and disengage a stationary contact mounted on a cover which is clampingly attached to the can but electrically insulated therefrom. The device is calibrated by applying a force to the can in the vicinity of the heater mount to change the prestress ofthe contacts.

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ZZy-Au MOTOR PROTECTOR APPARATUS AND METHOD This invention relates to motor protectors and more particularly to motor protectors suitable for use with high rate of temperature rise appliance motors.

The trend in recent years in the appliance field has been to make appliances ever smaller. Further, due to a highly competitive market place, manufacturers have increasingly striven to utilize cost savings wherever possible. This has also been reflected in the motors used to drive the appliances and has resulted in motors having a higher rate of temperature rise per unit time. Fifteen or twenty years ago, the temperature of a typical appliance motor would increase at a rate of in the order of to C. per second. Today, however, due to many factors such as making the motors smaller, substituting aluminum for copper in the motor windings, employing higher current densities, etc., a temperature increase of C. is now closer to the norm.

With slower heating motors, devices could be employed for protecting the motors having much greater response tolerances than would be possible with the typical appliance motor of today. While many of these prior art devices are still perfectly effective for such slow rate of rise motors, it is not feasible to use them for the high rate of rise motors due especially to the low yield in attempting to make such devices for the particular application. FIG. 1 shows temperature in degrees centigrade versus time in seconds with curve A reflecting a temperature rise of 20 C./sec. and curve B reflecting a temperature rise of 30 C./sec. The maximum winding temperature allowed in the industry is 200 C. for this type of application (class A). Since a three second time delay is required for motors of this type in order to prevent deenergization of the motor for transient overloads, it will be seen that 6 seconds are available with the curve A rate of rise and only 3 seconds with the curve B rate of rise.

An example of a protector which has been very successful in answering the requirements of the curve A motor is disclosed and claimed in US. Pat. No. 3,194,924 to Moksu et al., and assigned to the assignee of the instant invention. However, for the high rate of rise application, it has been found that too few of the devices have a first on time which falls within the three second range and therefore results in an unsatisfactory yield rate for a mass-produced device. It is therefore an object of this invention to provide mass produced motor protectors, substantially all of which having a response time which fall within a three second range, a protector having a typical standard of0.50 seconds or less.

Another characteristic of a protector in the appliance motor field which is of importance is the on-off cycling time of the protector versus temperature of the motor windings. It is important for the high rate of rise motor to be able to cool suffciently during the first ofF time to permit restarting of the motor. In FIG. 2, curve C reflects the cycling of a device made in accordance with the aforementioned patent.

It will be seen that curve C has a first on time somewhat below the safe maximum of 200 C., yet the temperature does not decrease sufficiently on the first off" time to prevent an even higher temperature being reached on the second on" time, nor does the protector give the motor a chance to cool until a substantial period of time has elapsed. Therefore, another object of the invention is the provision of a motor protector while has an improved off-on" ratio, and more specifically, a long first off time and a short first on" time. Another object is the provision of a protector having a ratio of off time to on" time of5:l to 20: l and preferably closer to 20:l. Another object is the provision of a motor protector which is so small that it does not add significantly to the size of the motor, one which is simple, reliable, yet conductive to mass production assembly procedures. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the constructions hereinafter described, the scope of the invention being indicated in the appended claims.

In the accompanying drawings, in which a preferred embodiment is illustrated:

FIG. 1 depicts temperature plotted against time and shows the rate of temperature rise curves of typical appliance motors;

FIG. 2 depicts motor winding temperature plotted against time of a motor protected by a prior art protector and one made in accordance with the invention;

FIG. 3 shows a top plan view of a protector made in accordance with the invention;

FIG. 4 shows a front cross-sectional view taken in lines 4-4 of FIG. 1;

FIG. 5 shows an exploded perspective of the heater and thermostatic blade assembly shown in FIG. 4; and

FIG. 6 shows a distribution curve of first on trip times for a device made in accordance with the aforementioned Moksu et al. patent and for a device made in accordance with the invention.

Numeral 10 in FIGS. 3 and 4 depicts a motor protector comprising a base or can 12 which may conveniently be formed generally as a parallelepiped defining a switch cavity 14 therein. Can 12 is preferably constructed out of a good heat conductive material such as cold roll steel and is formed with outwardly extending flanges 16 on two opposite ends and outwardly extending flanges 18 on the other two opposite ends. A cover 20 which also may be of cold roll steel, closes cavity 14 of can 12 and is electrically insulated from the

flanges

16,18 by a strip of electrically insulating material 22 such as a Dacron, Mylar, Dacron laminate. Dacron is a trade name of E. l. duPont deNemours & Co., Inc. and refers to textile fibers of polyester film. Mylar is a trade name of the same company for a polyethylene terephthalate resin. Strip 22 may also be provided with an adhesive layer, or adhesive thermoset material may be impregnated therein or layer 22 could be formed entirely out of adhesive material to enhance sealing of the protector from gross contaminants. Flanges 18 of can 12 are bent over to clampingly engage the cover member 20 through insulating layer 22 in much the same way as disclosed and claimed in US. Pat. No. 3,430,177, assigned to the as signee of this invention. Stationary contact 26 extends through aperture 24 provided in strip 22. Contact 26 is attached in a conventional manner to cover 20 as by welding at 28.

Terminal T extends from flange 18 of can 12 while terminal T extends from cover 20.

Heater and thermostatic element assembly 30, best seen in FIG. 5, comprises a heater 32 which is a rigid bar of nickeliron, nickel-chrome, nickel-copper, or a similar resistance material. Heater 32 is formed with an integral flange 34 which extends normal to the general plane of the heater and is relatively large to minimize the heat generated in the flange and to provide a rigid mount. Flange 34 is securely fastened to bottom wall 13 ofcan 12 as by welding at 36 to form a highly heat conductive juncture. Heater 32 is an elongated element having a restricted portion or neck 38 to cause increased heat generation at, this location. Immediately adjacent neck 38, a stop 40 protrudes from the heater element. As seen in FIGS. 4 and 5, the remainder 41 of element 32 is somewhat wider than neck portion 38. The distal end of element 32 is provided with a welding portion 42 having a curved surface 44. Surface 44 is curved to facilitate attachment of the thermostatic disc element 46 with the correct attitude or angle as set forth below in more detail. Element or disc 46 is preferably composed of a bimetallic material and is formed with a dished shape which causes it to snap from a convex configuration to an opposite concave configuration upon occurrence of predetermined temperature conditions. Movable contact 48 is attached in a conventional manner to one end of disc 46 while right angle bracket 50 is securely attached to the opposite end of disc 46, as by welding to form another highly heat conductive juncture. Bracket 50, mounting thereon thermostatic disc 46, is placed on surface 44 and angularly adjusted to effect desired alignment with respect to heater element 32 and is then welded to heater portion 42 as at 52 forming another highly heat conductive juncture.

Protector 10 is calibrated by applying a force on the bottom of can 12 where heater 32 is attached to the can, as indicated by arrow 49. This will adjust the prestress placed by contact 48 on stationary contact 26.

In the open contact position the disc is in close heat transfer relationship with and even rests against protrusion 40 in alignment with the line which the center of mass of the contact 48 makes as it moves from one configuration to the other. Protuberance 40 acts as a stop to prevent overtravel of disc 46, and more importantly, provides a conductive heat path to the disc in the contacts open or ofF configuration, thereby keeping the disc in that configuration much longer than previously obtainable. The off" time is further lengthened by the fact that the protuberance which contacts the blade is contiguous to that portion of the heater which generates an increased amount of heat and thus more heat will be conducted into the disc than in prior art designs.

One reason for a narrow spread of on" time for the first cycle compared to prior art devices is that the heater 32 is arranged so that the heat transfer to the disc is mainly conductive rather than radiative. The heater in Us. Pat. No. 3,194,924 referred to supra presents a relatively large area close to the thermostatic disc resulting in a significant portion of the heat received by the disc being transferred by radiation. The transfer of heat by radiation, however, varies with the fourth power of the distance between the heater and the disc. Thus, such a device is more dlffiCUlt to calibrate and to keep within desired parameters of first on time. in the instant invention, only the narrow thickness 35 of the rigid bar heater is in radiative heat transfer relation with the disc, thus the main portion of the heat produced by the heater is received by the disc by conduction through the heater element, bracket 50, to disc 46 and in the contacts open position from protuberance 40 directly to the disc.

Some prior art devices have used wire-type heaters which give relatively little radiative heat; however, they are not sufficiently rigid and strong to mount the disc, since their crosssectional size is limited by the amount of heat required to be generated.

Another significant factor in making the distribution of first cycle time fall within very narrow limits is the lack of mechanical joints which serve to impede heat transfer and are inconsistent in their effect. In the instant invention, as mentioned above, the heater is welded directly to the can on one end and a mounting bracket on the other end, and the mounting bracket is in turn directly welded to the disc to obtain highly heat conductive junctures. It is very important that these connections be securely made to optimize heat flow. It is found that the mechanical connection shown in US. Pat. No. 3,194,924, for instance, also with its limited area of contact, results in too great a variability in heat transfer. In the structure of the invention, the heater is made rigid while keeping radiative heat transfer to the disc to a minimum by employing a width 33 to thickness 35 ratio of the heater to greater than 1 and preferably within a range of 19%;] to l depending on the particular heater material chosen, The above structure allows the support of the disc by the heater with no other supports which would only impede heat transfer and make more variable the response time of the disc to heat generation by the heater.

As explained above in reference to FIG. 2, it is desirable for a protector for the high rate of rise motor application to have a short on time and a long off time. Curve C shows a typical on-off time for a prior art protector. lt can be seen that although the first on" time is quite short, the first off" time is also short with the result that the temperature of the motor winding rises considerably beyond the first cutoff temperature. Curve D, which represents the on-off" time for a protector made in accordance with this invention, shows that the protector opened quite close to the 200 C. limit and then stayed off for a comparatively long time until the temperature decreased significantly, and thereafter the average decreased gradually to a steady average temperature. Thus, the motor had considerably more time to cool during the firstoff. Although H0. 2 is not drawn to scale, specifically, the ratio of off' time to "on time is 20 to l for curve B, while curve A which represents prior art devices including the Moksu et al. structure, has been found to be approximately 5 to l. The im proved ratio is due to the particular mount of the heater which minimizes loss of heat from the heater element through its support and also the arrangement whereby the disc is in close heat transfer relationship with and even contacts a high heat generating portion of the heater when the disc is in the contacts open configuration so that the heat stored in the heater will be transferred to the disc keeping its temperature elevated, and hence, in the contacts open position for a longer period of time than previously obtainable.

Referring to FIG, 6, a standard distribution curve of first on times for the Moksu et al. device is shown in curve E with a standard deviation 0' equal to 0.75 seconds. Other prior art devices typically have standards of deviations of 0.75 seconds or greater. This represents the value in normal statistical analysis which, when multiplied by 6, defines a range in which 99.7 percent of the population in a normal distribution falls. Curve F represents a standard deviation curve for a device incorporating the improved heater/disc assembly previously set forth. The a for this device equals 0.25 seconds. The area enclosed by the abscissa and the curves E and F are equal so that the curves represent distribution for the same total number of devices. As mentioned above, the time available for high rate of rise motor is three seconds and specifically extends between three and six seconds to allow time for transient faults. Curve F is entirely within this range so that random sampling is sufficient as a quality control measure; however, curve E extends beyond these limits. This means that every device represented by the E curve must be checked to see whether it falls within the acceptable range. This, of course, adds considerably to the cost of the device and is very time consuming. Further, the yield is necessarily less since those devices which fall inside the range cannot be utilized for this motor application.

In addition to the minimal variability in the first on" time, the structure of the instant invention results in a long *off" time coupled with a very short on" time, as seen in FIG. 2, thereby giving the motor a chance to cool sufficiently to enable it to restart. With the improved structure, a range in ratio of 5:1 to 20:1 is possible with curve D of FIG. 2 being approximately 20: 1.

Another factor which contributes to the narrow distribution is the particular cantilever mounting of the heater coupled with the cantilever mount of the disc. The air between heater 32 and the can acts as excellent thermal insulation so that only the mount 34 would conduct heat away from the disc.

Although several devices have employed heater mounts which have certain similarities, they are not suitable for high rate of rise motor applications. For instance, the heater-element shown and described in US. Pat. No. 3,004,203, assigned to the assignee of the instant invention, shows a thermostatic disc cantilever mounted on a heater; however, due to the large area of contact between the heater and the element on which it is mounted, not enough heat is conserved in the heater for use for the disc. The aforementioned patent to Moksu et al. shows an arrangement which provides an air space between the heater and its support throughout a substantial portion of the length of the heater element. HOwever, even in that structure, not enough heat is conserved.

ln view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions 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.

lclaim 1. A motor protector comprising an electrically conductive base, a rigid elongated heater element having two opposite ends and a portion located intermediate the two ends which generates an increased amount of heat relative to the remainder thereof. one end formed with a relatively large flange integral with the heater, the flange secured to the conductive base so that the heater element is cantilever mounted on the base end having a first surface of greater width than a second surface, a thermostatic element cantilever mounted on the other end of the heater element in a plane generally normal to the first surface and extending generally along the length of the heat generating portions of the heater, a movable and a stationary contact. the stationary contact mounted on terminal means electrically insulated from the heater element. the movable contact mounted on the thermostatic element and adapted to move into and out of engagement with the stationary contact, and terminal means electrically connected to the heater element; the thermostatic element; in the vicinity of the movable contact, moves toward the said portion located intermediate theiends of the heater element when the disc is in the contacts disengaged position to enhance the heat transfer from the heater element to the thermostatic element.

2. A motor protector according to claim 3 in which the heater element has a first surface to second surface ratio greater than approximately 1% to l 3. A motor protector according to claim 1 in which the free distal end of the thermostatic element mounts the movable contact on one side and in which the other side of the thermostatic element is biased against a heat generating portion of the heater when the contacts are in the disengaged off position.

4. A motor protector according to claim 3 in which the heat generating portion of the heater against which the thermostatic element is biased comprises a protuberance extending from the heater.

5. A motor protector according to claim 4 in which the heater element includes a constricted portion contiguous to the protuberance.

6. A motor protector having a rigid barlike heater element one portion of which generates an increased amount of heat relative to the remainder thereof. the heater element having a first surface of greater width than a second surface, a thermostatic element cantilever mounted on the heater element remote from said portion in a plane generally normal to the first surface and extending generally along the length of the heat generating portions of the heater and movable from a first off position to a second on position and back. the free distal end of the thermostatic element being biased against a heat generating portion of the heater intermediate the ends thereof and contiguous with that portion of the heater which generates the increased amount of heat when the element is in the first off position the heater element being the sole support for the thermostatic element. whereby the standard deviation of first cycle on time is 0.5 seconds or less and the ratio of off to on" times is greater than about 5 to l.

7. A motor protector according to claim 6 in which the heat generating portion of the heater which is biased against the free distal end of the thermostatic element comprises a protuberance extending from the heater element.

8. A motor protector according to claim 7 in which the heater element includes a constricted portion contiguous to the protuberance.

9. A motor protector according to claim 6 in which the heater element is cantilever mounted on an electrically conductive support.

Claims

1. A motor protector comprising an electrically conductive base, a rigid elongated heater element having two opposite ends and a portion located intermediate the two ends which generates an increased amount of heat relative to the remainder thereof, one end formed with a relatively large flange integral with the heater, the flange secured to the conductive base so that the heater element is cantilever mounted on the base end having a first surface of greater width than a second surface, a thermostatic element cantilever mounted on the other end of the heater element in a plane generally normal to the first surface and extending generally along the length of the heat generating portions of the heaTer, a movable and a stationary contact, the stationary contact mounted on terminal means electrically insulated from the heater element, the movable contact mounted on the thermostatic element and adapted to move into and out of engagement with the stationary contact, and terminal means electrically connected to the heater element; the thermostatic element, in the vicinity of the movable contact, moves toward the said portion located intermediate the ends of the heater element when the disc is in the contacts disengaged position to enhance the heat transfer from the heater element to the thermostatic element.

2. A motor protector according to claim 3 in which the heater element has a first surface to second surface ratio greater than approximately 1 1/2 to 1.

3. A motor protector according to claim 1 in which the free distal end of the thermostatic element mounts the movable contact on one side and in which the other side of the thermostatic element is biased against a heat generating portion of the heater when the contacts are in the disengaged ''''off'''' position.

6. A motor protector having a rigid barlike heater element one portion of which generates an increased amount of heat relative to the remainder thereof, the heater element having a first surface of greater width than a second surface, a thermostatic element cantilever mounted on the heater element remote from said portion in a plane generally normal to the first surface and extending generally along the length of the heat generating portions of the heater and movable from a first ''''off'''' position to a second ''''on'''' position and back, the free distal end of the thermostatic element being biased against a heat generating portion of the heater intermediate the ends thereof and contiguous with that portion of the heater which generates the increased amount of heat when the element is in the first ''''off'''' position, the heater element being the sole support for the thermostatic element, whereby the standard deviation of first cycle ''''on'''' time is 0.5 seconds or less and the ratio of ''''off'''' to ''''on'''' times is greater than about 5 to 1.