US2293701A - Slow-acting electric switch with arc quencher - Google Patents

Description

SLOW ACTING ELECTRIC SWITCH WITH ARC QUENCHER Filed July 19, 1940 \J v /1// 1/ l/ m 3 '3 Patented Aug. 25,1942 I SLOW-ACTING ELECTRIC swlron WITH ARC ounnonna Jack Delmonte, Park Ridge,--lll., assignor to Chicago Flexible Shaft Company, Chicago, 111., a corporation of Illinois Application July 19, 1940. Serial No. 346,351

2 Claims.

This invention relates to slow-acting switches as distinguished from snap-acting switches and has more particular reference to switches of this kind especially adapted for handling electric currents such as experienced in fractional horse Dower motors wherein arc quenching is a problem.

This problem is particularly serious in the control of motor-driven household electric appliances and other motor-driven devices particularly of the small fractional motor type having small power consumption. In devices of this kind the rotor of the motor is apt to be stalled because of some mechanical defect or obstruction and at such time the magnitude of the peak currents resulting from thestalled motor condition causes arcing as an incident to opening of the contacts. This causes pitting or burning of the contacts and the condition is apt to result in fire hazard. The common practice in endeavoring to avoid this condition is the use of a snap-acting switch in which the rapiditi of separation of the two contact points eliminates or at least reduces the likelihood of drawing an arc. However, snap-acting switches of this kind are not altogether satisfactory for reasons such as their comparatively high cost of manufacture and also the possibility of mechanical defects due to their more or less multiple parts or complicated construction which tend to cause failure at some time during service. Furthermore, it is recognized that the effective life of a snap-acting switch is considerably less than that of a slow-acting switch. The problem, however, of introducing the desirable mechanical features of a slow-acting switch into small electric appliances of the kind mentioned and at the same time of attaining those desirable electrical qualities such as the ability to quench arcs under stalled motor conditions without increasing cost or impairing the mechanical desirability of construction has presented real difficulties. In this connection it should be observed that heretofore the problems of arc quenching have been dealt with mainly in connection with high current arcs and many theories have been evolved concerning the quenching of arcs in an enclosed chamber. However, I have found that such theories and structures usually applicable to high current are quenching are unsatisfactory when applied to low current conditions such as I have mentioned.

The primary object of the present invention is to provide an improved slow-acting switch having acceptable arc quenching properties.

I have also aimed to provide a slow-acting switch of the character described which may be produced at comparatively low cost and which will serve in a practical and effective manner the purposes intended.

My invention is characterized by the use of contact members arranged for operation within a semi-confined air space established by a sleeve structure of non-carbonization material, the parts being so constructed and arranged as to provide an effective are quenching media.

Other objects and attendant advantages will be appreciated by those skilled in the art as the invention becomes better understood by reference to the following description when considered with the accompanying drawing, in which:

Fig. 1 is a diagrammatic view of a slow-acting switch embodying my invention, partly in section, with the contacts in the normal open position;

Fig. 2 is an enlarged view of the contacts and are quenchingportion partly in section, with the contacts in the closed position; and

Fig. 3 is a cross section taken on the section line 33 of Fig. 2.

My invention is here illustrated as a switch applied in a motor circuit. The construction shown has been particularly designed fo appli cation to a small fractional horse power motor wherein the normal operation involves electric current of from 1 to 2 amperes and the peak stalling condition would probably involve as high as 8 amperes. In this particular case the motor is used for operating a household appliance such as a food mixer. I have also applied the invention to electric appliances such as small motordrlven shearing and clipping machines. My invention, however, is applicable to electric circuit controls which handle electric currents such as are experienced in fractional horse power motors.

In the drawing I have shown a stationary contact 4 fixed to a support 5 which serves as a conductor. A movable contact 6 is fixed to a yieldable spring support I which in turn is mounted on a fixed support}! which also serves asa conductor. The contacts are preferably annular in cross section and each has a convex contact face. This construction provides rounded contact points wherein the contact is theoretically at the highest center points. This type of contact serves particularly well with my invention because the contact occurs approximate- 1y at the longitudinal centerline of the contact bodies or in close proximity thereto with the result that if any arcing occurs it is localized centrally within the semi-confined chamber 9 formed by a housing or sleeve II which circumscribes the contacts when they are in the circuit closing position. These rounded contact surfaces are also desirable with my invention because of the tendency to rub one against another during closing and opening of. the contacts as a consequence of the yieldable spring mounting. Be-

cause of the rubbing action any oxide formed upon the contact surfaces would tend to be wiped off. Furthermore, contact points or this design do not necessarily require precise alignment of switch parts because there will be mutual contact of some points or the curved surface and, thiscontact with a mounting of the kind herein disclosed will be centrally localized.

Any suitable means may be employed for moving the contact 6 .into and out of the contact or circuit closing position such, for example, asa manually movable cam l2 adapted to act against a'plunger I3 which in turn engages the spring member 1. In this form the movable contact 6 is normally biased to the open position by means of the spring I and the cam H or any-suitable means is operable to close the switch against the tension of the spring. It will be apparent that in moving the contact member 6 into and out of contact the slight additional pressure required for maintaining contact will impart a slight rocking motion in one direction to said contact member when making contact and a reverse motion when breaking contact, thus giving the rubbing action above mentioned.

The sleeve It is preferably oi non-carbonization material or any material having good arcresisting qualities. More particularly, this material should be of such characteristics as not to establish a carbonized path when an arc is drawn across the surface thereof. For commercial purposes I have found red fibre to be satisfactory for this purpose, this having a paper fibre base. It will now be observed, viewing Fig. 2, that the sleeve H is of such diameter and length with respect to the contacts as to provide what may be termed a semi-confined chamber above referred to by the reference numeral 9. This chamber together with the confining wall of the fibre sleeve ll constitutes an arc quenching media which serves to efiectually quench any arcs resulting from stalled motor conditions such as above mentioned. As is well known, the arcing problem is of little concern where alternating current is used because the contact points will separate without establishing an arc due to the alternating current function. In the case of directcurrent and a stalled motor condition where a conventional slow-acting switch is used, the magnitude of the current at the contact points increases even though it may not be sufficiently high'to burn out an electrical fuse which may be connected with the power supply, with the-result that arcing and burning of the pointswilloccur. The intensity of this arcing and burning depends on various factors and may continue until the switch is moved to the off position or until the contact has melted away or the electrical wiring'within the motor burns out. In other words, at some point there will occur a burning out which will interrupt the current. However, with my invention arcing is effectually quenched by means of the construction describedand which I have referred to as the arc quenching media. This involves the semi-confined chamber provided by the sleeve H in coaction with the contact points.

In this connection the height of the sleeve with relation to the contact points is critical. I have discovered that the amount of current which can be broken upon separation of the contact points is dependent on the height of the fibre sleeve above the surface of the latter contact point. For example, where the contacts have a diameter of I have discovered the critical height of the sleeve to be not less than .072". In actual practice I have increased the length of the sleeve as an additional factor of safety, par;- ticularly since the motor may in some cases operate on higher voltages. In the present drawing the contacts shown in Fig. 1 are on a scale,

that there is a free outlet to gases generated within this enclosure as a consequence of arcing. I have also observed that the lower contact member 4 need not be of the same length as the opposite contact member and I have in actual practice shortened this and consequently the surrounding air chamber to approximately onefourth the length shown. I wish to point out at this time that the arc quenching function occurs under conditions contrary to conventional practice in this art so far as I am aware. The function does not occur in a closed chamber in which high pressures are generated from the heat of the arc sufiicient to blow out or quench the arc. In my invention I do not have a'confined area such as a closed chamber and consequently I do not rely on high pressure for quenching the arc. Furthermore, in the case of closed chamber arc quenching, I believe the theory has been advanced that moisture and volatile matter from the material in the surrounding chamber wall will volatilize due to the presence of intense heat of the arc and that this volatilization will produce a gas which is composed in the main of fresh non-ionized molecules which will serve to quench the arc. Since I do not employ a closed chamber and since I have further observed that separation of the contacts within a semi-enclosing fibre sleeve may be carried out an indefinite number of times, one operation following another in rapid sequence without any apparent diminuation of the effectiveness in quenching the arc. These observations lead me to believe that the arc quenching function is due to some other explanation. My invention is characterized, however, by the provision of a confining space around the contacts, this space being open at one end for free outlet of gases generated within this enclosure as a consequence of arcing, and the confining space having a critical length with respect to the contact which moves through the open end.

While I have illustrated my invention asapplied to a construction in which the fibre sleeve and one of the contacts are fixed, it should be observed that other embodiments may be used in actual. practice such as a moving sleeve and a moving contact with a fixed contact, and one or both moving contacts with a fixed sleeve. It I I claim:

1. A slow-acting switch comprising opposed contact members, a tubular sleeve of arc-resisting material having a bore of uniform diameter closed at one end and open throughout its opposite end, one of said contact members being fixed in the closed end of the sleeve bore, the other contact member having a. cylindrical body of uniform diameter smaller than the bore and of a length greater than the distance between the face of the fixed contact member and the open end of the bore, and means supporting the sleeve and the cylindrical contact member for relative movement of the latter member into and out of the bore through its said open end for making and breaking contact with the said fixed contact member, the sleeve forming an arc quenching chamber about said contact member body when the latter is in the contact position, the cross sectional area of said chamber around the said contact member body being approximately 25% greater than the cross sectional area of said contact member body, and the length of said chamber from the contact point of said fixed contact member to the open end of the sleeve bore being approximately equal at least to the radius of the contact member body.

2. A slow-acting switch as set forth in claim 1, in which each contact member has a convex contact face and at least one of the contact members has yielding support and is yieldingly urged into contact with the face of the other member in such manner as to impart a slight rocking motion of one member relative to the other when making and breaking contact.

' JACK DELMONTE.

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