US2583590A - Centrifugal switch for induction motors - Google Patents

Description

29, 1952 E. J. OSTERHUS CENTRIFUGAL SWITCH FOR INDUCTION MOTORS 2 SHEETS-SHEET 1 Filed Sept. 10, 1947 INVENTOR gRVlN J. OsTERHus W ATTORNEY Flai.

Jan. 1952 E. J. OSTERHUS CENTRIFUGAL SWITCH FOR INDUCTION MOTORS Filed Sept. 10, 1947 2 SHEETS-SHEET 2 INVEN TOR. ERVIN J. OSTERHUS ATTOPNFY Patented Jan. 29, 1952 UNITED STATES PATENT OFFICE CENTRIFUGAL SWITCH FOR INDUCTION MOTO Application September 10, 1947, Serial No. 773,278

2 Claims.

This invention relates to centrifugal devices of the type that are utilized to operate electric switches.

The invention has particular application to switches for starting single phase alternating current motors; but as will become apparent hereinafter, it is applicable to other uses.

The common commercial single phase motor has a stator field winding which develops full motor torque after the motor is started and brought up to running speed but little torque at rest and while accelerating. An auxiliary starting winding, and a circuit therefor, are commonly provided which, together with the main winding develop greater starting torque. The starting circuit is commonly opened and dispensed with after the motor has been accelerated up to a predetermined speed. A centrifugal switch driven by the motor is commonly used for this purpose.

The present invention is, in one aspect, an improved centrifugal device for operating a switch for this purpose.

In another aspect it is an improved centrifugally operated switch.

In general the invention comprises a springpressed switch-closing member; which normally holds closed a switch of the type that is springbiased to open position; and a rotary springopposed centrifugally operating device normally disengaged from the spring-pressed switch-closing member but moving into engagement with it at a predetermined rotary speed, and then retracting it to allow the switch to open.

The operation of the switch is thus rendered independent of the spring-force of the spring which opposes operation of the centrifugally operating device whereby more positive and reliable action by centrifugal force is provided.

It is the primary object of the invention therefore to provide an improved centrifugal device; and more particularly to provide an improved centrifugally operated switch having the above outlined characteristics of operation.

Another object is to provide improved means for adjustably predetermining the rotary speed of the centrifugal device (and therefore of the motor) at which the switch operates.

Another object is to provide an improved enclosing housing for the centrifugal device and switch which will be dust-proof; but which will allow ready access to the device and switch.

Another object is to provide a centrifugal device which is statically balanced in an improved manner, with respect to the axis of the motor 2 driving it, whereby, after the device has operated by centrifugal force it will be statically balanced and not tend to set up vibration at the high run ning speed of the motor.

Other objects will be apparent to those skilled in the art to which the invention appertains.

The invention is fully disclosed in the following description taken in connection with the accompanying drawing, in which:

Fig. l is a longitudinal sectional view of a part of a motor and showing an embodiment of the invention assembled therewith, as the parts appear when the motor is at rest;

Fig. 2 is a sectional view from the plane 2-2 of Fig. 1;

Fig. 3 is a fragmentary view of a part of Fig. 1

' showing operative parts as they appear when the motor has accelerated to and beyond a. predetermined speed;

Fig. 4 is a fragmentary sectional view from the plane 4-4 of Fig. 1.

Referring to the drawing, there is shown at I one end of the rotor of a motor mounted on a shaft 2 supported in a ball bearing 3 mounted in a bore 4 in an end bell 5 of the motor housing. These parts, or their equivalents, being well known, it is deemed unnecessary to illustrate the complete motor.

The end bell 5 is formed with an axially outwardly open chamber 6 beyond the bearing 3, having a cover plate 1 closing its outer open end and sealedly secured thereon in a dust-proof manner by screws 8. The inner end of the end bell as at 9 has, in a well known manner, a dusttight connection with the main motor housing 9A fragmentarily shown.

The shaft 2 extends through the ball bearing 3 and, for a short distance, into the chamber 6 terminating in a transverse end Ill. The ball bearing 3 is preferably a, unitary assembly fitting in the bore 4 in the end bell 5 and fitting on the shaft 2, and is itself of the lubricant-packed type, whereby dust that might find its way into the motor housing 9A in the vicinity of the rotor I, is blocked from entering the chamber 6 through the bearing or along the shaft 2 or through the bore 4.

The chamber 6 has a transverse shoulder l2 upon which is mounted a switch shown generally at IS, the terminals l4l5 of which extend outwardly radially into a terminal recess It in the end bell, and wires l|-l8 are connected to the terminals. The axially inner end of the recess is in the form of a cylindrical bore I9, and a plug 20 of rubber or like compressible material fits in the bore [9 and the wires 11-; go through tightly fitting holes in the plug, whereby dust cannot enter the chamber 6 by way of the terminal recess IS. The wires l'l-l8 go to the usual starting circuit, and starting winding in the stator of the motor not shown.

The switch 13 is of the spring-opened type and may be of any suitable commercial form, for example the snap switch illustrated and described in Patent No. 2,237,705; but since the detail construction of the switch, except as to its springopened feature, constitutes no part of the present invention, the switch has been illustrated in simplified form.

The switch comprises a depending rigid metal contact-support 2| having the terminal l at its upper end, and carrying a stationary contact 22 at its lower end. A movable switch element or switch arm 23 in the form of a leaf spring is held rigid at its upper end, and depends therefrom and carriesa movable contact 24 confronting the contact 22., The spring 23 is shaped to normally hold the contact 24 separated from the contact 22 (see Fig. 3) and to yield resiliently to close the contacts and to resiliently bend to open them again when released. The contact support 2| and the resilient switch arm 23 are insulated from each other and from the shoulder i2 by insulation 25, and mounted on the shoulder [2 by screws 26-26 in a well known manner.

The spring 23 extends below the contacts and has a button 21 of fibre or other electrical insulating material thereon, at substantially the axis of the motor shaft 2.

.The shaft 2 has a bore 28 extending thereinto from its end I0, along the axis of the shaft. A spring 29 in the bore abuts at one end on the bottom of the bore and at the other end on one end of a plunger 3llreciprocable in the bore; the plunger extending out of the ,bore axially, and at its .free end engaging the .button 21. The spring 29' holds the plunger 30 outwardly and pushes on the button 21, overcoming the contactopening tension of the spring 23 and resiliently holding the contacts 24-22 closed. The plunger an on the portion thereof beyond the-end of the shaft is reduced in diameter as at 3| providing an annular shoulder 32.

Now as to the centrifugal device. Upon the end of the shaft .is mounted 1a, frame base 33 by screws 34-34 projected through holesin the .frame and threaded into the end of the shaft;

and having a hole 35 therein through which the plunger passes with clearance. The frame has formed thereon a pair of transversely spaced bearing ears 36-31 extending in parallel relation away from the base33, and spaced from the shaft axis.

A bearing pin 38 extends transversely between the ears 36-31 being mounted insuitable axially aligned holes drilled therein.

A centrifugal actuator 39 is oscillatingly mounted on the pin 38. It comprises a pair of transversely spaced actuator arms 40-, having aligned holes drilled therein through which the pin 38 extends to provide an oscillation hearing support therefor. The arms 40- :extend upwardly and beyond thesmall diameter plunger portion 3|, and straddle it with clearance, and are normally spaced from theannular shoulder 32.

Below the bearing pin 38, the arms 4ll-:4I are rigidly connected to a weighted arm 42 extending toward and overlapping the end 10 of the shaft and spaced radially from the shaft. At

4 its inner. end, it carries a centrifugal weight 43, preferably extending toward and abutting on the shaft as a stop therefor, and of suitable length to dispose the arms 40-4! in a suitable spaced relation to the shoulder 32.

A coiled spring 44 is telescoped over the bearing pin 38 between the arms 40-4l and one free end 45 is hooked over an adjacent portion 46 of the base 33 at the inner ends of the ears 36-31; and the other end 41 overlaps the outer end 48 of the weighted arm 42 thereby reacting to yield ingly hold the arm 42 in normal position with the weight 43 stopped on the shaft 2 as aforesaid.

As to operation, the parts are illustrated in Fig. l .in their normal positions with the motor at rest and with the switch contacts 24-22 closed. It will be observed that the switch contacts are held closed solely by the spring 29, and that the spring 44 of the centrifugal device has no reaction on the plunger 30 or on the switch arm 23.

When current is supplied to start the motor the starting circuit comprising the wires 11-18 energized and the rotor I and shaft start to rotate and rapidly accelerate.

Centrifugal force on the weight 43 and arm 42 develops and at a speed predetermined by the spring 44, the weight v43 leaves the shaft and starts to move radially outwardly, moving the arms 4D-4l toward the shoulder 32. As the weight moves, the radius of its distance from the shaft axis increases and therefore the centrifugal force very rapidly increases. The spring 44 having a number of convolutions increases in tension very little. By the time the arms 40-4! reach the shoulder 32, the centrifugal force in excess of that necessary to overcome the spring 44 is sufficient to overcome the spring 29; and the arms acting on the shoulder 32 retract the plunger 30 and withdraw the end of the plunger from the button 21 and the spring arm 23 of the switch withdraws the contact 24 from the contact 22 and opens the starting circuit.

.By adjusting the tension of the spring 44 as will be described, this is caused to occur at a predetermined speed of the motor before it is completely accelerated to full speed, and at a speed at which the motor torque has risen to near maximum and will complete the acceleration.

Accordingly, the speed of the motor goes on increasingafter the-contacts open and the weight 43 keeps moving farther out radially and the centrifugal force .on it accordingly reaches a still higher value,-and the plunger continues to be retracted after the'contacts 24-22 are open, until the shoulder v32 is flush with the base 33 and the arms 40-41 are stopped on the base 33.

If now, as in many instances of motor power application, the motor should become over-loaded and slow down to said predetermined speed, it

.is desirable 'to prevent the starting circuit from being again closed at the predetermined speed to preventthemotor from running on the starting circuit at reduced speed and overload; and this is accomplished in the device. By allowing the weight '43 to continue 'to .move outwardly after "thesaid predetermined switch opening speed has been attained, the motor must slow down to a speed much lower than'this predetermined speed :before the spring '44 will be able to overcome above a much lowenspeed.

The advantages of this principle are well de veloped in the typical case of an induction motor driving a wood cutting circular saw to which the operator feeds the work. The characteristic torque of an induction motor is very low when at rest and at low speeds, rises slowly to a maximum as the speed increases, and then falls off toward full or synchronous speed. The range of torque and speed beyond this maximum is the normal working range for the motor.

The starting circuit is used to give high starting torque, to accelerate the motor from rest and is opened at a predetermined speed at which the motor, without it, develops somewhat less than the maximum torque, but enough torque to complete the acceleration and carry the speed on beyond the maximum torque speed and put the motor in said working range.

If the operator of such a saw-driving motor feeds the work to the saw too fast, it may overload the motor, and slow it down. As it slows down, the torque first rises to the maximum and then passes the maximum and thereafter dies out, and the speed falls and reaches the said predetermined speed.

If the centrifugal switch were allowed to close at the predetermined speed, or near it, it would cut in the starting circuit, raise the torque, and stop the slowing down, and the motor would go on driving the overload,"but with the starting circuit continuously energized. For reasons of economy among others, the starting circuit of themotor is conventionally designed to carry current only for a short time suflicient for motor acceleration or at most for intermittent starting operations. To drive the motor as aforesaid with the starting circuit continuously energized would damage it or burn it out.

As described herein, when the centrifugal switch once opens the starting circuit during the original acceleration, it holds it open down to a low speed; so that-if the operator, in the case under consideration, feeds the work to the saw too fast and overloads it beyond the maximum running torque, the torque falls, the motor slows down, the saw teeth become less and less able to cut the work and accordingly the overload increases still more; and unless the operator eases off on the work feed, the motor may completely stall and stop; and although, as the motor slows down, it may reach the low speed at which the starting circuit is again out in, the overload by this time will have become so great that the increase of torque produced by the cut-in starting circuit may not be enough to again accelerate the motor and prevent it from stalling.

Thus by the operation of the centrifugal device as described, upon overloading the saw, the motor suddenly slows down to a low speed, and the operator observes that it is about to stall, and he is induced thereby to relieve the feed and load to keep the motor running; and the device prevents damage to the starting circuit by preventing it from being cut in at or near the predetermined cutting-out speed at which the motor might pick up and carry the overload with the starting circuit energized.

In the case of an operator of a hand-fed motor-driven saw as referred to, who has acquired skill in its use, when the rate of feed to the saw slows it down below the maximum torque speed, he may ease off the rate of feed enough to keep the motor and saw running at the corresponding reduced speed, without causing it to stall or approach stalling; and since the starting circuit is held open at such reduced speed by the centrifugal device as described the saw to advantage may be operated in a wider speed range and torque range than merely that beyond the maximum range.

As an illustrative example, I have found the foregoing mode of operation satisfactorily performed with a motor having a synchronous speed of 3600 R. P. M. if the device is adjusted to open the switch at 2750 R. P. M. and close it at 2300 R. P. M. or lower.

The upper or predetermined switch opening speed may be adjusted to 2750 R. P. M. by adjusting the tension of the spring 44. To this end, a screw 49 is screwed into the end 48 of the weighted arm 42, and the end 41 of the spring engaging that arm is lodged in the screwdriver slot 50 of the screw head. To change the tension of the spring, the end 41 is removed from the slot 50 and the screw is turned one or more half revolutions and the end of the spring put back in the slot. The engagement of the spring end with the screw slot acts furthermore as a lock to lock the screw against unintended turnmg.

The lower or switch closing speed of 2300 R. P. M. or lower, is determined by the extent of travel of the arms 404| before being stopped on the frame base 33 as described; and the proportions of the parts in the drawing are suilicient for this purpose.

The end of the base 33 on the opposite side of the shaft axis from the pivoted actuator 39, has a weight 5| thereon, proportioned to counterbalance the weight of all of the parts on the other side of the axis, when in the running position of Fig. 3.

This avoids any static unbalance which might set up vibration when the motor is running at full speed.

The arms 40-4I at their point of contact with the shoulder 32, are convexly rounded as at 52, so that when the arms rock and propel the plunger 30 inward they push on the shoulder 32 always in the direction parallel to the axis of the plunger and therefore exert no lateral thrust on the plunger with attendant friction on the wall of the plunger and irregularity of operation as they otherwise would.

The plunger has a flat surface 53 formed on one side of it which provides an open passageway communicatin with the bore 28 inwardly of the plunger. This permits lubricant to be put in the bore to keep it lubricated, without liability that the lubricant would, by capillarity, seal the bore and cause air pressure or partial vacuum to be created behind the inner end of the plunger and interfere with its free movement.

The free end of the plunger portion 3| is rounded as at 54 where it engages the button 21, and the engaged face 55 of the button is preferably planar and at right angles to the plunger axis; whereby the point of engagement between the plunger end and the button may be made to occur on the rotary axis and therefore develop little or no wear.

The invention is not limited to the exact structure illustrated and described but comprehends all changes therein and modifications thereof that come within the scope of the appended claims.

I claim:

1. In a centrifugal switch mechanism, a rotary shaft having a free end portion; a switch operating element mounted to reciprocate axially of 76 the shaft; a switch comprising contacts, and a switch elementmovable ,in the axial direction of the shaft to engage and disengage the contacts, and spring-biased toward contact-open position; a portion of the switch operating element normally engaging theswitch element, and a spring holding the switch operating element in one reciprocatory direction and overpowering the spring-bias of the switch element and normally holding the contacts closed; shoulder means on the switch operating element; a frame mounted on the free end portion of the shaft and rotatable therewith; a centrifugal actuator pivoted on the frame and comprising a weighted arm at one side of the shaft axis and movable toward and from the shaft axis, and an actuating arm extending transversely of the shaft axis and movable axially, upon rocking of the actuator by centrifugal force on the weighted arm; an actuator spring opposing rocking of the actuator; the actuating arm being normally out of engagement with the shoulder means and movable into engagement therewith and then retracting the switchoperating element against the tension of its holding spring to permit the switch to open, upon pivoting movement of the actuator effected by centrifugal force on the weighted arm at a predetermined rotary speed of the shaft, determined by the tension of the actuator spring.

2. In a centrifugal switch mechanism, a rotary shaft having a free end portion; an axial bore extending into thefree end of the shaft; 2. switch operating plunger reciprocable axially in the shaft bore and having a free end portion projecting axially out of the bore; a switch comprising contacts, and a movable switch element having a portion movable in the axial direction of the shaft to close and open the contacts and springbiased to contact-open position; the end of the plunger normally engaging the movable switch element at a point substantially on the axis of the shaft; a spring in the shaft bore behind the plunger normally holding it reciprocated outwardly and overpowering the spring-bias of the switch element and holding the contacts closed;

a transverse annular shoulder on the lunger normally spaced from the end of the shaft; a frame mounted on the end of the shaft and rotatable therewith, and supporting a transverse bearing; ,a centrifugal actuator oscillatably supported on the transverse .bearing and comprising a weighted arm at one side of the shaft axis, movable by centrifugal force away from the axis; an actuator spring reacting on the weighted arm opposing centrifugal force thereon and normally holding the weighted arm toward the axis; ,a stop for normally positioning the weighted arm; the actuator comprising an actuating arm extending toward and beyond the plunger and spaced axially outwardly of the annular shoulder and movable to successively engage the shoulder and move the plunger inwardly to effect opening of the switch upon rocking of the actuator by centrifugal force on the weighted arm, effective at a predetermined rotary speed .of the shaft determined by the tension of the actuator spring.

ERVIN J. OSTERHUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,162,047 Freeman Nov. 30, 1915 1,208,421 Watts Dec. 12, 1916 1,554,694 Wolfe Sept. 22, 1925 1,926,339 ,Kindl et ,al Sept. 12, 1933 2,021,454 Levin Nov. 19, 1935 2,048,494 Diehl July 21, 1936 2,083,343 Naul June 8, 1937 2,187,207 McCabe Jan. 16, 940 2,199,232 "Taylor Apr. 30, 1940 2,225,144 Bassett Dec. 17, 1940 2,368,492 Ralston Jan. 30, 1945 2,394,283 Yost Feb. 5, 1946 2,416,973 Wright 'Mar. 4, 194.7 2,434,001 Moody Jan. 6, 1948

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