US2417710A - Starter for electric motors - Google Patents

Description

March 18, 1947. 5, 5M|TH 2,417,710

STARTER FOR ELECTRIC MOTORS Filed Feb. 2, 1944 A 1- Hz "F2 A2 Al as 175 CB 2 FZ AZ Al F'i Zmomto: I F 16. 5 ml EDWARDR SMITH E] I W I I,

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Patented Mar. 1e,- 1e47 STARTER FOR ELECTRIC MbTORS Edward P. Smith, Mansfield, Ohio, asslgnor to The Ohio Brass Company, Mansfield, 01110, a corporation of New Jersey Application February 2, 1944, Serial No; 521,003

My invention relates to starters for electric motors and has particular reference to starters oi the type employing a starting resistance.

One object of my invention is to provide a mo-- tor starter employing a starting resistance and a shorting switch to shunt out the resistance after a lapse of time.

Another object of my invention is to provide a means which will insure the shunting out of the starting resistance and a completion of the starting cycle.

Another object of my invention is to provide a motor starter in which the acceleration of the motor armature is controlled through variation of armature voltage or armature current or both including means to effect completion of the timing cycle under adverse starting conditions.

Another object of my invention is to provide a motor starter having thermally controlled means to increase the voltage applied to mechanism for shunting out the starting resistance after a period of time.

Another object of my invention is to provide an automatic starter including a resistance shunting switch with an operating coil including means to automatically control the energization of the coil after a time lag.

Another object of my invention is to provide an automatic starter in which the operating coil for the resistance shunting switch is in series with a resistor in which the resistance decreases as the temperature of the resistor increases, to bring the coil up to "its operating strength after a time lapse.

Another object of my invention is to provide an automatic motor starter having a shorting switch for the starting resistance and with a holdout coil in series with thestarting resistance and an operating coil for the switch provided with means to increase the-operating strength of the coilto overcome the holdout coil after a lapse of time.

Still another object of my invention is to provide a motor starter having a control resistor with a negativecoeificient of resistance and means to heat the same whereby its resistance is reduced accordingly.

Other and further objects of my invention, as well as advantages and uses, will become appar: ent as the description and claims are read and upon examination of the accompanying drawings, in which- Fig. 1 is a schematic diagram of one form of my invention.

27 Claims.- (01. 172-288) Fig. 2 is a line diagram 01' a modification c Fig. 1. Fig. 3 is a schematic diagram of another to oimy invention.

Fig. 4 is a line diagram 0! a modification of Fig. 3.

Fig. 5 is a schematic. diagram of still another form of my invention.

Fig. 6 is a, line diagram oi a modification oi Fig. 5.

All three forms of my invention shown by Figs. 1, 3 and 5 are related types oi direct current motor starters and omitting certain features, which are new and novel with me, they are in general forms familiar to those skilled in the art.

However, under conditions of abnormal overload or low main line voltage such prior art starters are not dependable as the starting resistance is not necessarily shunted out and the starting cycle is not complete, thereby causing the starting resistance to remain as a part of the starting circuit, resulting in either damage to the motor starter or inefficient operation of the motor.

During the starting cycle,,the acceleration of the motor speed may be very slow or acceleration may not take place at all if the load-is too great or the line voltage low in which case the starting current will decrease very slowly or remain constant and not decrease to normal as it should, and at the same time the counter E. M. F. of the motor armature will not increase as it should under normal starting conditions.

I depend upon the decrease in the starting current and the increase in the counter E. M. F. of the motor armature to control the normal operation of the shunting switch and upon auxiliary means associated with the shunting switch to effect operation thereof under adverse starting conditions, to shunt out the starting resistance;

The main object of my improvement is to provide means which will insure the completion of the starting cycle without materially complicating the starting circuit.

In the embodiment of my invention, disclosed schematically in Figs. 1 and 2, A designates the armature of a direct current motor, the terminals of which are connected to posts Al and A2 of the starter. The motor has a field winding W having its terminals connected to. the posts Fl and F2 of the starter.

Current is supplied to the starter and motor I through the supply line I and return conductor 2. The current from the supply line I is controlled by a. main or disconnecting switch DS which closes the circuit to the conductor 3 and which includes the fuse F and is connected to the circuit breaker CB having contacts 4 and 5. The contact is operated by the armature I actuated by the solenoid or electromagnet coll M.

The contacts 4 and 5 are normally held open as by a spring or gravity when the solenoid M is not energized.

The armature 6 is connected to the starting resistance SR by the conductor 1 and to one terminal 8 of a shorting switch SS by conductor 9. The other terminal or contact ID of the shorting switch SS is operated by the armature H, actuated by the solenoid or electromagnet coil C.

The other terminal of the starting resistance SR is connected to the post Al by the conductor l2 and to the armature II by the conductor I3.

The coil M is energized while the control switch CS- is closed thus connecting the coil M to the conductor 3 and to the conductor 2 through the conductor H. The switch CS is usually of the push-button type. When the coil M is de-energized the contacts 4 and 5 are open. The field winding W is connected to the posts Fl and F2 which in turn are connected to the power lines 1 and 2 by the conductors l5 and I8 respectively.

One terminal of the coil C is connected to conductor H and the other terminal is connected to conductor l3 through an interposed control resistor CR. It will be noted that the coil C in series with resistor CR is thus connected across the terminals of the motor armature A, and the energization of the coil C therefore depends upon the operation or acceleration of the motor in starting, in other words the closing of the shorting switch SS and the cutting out of the starting resistance SR depends upon the counter E. M. F. generated by the motor.

Closing the control switch CS after closing the disconnecting switch DS, energizes the coil M which in turn closes the circuit breaker CB therefore establishing a circuit including conductor i, switch DS, fuse F, circuit breaker CB, conductor 2, starting resistance SR, conductor l2, armature A and conductor 2.

Most resistance elements have a positive temperature coefficient of resistance, that is the resistance increases as the temperature of the element increases but I use a resistance element CR having a negative temperature coefllcient of resistance, that is for a given E. M. F. across its rminals the resistance of the element decreases as its temperature rises thereby allowing more current to flow as the temperature of the element CR increases.

Such a resistance element may be one of carbonaceous material or of carbonaceous material combined with other materials whereby the characteristics of the element may be made to meet requirements.

I am unable to say just what the composition of such an element is or should be to meet given requirements or what treatment is applied to the material to produce such elements having a negative temperature coefficient of resistance as this is within the knowledge of the manufacturer.

The characteristics of such elements may be greatly varied by a chang in the composition and treatment but such elements are obtainable on the market from certain manufacturers of carbon products. The characteristics of a CR element must be such as to harmonize with the other elements of the starter with which it is used and with the characteristics of the motor and therefore I am unable to give any definite 4 characteristics which will meet all the various conditions.

As stated before, and still referring to Figs. 1 and 2, the coil C and control resistor CR are connected in series and across the motor armature A and the energization therefore of the coil C depends upon the counter E. M. F. generated by the motor and upon the performance of the control resistor CR.

The coil C and control resistor CR are so designed and proportioned as to permit an elapse of time after the motor starts before the shorting switch SS operates to cut out the starting resistance SR but later insures the completion of the starting cycle if normal predetermined conditions of operation of the motor in starting are not attained:

The coil C and resistor CR are designedand proportioned to function properly with the motor with which the starter is to he used and is such that when the armature voltage, increasing as the armature accelerates, reaches a predetermined value under normal starting, say one-half that of the supply voltage, the shorting switch S8 will close thus cutting out the starting resistance SR and thereby completing the running circuit.

The cold resistance of the control resistor CR may be made substantially that of the coil C.

The wattage rating of the control resistor CR is such that the current flowing therethrough will cause the resistor to gradually heat and simultaneously the resistance will decrease until it reaches its normal operating temperature at which it resistance may be about 6 that of its cold resistance.

Under normal starting operation, in which the load on the motor is not too high, the speed oi the motor increases gradually until the E. M. F. of the motor reaches a predetermined value at which the switch SS closes; in this case the effect of the control resistor CR is of minor importance. As the motor continues to build up speed the armature or motor operating current will gradually decrease and the counter E. M. F. of the motor will gradually increase, but if the load on the motor is too great, it will prevent the arms.- ture speed accelerating properly and therefore prevent the counter E. M. F., reaching a. value suflicient to cause the necessary energlzation oi the coil C and to closethe shorting switch S8 at the adjusted value thereof.

In this latter case, as the current continues to flow through the control resistor CR, the resistor will gradually increase in temperature and simultaneously its resistance will decrease, thus allowing an increasing flow of current through the coil C until after a lapse of time the current flow through the coil C is sufilcient to energize the coil to close the shorting switch SS and cut out the starting resistance and thus subject the motor directly to the full line voltage.

If in the latter case the speed of the motor should simultaneously increase somewhat with th heating of the control resistor CR this will also assist in assuring a closing of the shorting switch SS at an earlier period as the current flow through the coil C will increase that much faster.

From the above it will be evident that the control resistor CR will automatically permit an increasing amount of current to how through the coil C if the acceleration of the motor armature is abnormally slow until a condition is reached wherein the current flow through the coil C is sufficient to operate the shorting switch SS and this is true even though the counter E. M. F. of

the motor does not increase.

If desired, a. shorting switch a: operated by the armature ll may be used whereby the control resistor CR is short-circuited while the switch SS is closed during normal operation of the motor. This permits the operation of the control ,resistor CR at atemperatur above that which 'means of heating\the resistor, depending upon the design. In Fig. 2 the switch a: is omitted butmay be included if desired.

In Fig. 3 and its modification shown in Fig. 4, the arrangement is thesame as in Figs. 1 and 2 respectively exceptior the addition of a hold-out solenoid or electromagnet coil HO and the omission of the switch a: in Fig. 4, therefore I have designated the other partsoi the circuits of Figs. 3 and 4 by the same characters as used in Fig. 1 and 2 and in Figs. 5 and 6 forthe same reasons. In Fig. 4, the resistor CR is supplied with an auxiliary heater H.

The hold-out coil HC is connected in series with the starting resistance SR and acts upon the armature to hold the shorting switch SS open against the pull of the coil C which is simultaneously energized. It will be noted that the coil C and the resistor CR are connected to conductors 2 and l and therefore directly across line, hence are energized independently of the counter E. F. of the motor.

When the switches D8, CS and CB have been closed the motor will start accelerating with the -starting resistance SR in series therewith and with the coil HC, therefore the current in the starting circuit will energize the hold-out coil H0. The construction and relation of the parts of switch SS are such that its coil HC controls and holds the switch SS open until such time as the acceleration of the armature A has reached a speed at which the current therethrough has gradually decreased thereby gradually de-ener-' gizing the coil I-IC until the current has reached a predetermined value of possibly 125 percent of the rated full load current at which point the energization of coil C either remains constant or has increased due to resistor CR and become superior to that of coil HC, at which time the coil C acting upon the armature it closes the short' ing switch SS. throwing the motor directly upon full line voltage.

Since the coil C and resistor CR are connected to a source of constant voltage, any variation in the energization of the coil C is due to variation of the resistance of resistor CR.

The combined operation of the coil C and control resistance CR in Figs. 3 and 4 is the same as explained above in the case of Fig. i should the acceleration of the motor be unduly delayed as in case of abnormal starting.

The control resistor CR in Figs. 3 and 4 may he short-circuited by the switch or operated by the armature H, as in the case of Fig. 1.

In the starter shown in Figs. 5 and 6 which are similar to the starters of Figs. 3 and 4 respectively there is a differential action between coil 0 and the hold-out coil HC, the acceleration of the armature affecting the energization of both coils in that as the acceleration of the armature increases, the starting current which is normally high, decreases, thereby decreasing the energization of coil HC and at the same time the counter E. M. F. of the armature increases, thereby increasing the energization of the coil C.

The coil C and its control resistor CR in the case of Figs. 5 and 6 are connected across the armature A and therefore the energization of coil C and the operation of the resistor CR are affected by the counter E. M. F. of the armature, while in the case of Figs. 3 and 4 the coil C and resistor CR are subject to a constant line voltage.

In case of Figs. 5 and 6, in starting under an overload or a low line voltage in which case the acceleration of the motor is very slow or absent, control resistor CR tends to heat due to the prolonged iiow of current therethrough and its resistance drops thus permitting more current to flow through the coil C as a result of the counter E. M. F. of the armature which may remain constant or increase slightly until the energization of the coil C' is superior to that of coil HC, whereupon the switch SS is closed and the starting resistance shunted out. The combined action of the coil C and of the control resistor CR is the same as in Fig. 1.

If in Figs. 5 and 6 the acceleration of the motor is continuous after the closing of the control switch CS, then the energization of the coil HC will gradually decrease and the energization of the coil C will gradually increase until the pull of coil C becomes superior to that of coil I-IC thereby closing switch SS without the control resistor heating and playing much part in the operation.

In Fig. 6 the resistor CR is shown adjacent the starting resistance SR which may become very hot and the radiated-heat acts as auxiliary heating means for the resistor.

In Figs. 5 and 6 the use of switch :1: operated by the armature 6 is optional as. in the other cases.

In Figs. 1, 2, 5 and 6 the energization of the coil C and the functioning of the control resistor CR depends upon the counter E. M. F. of the armature which under normal startin 'conditions will gradually increase until the current flowing through coil C energizes the coil sufiiciently to close the switch SS. I

If, however, there is a prolonged period of starting as in the case of an overload or low line voltage, then the counter E. M. F. of the armature may remain constant or increase very slowly, resulting in the heating, assisted by the auxiliary means, of the control resistor CR, the resistance of which decreases, permitting more current to flow through the coil C until its energization is sufiicient to operate the switch SS. With a given counter E. M. F. of the motor armature, the voltage drop'across the coil C will increase as the resistance of the control CR decreases.

As before stated, in the case of Figs. 3 and 4 the coil C and its control resistor CR are connected directly across the line and are therefore subject to full line voltage which is con stant and are not dependent upon the variable counter E. M. F. of the armature, therefore any variation in the energization of coil C is due to variation of the resistance of resistor CR.

. i 7 ergization of coil C will-be superior-to that of coil HC due to the armature current decreasing as the armature speed increases until the pull of coil HC is weaker than that 01' coil C, whereupon the switch SS will close.

There is an advantage in the use of my improved control element CR over that of the prior art in that it is simpler, has no movin parts,

nothing to get out of adjustment, functioning entirely different from devices of the prior art for the same purpose in motor starters, viz., to control the operation of the shunting switch for the starting resistance and last but not least the control element CR is quite uniform in operation especially when inclosed and unaffected by air currents and temperature changes from without.

There might be conditions when it would be desirable to cut-out only a part of the starting resistance, in which case the contact 8 would be connected to the starting resistance SR at some point between the terminals of the starting resistance and not connected to the conductor 1, as will be evident to those skilled in the art.

While I prefer to use a resistor CR which is self-heating, that is by the current passing therethrough and through the coil C, under some conditions the resistor CR may have such characteristics as not to be self-heating or not sumciently self-heating and in that case, heating must result indirectly from an auxiliary source a H or SR. The heater H may be applied to Fig, 3 in the same manner as in Fig. 4 in which case the heater will be shorted when resistor CR is shorted.

If desired the auxiliary heater shown in Figs.

' 1 and 2 may be connected directly across the starting resistance.

The armature H in each starter is held in its open position by gravity or a light spring when coils C and K are not energized.

The spring a in Figs. 1, 4 and 6 are of the compression type and in Figs. 2, 3 and are of the tension type and may be used to adjust the time of closing of the shunting switch SS.

Starters have been produced employing a closing coil and a lock-out coil acting upon a shorting switch with a thermal or bi-metal switch to short the lock-out coil after a lapse of time, but I employ entirely diiierent means acting upon th closing coil whereby the starter shunts out the starting resistance under normal starting of the motor and insures the shunting of the starting resistance under abnormal starting conditions.

In Figs. 1 and 5 the resistors are entirely selfheating and dependent upon a variable source of E. M. F. Therefore the time lag is not as fixed as in the case of Fig. 3 where the control circuit is connected directly to the supply lines where the E. M. F. is constant.

In Fig. 3 there is a definite maximum lapse oi time due to the line voltage and the characteristics of the coil C being constant and the resistance of the resistor CR becoming constant when the temperature of the resistor reaches a predetermined value. With proper selection and use of the resistor CR, the time lag in which the shunting switch SS may be made to close against resistance to closing may be brought into reasonably close limits desired within the said maximum definite lapse of time;

If the resistance to closing i v the coil K0, the pull of this coil will decrease as the motor speeds 8 up until its pull is just overcome by the Pull 0! the coil C and the switch 88 will close.

That which is required as new and useful and which is sought to be protected by Letters Patent 01! the United States is as follows:

1. In combination, a motor armature, a starting resistance in series with said armature, operable means for effectively eliminating said resistance from the circuit oi. said armature when the counter E. M. F. of the motor armature attains a predetermined value, and supplemental means having a time lag in its operation thus insuring the functioning of the operable means and the elimination of said resistance from said armature circuit in case said counter E. M. F. of the motor armature fails to attain said predetermined value within the time lag period 01' said sup- .plemental means, the said supplemental means comprising a resistance having a negative coeflicient of resistance in series with said operable means whereby the operable means becomes eifective at a lower counter E. M. F. of the motor than said predetermined value.

2. The combination with a motor armature of a starter resistance in series with said armature, a shunting switch for said starter resistance, a coil connected across said armature for closing said switch when the counter E. M. F. 0! the armature reaches a predetermined value. and supplemental means having a time lag in its operation thus insuring the functioning of the coil to close the switch and effectively eliminate said resistance from the said armature circuit in case the counter E. M. F. of the armature fails to attain said predetermined values within the time lag period of said supplemental means, the said supplemental means comprising a resistance having a negative coefficient of resistance in series with said coil whereby the coil becomes efiective at a lower counter E. M. F. of the armature than the said predetermined value to close the switch.

3. The combination with a motor having an armature and a field winding, of a starting resistance in series with the armature, a switch for shunting out the starting resistance, a solenoid for closing the switch, control means connected in series with the said solenoid, the said solenoid and control means connected across the armature whereby they are subject to the counter E. M. F. of the armature to close the switch when the counter E. M. F. of the armature reaches a predetermined value under either a, normal starting condition or under a lower counter E. M. I". of the armature providing the armature fails to attain said predetermined counter E. M. F. after a lapse of time in starting, the said control means comprising a resistance having a negative temperature coefficient of resistance.

4. In an automatic starter for electric motors having an armature, the combination of a starting resistance arranged to be connected in series with the armature, means for automatically shunting out the starting resistance, the said means actuated by means controlled by the counter E. M. F. of the armature to shunt the starting resistance when the counter E. M. F, of the armature increases to a predetermined value and to shunt the starting resistance after a lapse of time regardless of the failure of the counter E. M. F. of the armature to increase.

5. In an automatic starter for electric motors having an armature, the combination of a starting resistance arranged to be connected in series with the armature, means for automatically starting resistance arranged to be connected in series with the armature, a shunting out switch provided with an operating coil to close the switch after a lapse of time and the current through the operating coil has reached a value to sumciently energize the coil due to the counter E. M. F. of the armature having reached a predetermined value and control means associated with the coil and arranged to gradually increase the current through the coil when the counter E. M. F. of the armature fails to reach a predetermined value.

'7. An automatic starter for electric motors comprising a starting resistance, means including an operating coll for shunting out said resistance after a lapse of time the energization of said coil for operating the shunting out means depending upon the counter E. M. F. of the motor increasing to a predetermined value and means in series with said operating coil to gradually effect an increase of current flow through said shunting out coil to energize the coil sufficiently to operate the shunting out means when the counter E. M. F. of the motor fails to increase sufiiciently.

8. An automatic starter for an electric motor having an armature comprising, a starting resistance, switch means for shunting out the starting resistance, the said switch means including current responsive operating means, and control means associated with the operating means the control means increases due to the current, flow through the coil and control means, the said coil operating to actuate the shunting switch if and when the speed of the motor reaches a predetermined value, the control means operating to effect sufficient energ zation of the coil after a lapse of time to actuate the shunting out switch regardless of the starting condition of the motor.

10. An automatic starter foran electric motor comprising, a starting resistance, means for shorting the starting resistance, the said shorting means including current responsive means and its connections to a source of power to actuate I the shorting means to its shorting position after a time lag and to maintain said position, the current responsive means including control means so constructed and so related to the current responsive means as to effect. a decrease in the resistance of the circuit of the current responsive means whereby the current through the current responsive means will gradually increase until the energization of the current responsive means is sumcient to actuate the shorting means regardless of the failure of the motor speed to increase to a predetermined value.

11. In an automatic starter for electric motors having an armature. the combination of a starting resistance arranged to be connected in series with the armature, a current controlled switch having a definite time limit delay in closing after energization of the motor for shunting out the starting resistance, the said switch including a coil for operating the switch when sufiiciently energized to overcome resistance to its closing, and current control means having a negative temperature coefilcient of resistance associated with the coil in such manner as to gradually increase the current value through the coil until it reaches a predetermined maximum value after a definite lapse of time, the current value necessary to sufficientl energize the coil to effect closing the switch being less than said maximum value.

12. The combination with a motor armature of a starter resistance in series with said armature, a shunting switch for the starter resistance,

means for closing the switch, holding means actuated by the starting current resisting the closing of the switch when the current is above a predetermined value and a time lag means associated with the closing means to control operation of the closing means when the starting current is sustained above said predetermined value for a lapse of time, the said time lag means operat ng to reduce the resistance of the closin means circuit until the current value therethrough is sufficient to energize the closing means to a point where it is superior to the holding me ns and .thereupon close the shunting switch and shunt means being so construct d and so operatin that the current value through the first solenoid will increase gradually as the armature current pers sts for a time above sa d predetermined value until the energization of the first solenoid is sufficient to overcome the energization of the second solenoid and operate the shunting means to shunt out the starting resistance.

14. An automatic starter for an electric motor comprising a starting resistance, a normally open shunt ng switch arran ed to shunt out the starting resistance, a holding coil connected in series with the starting resistance and with the armature circuit to restrain the closing of said switch, a solenoid for closing sa d shunting switch when said solenoid is sufficientlv energized and means in series with said solenoid to control the current flow through the solenoid whereby the current value will gradually increase until the pull of said solenoid is superior to that of the holding coil.

15. An automatic starter for electric motors comprising, a starting resistance, a switch for shunting out the starting resistance, a series concomprising, a starting resistance, a normally open shunting out switch arranged to short-circuit at least a part of said starting resistance, a holding coil responsive to the starting current for resisting the closing 01' said switch, a closing coil connectable in shunt with the motor armature for actuating the switch to its closed position, and

a time lag means associated with the closing coil and its shunt connection whereby the energizing current through said closing coil will in--' crease sufliciently to eiiect the closing of the switch ii. it does not close within a time interval after starting the motor.

17. An automatic starter for electric motors comprising, a starting resistance connectable in series with the armature of said motors, switching means for shunting said starting resistance, a control coil responsive to the starting current of the armature of the connected motor to res st the operation of the switching means, a second control coil responsive to the counter E. M. F. of the connected motor to operate the switching means to shunt the starting resistance, and means associated with the second control coil to assure sufiicient current through said second coil to eiIect operation of the shunting switch after a lapse of t me after starting the motor.

18. In an automatic starter for electric motors compr sing, a starting resistance and a switch adapted to shunt out at least a part of the st rting resistance, and a current responsive coil with an energizing circuit to operate the switch to its shunt ng position, the combination with the said coil 01' means to effect a current increase throu h said coil whereby the current will be made sufficient for the coil to overcome resistance to the operation of the switch.

19. In an automatic starter for electric motors compris ng a starting resistance and a shunting switch to shunt out at least a art of the resistance and a holdout coil res onsive to the start ng current throu h the starting res stance ur ing said switch to maintain its non-shunting position and a second coil connected in shunt to a source of current o os n the holdout c l and operating the swi ch to its shunting position, the combination with the said second coil of control means whereby the eneraization. oi the second coil is automatic lly and gradually increased until the pull of the second coil is superior to that of the holdout coil.

20. In an automatic starter for electric motors comprising a starting resistance and a shunting switch to shunt out at least a part of the resistance and a holdout coil responsive to the starting current through the starting resistance urging said switch to maintain its non-shunting position and a second coil connected in shunt to a source of current opposing the holdout coil and operating the switch to its shunting position, the combination with the said second coil of control means whereby the energization of the second coil is automatically and gradually increased until the pull of the second coil is superior to that of the holdout coil, and means associated with the shunting switch and operated thereby 12 to shunt the control means when the starting resistance has been shunted.

21. In an automatic starter for electric motors comprising, a starting resistance and a switch adapted to shunt out at least a part of the starting resistance, and a current responsive coil with an energizing circuit to operate the switch to its shunting position, the combination with the said coil oi means to eiiect a current increase through said coil whereby the current will be made suflicient for the coil to overcome resistance to the operation of the switch, and means associated with the said switch and operated thereby to shuntout the said means upon the closing of said switch.

22. An automatic starter for electric motors comprising, a starting resistance, a switch ior shunting out the starting resistance, a series connected holding coil resisting the closing of the switch, a shunt connected coil for closing the switch and a current responsive resistance element connected in series with said shunt coil whereby the current through the shunt coil will automatically increase when the shunt coil fails to close the switch after a lapse of time, and means to shunt the current responsive resistance element after the switch has shunted the starting resistance.

23. In an automatic starter for electric motors comprising, a starting resistance and a switch adapted to shunt out at least a part of the starting resistance, and a current responsive shunt connected coil to actuate the switch to its shunting position and connected to an energizing circuit, the combination with the said coil of means having a negative temperature coefficient of resistance to eflect an increase of current through said coil whereby the current will be made suiiicient for the coil to overcome resistance to the operation of the switch and means to shunt the said means after it has effected said current increase.

24. In an automatic starter for electric motors comprising, a starting resistance and a switch adapted to shunt out at least a part of the starting resistance, and a current responsive coil to actuate the switch to its shunting position and connected to an energizing circuit, the combination with the said coil of self-heating means having a negative coefllcient of resistance to eiIect an increase of current flow through the coil alter a time lag whereby the current will be made suiflcient for the coil to overcome resistance to the operation of the switch, and means to shunt the said means after it has ellected said current increase.

25. An automatic starter for an electric motor having an armature, comprising, a starting resistance, switch means for shunting the starting resistance, the said switch means including current responsive means to operate the switch means, control means associated with the current responsive means and having a negative coethcient of resistance whereby the current through the current responsive means will gradually increase as the temperature of the control means increases until the current is sufiicient to energize the current responsive means suilicient to operate the shunting means and other current responsive means positioned adjacent the control means to gradually heat the control means to gradually reduce its resistance.

26. An automatic starter for an electric motor having an armature, comprisin a starting resistance, arranged to radiate heat and prevent a rush of starting current through the motor armature, a switch for shunting the starting resistance, the said switch including current responsive means to operate the switch to its closed position, control means having a negative coefficient of resistance associated with the current responsive means to efifect a gradual increase of current through said current responsive means until sufficient to efiect operation of the switch to shunt out at least a portion of the resistance, the control means positioned adjacent the starting resistance to be affected by the heat radiated therefrom and thereby effect a, decrease in its resistance and a corresponding increase in current flow through the current responsive means.

27. In an automatic starter for electric motors comprising a starting resistance and a switch adapted to shunt out at least a part of the starting resistance and a current responsive coil to 29 actuate the switch to its shunting position and connected to an energizing circuit, the combination with the said coil of control means having a negative coefficient of resistance, current responsive means to gradually raise the temperature of .the control means whereby the resistance i4 of the control means will gradually decrease and the flow of current through the control means increase to effect operation of the switch and means to shunt the control means upon operation of the switch to its shunting position.

EDWARD P. SMITH.

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

UNITED STATES PATENTS Number Name Date 1,149,150 Steen Aug, 3, 1915 1,325,048 Simon Dec. 16, 1919 1,728,551, Jennings Sept, 17, 1929 1,516,646 Roseby Nov. 25, 1924 2,123,063 Peters July 5 1938 2,185,130 Morrill et a1. Dec. 2c, 1939 1,136,312 Cartwright Apr. 20, 1915 1,691,735 Oberschmidt Nov. 13, 1928 FOREIGN PATENTS Number Gountry Date 1 440,392 German Feb. '7, 1927

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