US2636153A - Starting relay circuit for single phase motors - Google Patents

Description

Aprll 21, 1953 R RECH 2,636,153

STARTING RELAY CIRCUIT FOR SINGLE PHASE MOTORS Filed April 5, 1950 INVENTOR. Robert H.Rech

HMQ M ATTORNEYS.

Patented Apr. 21, 1953 STARTING RELAY CIRCUIT FOR SINGLE PHASE MOTORS Robert H. Koch, Milwaukee, Wis., assignor to A. 0. Smith Corporation, Milwaukee, Wis., a

corporation of New York Application April 5, 1950, Serial No. 154,149

7 Claims. 1

This invention relates to relay circuits for controlling the energization of the starting windings of single phase motors.

In single phase motors of the type wherein a relay is used to open the starting circuit of the motor as running speed is approached, it is customary to connect the relay coil either across the series combination of the starting winding and the relay contacts or across the starting winding alone. The former of these alternatives is undesirable in many applications because a line derived voltage is impressed across the relay coil during the entire period of motor operation and the relay contacts, once open, will not close again and re-energize the starting circuit even though the load is sufficiently great to slow the motor to a very low rate of speed.

The latter alternative, that of connecting the relay coil across the starting winding alone, has also been found to be disadvantageous in that the relay contacts tend to rapidly open and close before they finally remain open as a result of the increasing motor speed. This flutterin or chattering of the contacts causes them to heat excessively and possibly to weld together and maintain the starting winding in the line circuit while the motor is operating at running speed. Since a starting winding is normally not rated for use over a long period of time, the retention of the starting winding in the line circuit may result in serious overheating.

The principal object of the invention is to provide a simple and inexpensive means for preventing the above described relay contact fluttering, with consequent elimination of the danger of overheating the starting winding.

Another object of the invention is to promote positive action of the relay contacts and thus to reduce the arcin therebetween and lengthen the life thereof.

Another object of the invention is to increase the differential between the motor speed at which the relay contacts open and the speed at which they close, so that the contacts will have the benefit of a relatively long cooling interval in the event of cylic operation of the motor.

The circuit, in general, comprises a relay coil connected across the starting winding of a single phase motor and having contacts arranged to break the starting circuit of the motor as run ning speed is approached, a half wave rectifier connected in series with the relay coil to rectify the current flowing thereto, and a filter capacitor shunted across the coil to maintain the energization thereof during the interval when current flow is blocked by the rectifier. In a separate embodiment of the invention, wherein the filter capacitor is eliminated, one side of the relay coil is tapped to the center of the starting winding and the other side thereof is connected through separate leads and rectifiers to opposite ends of said winding, so that the energization of the coil is maintained in one direction by half wave current pulses from the respective recti fiers.

Other objects and advantages of the invention will be set forth more fully in the followin description of embodiments of the invention illustrated in the accompanying drawings.

In the drawings:

Figure 1 is a schematic wiring diagram of an embodiment wherein a rectifier and filter capacitor are used in combination with a capacitor start single phase induction motor; and

Fig. 2 is a diagram of another embodiment of the invention, illustrating the use of two rectifiers instead of a rectifier and a filter capacitor.

The invention is illustrated in the drawings as applied to a capacitor start single phase induction motor adapted to be energized through lines I and 2 and a line switch 3 by a suitable source of volt single phase alternating current supply. The motor comprises, in addition to a squirrel cage rotor, not shown, a running circuit which is operative during the entire period when the switch 3 is closed and a starting circuit which is only operative to create starting and low speed torque and is consequently disconnected as running speed is approached.

The running circuit of th motor includes a running winding 4 which is connected across lines I and 2 and is shown for purposes or" illustration as split into a pair of series arranged segments 5 and 6.

The starting circuit comprises a starting winding 1 connected in series combination with a phase shifting starting capacitor 8 across one of the segments 5 of the running winding 6. The winding 1 and capacitor 8 could also be connected, without changing the invention, across the entire run winding 4, in which case the splitting of the winding 4 into the segments 5 and 6 would be unnecessary.

Referring to Fig. 1, the starting circuit of the motor is broken, as runing speed is approached, by means of a relay 9 having a coil It and a pair of normally closed contacts H. The coil I0 is connected in parallel with the starting winding 1 and is consequently subject to substantially the same voltages. The contacts II, which may be arranged at any point in the starting circuit, are shown as located between the starting winding '5 and the capacitor 8.

When the relay contacts H are closed, the voltage across the coil H1 is the resultant of the voltage derived from the lines I and 2 through the starting capacitor 8 and the voltage induced in the starting "winding 7 irorn'the rotor. The opening of the contacts l reduces the voltage in the coil Iii to the amount of the induced voltage alone and may operate, because of the inherent relay characteristics next to be" described, to cause the contacts Ii to reclose and initiate a fluttering action.

As the result of variations in the length or the air gap between the armature and core ofthe relay 9, the coil voltage necessary to actu ate the contacts H varies according to whether the contacts are initially in a closed, open, or intermediate position. For example, the design or the relay maybe such that a 159 volt. pickup. or contact opening voltage is'required to create sufiicient magnetism to overcome the large initial airv gap, whereas only 45 volts is necessary,'once' the armature is sealedagainst the coil Ill; toprevent, the relay from dropping out. and allowing the contacts H to close. 'When theair gap'has'fbc en onlypartiallyclosed, as is the case immediately. after. the relay. picks up and the contacts 1 i open a slight distance, approximatee 1y 70 volt s.wil1 be required to. seal in the armaturefan'dopen the .contactsthe remainder of the Wa Under the aboyeillustration, wherein the relay 9 is assumed to have a 150"vol t.pick' -up voltage :l,

and a 'ZOvolt seal in voltage'ihef contacts II will, not beheld 'openaiter pick up unless the inducedfvoltage. startingwinding is at least 'ifivolts'when' theiresultant, of the induced voltage, and the .line derived voltageis 150 volts. rents; bec aus'ethi" voltage across the. windin l al'n'dthe relay coil it is instantly reduce upon the opening or the contacts li',from.1'5il'volts to the value of. the induced voltage alone. Assuming thatfthe induced voltagefis less than 70 volts at the instantwh'en the resultant or the induced andlline. derived. voltag'esis the. 15,0 -.volt pitch-up value, the relay '9 will'notsealin afterfthe contacts. ll have opened. butinstea d will allow the contacts. to close'and cause, theyoltage across th'relay coil to. be increased to 159 volts again. The contacts M will then reopen andthe cycle willberepe'ated, in a. rapid fiuttering or chat:- tering action, until the motor speed has increased sufiiciently to create an induced voltagein excess of 70 volts.

The, describedfluttering, whichoccurs in the majority of relays used with single phase motors as herein described, has been a causeiof excessive heating of the. contacts I! an d exces sive wear thereon. In certain cases, the heating .may be suificient to weld the contacts H ,together maintain the fio w or line current through the starting winding while the rnotor is operatingat runningspeed, which in turn may causethewinding 1 to overheat and burn out According to the invention, a halfwave rectifier I2 is connected in series with tlre relay coil it, so that the currentflowingto the coil 10 from the winding .l or the lines I and Zialres the form of a unidirectional pulse which flowscluring half of each wave cycle. The energization of the coil i0 is maintained during the other half of each' wave cycle'by'a filter capacitor l3 which .is ary ranged in parallel with. the coil a} I ll -to .dis charge greatly reduced.

current therethrough in the same direction as the current flow from the rectifier [2.

The rectifier I2 is preferably of the barrier type, comprising discs coated with selenium, copper oxide, or other material having the proper one-way conductance characteristic.

Because ofthe describedcontinuous.direct current energization of the. coil. Hi, the voltage values necessary to actuate the relay contacts II are It is therefore necessary to adjust the relay t or to add resistance in series withthe relay coil ill in order to cause the startingcircuit of the motor to be opened at the same motor speed as in cases where the invention is not used. The resistance may be incorporated in the. coil itself, as by increasing the number of turns, or added in the form of a fixed or variable resistor.

Assuming, for the particular filter capacitor used, that sufiicient resistance has been added to increasethe pick-up voltage to volts as; in theformer illustration wherein the inventionwas not employed, the seal-in and drop-out voltages will not be thesame as their former values of wand 45 volts, respectively. Instead, theseal in voltage will be decreased toapproximately-5O volts and the drop-out-valueas. low as. lllvolts. i e. re w th respett e he Pick-1 p value, is a result of. the disproportionately. in, creased effectiveness of r tifica n at lower p s s. s i shee mrgtlzehieher olta e values.

For the great majorityorirnotors thesubstan tial decrease in s eal in voltageunder theinveniionis ilfiici et event. l ps bility Qfrel wni st fiuii n hus, n, l1ee a rle i h voltage induced in the startingwinding, 111$- 65 volts. win- 3n the resultant. oij 1 the induced anfiiline derived voltage is the, 150 volt. piclr-upwalue, there will be an e cess. of.-15 volts. over the. scale in. voltage necessary to preyentthe, contacts, H from rec losing. once, they. have-.opened initiallyl In at d i p o. heir. unct on. of. lim natin relay contact fluttering, the-rectifier andfilter are r??? re. enefi a n. he r ven on ionrst lle lmgarhen; the mo r sope ting. yclical- 1y.' cyclic operation;occurs when the load is suit; fici ntly great tostop the motor whenaonly; the running Wil'ldillg is in use. but. insufiicient to prevent substantially full speed-operation when both the running andstartin windingszarein the linecircuit and producingtorque. The motor thus increases in speed until; the @relay,9.-.picks.up and opens thestarting, circuit, aiitenwhichthe speedis reduced until the inducedwoltage.reaches. the drop-out vaiuelancl the. contacts-ll .areaal-e w st? lo e bsent in th t rtin windineror 'e i ie r. de r e,

When the drop-out value, is relatively, high; the time interval between pick-unand; droprout il i, zfihq t??? the C nta t wi l onenand close frequentlyand; tend to. overheat; The sub, stantial change in; the drop out voltageunder the present invention, illustrated in. theexample .-.as a; 3 5. volt decrease, greatly. incleases the time interval between picks-up and -droprout. and: allows h OIUIZ P u Q. have; a... relatively long. cooling period. Thewdegreeofl contact heating is therefore substantially.diminished. and. the possibility ftXQdfiiVfifiOntactwear is decreased accord nelr- Althoueh he use of. a, singleuhaif...w l flel'. nd filt r fip itorlis probably theisime ea n astine rensiremeans otxelim-inating.

l rflo iact flut erin it. .WQuldcbe lpossiblelto achieve substantially the same results by using other rectifier circuits. A relatively expensive full wave bridge rectifier, for example, could be used in place of the half wave rectifier l2, in which case the condenser 13 would be unnecessary.

The embodiment of the invention diagrammed in Fig. 2 illustrates a method of achieving full wave rectification without the necessity of using the usual bridge combination of four half wave rectifiers. In this embodiment, one side of the relay coil II! is connected to the center of the starting winding 1 and the other side of coil I is connected to a lead M which extends between a pair of opposed half wave rectifiers l and H5. The rectifiers l5 and iii are then connected, respectively, to opposite ends of the starting winding 1, so that the coil III is arranged across a bridge circuit comprising the segments of the winding 1 and the rectifiers l5 and 16.

The effect of this arrangement is to cause a direct current to fiow through the coil from one or the other of the segments of winding 7 and the associated rectifier or [6. During the first half cycle of the voltage wave in the winding 1, the rectifier IE will be operative to block the fiow of current but the rectifier l5 will permit current to flow through the relay coil in a certain direction. During the second half cycle the action is reversed, with the rectifier l6 permitting current to fiow through coil ID in the same direction as during the first half cycle. The resulting continuous fiow of direct current in the coil reduces the voltage values necessary to actuate the contacts II and operates to eliminate fluttering in the same manner as was described in connection with the first embodiment of the invention.

Due to the fact that the coil I0 is connected to the center of the starting winding I, it will be subject to only half of the voltage impressed upon or induced in said winding. It may therefore be unnecessary to adjust the coil [0 or the resistance of the coil circuit in order to cause the relay to pick up and open contacts II at substantially the same voltage and motor speed as in motor circuits not employing the invention.

It maybe desirable, in certain cases, to connect a capacitor across the relay coil ID. The capacitor, although not necessary to th invention, has the effect of eliminating the possibility of hum in the relay 9.

The circuits described in connection with the embodiments of the invention afford simple means of eliminating the relay contact fluttering which frequently occurs when the relay coil is connected across the starting winding of the motor instead of across the series combination of the starting winding and the relay contacts. The invention involves the use of standard circuit elements which may be wired into a single phase motor circuit with a minimum of time and effort. The cost of applying the invention to a motor is therefore very small, especially in view of the decreased relay contact wear and the improved performance of the motor.

Various embodiments of the invention may be employed within the scope of the accompanying claims.

Iclaim:

1. In a rectifier system for use with a single phase motor having a running circuit and a parallel connected starting circuit, a relay connected for energization by the starting winding of said starting circuit and operable to open said starting circuit as running speed is approached, and

a rectifier connected in the circuit of said relay to rectify the current flowing therethrough and increase the differential between the pick-up and seal-in voltages thereof.

2. In a rectifier system for use with a single phase motor having a running circuit, a parallel connected starting circuit, and a relay connected across the starting winding of said starting circuit to open and close said starting circuit as the motor accelerates to and decelerates from running speed, respectively, a rectifier arranged in series with said relay across said starting winding to rectify thecurrent flowing through said relay and increase the differential between the pick-up and seal-in voltages thereof.

3. In a rectifier system for use with a single phase motor having a running circuit, a parallel connected starting circuit, and a relay connected across the starting winding of said starting circuit to open and close said starting circuit as the motor accelerates to and decelerates from running speed, respectively, a rectifier arranged in series with said relay to rectify the current flowing therethrough, and a capacitor shunted across said relay to maintain the energization thereof during intervals of diminished current fiow from said rectifier, said rectifier and said capacitor cooperating to cause continuous direct current energization of said relay and thus to increase the differential between the pick-up and seal-in voltages thereof.

4. In a circuit for use with a single phase motor having a running winding and a parallel connected starting winding, a pair of opposed half wave rectifiers connected in series with each other across said starting winding, a relay coil connected at one end to the center of said starting winding and at the other end to the electrical connection between said rectifiers, and a pair of normally closed relay contacts arranged in series with said starting winding and operably associated with said relay coil to open the starting circuit of said motor as running speed is approached, said rectifiers cooperating to effect continuous direct current energization of said relay coil and increase the differential between the pick-up and seal-in voltages thereof to prevent fiuttering of said relay contacts.

5. A single phase motor adapted to be connected to a source of alternating current and comprising a rotor, a running winding, a starting winding connected in parallel with said running winding, a reactive element connected to shift the phase of the current in said starting winding with respect to the current in said running winding, a relay coil connected across said starting winding and energized by voltage induced in said starting winding from said rotor, a relay contact arranged in the circuit of said starting winding and operable when actuated by said relay coil as the speed of said rotor increases to disconnect said starting winding and said relay coil from said source of current of said motor, and a rectifier connected in the circuit of said relay coil to rectify the current flowing therethrough and increase the diiferential between the pick-up and seal-in voltages thereof.

6. A single phase motor adapted to be connected to a. source of alternating current and comprising a rotor, a running winding, a starting winding connected in parallel with said running winding, a reactive element connected to shift the phase of the current in said starting winding with respect to the current in said run- "Lia-88351153 "nine Minding, a irelay 'noiliconnected across said "starting winding and energized :by voltage induced saidstarting winding-from said motor, -:a

relay-contact arranged in series with said starting winding and operable-:when aehuated by said :rela-y eoil as the speed-nfv-said :rotor increases to disconnect said starting-winding and said relay coil :from saidsource o'f curren't ofsaid-motor, a rectifier connected in "the circuit of :said relay coilto-meotify the current flowing therethrough, and al-vcapaoitor shunted across said relay coil to xmaintam the energization thereof .during in :fiervals ofdimi-nished enrrent fiow from said reotifier, :sa'idrectifienand :saideapacitor cooperating to oauseicontimious direct currentenergization of said relay coil-and increase-the difie'rential Haetween the*pi'ckup andsealein voltages thereof.

7. A single phase motor adaptedto be conneo'ted to .a source aof alternating current and comprising arotor, a running winding; a starting winding connected in parallel relation-with said running winding, a reactive element connected to shift the phase of the current in said starting windingwith respectto the current in saidrunsling winding, 'a male of opposed half'Wave recti- :fierseonnected series *with each other and in 8 parallel with said starting winding, :a :relay coil connected at one end to the center of saidstartingWinding--and:at the other end to the electrical -connec'tion between said rectifiers, and a pairof ROBERT H. RECH.

,References Cited the .file of this patent UNITED 'STATES PATENTS Number Name .Date

995,646 Willis June 20, 1911 19433524 Godsey Jan. 16, 1934 1 97-4989 I-Iamm Sept. 25, 1934 2,296,123 Stimson Sept. ,15, 1942 2,455,690 Matthias Dec. 7, 1M8

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