US3695006A

US3695006A - Vacuum cleaner

Info

Publication number: US3695006A
Application number: US83504A
Authority: US
Inventors: Bruno M Valbona; William J Conlon; Harry M Voglesonger
Original assignee: Dynamics Corp of America
Current assignee: Dynamics Corp of America; Ryobi Motor Products Corp
Priority date: 1970-10-23
Filing date: 1970-10-23
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03

Abstract

A multi-speed electrically driven, lightweight vacuum cleaner with a switch manually held closed for momentary extra high speed and air volume surge. A warning light responsive to rising temperature of motor cooled by filtered exhaust air passing through the motor indicates when a filter bag should be emptied.

Description

United States Patent 1 3,695,006

Valbona et al. 1 51 Oct. 3, 1972 [54] VACUUM CLEANER References C t d 72 Inventors: Bruno M. Valbona, Avon; William UNITED STATES PATENTS i a" g g 2,789,660 4/1957 Brown ..15/319 x 3,232,030 2/1966 Owenmark ..55/217 [73] Assignee: Dynamics Corporation of America, 3,319,282 5/1967 MacFarland ..l5/3l9 New York, NY. Primary Examiner-Dennis E. Talbert, Jr. [22] Flled' Oct. 1970 Attorney-Harbaugh and Thomas [21] Appl. No.: 83,504

[57] ABSTRACT [52] US. Cl. ..55/274, 15/319, 15/344, A multi speed electrically driven, lightweight vacuum 15/347 15/412 55/357 55/361 55/467 cleaner with a switch manually held closed for mo- SS/DIG' 3 mentary extra high speed and air volume surge. A cL n u ni ..B0ld respo s e to temperature of [58.] new of 1 motor cooled by filtered exhaust air passing through 3 339, 412, 344:, the motor indicates when a filter bag should be emptied.

19 Claims, 12 Drawing Figures VACUUM CLEANER CROSS REFERENCE OF RELATED APPLICATION Valbona et al., filed of even date Oct. 23, 1970, Ser. No. 83,300 for Lightweight Vacuum Cleaner.

BACKGROUND OF INVENTION In the general field and use of a household vacuum cleaner for floor coverings, two speeds, HI and LO, generally provide results satisfactory to a housewife, but very often conditions arise where something is on the floor that requires extra time and attention for its removal, such as threads, tiny pieces of tape and paper, animal hair, small pebbles, etc., which heretofore require extra and burdensome manipulation of the pickup nozzle and generally along with it the manual lifting of the vacuum cleaner body itself. This is particularly the situation where the vacuum cleaner is a hand supported unit. Under these special conditions a surge of volume air flow, also increases vacuuming conditions at the nozzle and saves time and user fatigue.

Another problem that repeatedly occurs is the inattentiveness of the user regarding the emptying of the filter bag as often as it should be emptied to maintain a proper efficiency for both HI and LO speeds that is desired for optimum operating conditions.

SUMMARY OF THE INVENTION These problems are met by an extra surge of airflow and vacuum when desired and by a signal indicating when a filter bag should be emptied. It is found that both of these cooperate for the user to maintain the proper efficiency of the vacuum cleaner at all speeds.

The present invention relates to providing and maintaining the speed and efficiency of a vacuum cleaner in which a hand set switch for the motor selects an airflow and vacuum volume speed within a speed range that a vacuum cleaner will be operated during sustained use, and a normally open hand switch, momentarily held closed, is provided to override the hand set switch for momentary extra performance results at an increased airflow volume and vacuum surge speed. An automatic switch not only indicates the existence of a clogged or filled condition of the filter bag that diminishes its potential efficiency for the sustained operation, but if potential motor temperature may be different fro HI and surge speeds it also indicates if the momentary extra performance is being resorted to more often than would be expected as when the bag should be emptied more often for surge operation. Overuse of the surge speed thereby indirectly indicates when a bag should be emptied for that purpose. Therefore, by the unconscious reaction of the user resulting in an overuse of the surge button, the user appreciates the situation and restores normal efficiency of the vacuum cleaner by emptying the bag. This reduces the need for excess surge use. The end result is a better overall efficiency, less manipulation fatigue and a saving in cleaning time. This warning is provided by a simple warning light responding to the heat of the blower motor that is located where it is, preferably cooled by the exhaust filtered air going through it. The less air passing through the filter bag due to retardation, the less air there is to cool the motor. This results in a greater accumulation of heat in the motor stator, and a higher temperature 5 desired, if there is delay in emptying the bag.

The speed control in the present invention is accomplished in several ways usable with several types of motor circuits, namely, motors having a plurality of single or cowound field coils on the stator poles which are of the same or different impedances; field coils connected in series or in parallel, or permutations of both; or, the electrical power potential is changed by a rectifier or resistance.

It is preferred that all field coils share energization of the motor at all speeds and both stator poles are energized at each speed to assure temperature increase.

Another object of the invention is to utilize a separate power surge switch that has to be manually held while it is closed so that it is not inadvertently or easily left on.

A further object is to provide an airflow vacuum surge speed that is attained without changing the manually set operating speed, either HI 0r LO, at which the vacuum cleaner was or will be operating when the surge is turned on or off, respectively.

Another object of the invention is to provide a lightweight vacuum cleaner that is weight-supported essentially by its nozzle on the floor at the lower end for full weight agitation of the nap of carpeting, supported manually at the other end by the user, and is controlled to effectively reduce the need for repeated passes of the nozzle over particular areas of the carpeting whereby fatigue is reduced while improving performance and saving time in a cleaning operation.

Other and further objects and advantages will appear from the description and claims which follow.

IN THE DRAWING FIG. 1 is a side elevational, partially cut away view of a lightweight, manually supported vacuum cleaner embodying the invention;

FIG. 2 is a top plan view of a portion of the handle showing the location of the switches and indicator light;

FIG. 2A is a side elevational cut away view showing the intimate contact of a signal lightthermostat with the outer surface of the motor stator as shielded from surrounding air in the motor compartment;

FIG. 3 is a diagrammatical view of a motor control circuit as controlled by a slide switch and push button switch for a two pole universal motor stator having four distinct permutatable field coils;

FIGS. 3A and 3B along with FIG. 3 indicate the three switches positions for OFF, L0 and HI;

FIG. 4 indicates the coil permutations involved with the three position switch and the surge speed;

FIG. 5 is a diagrammatical view of a single wound coil on each stator pole utilizing a rectifier with the armature electrically connected at one side of the field;

FIG. 5A indicates the coil permutations for a surge for each HI and LO speeds;

FIGS. 6 and 6A are views similar to FIGS. 5 and 5A showing the permutations of four coils in which all connections are series connections with a diode in one permutation circuit of the field coils;

FIG. 7 shows a circuit like that shown in FIG. 6 with the diode in another permutation circuit of the field coils wherein three speed choices are available merely by changing location of one of the switch relations and diode can be used with either one of two of the speeds and the motor armature is located electrically to one side of the field coils.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 is illustrated a lightweight portable vacuum cleaner having an elongated body 10 with a carrying handle 12 on the main housing 15, a floor engaging nozzle 14 supported on a tube 16 adjustably telescoping into the body at the lower end, and a removable motor or upper housing 18 at the other end. The upper housing 18 has a working handle 20 that curves downwardly for comfortable use and is provided with a narrow neck portion 22 which can be gripped by hand as a secondary handle, which terminates in an enlarged portion 24 having a closed opening 26 therethrough to receive the fingers of a users hand to serve as a primary handle 28 by which the body and nozzle can be manually moved easily back and forth over a floor covering by the users fingers engaging either end of the handle opening 26. g

In front of the primary handle 28 and above the secondary handle 22 are the buttons of a main switch 30, a spring return push button or slide button for the surge" switch S, and the bezel of a reminder red light 32 controlled by a thermal responsive switch 34 (FIG. 2A). The main housing comprises a vacuum compartment 36 which loosely receives a filter bag 40 therein having porous stiff walls. The opening at the lower end of the filter bag closed by a plate 42 having an opening receiving the upper end 16A of the tube 16 therethrough opening into the bag. Air filtered through the wall of the filter bag moves longitudinally through the flutes defined by the ribs 38 to the open end of the compartment 36 which is closed by the removable upper housing 18 held by trunk snap fasteners 44 which provide for emptying or renewing the bag 40. The motor 46 is mounted in the upper housing and drives a blower 48 which draws air from the compartment 36, and as constrained by a baffle 38 forces the air through the open space within the motor housing and around the stator poles of the motor and ejects it through a grille 49 as indicated by the air flow arrows in FIG. 1.

The thermally responsive switch 34 is preferably a single pole signal throw switch mounted on the side of the motor, preferably in direct contact with the field core, and shielded from motor compartment air by a shield 50 so that the bimetal blade thereof is influenced almost entirely by the temperature of the motor housing 49 where it is in direct heat exchange contact with the stator core 52 of the motor.

Referring to the motor circuit diagrams of the respective FIGS., the circuits illustrated are self explanatory except possibly in FIG. 5 where the coils C and D are shown electrically connected in series at one side of the armature. They are physically on separate poles of the stator and the armature E is physically between them. Otherwise, the coils are physically associated the same as they are shown circuitwise.

The

switch numbers

1, 2, 3, 4, 5, 6 and 7 in the diagrams are disposed between the two switch contacts that are closed by the respective movable switch conductors. The field coils that are wound on the stator poles are indicated by letters A, B, C and D and are designed and of impedances which provide at least the HI and LO speeds of conventional vacuum cleaners in the several embodiments. In the chart FIGS. 3A, 5A and 7 the permutations of the coils are set forth to represent a series connection (FIG. 4) such as AB, and a parallel configuration as by A/B. The switches l, 2 and 3 that are shown closed as by 2 for series coils AB and switches 13 for A/B where the coils are connected in parallel (FIG. 4). See

switches

4, 5 and 6 similarly arranged (FIGS. 3A and 5A).

As diagrammatically shown in FIG. 3 the air volume surge switch S is a combination of a single pole single throw for switch 2 and a double pole single throw switch for switches 1 and 3 in which slidable bridging conductors 1A and 3A are in continuous contact with the respective upper terminals of switches l and 3 and when carried downwardly by the dielectric plunger S1 as manually actuated by push button S2 the terminals of switch 2 are closed. The plunger movement first moves the bridging conductor 2C away from and opens the terminals of switch 2. Then, secondly the conductors 1A and 3A bridges and close Switches 1 and 3 respectively which involve the terminals of switch 2 separately.

A spring SS urges the plunger S1 to its original position shown in FIG. 3 and the manually induced action is to move the switch manually from a position connecting coils in series AB by switch 2 where the coil impedances are additive for lesser wattage to a parallel configuration A/B by switches l and 3 where the composite coil impedance is reduced and provide greater wattage for the increased blower load that occurs at higher speeds.

The same operational effect is provided to vary the impedance and resulting wattage for coils C and D by a three position switch 30 which comprises a double pole double throw slide switch having sliding conductors 30A and 303 whose

switch contacts

4 and 6 are open when switch 5 contacts are closed to connect coils C and D in series CD (FIG. 3B). Switch 5 contacts are opened when

switches

4 and 6 contacts are closed to connect coils C and D in parallel C/D (FIG. 5A) when the slide button 30S is manually actuated to the position illustrated.

With these two switches there are four permutations with four coils as shown in FIG. 4, plus the OFF position shown in FIG. 3. The switches S and 30 can be interchangeably used and also the impedances of the coils in FIG. 3 can be different and designed to provide optimum results in the respective permutations. By way of example, with an inside opening of 1.250 inches in diameter for the conduit 16 and with a filter bag in place and the nozzle 14 wide open, the motor, blower and circuitry are designed to rate 15,000 R.P.M. at surge speed, 13,000 R.P.M. at HI and 10,000 R.P.M. at LO speeds.

In FIGS. 5 and 6 a rectifier R is employed in the motor circuits for the LO speed settings. These are connected in series with the motor and shunted by a single pole, single throw switch SR whose conductor 26C is normally held open by spring SS. Whenever the switch SR is closed the current applied to the motor is changed from half wave rectified current to full wave A.C. current to provide higher wattage and thereby higher speeds.

In FIGS. 5 and 6 there is a surge speed for both HI and LO set speed operations since the rectifier is controlled by a separate switch SR and although represented as a push-pull with a center OFF it, along with switch 30A of FIG. 6, can be a standard single pole-double throw switch. Switch 30A is urged by spring SS to a REG position while switch SR has a center OFF position.

In FIG. 6 the surge speed switch 30 is related to switching the coil permutations from ABCD (switch 1 indicated by Reg) Reg to BD (switch 2). With switch 30A in Reg position the SR switch controls HI and LO operations with coils ABCD in series.

Referring to FIG. 7, although the parallel connected switch SR and rectifier R can be connected as a unit in series with coil B in the connection Z between and the common pole CP of switch 1 and 2 would provide an embodiment in which the surge switch 30 can only be effective if the vacuum cleaner is in HI or L operation, it is preferred to connect the switch SR and switch 30 as units in series with coil A instead of coil B through the connection Y between the other pole of switch 1 and coil A as shown in FIG. 7. Then the surge switch 30 by

switches

2 and 3 and coils BC supersedes any operation of the OFF, LO, l-II switch SR. In both instances, however, of FIGS. 6 and 7 the SR switch unit serves as a speed selector and switch 30 operates as the spring return surge switch. This is a modification of the other embodiments where the speed selector switch 30 is supplied with the spring SS to serve as the surge switch which normally connects coils A and B and coils C and D in series through

switches

2 and 3, respectively. Thus, in these embodiments, a closing of the switch 2 (FIG. 6) of switch 30A or

switches

2 and 3 of switch 30 at any time energizes coils BC at surge speed including the condition when the vacuum cleaner is not otherwise energized.

Having thus described the invention and various embodiments thereof it will be appreciated how the stated objects and advantages are attained, and how various and further modifications can be made including combining both switches in housing for single installation purposes or single button control.

What is claimed is: l. A portable electrically powered vacuum cleaner for household use having a variable speed motorblower with a stator utilizing a plurality of field coils, an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment, the combination of means for conducting flow of air from the filter bag in the compartment through the motor to cool the stator of the motor,

manually settable switch means connected to said field coils for varying the speed of the motorblower to control the volume flow of air through said bag and motor stator within a range of working speeds,

manually held switch means including a switch and impedance varying motor circuit connections to increase the wattage of electrical current through said motor to surge the volume flow of air through said bag and motor stator at a speed higher than said range of speeds, and

means responsive to the temperature of said stator core for indicating critical changes of permeability of said bag to air flow therethrough for all speeds respectively.

2. The vacuum cleaner defined in claim 1 in which said settable switch means includes a half wave electrical current rectifier and said manually held switch means includes a switch shunting said rectifier.

3. The vacuum cleaner defined in claim 1 in which said manually held switch means includes permutable switches changing motor-blower field coil connections between series and parallel configurations to change their composite impedance.

4. The vacuum cleaner defined in claim 1 in which said settable switch means includes permutation switches selectively connecting in series with each other two separately operative circuits of series connected pairs of field coils to provide a high impedance field, one of the series circuits including a half wave rectifier shunted by a switch controlled by one of said switch means.

5. The vacuum cleaner defined in claim 4 in which said one of said switch means is the settable switch means.

6. The vacuum cleaner defined in claim 1 including resilient means urging said manually held switch means to a resting position in which the impedance controlled thereby is the highest.

7. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment and including 1. a variable speed motor-blower whose field core includes a plurality of stator poles and a plurality of field coils thereon and an armature driving a blower to force air through said filter bag,

2. manually operated switch means including a. an electrical connection including a first switch to connect two field coils in series electrically to provide a high impedance field on the field core for a low speed performance,

b. an electrical connection including a second switch shunting one coil and said first switch,

. an electrical connection including a third switch shunting the other coil and said first switch, said second and third switches connecting the coils in parallel to provide a low impedance field on the field core in series with the armature for high speed performance,

3. an electrical current flow reducing means in series with the armature and said two field coils,

4. manually operated switch means controlling the current flow reducing means and movable alternately to two positions in one of which a low impedance is presented to the current flow for a high speed performance and the other position a high impedance is presented to the current flow in the circuit for low speed performance, and

5. resilient means urging one of said switch means to a resting position of high impedance and manually movable in coaction with the other switch means to provide a composite low impedance speed surge flow of air through said filter bag.

8. The vacuum cleaner defined in claim 7 in which said current flow reducing means includes a half wave rectifier and said switch means controlling the current flow shunts the rectifier in its low impedance position.

9. The vacuum cleaner defined in claim 8 in which said resilient means actuates the half wave rectifier switch means.

10. The vacuum cleaner defined in claim 7 in which said resilient means urges the first mentioned switch means to its high impedance position.

11. The vacuum cleaner defined in claim 7 in which said flow reducing means is a second pair of field coils inductively coupled with said stator core and connected in series with the armature, and

said switch means controlling said flow reducing means including a fourth switch connection to connect the second pair of field coils in series, a fifth switch connection to shunt one of the second pair of coils and said fourth switch, and a sixth switch connection to shunt the other of the second pair of coils an said fourth switch, said fifth and sixth switches connecting the second pair of coils in parallel and opening said fourth switch to provide a low field impedance. 12. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment and including a variable speed motor-blower whose field core includes a plurality of stator poles and a plurality of field coils thereon connected in series circuit with an armature between two power connections to drive a blower and force air through said filter bag,

an electrical connection including a first switch to connect two of said field coils electrically in series,

an electrical connection including a second switch to connect two other field coils in series and in series with the armature,

an electrical current flow reducing means in series with one of said electrical connections,

manually operated switch means controlling the current flow reducing means and being movable alternately between two positions in one of which a low impedance is presented to the flow of current in said one of the connections and the other of which a high impedance is presented to the flow of current in the circuit,

an electrical connection including a third switch to connect one of said two field coils in series with one of the power connections, an electrical connection including a fourth switch to connect the other of said two field coils in series with the other one of said power connections,

manual means for alternately closing said first and second switches as a unit and said third and fourth switches as a unit,

means urging the last said manual means to close the switches in the connections having said flow reducing means in series therewith.

13.The vacuum cleaner defined in claim 12 in which said four switches constitute a double pole double throw type switch.

14. The vacuum cleaner defined in claim 12 in which the first mentioned switch is a double pole single throw switch with an OFF position.

15. The vacuum cleaner defined in claim 12 including a rectifier connected in series in said series circuit,

switch means connected in said series circuit in parallel with said rectifier, and

manual means for actuating the last mentioned switch means.

16. The vacuum cleaner defined in claim 15 in which the said two field coils are connected at one end to the armature, and

said two other field coils are connected in series with said rectifier.

17. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filler bag in the compartment and including a variable speed motor-blower whose field core includes a plurality of stator poles and a plurality of field coils thereon collectively connected in series circuit with an armature between two power connections to drive the motor blower and force air through said filter bag,

an electrical connection connecting two of said field coils electrically in series as a unit,

an electrical connection connecting two other field coils in series as a unit an electrical connection including first switch means to connect said units in series with each other in said series circuit,

an electrical connection including second switch means to connect one of said units to shunt the other unit in said series circuit,

manual means for alternately opening and closing said switch units oppositely and simultaneously,

means urging the last said manual means to close the first switch and open said second switch.

18. An electrically powered appliance for household use and including a variable speed motor and a blower driven thereby for cooling said motor,

said motor having an armature and a field core including a plurality of stator poles and a plurality of field coils thereon, said field coils being collectively connected in series circuit at one end with said armature,

a first connection connecting said armature to one side of a source of electrical current and said coils to drive said motor, said armature being mechanically located between said field coils and said one side of said source of electrical current,

an electrical connection connecting two of said field coils on opposite stator poles electrically in series as a unit,

another electrical connection connecting two other of said field coils on opposite stator poles electrically in series as a unit,

a third electrical connection including a first switch means to connect said coil units in series with each other in said series circuit,

a fourth electrical connection connecting the other end of said series circuit to the other side of a source of electrical current and including a second switch means to connect one of said coil units to shunt the other unit in said series circuit to said second source of electrical current,

manual means for alternately opening and closing said switch means oppositely and simultaneously,

Claims

1. A portable electrically powered vacuum cleaner for household use having a variable speed motor-blower with a stator utilizing a plurality of field coils, an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment, the combination of means for conducting flow of air from the filter bag in the compartment through the motor to cool the stator of the motor, manually settable switch means connected to said field coils for varying the speed of the motor-blower to control the volume flow of air through said bag and motor stator within a range of working speeds, manually held switch means including a switch and impedance varying motor circuit connections to increase the wattage of electrical current through said motor to surge the volume flow of air through said bag and motor stator at a speed higher than said range of speeds, and means responsive to the temperature of said stator core for indicating critical changes of permeability of said bag to air flow therethrough for all speeds respectively.

2. manually operated switch means including a. an electrical connection including a first switch to connect two field coils in series electrically to provide a high impedance field on the field core for a low speed performance, b. an electrical connection including a second switch shunting one coil and said first switch, c. an electrical connection including a third switch shunting the other coil and said first switch, said second and third switches connecting the coils in parallel to provide a low impedance field on the field core in series with the armature for high speed performance,

7. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment and including

11. The vacuum cleaner defined in claim 7 in which said flow reducing means is a second pair of field coils inductively coupled with said stator core and connected in series with the armature, and said switch means controlling said flow reducing means including a fourth switch connection to connect the second pair of field coils in series, a fifth switch connection to shunt one of the second pair of coils and said fourth switch, and a sixth switch connection to shunt the other of the second pair of coils an said fourth switch, said fifth and sixth switches connecting the second pair of coils in parallel and opening said fourth switch to provide a low field impedance.

12. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filter bag in the compartment and including a variable speed motor-blower whose field core includes a plurality of stator poles and a plurality of field coils thereon connected in series circuit with an armature between two power connections to drive a blower and force air through said filter bag, an electrical connection including a first switch to connect two of said field coils electrically in series, an electrical connection including a second switch to connect two other field coils in series and in series with the armature, an electrical current flow reducing means in series with one of said electrical connections, manually operated switch means controlling the current flow reducing means and being movable alternately between two positions in one of which a low impedance is presented to the flow of current in said one of the connections and the other of which a high impedance is presented to the flow of current in the circuit, an electrical connection including a third switch to connect one of said two field coils in series with one of the power connections, an electrical connection including a fourth switch to connect the other of said two field coils in series with the other one of said power connections, manual means for alternately closing said first and second switches as a unit and said third and fourth switches as a unit, means urging the last said manual means to close the switches in the connections having said flow reducing means in series therewith.

13. The vacuum cleaner defined in claim 12 in which said four switches constitute a double pole double throw type switch.

15. The vacuum cleaner defined in claim 12 including a rectifier connected in series in said series circuit, switch means connected in said series circuit in parallel with said rectifier, and manual means for actuating the last mentioned switch means.

16. The vacuum cleaner defined in claim 15 in which the said twO field coils are connected at one end to the armature, and said two other field coils are connected in series with said rectifier.

17. An electrically powered portable vacuum cleaner for household use having an accessible compartment for a filter bag and means for conducting the flow of air through a filler bag in the compartment and including a variable speed motor-blower whose field core includes a plurality of stator poles and a plurality of field coils thereon collectively connected in series circuit with an armature between two power connections to drive the motor blower and force air through said filter bag, an electrical connection connecting two of said field coils electrically in series as a unit, an electrical connection connecting two other field coils in series as a unit an electrical connection including first switch means to connect said units in series with each other in said series circuit, an electrical connection including second switch means to connect one of said units to shunt the other unit in said series circuit, manual means for alternately opening and closing said switch units oppositely and simultaneously, means urging the last said manual means to close the first switch and open said second switch.

18. An electrically powered appliance for household use and including a variable speed motor and a blower driven thereby for cooling said motor, said motor having an armature and a field core including a plurality of stator poles and a plurality of field coils thereon, said field coils being collectively connected in series circuit at one end with said armature, a first connection connecting said armature to one side of a source of electrical current and said coils to drive said motor, said armature being mechanically located between said field coils and said one side of said source of electrical current, an electrical connection connecting two of said field coils on opposite stator poles electrically in series as a unit, another electrical connection connecting two other of said field coils on opposite stator poles electrically in series as a unit, a third electrical connection including a first switch means to connect said coil units in series with each other in said series circuit, a fourth electrical connection connecting the other end of said series circuit to the other side of a source of electrical current and including a second switch means to connect one of said coil units to shunt the other unit in said series circuit to said second source of electrical current, manual means for alternately opening and closing said switch means oppositely and simultaneously, means urging the last said manual means to close the first switch means and open said second switch means.

19. The combination called for in claim 18 in which said third electrical connection includes an electrical current flow limiting device, and switch means for shorting out said device.