US3541364A

US3541364A - Electric motor starting circuit

Info

Publication number: US3541364A
Application number: US673560A
Authority: US
Inventors: Ernst Bosch
Original assignee: Metabowerke GmbH and Co
Current assignee: Metabowerke GmbH and Co
Priority date: 1966-10-15
Filing date: 1967-10-04
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17
Also published as: GB1151397A; CH455916A

Description

Nov. 17, 1970 E. BOSCH 3,541,364

ELECTRIC MOTOR STARTING CIRCUIT Filed Oct. 4, 1967 ATTORNEY United States Patent O US. Cl. 31068 7 Claims ABSTRACT OF THE DISCLOSURE A switching arrangement for limiting the starting current when operating portable power tools. A manually actuated switching arrangement is provided within the portable tool so as to connect a resistor into the circuit and in series with the power tool, during the transient period when the tool is being started. The switching arrangement is actuated through the manual movement of a lever between two end positions. This lever located within the gripping area of the handle associated with the portable tool, has also an intermediate position between the two end positions. When moving the lever to start the power tool, this lever is first moved to the intermediate position at which the current limiting resistor is connected into the circuit. This resistor then serves to attenuate the starting peak current. When the lever is moved further towards the opposite end position or on position, the resistor is short-circuited and is effectively out of the power circuit. The resistor protects the power line from large peak currents which may result in the burnout of fuses or circuit breaker drop-out.

BACKGROUND OF THE INVENTION It is well known in the art, that the electrical machine tools with series wound motors give rise to large starting currents, when they are designed for higher power requirements. This starting current can, at the instant that the machine tool is switched on, cause the fuse of the power feed line to burn out. This situation may prevail even though the power line and associated fuse may be more than adequate for normal operation of the electrical machine tool or power tool.

Accordingly, it is an object of the present invention to provide an electrical power tool, in which the undesired effects arising from the burn-out of fuses when starting the power tool, is eliminated.

To achieve the objects of the present invention, a switching arrangement is provided in which a contact closes a circuit when a switching lever is midway between two end positions. In this midway position of the switching lever, a resistor is connected between the motor and the power line feeding the motor. The significance of this arrangement resides in the condition that the resistor which is thus connected into the circuit, is designed to adequately attenuate the peak of the current during the starting or transient phase. It is sufiicient that this intermediate swiching position in which the resistor is connected into the circuit, takes place for only a few hundredths of a second. This time interval is readily achieved when moving the switch lever from the off position to the on position. At the same time, the movement of the switch lever may be designed to have a delay in it so that a predetermined amount of delay is encountered at the intermediate or 3,541,364 Patented Nov. 17, 1970 midway switch position. This delay may be designed in the form of a mechanical resistance constructed as a detent which must be passed by the switch lever in its motion from the on position to the off position.

The switch arrangement, in accordance with the present invention, is designed in a particularly advantageous manner, whereby two further switching contacts are located adjacent to the switching contact which connects the resistor into the circuit. The separations between the contacts which are movable and their corresponding contact position, are arranged so that when the switch lever is actuated or moved, the contact connected to the resistor and one of these further contacts first arrive at their contact resting position. The second further contact, on the other hand, arrives at its resting position, only when the end position of the switch lever is attained, corresponding to the on position of the power tool. At this point, in the switching arrangement, the current limiting resistor is short circuited. The switching arrangement is of particularly simple construction when the movable contacts are secured to a switch rod which is, in turn, movable or operable against the action of a spring.

SUMMARY OF THE INVENTION A switching arrangement for limiting the starting current of electric motors. The normal electric circuit path between the power line feeding the motor and the electromagnetic fields of the motor, is subjected to a switching phase in which a resistor is connected between the motor and the power source. The switching arrangement includes a switch lever having two end positions for turning the motor on and off. When the switch lever is brought to an intermediate position from the on position, and on its way to the off position, the current limiting resistor is connected into the circuit for limiting the current flow to the motor during its start-up phase. As the switch lever is moved further to the on" position, the current limiting resistor is short circuited and full current flow is permitted to the motor for normal operating conditions. The current limiting resistor is connected in the circuit for only a short interval of time to limit the current during the transient period in which the motor experiences a current peak. The arrangement is particularly adaptable to a portable power tool in which the switch lever is connected or located in the handle of the tool. The switching arrangement is thus a safety device for assuring that fuses will not burn out when the power tool is connected to the line and started.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and'its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of a portable power tool shown in the form of an angular grinder;

FIG. 2 is an electrical schematic diagram of the switching arrangement, in accordance with the present invention, in which the switching contacts within the circuit are in their off position;

FIG. 3 is an electrical schematic diagram of the switching arrangement of FIG. 2, and shows the position of the switching arrangement at an intermediate stage in which 3 the current to the motor is limited during a start-up phase;

FIG. 4 is an electrical schematic diagram of the switching arrangement of FIG. 2, in which the latter is in the on position and wherein full power is applied to the motor controlled by the switching arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A grinding disc 1 of an angular grinder 2 is driven by an electrical motor 3, through means of mechanical linkage (not shown). The driving motor 3 is, in particular, a series wound motor. The handle 4 of the grinding machine includes a switch 5 for the purpose of turning the machine on and off. The switch 5 is located in the gripping area and is readily accessible. The switch 5 is actuated by means of a switching lever 6 which is movable between one end position 7 and a second end position 8. The end position 7 corresponds to the ofi position on the switch when referring to the circuit arrangement of FIG. 2. As shown therein, a switch rod 9 has a first contact 10, an auxiliary contact 11 and a second contact 12, all movable against their

corresponding contact positions

13, 14 and 15, respectively. A resistor 16 is connected between the motor 3 and the contact position 14.

When the switch lever 6 is moved from the end position 7 to the end position 8, the switch rod moves in a downward position, in FIG. 2, so as to close the circuits. As a result of this motion of the switch rod 9,

contacts

10, 11 and 12 are brought into contact with the

positions

13, 14 and 15. The

contacts

10, 11 and 12 are moved in this position against the action of the

springs

17, 18 and 19. When the switch lever 6 arrives at an intermediate position midway between the end positions 7 and 8, while being manually moved, the contacts and 11 assume the position as shown in FIG. 3, wherein they rest against the

contact positions

13 and 14. In this particular position the contact 12 still remains separated from its contact position 15.

As a result of this preceding switching arrangement, the connection to the network 20 is established. However, since the resistor 16 is connected into the circuit, any transient peak currents, during the starting phase, are attenuated. The network 20 is a power source from which the motor 3 draws its power and energy for performing its intended purpose, as, for example, a grinding operation.

At this midway position a further detent (not shown) can be provided so as to present a definite indexed or detented position for the movement of the switch rod 9. The arrangement may be such that this detent gives rise to a certain amount of mechanical resistance which must be overcome before moving the switch rod to its other end position 8.

As soon as the switch lever 6 is located at the end position 8, the circuit is also closed through the movable contact 12. Thus, in this position the movable contact 12 rests against the switching contact 15, as shown in FIG. 4. At this point the current no longer flows to the motor via the resistor 16. This is due to the fact that when the switch lever 6 is in the end position 8 and the movable contact 12 rests against the contact position 15, the resistor 16 is short circuited through the path 21. Thus, in this switching configuration, the power flow takes place directly from the power source 20 to the motor 3, by way of the movable contact 12. In this particular circuit position, the resistor 16 is superfluous, since the current flow under normal operating conditions will not cause the circuit breaker in the power feed line 20 to drop out.

The advantages achieved by the present invention resides particularly in the condition that operating current when starting the electrical machine tool, do not result in the burn-out of fuses or the dropping-out of circuit breakers. Furthermore, the present invention achieves this advantageous condition with the use of only a single switch in a simple circuit arrangement. The manual operator or user of the portable machine tool cannot do anything wrong when applying the structural arrangement of the present invention. The handle or grip of the machine tool is, furthermore, substantially equivalent to the handle associated in other conventional apparatus, and is particularly comfortable. The switching arrangement, in accordance with the present invention, is especially useful and of advantage when large starting currents may arise as, for example, in portable electrical saws.

It will be understood that each of the elements de scribed above, or two or more together, may also find a useful application in other types of starting circuits in electrical machine tools differing from the types described above.

While the invention has been illustrated and described as embodied in a starting circuit in electrical machine tools, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A switching arrangement for controlling the starting current of an electric motor, comprising, in combination, a switching member movable between an initial, an intermediate, and a final position, said switching member being constructed so as to stay in said initial and said final position, respectively, when moved into one of said positions; first and second movable contact means arranged at a predetermined distance on said switching member in direction of the movement thereof; first and second stationary switching means arranged along the path of movement of said switching member, the distance between said stationary first and second contact means being greater than the predetermined distance between said first and second movable contact means so that upon movement of said switching member from said initial into said final position thereof said first movable contact means will contact said first stationary switching means before said second movable switching means contacts said second stationary switching means; and resistor means connected in circuit with said first stationary contact means so that the motor connected in circuit with said stationary contact means will, during movement of said switching means from said initial to said final position thereof first be energized through said resistor means while said second contact means are spaced from each other and then energized without interposition of said resistor means when said second contact means are in contact with each other.

2. The switching arrangement for controlling the starting current of an electric motor as defined in claim 1 including mechanical resistance means associated with said switching member for increasing the duration of motion of said switching member.

3. The switching arrangement for controlling the starting current of an electric motor as defined in claim 2 wherein said mechanical resistance means is a detent means.

4. The switching arrangement for controlling the starting current of an electric motor as defined in claim 1 including switch lever means linked to said switching mem ber for manually operating said switching member between said initial and final positions.

5. The switching arrangement for controlling the starting current of an electric motor as defined in claim 1 including spring means acting on said switching member for providing mechanical resistance when said switching member is moved between said initial and final positions.

6. The switching arrangement for controlling the starting current of an electric motor as defined in claim 5 wherein said switching member comprises switch rod means; and switching contact means secured to said rod means, said switch rod means being surrounded by said spring means and moving said switching contact means between said initial and final positions.

7. The switching arrangement for controlling the start- 2,554,506 5/ 1951 Schwarz. ing current of an electric motor as defined in claim 6 in- 2,373,256 4/ 1945 Miller 318-508 XR eluding lever operating means linked to said switch rod 3,260,873 7/1966 Wilson et a1. 31072 XR means for manually moving said switch rod means be- 2,348,656 5/1944 Scofield et al. 310-68 tween said initial and final positions. 5 2,103,922 12/ 1937 Van Guilden 31068 XR References Cited MILTON O. HIRSHFIELD, Primary Examiner UNITED STATES PATENTS B. A. REYNOLDS, Assistant Examiner 834,899 11/1906 Freeman 310-681 XR 1,827,372 10/1931 Riddell 310-681 XR 10 2,737,371 3/1956 Gerry. 31072;318421