GB1599202A - Suppressor-insulator member and circuit arrangement therefor - Google Patents

Description

PATENT SPECIFICATION

( 11) 1 599 202 ( 21) Application No 21807/80 ( 22) Filed 3 Jan 1978 ( 31) Convention Application No 2700388 ( 32) Filed 7 Jan 1977 in ( 33) Fed Rep of Germany (DE) ( 44) Complete Specification published 30 Sept 1981 ( 51) INT CL 3 B 23 B 45/02 H 02 K 11/00 H 04 B 15/02 ( 52) Index at acceptance H 2 A TL WN ( 54) SUPPRESSOR-INSULATOR MEMBER AND CIRCUIT ARRANGEMENT THEREFOR ( 71) We, BLACK & DECKER I Nc, a corporation organized and existing under the laws of the State of Delaware, United States of America, whose address is Drummond Plaza Office Park, 1423 Kirkwood Highway, Newark, Delaware 19711, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following

statement:-

The invention relates to a hand-held, electric power tool including a suppressorinsulator member for conducting higherfrequency noise from a voltage-carrying portion of the tool to an exposed-metal part of the tool accessible to the operator The invention is especially applicable to tools which are double-insulated and have housings which are made in part of metal.

Double insulation is a term used to characterize electric motor driven devices that have been equipped with supplementary insulation as well as functional insulation.

Functional insulation denotes the insulation necessary for the proper functioning of the tool and for basic protection against electric shock as for example the conventional winding insulation of the pole pieces; whereas, supplementary insulation denotes an independent insulation provided in addition to the functional insulation, in order to ensure protection against electric shock in the event of failure of the functional insulation Thus, in a double-insulated tool, if the functional insulation breaks down or is by-passed for some reason, a protective barrier remains to prevent injury to the user For example, an electric tool can have an electric motor whose armature shaft is normally connected directly to an output element by means of which the tool produces useful work In addition to the insulation on the wires which are on the armature, the double insulated tool would have a second barrier of insulation between the armature lamination stack and the tool element of the device The second barrier of insulation provides the supplemental protection against possible injury to the user of the tool and can for example be provided by means of a sleeve positioned on the armature shaft In other cases, the insulated gears can be provided by building insulating members into the 55 armature shaft There are of course still other arrangements which can be utilized to provide double insulation.

Further information relating to the insulation terms described herein may be had by 60 making reference to the handbook entitled:

"Specification for Hand-Held Motor-Operated Tools" published for the International Commission on Rules for the Approval of Electrical Equipment by Nederlands Comite 65 voor de CEE, Utrechtseweg 310, Arnhem, Netherlands.

Double insulated tools like other tools generate high-frequency interference which is sometimes referred to as elec 70 tromagnetic or radio frequency interference and is hereafter referred to as highfrequency noise This high frequency noise generated in a power tool has generally two ( 2) components, namely: a symmetrical 75 component generated internally such as from the brush action; and, a asymmetrical component also generated within the tool by brush action and becomes a problem as a consequence of the impedance to ground 80 of a person holding the tool.

Electric apparatus such as electric power tools driven by electric motors such as drills, circular saws, and the like must be provided with noise suppression means and for this 85 purpose generally capacitors and/or chokes are utilized for short-circuiting or conducting away the high-frequency noise It is also known to combine capacitors and coils to form a filter which can affect the symmetri 90 cal noise component.

However, it is still more difficult to conduct away noise caused by inductive or capacitive action in the machine and wherein the noise appears as asymmetrical For this pur 95 pose, it is already known to use so-called x-y capacitors; that is, a circuit configuration of three capacitors of which x capacitor is connected between the lines of the source voltage; whereas the two y capacitors have 100 ( 19) 1,599,202 respective terminals connected to corresponding ones of the lines and with the other respective terminals of these y capacitors mutually connected at a common terminal.

This common terminal is then, in turn, connected to a mass such as the laminated stator stack of the field if this stack is electrically insulated with respect to metal parts touchable by the operator of the tool.

It is, however, not permitted to use such an x-y capacitor for short-circuiting noise on operator-accessible metal parts such as the gear housing of a double-insulated drill, since the danger exists that a capacitor can be defective and lead to a voltage breakdown from the voltage conducting parts to the operator-accessible metal parts In order to eliminate this danger, German and international safety requirements call for providing a minimal isolation between the voltageconducting parts and the operator-accessible metal parts According to these requirements a definite isolating spacing is required for maintaining a voltage break-down gap of a predetermined length as well as a minimal creep length Furthermore, the conditions for the leakage current and the resistance to high-voltage must be maintained.

Accordingly, up until now, it was impossible to bleed away high-frequency noise on operator-accessible metal parts or to short circuit the same in a simple manner without clashing with the safety standards.

In view of the foregoing, it is an object of the invention to provide, in an electric power tool, a means for enabling this noise voltage to be conducted away while, at the same time, enabling adequate electrical insulation to be provided between the voltage conducting parts and the operator-accessible parts.

The present invention provides a handheld, electric-powered tool including a suppressor-insulator member for conducting higher-frequency noise from a voltage-carrying portion of the tool to an exposed-metal part of the tool accessible to an operator, the suppressor-insulator member comprising:

two connecting plates connected to the voltage-carrying portion and the exposedmetal part respectively: and, an electrically-insulating structure disposed between said plates, said structure having a predetermined thickness and extending outwardly beyond the periphery of said connecting plates so as to cause said structure to define a creep-path between said plates of sufficient length to electrically insulate the exposed-metal part from the voltage carrying portion of the tool.

According to an example of the invention the electrically-insulating structure comprises a plate-like element of insulating material.

The plate-like element may define a creeppath between said connecting plates, which has a length at least four times greater than the thickness of the element: the plate-like element may, for example, have a thickness 70 of at least one ( 1) millimeter, and the said creep-path extends out over the surface of the element from the peripheral edge of one of the connecting plates to the peripheral edge of the other connecting plate, said 75 creep-path having a length of at least four ( 4) millimeters.

In an embodiment of the invention to be described herein, the suppressor-insulator member is, on the one hand, configured es 80 sentially in a similar manner to a capacitor but, on the other hand, meets the insulating standards for joining operator-accessible metal parts with parts which are already insulated once, for example, windings Further 85 more the suppressor-insulator member, in this embodiment, provides a capacitive effect so that a complete blockage for the lowfrequency supply voltage is provided while at the same time the high-frequency noise 90 can be led away over the suppressor-insulator member.

Should the suppressor-insulator member of a tool in accordance with the invention be utilized to connect such parts of the tool 95 which are not provided with electrical insulation (as for example the carbon-brush holder of an electric motor) with the operator-accessible metal parts, then in this instance too, the standards for a strength 100 ened isolation must be observed and the material thickness of the insulating plate should be at least 2 mm while the creeplength has a length of at least 8 mm.

When the electrically-insulating structure 105 comprises a plate-like element of insulating material then, in order that the suppressorq"-ulator member has a sufficient capacitance, the thickness of the material making up the plate-like element should be selected 110 so as to be as thin as possible consistent with providing the desired creep-path.

In order to prevent mechanical damage to the suppressor-insulator member and to increase further its isolating capability, the 115 insulating structure and the connecting plates can be cast into a body of insulating material.

By way of example, an illustrative embodiment of the invention will now be des 120 cribed with reference to the accompanying drawings, of which:FIG 1 is a schematic diagram illustrating the capacitance present in an electrical machine having operator accessible parts; 125 FIG 2 is a schematic diagram showing the portable tool of FIG 1 equipped with a suppressor-insulator member in accordance with the invention; FIG 3 illustrates an elevation view of the 130 3 1,599,202 3 suppressor-insulator member of the tool of FIG 2, and FIG 4 is a plan view of the suppressorinsulator member shown in FIG 3.

High-frequency noise is generated in a portable tool by the commutating action of the brushes and commutator of the drive motor of the tool This noise includes two primary components One of these components is a line to line component and is known as the symmetrical component of noise If it is not blocked, the symmetrical component would flow out onto the supply mains supplying the tool The other primary component is a so-called asymmetrical component which is always present in the tool.

The asymmetric component becomes a problem when an operator of the tool grasps the tool at a metal part thereof accessible to the operator because a path to ground is thereby established by the operator's body The asymmetrical noise path extends through the operator's body impedance to ground and returns back through the ground of the power supply.

The circuit is completed back to the tool through the supply mains The asymmetrical and symmetrical components of noise are disturbing to other consumers such as television sets supplied by the same supply mains feeding the tool.

In a double-insulated tool, wherein at least a portion of the housing grasped by the operator is made of metal and is ungrounded, there are inherent stray capacitances within the tool that provide paths for the noise generated by the brushes to reach the metal portion of the case grasped by the operator.

FIG 1 represents a double-insulated tool equipped with a conventional x-y capacitance arrangement for suppressing symmetrical and asymmetrical noise components.

The arc 1 represents the metal part of the tool accessible to the operator of the tool.

A motor armature 2 is supplied from a source such as a 110 volt, 60 Hertz power source 3 through supply mains 16 and 18 and brushes 4 The field coils 5 are also excited by the power source 3 The field coils 5 are wound on a laminated stator stack 6 Reference numerals 7 and 8 represent capacitance in the motor between the field coils 5 and the laminated field stack 6 which are inherent from the construction of the motor, and have been shown in brokenline form to emphasize that they are not actual components The capacitance 9 is likewise an inherent capacitance and exists between the motor armature 2 and the operator-accessible metal housing 1 The impedance 14 represents the impedence of the body of an operator grasping the tool at the housing 1 Reference numeral 110 designates the stray capacitance between the field 65 stack 6 and the armature 2.

The high-frequency noise generated by the motor brushes can be eliminated by providing an open-circuit path for the noise such as a large choke in series with the supply 70 Another alternative is to provide a highfrequency short-circuit path by connecting the capacitor 11 across the supply terminals of the motor and thereby provide a path wherein the symmetrical component can 75 circulate Capacitor 11 is known in the art as an x capacitor.

The capacitors 12 and 13 are known as y capacitors and are components added to provide a path for circulating the asym 80 metrical component of noise within the tool thereby blocking this component from reaching the supply mains The common node 15 of the y capacitors is connected directly to the field stack 6 The field stack is not acces 85 sible to the operator and can cause him no injury should one of the y capacitors fail.

The asymmetrical component circulates in the tool through the y capacitors, the field stack 6, the stray capacitance designated by 90 reference numerals 10, 7 and 8 and the field coils 5 The asymmetrical current will circulate within the tool in this manner so long as the operator has not grasped the tool at the exposed metal part 1 95 When the operator grasps the tool at the metal portion 1, a path is placed in parallel to the paths for the asymmetrical component within the tool Thus, an additional path is provided which circulates a portion of the 100 asymmetrical component away from the tool through capacitance 9, the metal part 1, the impedance 14 of the operator's body, through earth return to the power supply and from there through the supply mains 105 back to the tool The impedance of the additional path 14 is less than the impedance path within the tool from the common node through the field stack 6 and the set of inherent capacitance 7, 8 and 10 to the field 110 coils 5 Accordingly, the ground return portion of the asymmetrical component flowing in the supply mains 16 and 18 of the power source can be substantial.

It is this ground return current portion 115 of the asymmetrical component which is minimized in the tool embodying the invention and described hereinafter.

It was discovered that if the inherent capacitances 7, 8 and 10 could be by-passed, 120 the impedance of the circulating path within the tool would be much lower than that of the additional path introduced by the operator.

In the tool embodying the invention and 125 shown in Fig 2, a circuit arrangement is provided wherein a suppressor-insulator member 17 is connected from the common node 15 directly to the operator-accessible 1,599,202 1,599,202 metal part 1 In this way, the inherent stray capacitances 7, 8 and 10 are by-passed The inherent capacitance 9 is much larger than the set of inherent capacitances 7, 8 and 10 and impedance of the circulating path within the tool is far less than the impedance through the additional path thereby causing the ground return portion of the asymmetric component to be greatly reduced.

The suppresor-insulator member 17 must also provide adequate electrically insulating qualities in the event that one of the y components shorts and yet provide an impedance sufficiently low so that noise circulating via the ground return is kept low.

The suppressor-insulator member 17 shown in FIG 2 is illustrated in detail in FIGS 3 and 4 The suppressor-insulator member 17 is a capacitive device and is provided with two connecting leads 23 and 24 as well as two metal terminal plates 21 and 22 Between these two plates, an electricallyinsulating structure, comprising a plate 25 made of insulating material, is arranged.

The plate 25 is in the form of a circular disc and extends outwardly beyond the connecting plates The thickness of the material of the insulating plate 25 is so selected that a voltage break-down gap of the described magnitude is obtained, whereas, the lateral projection of the insulating material 25 beyond the terminal plate 21 and 22 together with the thickness of the insulating plate results in the prescribed creep length: that is, the distance between an edge location on the terminal plate 21 and an edge location on the terminal plate 22 is made up of the sum of the two lateral projecting distances of the insulating plate beyond the terminal plates and the thickness of the insulating plate itself.

In one particular form of the suppressor insulator member 17, the terminal plates 21 and 22 are made of copper and have a diameter of 5 mm; whereas, the insulating plate 25 has a thickness of 2 1 mm and a diameter of 12 mm For these dimensions, a ceramic can be utilized which is available as Type K 4000 ceramic from the Resista Company of 8300 Landshut, Federal Republic of Germany This configuration achieves a voltage break-down length of 2 1 mm and a minimal creep length of 9 1 mm so that the prescribed values for reinforced insulation are not only maintained but are exceeded Reinforced insulating denotes an unimproved functional insulation with such mechanical and electrical qualities that it provides the same degree of protection against electric shock as double insulation.

Referring to FIG 2, suppression is provided by an x-y capacitor whose x capacitor 11 is connected between lines 16 and 18 for short-circuiting hiah-frequency noise; whereas, the y capacitor 12 is conected to line 16 and the y capacitor 13 is connected to the line 18 Both y capacitors 12 and 13 are connected to each other as shown Highfrequency noise occurs at the operatoraccessible metal parts such as at the metal 70 housing 1 In order to circulate this highfrequency noise within the tool, the common node 15 of the y capacitors 12 and 13 is connected through connecting lead 23 to terminal plate 21 of the suppressor-insulator 75 member 17, whereas, the terminal plate 22 is connected via the connecting lead 24 to the metal housing 1 so that the insulating plate 25 lies between the x-y capacitor and the metal housing 1 The suppressor-insula 80 tor member 17 in the illustrated embodiment affords increased insulation and the safety standards for the insulation of operator-accessible metal parts with respect to voltage conducting parts can be main 85 tained and there is nonetheless a capacitive connection established between the x-y capacitor and the metal housing 1 in order to direct away high-frequency noise.

The insulating plate 25 is an insulating 90 material such as a ceramic having a very high dielectric constant which permits the 2 mm thickness to be maintained in combination with a capacity of around 700 p F with small dimensions If desired, the suppressor 95 insulation member comprising terminal plates 21, 22 and insulating plate 25 can be cast in insulating material (not shown) to prevent mechanical damage and increase the isolating capability 100 The suppressor-insulator member and circuit arrangement described above are also described in our co-pending Application No.

56/78 (Serial No 1599201) which claims a hand-held, electric-powered tool including 105 an arrangement for suppressing asymmetrical noise in the tool and for electrically insulating an electrically-conductive part within the tool from an exposed-metal part of the tool accessible to the operator, the 110 arrangement comprising:

filter means, for conducting the asymmetrical noise, connected across input current supply means of the tool; and a suppressor-insulator member for con 115 ducting said asymmetrical noise away from said filter means, said suppressor-insulator member including:

two electrodes and an electrically insulating structure disposed therebetween, a first 120.

one of said electrodes being connected into said filter means and the second one of said electrodes being connected to the exposedmetal part of the tool:

said insulating structure defining a creep 125 path between said electrodes of sufficient length to electrically insulate the exposedmetal part from the input-current supply means in the event of a short-circuit failure in said filter means 130.

1,599,202

Claims (8)

WHAT WE CLAIM IS: -

1 A hand-held, electric-powered tool including a suppressor-insulator member for conducting higher-frequency noise from a voltage-carrying portion of the tool to an exposed-metal part of the tool accessible to an operator, the suppressor-insulator member comprising:

two connecting plates connected to the voltage-carrying portion and the exposedmetal part respectively; and, an electrically-insulating structure disposed between said plates, said structure having a predetermined thickness and extending outwardly beyond the periphery of said connecting plates so as to cause said structure to define a creep-path between said plates of sufficient length to electrically insulate the exposed-metal part from the voltage carrying portion of the tool.

2 A tool as claimed in claim 1, in which the electrically-insulating structure comprises a plate-like element of insulating material.

3 A tool as claimed in claim 2, in which the two connecting plates are similar and the plate-like element extends outwardly beyond the periphery of both connecting plates around the whole periphery of both plates.

4 A tool as claimed in claim 2 or claim 3, in which the element defines a creep-path between said connecting plates, which has a length at least four times greater than the thickness of the element.

A tool as claimed in any one of claims 2 to 4, in which the said plate-like element has a thickness of at least one ( 1) millimeter, and the said creep-path extends out over the surface of the element from the peripheral edge of one of the connecting plates to the peripheral edge of the other connecting plate, said creep-path having a length of at least four ( 4) millimeters.

6 A tool as claimed in claim 5, wherein said thickness is at least two ( 2) millimeters and said creep-path has a length of at least eight ( 8) millimeters.

7 A tool as claimed in any one of the preceding claims, in which the said connecting plates and the insulating structure are cast into a body of insulating material.

8 A tool as claimed in any one of claims 1 to 7, in which the suppressor-insulator member is substantially as described herein with reference to, and as shown in, Fnigs 3 and 4 of the accompanying drawings.

ABEL & IMRAY, Chartered Patent Agents, Northumberland House, 303-306 High Holborn, London WC 1 V 7 LH.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon), Ltd -1981.

Published at The Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.