CA1118834A - Electromagnetic interference suppression system for electrical equipment - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to electromagnetic interference emission suppression systems and, more particularly, to such a system for suppressing such interference in small, electrical equipment such as portable hand tools. Differential mode conducted inter-ference is conventionally suppressed by connecting a capacitor between the A.C. power leads. One prior art technique for suppressing common mode interference is connecting a capacitor between each of the two A.C.
power leads and the ground lead to short out or by-pass the interference. The required capacitance value are empirically determined by the type of equipment and the suppression standard to be met. If the ground connec-tion is partly or wholly defective, the case becomes electrically energized and an equipment user experiences a low energy shock. If one of the common mode sup-pression capacitors shorts, the appliance is then elec-tively connected to the A.C. power leads and if the ground connection should be defective, an equipment user may experience a very serious, and occasionally fatal, shock According to the present invention, differen-tial mode interference is suppressed by connecting a capacitor across the power lines and common mode inter-ference is suppressed by an inductor inserted in the ground lead, preferably within the equipment housing.
The capacitor and inductor have substantially fixed values regardless of the type of equipment. In the preferred embodiment, inductor is constructed by winding a few turns of the ground lead itself through a high permeability, ferrite toroid positioned within the housing.

Description

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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
SYSTEM FOR ELECTRICAL EQUIPMENT
TECHNICAL FIELD
This invention relates to electromagnetic inter-ference emission suppression systems and, more particu-larly, to such a system especially adapted for economically and safely suppressing such~inter~erence in a relatively small, portable electrical equipment such as portable hand tools and the like.
:BACKGROUND OF ~ THE :PRIOR ART
Portable electrioal equipment, typically electric~
drills, electric mixers, hand-held saws and the like, generate electromagnetic~interference emlssion during the course of their operation~ principally due to com-mutatlon of t~he electric motors~used i~n~such equipment ;and, also, becaus~e of generally periodic electrical transients~generated~by the~type~of motor speed controls~ -employed. The interference may~ be~both~radiated~from the~equipment itself~and~also c~onducted~back through the A.C. power~to other~ equlpment with~the power lines serving to~further radiate the inter~erence. It is the conducted interference which appears to be most ob~ectionahle and which is the subj~ect o~ suppression regùlations for some applications in vari~ous areas of the world lncluding the United States.
Conducted electromagnetic inte~ference is~gener-ally considered to be generated in two distinct propa-gation modes. Differential mode eleotromagnetic interfer~nce is generated largely by motor commutation :, :
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current impulses or motor control transients and is propagated on the A.C. power leads between the motor and the impedance o~ the power source due to phase~and voltage differences between the leads. Commo~ mode interference~ on the other hand, is generat,ed between both A.C. power leads acting together with a common phase and the ground lead. Common mode electromagnetic interference problems are most signif'icant in~a fre-quency range between approximately l MHz and~30~MH in which the interference frequencies are~low enough to be conducted in-phase through the two powe~r leads toward the impedance o~the power source and returned through the ground lead to the case or housing o~ the equip~
ment, and high enough to be capacitively and ~nducti~vely coupled from the source of its inter~erence back to~the case which is connected~to the~ground lead. The elec~
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trical circuit for the interference;~is thereby com-~
pleted, allowin~g transmission;~of the~interference through the power lines.
Various suppres~s1on systems~are known ln the~
prior art for conneckion to the power terminals of ~ ;
electrical equlpment to suppress conduction of elec-tromagnetic interference;through the power leads that supply~ the equipment. ;However, most of these systems are~deficient due to introducing a shock hazard by connecting active~power leads to the equipment case through the suppression el~ements, or are complex and require mounting space which normally is available only in or around relatively largeg s~tationary equipment,~
or both~
Of course, creation of~the aforementioned shock hazard ls undesirable in either stationary or portable electric eqUipment. Unlike stationary equlpment, though, portable electric equipment is continually being~connected and disconnected by the user frorn its souroe of power.
It is believed that this situation is more likely to resuIt in a fefective ground connection to the equipment case and, therefore~ a greater likelihood of shock hazard.
To some extent, the equipment user determines whether - :, .
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81~34 a defective ground connection is created, for instance, in the case of single-phase equipment th:at employs a three-prong power plug and where only a two-prong ~;
power outlet is available. Under these circumstances,~
it is not ur.common ror the~equipment user to frustrate the special three-prong safety feature and operate the~
equipment by leaving the case ungrounded.
In addition, special problems exist with rè'gard to portable electric equipment in terms of space and cost considerations. I~eally, incorporation o~ an ;
electromagnetic suppression system into portable electric equipment should not detract from its portability~or ease ;
of use. Yet, stringent space limitations are inherent in ~
~;~ portable electric equipment, and render~inreasible the ~ ;
use of bulky filter elements or~systems. This problem~
can be especially severe when the original~manU~aoturer~
~of the;equipm~ent, apparent~ly, did not consider ~t~hat the equipment~ultimately~would have to be equipped~
with a suppression system to satis~y particular~
governmental suppress~lon regulations.~ Also, the~cost of a complex filterin~g system~can be~come a prohibitive factor when equipping relatively inexpensive portable ;~
equipment.
Differential mode~conducted interference is ; ~ 25 conventionally suppressed by~connecting a capacitor of approximately O.047 to 0.22 mfd between~thé A.C.
power leads. Di~erential mode interrerence is con-;~
duct~ed between the two power leads by the capacitor~and effectively shorted out~or`by-passed between the;'leads~
' 30 preventing flow down the lines~.~ The~actual value;~of~
~capacitance value nee~ded is t~pically empirically~
determined~ror each type o~ equipment and the sup-pre~sion standard to be met. ;~
One prior art technique for suppressing common mode interrer~nce is connectin~ a capacitor between each o~ the two A.C. power- leads and the ground lead to short out or by-pass the inter~erence. Again, the required capacitanoe values vary over a wide range (0.001-0.047 mfd) and are empirically determined by the : ~ ~

1~18~334 type of equipment, suppression standard to be met and the value of the dlfferential mode suppression capacitor.
Thus, with this suppression technique, the di~ferential and common mode suppression capacitor$ must usually be experimentally determined for each type of equipment for optimum results and many different capacitor~values are needed when providing suppression components for a number of different types o~ equi~pment.
These capacitors provide a relatively low radio ~requency impedance but a relatively high power ~requency impedance path ~or current between the power lines and the equipment case. However, lf the ground connection is partly or wholly defective the case becomes elec-trically ènergized and an equipment~uæer exp;eriences~ ~ ;
a low energy shock. Though not dangerous~,~the~shock ~ is continuous and uncomfortable and it is common;;~for ;~ ~ the user to respond bD removing the interference ~
suppression capacitors. The suppression of electro-magnetic interference is thUs defeated. An even~more ~; ~ 20 serious problem arises from a possible failure of one o~
the common mode suppression capacitors by shorting~
The applicance i8 then~effectively connected to the~A.C.
power leads and, if the~ground connection should be defective, an equipment user may experience a ~ery~serious,~
and occasionally~fatal shock, shock upon ¢ontact with~
the case and any ad~acent, substantially grounded object.
Another, and safer, prior art technique for, ~; ~ suppressing common mode interference is to insert~an inductance in the common mode transmission circui~t to effectively block conduction of the interference signals.
There is then no possible connection between the power lines and the case of a piece o~ equipment, eli~inating the shock hazard described above. However, due to the mu¢h higher cost and space requirements o~' inductors, as compared to capacitors, this suppression technique has heretofore been u~ed only for relatively sohisti-cated and costly equipment typically designed ~or inter-connection with other such equipment and has utllized speclalized components such as a du`al winding inductors .~ :

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- -~1~8834 for common insertion in both power line leads to avoid placing inductive inpedances in ground wires creating ground loops between interconnected pieces of ; equipment.
Thus, from the foregoing, it can be seen that there has long been a need for a technique for the suppression of electromagnetic interference in small, portable equipment which would not pose a shock ha~ard and yet is compact and inexpensive for incorporation in such equipment~ The present invention satisfies that need.
BRIEF SUMMARY OF THE IN~ENTION
The present invention provides an electro-magnetic interference suppression system, particularly for small, portable electric equipment and the like~, which is both sa~e ~rom shock hazard and i9 economlcal in its use. The suppression system includes a differen-tial mode suppression capacitor connected across the power line leads within the case or housing of the equipment with the capacitor having only one, or very ;20 few, capacitance ualues for a wide range o~ types of equipment. The syste, also includes a common mode suppression inductor in the ground lead o~ the equip-ment, preferably also within the case of the equipment.
Only one inducta~ce value is typlcally requlred for~a wide range of equipment so that~a single set of capaci-tance and inductance values may serve a great numb~er of different types Or electrical equipment.
In the presently preferred embodiment, the inductor may be economicàlly~ fabricated by winding a 3o relatively few turns of an existing ground lead around or through a high relative permeability magnetic material, such as a ferrite torold. Thus, a separate induckor and lts mounting are not needed. Addition-ally, a relatively ~ew turns through a toroid greatly reduces interference transmlssion through the para-s1~tic capacitance. Thus, the electromagnetic interference suppression system o~ the present invention may be in-corporated into electrlcal equipment economically and without shock hazard.
In accordance with one braad aspect, the invention : : , ~"' ' ~ - ' : ~
: -`' ~ ' - : , :L~18~334 -5a-relates to an electromagnetic interference suppression system for use with electrical equipment having a surrounding case, said case receiving a power cable comprising two alternating current power lines connected for operably energizing said equipment and insulated electrically to said case, and a ground lead connected electrlcally to sald case, said system consisting essen-tially of: a capacitor shunting said power lines; and an inductor connected in series with said ground lead, said inductor formed by a toroidal core through which said ground lead is wound a relatively few number of turns, said capacitor and said inductor both being posi-tioned within said case.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is an electrical diagrammatic view of the power line circuitry illustrating the differential and common mode interference sources and the sup-pression system of the present invention;
FIGURE 2 is a combined electrical schematlc and diagrammatic illustration of a presently preferred embodiment of the invention installed in an electric drill such as that shown in FIGURE 3;
FIGURE 3 1s a partially phantomed view of an electric drill with the components of a presently preferred embodiment of the invention shown installed;
and FIGURE 4 is a graphical presentation of the suppression efficiency of the~system of the invention.
DETAILED DESCRIPTION OF THE INVENTION
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Turning now to the drawings, particularly FIGURE 1 thereof, the electric circuit of a piece of electrical equipment is diagrammatically illustrated as being powered from a pair of A.C. power lines 10, 12 connected across a representative load RL (14). The power lines 10, 12 include a ground lead 16 whi.ch is conventlonally connected to a case 18 o~ the equipment. However, in the presently pre~erred embodiment of the suppression ~ystem o~ the invention, the ground lead 16 is first connected through an inductor LC (20) and then through ,~

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-5b-a line 22 to the case 18. In addition~ the suppression system includes a capacitor CD (24) connected across the power llnes 10, 12 as close to the load RL as possible.
In its operation the electrical equipmen~ produces differential and common mode electromagnetlc interference.
The di~ferential mode interference appears across the power lines 10, 12 and is represented in FIGURE 1 as a generalized, high frequency voltage source VD (26) across the lines. The common mode inter~erence generally appears between each power line - ~

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--6--10~ 12 an~ ~he~ casa lv Or t'Qe eC~Uipm~ill; and iS reprc-~ente~ UN3 1 b~ a palr o~ Penerall~ed capaci'clve impedailc~s Zc (28, 30) between the lines and the::case.
Additionally~ the interference may be induGtively coupled to the case 18 through a generalized inductive imped-ance Zm (31). r1'rom the case 18j the di~ferential mode voltage source Vc (28), and common voltages across the generalized i~pedances Z0 (28, 30) and inducti~e im-pedance Zm (31) are ef'f'ectively connected to the line 22.
With the dif'f'erential mode interference sup-pression capacitor CD (24) in place acros~ the power lines 10, 12, a relatively low capacitlve impedance a.t the f~requencies involved) the dif'ferential mode lnter-~:. 15 f'erence voltage source ~ (26) is e~f'ectively shorted ~ :
:out or by-passed preventing the interf~erence f'rom~propa-gating down the power lines~. Similarly, with the common : mode interf'erence suppresaion inductance ~c (20)~in ;~ place in the ground lead 16g the high inductive im- :
20 pedance a~ the~inter~erence f'requencies: involved ~ : :
creates a~.s~bstantially open c~ircuit:f'or the common :
mode inter~erence through thé:capacitive and inductive impedances Zc (28~ 30) and Zm ~31)~ respective:lyg pre~
venting propagation o~ the common mode inter~erence :
between either of the power:lines 10, 12 and the~gro:und :lead 16. ~ : :
~ It should be appreciated ~rom a consideratior :~ : o~ FIGURE 1 that there is, as a pract~cal matter,~:no impedance path, capacitive or otherwise between~:either :
o~ the power lines 10~ 12 and the ground lead ~or the usual power llne ~requenc~ Or 60 Hz. Thus, there ~s no possibllit~ o~ a shock hazard due ko the ~uppres~ien components themselves, The di~erentlal mode voltage source VD (26) and the voltages across the capacitlve 3~ anc~ inductance impedan¢es Zc (28J 30) and Z (31)~
respeckively) are o~ low potential and relatively high ~requency (1-30 M~Iæ) and pose no shock ha~ard them-~elves) only an inter~erence problem.
The v~lues o~ the di~erential mode suppression ~ :

capacitor CD (24) is empirically selected in accordance with the type of equip~ent and the suppre sion ~tandard to be met but it has been ~ound that one value will ~erve a number or di~erent modelæ o~ the ~a~e general ~ type o~ equipment. For example, one capacltor value ~
; may be utllized in a number ~ dif~erent sized electr~ic drill~ ~ the same general mechanical and ele¢trlcal ¢onfiguration. Thus empirical testing need only be performed once and only Gne value o~ capacitor need be ~tocked in order to suppres~ as desired any o~ a number o~ di~erent models of the tested type of equipment.
This re~ults in the saving ~ a great deal o~ te~ting~
time and elimlna~eR the need ~or stocking a large number ~ component values, reæulting in a cost ~aving.
Similarly, it has been found that common;mode~lnte~
~erence suppression with the suppress~ion inductor ~
(20) may be accomplished with a single~va~lue ind~ùc~tance for a number of di~erent models of the same type of~
equlpment, again as an example, dl~ferent sizes of 20 electric drill. An inductor havlng the requi~ed induc- ;
tance value may then be;deslgned to fit within the~case of a wlde variety~of sim~lar types 0f equipme~nt.
FIGURE 2 is a diagrammatlc representatlon of a partlcular type of equlpment~in whlch the presently ?5 pre~erred embodlment of~the ~nvention is utilized.
~he piece ~ e`qulpment Is a~portable electric drIll (FIGURE 3) includlng a motor 32 conventionally mounted ln a case 18. The ~ot~or 32 has a conventional rleld w~nding supplied with ourrenb from one of the powe`r~
lines 12 through a connecting l~e 34 to a ~leld~windtng termlnal 36. The motor 32 also has a conventional armature 38 supplied with power through a pair o~ !
brushes 40 and 42. The brush 40 i~ ¢onnected directly to the poWer llne 10 but the bru~h 42 is supplled by a oontrolled power llne 44 which is the output o~ a con-ventional speed control clrcult 46. ~he operation o~
the speed control clrcult 46 is diagrammatioally repre~ented by a tr~g~er ¢ontrol input 48.
Fo~ the con~iguration o~ the electri¢ drill, ~ ' - .
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the c1if~erential mo~e inter~ererlce suppresslon capacltor CD (24) l~ connected across the power line ,lO,~ 12 between the lines and the control circuit 46 in order to .~uppress di~ferentlal mode inte~erence generated both by the commutation and the armature 38 o~ the motor 32 and the lnterference generated by the opera-tion ~ the control clrcuit 46.
The common mode interference suppres~ion lnductor LC (20) ls connected ln tha ground lead l6 lO which is connected to the case 18 of;the~drill through the line 22. In the presentl~ preferred embodiment of the invention, the 3uppression inductor LC (20)~ is provided by a ~ew turns of' wire around or thr~ough a toroid 50 or other physioal con~lgurat~on o~ a high~
relative permeability. Whlle a separate inductor,~may ~;` be used~ it has been~found~that the needed inductance~
v~lue~ may be provided~by;cohstructing the induot~or by winding~the ground lead ~ itsel~ through the~tor~oid 50 be~ore attaching lt to ~the case ~ o~ the drill.
0 The physical slze of the~torold 50 required and the~
number o~ turns typical~ly~required to create the cor~
rect inductanoe are such~that the~oonstructed ind~uctor~
will normally;~fit within~the conrines of the open spaces~
` ; o~ the case ~o~ the drill~. Since the insulatlon'~on~
the ground lead 16 remains~in plaoe, no further~pecial insulation is required nor~are~mounting braokets~or~
the lnductor LC~(20) needed.~ Thusj khe required~,oommon interference suppre~sion~lnductor LC (20) may be ec~onomically constructed and ~nstalled.
30~ FI~URE 3~ illuætr~te~ the placing of the;di~
ferential modes o~ inter~erence ~uppre~ion capacitor 24 a~d the common mode inter~erence suppres~ion ln-ductor LC (20) within the ca~e l8 o~ the drill. The~
~uppressl~n capacitor CD (2L~ normally o~ rç}atively ~mall ~l~e and may be po~ltioned within the drill wlthin the vicinit~ o~ the brushes 40~ 1~2, The capacltor CD (2l~) is shown in phantom and without assocla~ed connecting clrcultry ~or clarity. The power line~
lO~ 12 are lllustrated as merely enterlng the dri~

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and no further circultry i3 ~hown. The ground lead 16 is shown in phantom as being wound through the toroid 50 a number of times and the ~ree end cf the line 22 being connected to a conventional screw ~erminal 52 on the case 18. A trigger 54 is shown operatively con-nected to a representative control circuit 46 which 1 shown without connectlng circuitry~ It can be seen that the size o~ the toroid 50 formlng the induotor LC (20) easily fits wi~hln the con~ines o~ the handle Or the drill, which i8 normally holl~w. It should be appreciated th~t the position o~ the toroid 50 shown in FIGURE 3 is representatlve onl~`and that the inductor LC (20) may be located anywhere within the case 18 of the drill depending upon its physical con~iguratlon.
It has been found that ~or a large variety of particular types of equipment~ such as the illus-~
trated electric dril}, a ~ingle set o~ capacitor and lnductor component values can be used so that the electromagnetic interference suppression can be eoonom-ically accomplished. For the illustrated electric drill, the presentl~- preferred embodiment o~ the lnvention includes a 0.1 m~d capacltor and an inductor LC (20~
formed by between 5 and 15 kuras of the ground lead 16 ~ through a toroid having a relative permeability of approxlmately 1,000 to 5,000. The exact number o~
turns is~normally not crltical and depends upon the size o~ the ground lead 16 and the siæe o~ the torold 50. It should be appreclated that the number o~ turns should remain reIatlvely low due to the fact that, increasing the number o~ turns also increases the parasltlc capacltance o~ the lnductor LC ~20) which permits passing o~ the common mode inter~erence through that parasitic capacitance and also increases co~ts.
Other type~ o~ equipment may reqUire di~erent aombina-tions o~ capacltance and inductance which should beempirlcally determined ~rom the type o~ equipment and the suppre~ion standard which must be met.
With respect to a partlcular suppression standard ~or which the presently preferred embodiment B g .

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of the invention was designedj FIGURE 4 illustrates a graph of electromagnetic int0r~erence energy vérsus a logarithmic frequency scale. ~ particular emission standard is shown b~r the llne 56 which approx~mates~a partlcular standard. A line 58 represents the emissions from a typIcal electrical drill which is not suppressed and a line 60 represents that same electric drill with the suppresslon system o~ the present invention in- -~skalled, It can be seen that the emission standard is easily met by the suppression syetem.
Whi:le a particular presently preferred embodl~
men~ o~ the inventlon has been described and illuetrated in dekail it should be apprec-lated that ~he eleckro~
, magnetic interference suppression system of the~preeent ;
inve~tion~ma~J be utilized ln a~wide variety o~eIeotrlcal e~quipment and that man~ variations and modifications of particular applioations ma~ bè utilized by t~hose~having~
ordinary skill in`the~ art. Therefore~ the~invention is~
not to be limited except by the ~ollowlng claims.

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Claims (6)

1. An electromagnetic interference suppression system for use with electrical equipment having a surrounding case, said case receiving a power cable comprising two alternating current power lines connected for operably energizing said equipment and insulated electrically from said case, and a ground lead connected electrically to said case, said system consisting essen-tially of: a capacitor shunting said power lines; and an inductor connected in series with said ground lead, said inductor formed by a toroidal core through which said ground lead is wound a relatively few number of turns, said capacitor and said inductor both being positioned within said case.

2. The electromagnetic interference sup-pression system defined in Claim 1, wherein: said toroidal core comprises a magnetic material having a high relative permeability.

3. The electromagnetic interference sup-pression system defined in Claim 2, wherein: said toroidal core has a permeability of between 1,000 and 5000.

4. An electromagnetic interference suppression system for use with electric equipment having a motor and a surrounding case, said case receiving a power cable comprising two alternating current power lines connected for operably energizing said equipment and insulated electrically from said case, and a ground lead connected electrically to said case, said system consisting essentially of: a capacitor shunting said power lines; and an inductor connected in series with said ground lead, said inductor comprising a toroid formed of a ferrite material through which said ground lead is wound between 5 and 15 turns, said capacitor and said inductor both positioned within said case.

5. The electromagnetic interference sup-pression system defined in Claim 4, wherein said toroid has a high relative permeability.

6. The electromagnetic interference sup-pression system defined in Claim 5, wherein said toroid has a relative permeability of between 1,000 and 5,000.