Note: Descriptions are shown in the official language in which they were submitted.
~ 3EJ3~L
:
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
:,
:
~ , : : '
jl
": ': ' . : ~: , ' ' ~,
, ~
33~
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~
~ ~ :
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
- :, .
,
.
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
--4--
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
.~ :
.
:
, ~ . . : , :, :
..
- -
~1~8834
--5--
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
_ . _ .
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
,~
~ ', ' '~
: ,
'~ ,
" ~ ~
383~
-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 - ~
:
: ~ : : :::
::
: :
' ~ :
,
.
,~ :
- -
334
"~..
--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
~ :
3834
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,
~ '
- .
- : .
~883~
. ~
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~
,,, , , : :
,
~-.:.
. : .:
": .: : : . ' ',
.. . - ~
~ 3 ~
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
.
l33~
:
--10--
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.
~::
~;:: : : : :
'
:: : ::
::
.
.
. :.- - - :
. - . ~ . . :
