1927 PMG Means of suppressing radio inductive interference
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Means of suppressing radio inductive interference
The obvious and only satisfactory method of suppressing radio
inductive interference which is caused by electrical apparatus which is
defective is to put this apparatus in good condition. The owners of such
electrical apparatus are always very pleased to have their attention drawn to
the fact that their apparatus is in need of repair, in having been found many
times that this radio interference is the first indication of a fault, which is not
attended to, may cause serious damage.
In cases where the electoral apparatus appears to be good mechanical
and electrical condition is very often possible to supply some means of
preventing electrical surges, originating in the apparatus from getting out to
the power line where they would radiato and caused radio interference.
These electrical surges have the property of passing through
condensers more readily than through inductances. The method employed
therefore, is preventing electrical surges from travelling along the power lines
and thus causing radio interference, is to provide a path to ground in the form
of a condensor to filter or drain off this surge. In order to make this filter more
effective it is often advisable to introduce between the lines and the source of
the disturbance a trap, which will make the passage of the surge more
difficult. This trap preferably takes the form of choko coil, which consists of a
number of turns of wire of sufficient size to carry the required current without
overheating and sufficient insulation to withstand the voltage on the line.
These installations should be approved by the local electrical inspector to
ensure that there are no fire or accident hazards introduced by the installation.
It is important in the design of those choke coils that they should have
low distributed capacity in order to prevent the electrical surge passing
through the choke coil by means of this capacity.
CHOKE COIL.
A type of choke coil recommended for cases where the current is less
than 3 amperes is constructed as follows. It consists of 100 turns of No. 18
double cotton- covered wire in a single layer wound on a fibre tube 3 inches in
diameter, 1/8 inch thick. Leads are soldered to each end and make from No.
14 flexible rubber- insulated braid- covered copper wire, making one turn
around the tube them through a hole in the tube and extending through the
inside of the tube a length of one foot. The burrs are carefully removed from a
hole in the fibre tube so as not to cut the insulation. End rings or filters are
mounted at the end of the winding to build up the same diameter as the
outside diameter of the winding. These may be made of taps or a suitable
size of fibre tube. The whole coil is then covered with five layers of Empire
cloth and then taped with black friction tape and painted with insulation
varnish. Mounting lugs made of fibre are then attached to each end of the
coil.
This coil should be mounted on board covered with asbestos 1/8 inch
thick and the leads soldered and thoroughly taped according to standard
wiring practices.
Instructions for mounting condensers.
Condensers which will stand a test voltage of 1,000 volts d.c. may be
connected across an alternating current or direct current circuit of 250 volts or
less. On circuits which are protected by fuses is of no more than 15 amperes
capacity, no additional fuses is required for the condenser. On circuits
protected by fuses of greater than 15 amperes capacity a separate out
cut base and a small fuse (of approved type) not exceeding 15 amperes,
must be installed between the condenser and each unground power wire.
Where condensers are not installed in metal boxes and are to be placed on
wooden surfaces they should be mounted on pads of asbestos at least 1/8
inch thick and these pads should be sufficiently large extent beyond the
clamps used for holding the condensor in place.
Where condensers are to be used on 550 volt circuits two condensors
of the approved type must be connected in series between the lines and the
common point may be grounded. In such installations the condensors are to
be protected by 600 volts fuses no greater than 10 amperes in each live line,
and both the condensors and fuses are to be enclosed in a grounded metal
case where are the condensors are to be connected to two-phase or three-
phase circuits no greater than 600 volts, one condenser may be connected
from each live line to ground and installed with fuses in boxes as stated
above.
As these condensors contain wax. they should not be placed where
they may be subject to excessive heat.
Condensors when connected to a circuit as stated above have no
objectionable effect on the circuit, all the operation of an electrical apparatus
and their power consumption is negligible.
Treatment of typical cases
In most cases it is possible to make a slight changes in the connections
of the electrical apparatus causing the surge in order to use some existing
apparatus as a choke coil and thus prevent the necessary of adding additional
choke calls to the system.
A series of commutator motor causing a surge by sparking at the
brushes may have its leads reversed to reduce the radio interference when
one wire is grounded, radio interference from such a motor is sometimes
reduced by reversing the leads supplying the motor, so that one of the
brushes is connected to the ground side of the line and the field coil is
connected to the live side of the line. In this case the field coil is used as a
choke. It may also be necessary to place a condensor of one or two
Microfarads capacity across the brushes. The suggested connections are
shown in fig. 1.
In cases where neither side of the line is grounded a choke may be
inserted on the line connected directly to one of the bushes while the field coil
may act as a choke in the other line. In this case it is recommended to use
two 2-microfarad condensors in series and ground the middle point according
to the diagram. The suggested connections are shown in fig. 2.
Iin the case of shunt connected motors, and also in other cases where it
may not be convenient to make connections with the brushes of a motor the
condensor may be placed across the line is near as possible to the motor and
between the motor and the choke coil or choke coils.
The live side of a low voltage lighting circuit may be determined by
means of a test lamp connected from ground first to one wire the then to the
other. The lamp will light when connected from the live line to ground.
Battery charger of the vibrator type may be prevented from causing
radio interference by connecting the condensor of 1/2 microfarad capacity
across the vibrating contacts. In the case of battery charger is useless to put
condensors across the mains, as it is necessary to make use of the choke
consisting of the wire and coil within the battery charger, to prevent this surge
from getting out on the line.
Flashing electrical signs may usually be prevented from causing radio
interference by connecting condensors of from 1 to 2 microfarads capacity
across the contacts of the circuit breaker. It is sometimes necessary, however
to add a choke coil at the line side of the circuit breaker and also connect the
condenser across the contacts of the circuit breaker. As the radio interference
from such sources depends upon the condition of installation it is necessary to
make these few experiments are suggested above in each case to reduce the
radio interference most effectively.
Sometimes the radio interference from a flashing sign installation is
caused by sparking at the commutator of a motor drives the flasher. This may
be readily determined by the nature of the sound in the radio receiver and
may be remedied by the method described for dealing with interference from
commutator motors.
Internal combustion engines ignition systems make calls radio
interference, but this is usually of a very local nature. This however may be
considerably reduced by making the leads from the magneto or spark coil to
the engine as short as possible and running them in a grounded shield such
as metal conduit or lead covered cable. The frame of the engine magneto and
all shields should be thoroughly grounded.
Small rotary converters such as those used to energise some forms of
medical apparatus, sometimes cause radio inductive interference by
producing a surge, which travels out both on the alternating and direct current
line. In such cases it may be necessary to introduce choke coils into the
alternating current lines and put condensors across the lines between the
choke coil and the converter. Before putting these choke coils in the
alternating current lines it is recommended to try the effect of the condensors
across the brushes as described previously from the case of commutator
motors.
Electric ozonators which are used for purifying the air in large buildings
and for bleaching purposes in flour mills sometimes cause radio inductive
interference by producing a surge which travels along the primary line
supplying the ozonator.
This interference by easily be eliminated by connecting two choke coils
of the cylindrical type previously described one in each of the low voltage lines
placed near as possible to the ozonator.
The suggestions of the circular may be summarised into following
general rule:-
(a) Put condensers across the actual source of disturbance in the case
of motors, for the condensors across the brushes; in the case of
make and break contacts, put the condensors across the contacts
themselves.
(b) When choke coils are used in conjunction with condensors put the
choke's between the apparatus and the main supply and put the
condensors between the apparatus and the chokes.
© Where the power supply system has one of its wires grounded at the
power station or other supply point, put choke coils in the
ungrounded or live wire and put the condensors between that wire and
ground.
(d) Where the supply system is ungrounded, put a choke coil in each
supply wire and use two condensors in series across the mains with
the centre point between the two condensors connected to ground.
(e) If the apparatus already contains electro- magnetic windings,
endeavour to make those windings serve also as anti- interference
choke coils.
(f) Always inserted fuses between the condensors and the mains.
Recent investigations by this department indicates that radio
interference is almost a common in those country centres where the local
electric supply is a direct- current system. Experiments have shown that most
of the trouble is due to one or more of the following causes. The remedies are
given for each particular case.
(a) Sparking commutators due to accumulation of carbon dust between
the commutator segment, the dust accumulating on account of the
mica insulation being below the commutator segments.
Remedy:- Re-insulate the segments.
(b) Low insulation between the commutator and the machine shatft.
Remedy:- Re-insulate with good quality micanite.
© Carbon brush use of unsatisfactory grade.
Remedy:- Use a hard grade carbon brush of good Quality.
(d) If leather belting of a copper-laced type is used then continuous
spark discharges may occur between the copper lacing and the
machine pulley.
Remedy:- Use camel hair belting or effectively earth the copper- lacing
by means of arm brush contact, this brush contact may be improvised
for the test by fanning out a piece of 7/20 copper wire and placed in
such a position as to give light contact on the copper lacing.
Should any of the means mentioned in this communication fail to
overcome the difficulty the Department’s officers will be only too glad to give
any further information regarding the case that may warrant special
investigation.